Exhibit 99.1

Filed by First Interstate BancSystem, Inc.

pursuant to Rule 425 under the Securities Act of 1933

and deemed filed pursuant to Rule 14a-12

under the Securities Exchange Act of 1934

Subject Company: Great Western Bancorp, Inc.

Commission File No. 001-36688

Date: October 26, 2021

For Immediate Release

First Interstate BancSystem, Inc. Reports Third Quarter Earnings

Billings, MT - October 26, 2021 - First Interstate BancSystem, Inc. (NASDAQ: FIBK) today reported financial results for the third quarter of 2021. For the quarter, the Company reported net income of $47.1 million, or $0.76 per share, which compares to net income of $42.5 million, or $0.69 per share, for the second quarter of 2021, and $48.3 million, or $0.76 per share, for the third quarter of 2020.

The third quarter 2021 earnings include pre-tax acquisition costs of $6.6 million related to the pending acquisition of Great Western Bancorp, Inc. (“Great Western”), the parent company of Great Western Bank (“GWB”), and accrued expenses of $1.2 million related to a settlement of a legal claim that was resolved in the fourth quarter. Comparable quarters do not include these third quarter 2021 costs, which reduced earnings by $0.10 per common share for the third quarter.

HIGHLIGHTS

•Entered into a definitive agreement to acquire all of the outstanding stock of Great Western, the parent company of GWB.

•Loans held for investment decreased $212.2 million, or 2.2%, to $9,622.5 million as of September 30, 2021, from $9,834.7 million as of June 30, 2021. This decrease included Payroll Protection Program (PPP) loans, net of deferred fees, which declined $265.4 million, or 48.6%, during the period to $280.5 million, from $545.9 million as of June 30, 2021. Excluding PPP loan activity, loans held for investment increased $53.2 million, or a 2.4% annualized growth rate, as of September 30, 2021 compared to June 30, 2021.

•Total deposits increased $441.6 million, or 2.8%, to $16,007.3 million as of September 30, 2021 from $15,565.7 million as of June 30, 2021, resulting in a 11.1% annualized growth rate in deposits. Non-interest bearing deposits increased $201.1 million, or a 14.7% annualized growth rate, to $5,617.9 million as of September 30, 2021, from $5,416.8 million as of June 30, 2021.

•Asset quality further improved in the third quarter of 2021. Criticized loans were $251.2 million, as of September 30, 2021, resulting in a $22.2 million, or 8.1%, decrease from $273.4 million as of June 30, 2021, and a decrease of $128.1 million, or 33.8%, from $379.3 million as of September 30, 2020.

•Net charge-offs decreased $0.5 million, or 45.5%, to $0.6 million, or an annualized 0.02% of average loans outstanding, as of September 30, 2021, from $1.1 million, or an annualized 0.04% of average loans outstanding, as of June 30, 2021, and decreased $4.0 million, or 87.0%, from $4.6 million, or an annualized 0.18% of average loans outstanding, as of September 30, 2020.

•Book value per common share increased $0.22 per common share, or 0.7%, to $31.89 as of September 30, 2021, compared to $31.67 as of June 30, 2021, and $31.33 as of September 30, 2020.

•Tangible book value per common share increased 1.3%, to $20.75, as of September 30, 2021, compared to $20.49 as of June 30, 2021, and increased 2.8%, from $20.19 as of September 30, 2020.

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“We continued to deliver strong financial results driven by quality balance sheet growth, higher revenue, and disciplined expense management,” said Kevin P. Riley, President and Chief Executive Officer of First Interstate BancSystem, Inc. “While our loan pipeline continues to be healthy, we are seeing more projects and planned investments among our commercial clients being delayed due to supply chain disruptions and the tight labor market, which impacted our total loan growth in the third quarter. Despite these delays, economic activity remains very healthy in our markets. This is resulting in continued strong deposit inflows, which enabled us to continue growing interest earning assets and net interest income, while also having a positive impact on all of our major areas of non-interest income.

“During the third quarter, we also entered into a transformative merger agreement with Great Western Bancorp and the integration planning is proceeding well. The combination of our two highly compatible organizations will enable us to increase scale and efficiencies, improve our diversification, and provide exposure to many attractive markets that are experiencing strong growth. In addition to significant earnings accretion and improved returns resulting from the synergies projected from the merger, we believe the addition of Great Western Bancorp will enable us to generate a higher level of organic growth over the long-term than we have historically experienced, which should improve our ability to consistently enhance the value of our franchise in the years to come,” said Mr. Riley.

DIVIDEND DECLARATION

On October 25, 2021, the Company’s board of directors declared a dividend of $0.41 per common share, payable on November 18, 2021, to common stockholders of record as of November 8, 2021. The dividend equates to a 3.9% annualized yield based on the $42.37 per share average closing price of the Company’s common stock as reported on NASDAQ during the third quarter of 2021.

PENDING ACQUISITION

On September 16, 2021, the Company entered into a definitive agreement (“Agreement”) to acquire all of the outstanding stock of Great Western, the parent company of GWB, a Sioux Falls, South Dakota based community bank with 174 banking offices across Arizona, Colorado, Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, and South Dakota. Under the terms of the Agreement, each outstanding share of Great Western will convert into the right to receive 0.8425 shares of the Company's Class A common stock in an all-stock transaction valued at approximately $2.0 billion in aggregate based on the closing price of the Company’s Class A common stock on September 15, 2021. Upon completion of the merger, which is subject to regulatory and shareholder approvals and expected to close during the first quarter of 2022, the Company will become a regional community bank with over $32.0 billion in total assets (based on June 30, 2021 financial information) with a geographic footprint spanning 14 States.

NET INTEREST INCOME

Net interest income increased $8.1 million, or 6.8%, to $126.9 million, during the third quarter of 2021, compared to $118.8 million during the second quarter of 2021, an increase of $1.7 million attributable to organic balance sheet growth and an increase of $6.4 million in PPP loan income. Net interest income increased $3.9 million, or 3.2%, during the third quarter of 2021, from $123.0 million during the third quarter of 2020.

•The Company earned a total of $14.2 million of interest income, including loan fees, on PPP loans with average balances of $462.1 million during the third quarter of 2021 as compared to $7.8 million of interest income, including loan fees, on PPP loans with average balances of $750.4 million during the second quarter of 2021 and $10.6 million of interest income, including loan fees, on PPP loans with average balances of $1,161.8 million during the third quarter of 2020. The Company had $13.1 million of unearned deferred fees related to PPP loans accrued as of September 30, 2021.

•Interest accretion attributable to the fair valuation of acquired loans from previous acquisitions contributed $2.3 million to net interest income during the third quarter of 2021, of which approximately $1.4 million was related to early payoffs. This compares to interest accretion of $2.5 million in net interest income during the second quarter of 2021, of which approximately $1.4 million was related to early payoffs, and $3.2 million in net interest income during the third quarter of 2020, of which approximately $1.4 million was related to early payoffs.

The net interest margin ratio was 2.91% for the third quarter of 2021 compared to 2.82% reported during the second quarter of 2021 and 3.29% during the third quarter of 2020. Excluding PPP income, on a linked quarter basis the net interest margin declined 10 basis points primarily as a result of a shift in mix of earning assets toward investment securities at modestly lower yields. Year-over-year, the decrease in net interest margin was the result of a reduction in yields on earning assets due to the current low interest rate environment and a shift in mix of earning assets toward both investment securities and higher cash balances, the impact of which was partially offset by lower deposit costs.

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PROVISION FOR (REDUCTION OF) CREDIT LOSSES

The Company had no provision for credit losses during the second quarter of 2021 or the third quarter of 2021, compared to a provision for credit losses of $5.2 million during the third quarter of 2020.

The allowance for credit losses is updated quarterly based on the results of the current economic outlook for our clients and the market generally and includes the impact of net charge-offs of $0.6 million, or an annualized 0.02% of average loans outstanding, for the third quarter of 2021, compared to net charge-offs of $1.1 million, or an annualized 0.04% of average loans outstanding, for the second quarter of 2021, and net charge-offs of $4.6 million, or an annualized 0.18% of average loans outstanding, for the third quarter of 2020.

The Company’s allowance for loan losses as a percentage of period-end loans was 1.40%, 1.38%, and 1.43% at September 30, 2021, June 30, 2021, and September 30, 2020, respectively. Coverage of non-performing loans increased to 384.90% at September 30, 2021, compared to 380.62% at June 30, 2021 and 267.46% at September 30, 2020, primarily as a result of lower levels of non-performing loans.

While the allowance for credit losses on loans held for investment of 1.40% includes PPP loan balances, the allowance for credit losses does not include a reserve on the PPP loans which are 100% guaranteed by the Small Business Administration. The allowance for credit losses on loans as a percentage of period-end loans held for investment would have been 5 basis points higher had the PPP loan balances been excluded at September 30, 2021.

NON-INTEREST INCOME

Total non-interest income increased $4.4 million, or 12.5%, to $39.7 million during the third quarter of 2021, as compared to $35.3 million during the second quarter of 2021, primarily driven by an increase in mortgage banking and payment services revenues. Total non-interest income decreased $5.0 million, or 11.2%, from $44.7 million during the third quarter of 2020, driven by other service charges, commissions, and fees and mortgage banking revenues.

Payment services revenues increased $0.8 million, or 7.0%, to $12.2 million during the third quarter of 2021, when compared to the $11.4 million earned during the second quarter of 2021. Payment services revenues increased $1.7 million, or 16.2%, during the third quarter of 2021, when compared to the $10.5 million earned during the third quarter of 2020. These increases are mainly the result of higher business credit card volume during the third quarter of 2021 as compared to the second quarter of 2021 and the third quarter of 2020.

Mortgage banking revenues increased $2.0 million, or 20.8%, to $11.6 million during the third quarter of 2021, as compared to $9.6 million during the second quarter of 2021. The increase was driven by gains realized on the sale of mortgages. Despite a decline in originations volume relative to the second quarter of 2021, the Company returned to a more historical mix of volume sold on the secondary market, which when combined with wider sale margins drove increasing revenues. Mortgage banking revenues decreased $2.7 million, or 18.9%, during the third quarter of 2021 from $14.3 million during the third quarter of 2020. The decrease was primarily driven by a decline in total origination volume compared to the third quarter of 2020. During the third quarter of 2021, loans originated for home purchases accounted for approximately 65.8% of loan production, as compared to 60.2% during the second quarter of 2021 and 48.6% during the third quarter of 2020.

Other service charges, commissions, and fees decreased $0.2 million, or 12.5%, to $1.4 million during the third quarter of 2021, when compared to $1.6 million during the second quarter of 2021, and decreased $3.6 million, or 72.0%, during the third quarter of 2021 from $5.0 million during the third quarter of 2020. This decrease from the third quarter of 2020 is primarily due to a decrease in swap fee revenues.

Other income increased $0.7 million, or 26.9%, to $3.3 million during the third quarter of 2021, as compared to $2.6 million during the second quarter of 2021. Other income decreased $1.3 million, or 28.3%, during the third quarter of 2021 from $4.6 million during the third quarter of 2020, partially due to a decrease in life insurance benefits.

NON-INTEREST EXPENSE

Non-interest expense increased $6.9 million, or 7.0%, to $105.9 million during the third quarter of 2021, as compared to $99.0 million during the second quarter of 2021 and increased $6.4 million, or 6.4%, from $99.5 million during the third quarter of 2020, primarily due to $6.6 million in acquisition related costs connected to the pending acquisition of GWB and $1.2 million in accrued expenses related to the settlement of a legal claim. Exclusive of acquisition and settlement related expenses, non-interest expense decreased to $98.1 million during the third quarter of 2021.

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Salaries and wages expenses increased $0.4 million, or 1.0%, to $42.0 million during the third quarter of 2021, compared to $41.6 million during the second quarter of 2021 and decreased $4.0 million, or 8.7%, from $46.0 million in the third quarter of 2020. The decrease from the third quarter of 2020 was primarily the result of lower commission expenses and lower incentive-based payment accruals during the third quarter of 2021.

Employee benefit expenses decreased $1.8 million, or 12.2%, to $12.9 million during the third quarter of 2021, compared to the $14.7 million incurred during the second quarter of 2021, primarily due to lower long-term incentives. Employee benefit expenses increased $1.1 million, or 9.3%, from $11.8 million during the third quarter of 2020, primarily due to an increase in health insurance costs.

Other expenses increased $1.2 million, or 4.1%, to $30.2 million during the third quarter of 2021, as compared to $29.0 million during the second quarter of 2021, primarily due to an accrual for a settlement of a legal claim of $1.2 million. Other expenses increased $2.5 million, or 9.0%, during the third quarter of 2021 from $27.7 million during the third quarter of 2020. The increase from the third quarter of 2020 is primarily due to an accrual for a settlement of a legal claim and increased loan expenses during the third quarter of 2021.

BALANCE SHEET

Total assets increased $431.7 million, or 2.3%, to $19,372.2 million as of September 30, 2021, from $18,940.5 million as of June 30, 2021, primarily as a result of higher levels of deposits which were deployed primarily into the investment securities portfolio. Total assets increased $2,302.7 million, or 13.5%, from $17,069.5 million as of September 30, 2020. The increase from the comparable prior year period was primarily a result of higher levels of deposits, which increased 15.3%, resulting in higher levels of cash and cash equivalents and investment securities.

Investment securities increased $378.4 million, or 6.7%, to $6,021.7 million as of September 30, 2021, from $5,643.3 million as of June 30, 2021. On August 2, 2021, the Company invested $200.0 million in seven-year U.S. Treasuries at 99 basis points, while simultaneously entering into a three-year forward starting, four-year pay-fixed interest rate swap arrangement on $200.0 million in notional amount. Beginning on August 31, 2024, the Company will begin receiving the effective federal funds rate, and will pay 1.22% interest on such funds. Within the securities portfolio, available-for-sale securities increased $445.8 million, or 11.3%, and held-to-maturity securities decreased $67.4 million, or 4.0%, during the third quarter of 2021. Investment securities increased $2,513.2 million, or 71.6%, from $3,508.5 million as of September 30, 2020.

Mortgage loans held for sale decreased $6.3 million, or 12.9%, to $42.5 million as of September 30, 2021, from $48.8 million as of June 30, 2021, primarily as a result of a decrease in originations of mortgage loans held for sale related to lower refinance activity. Mortgage loans held for sale decreased $59.5 million, or 58.3%, as of September 30, 2021, from $102.0 million as of September 30, 2020, primarily as a result of a decrease in originations of mortgage loans held for sale related to lower refinance activity, which was elevated in 2020, as a result of the low interest rate environment.

Loans held for investment decreased $212.2 million, or 2.2%, to $9,622.5 million as of September 30, 2021, from $9,834.7 million as of June 30, 2021, and decreased $529.7 million, or 5.2%, from $10,152.2 million as of September 30, 2020. Loans held for investment included originated PPP loans, net of deferred fees, of $280.5 million, $545.9 million, and $1.1 billion as of September 30, 2021, June 30, 2021, and September 30, 2020, respectively. Excluding the impact of PPP loans, loans held for investment as of September 30, 2021, increased from June 30, 2021, primarily due to an increase in the real estate loan portfolio that was partially offset by a decline in consumer loans, and increased from September 30, 2020 due to increases in the real estate loan portfolios that were offset by declines in commercial, consumer, and agriculture loans.

Total real estate loans increased $95.6 million, or 1.4%, to $6,806.8 million as of September 30, 2021, from $6,711.2 million as of June 30, 2021, primarily driven by increases in commercial loans of $129.8 million, or 3.5%, residential construction loans of $4.9 million, or 1.9%, and agricultural loans of $6.4 million, or 2.9%. These increases were offset by a decrease in residential loans of $22.8 million, or 1.4%, commercial construction loans of $21.8 million, or 3.4%, and land acquisition and development construction loans of $0.9 million, or 0.3%.

Total real estate loans increased $543.5 million, or 8.7%, from September 30, 2020. Growth within the real estate loan portfolio is attributable to increases in residential loans of $243.7 million, or 18.6%, commercial loans of $192.3 million, or 5.2%, commercial construction loans of $79.4 million, or 15.0%, residential construction loans of $40.5 million, or 17.8%, and agricultural loans of $2.2 million, or 1.0%. Growth was offset by a decrease in land acquisition and development construction loans of $14.6 million, or 5.3%.

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Total consumer loans decreased $19.1 million, or 2.0%, to $953.8 million as of September 30, 2021, from $972.9 million as of June 30, 2021, with the decrease occurring across all consumer loan portfolios. Indirect loans decreased $16.9 million, or 2.2%, direct loans decreased $1.9 million, or 1.4%, and credit card loans decreased $0.3 million, or 0.5%, as of September 30, 2021 from June 30, 2021. Total consumer loans decreased $91.0 million, or 8.7%, from $1,044.8 million as of September 30, 2020 across all consumer loan portfolios. Indirect loans decreased $56.0 million, or 6.9%, direct loans decreased $29.2 million, or 18.0%, and credit card loans decreased $5.8 million, or 8.3%, as of September 30, 2021 from September 30, 2020.

Commercial loans decreased $290.7 million, or 14.8%, to $1,668.7 million as of September 30, 2021, from $1,959.4 million as of June 30, 2021. Commercial loans included $293.4 million of PPP loans as of September 30, 2021. During the third quarter of 2021, $278.6 million of PPP loans were forgiven by the Small Business Administration. Net of the impact of PPP loans, commercial loans decreased $12.2 million, or 0.9%, from June 30, 2021. Commercial loans decreased $930.9 million, or 35.8%, from $2,599.6 million as of September 30, 2020. The decrease from September 30, 2020 is primarily due to a decrease of $870.5 million of PPP loans and pay-downs within the portfolio.

Agricultural operating loans decreased $5.3 million, or 2.4%, to $212.4 million as of September 30, 2021, from $217.7 million as of June 30, 2021. Agricultural operating loans decreased $62.3 million, or 22.7%, from $274.7 million as of September 30, 2020, primarily due to client payoffs and pay-downs within the portfolio.

Other real estate owned increased $0.3 million, or 15.0%, to $2.3 million as of September 30, 2021, from $2.0 million as of June 30, 2021. Other real estate owned decreased $3.4 million, or 59.6%, as of September 30, 2021, from $5.7 million as of September 30, 2020, as a result of the disposition of properties in the normal course of business.

Total deposits increased $441.6 million, or 2.8%, to $16,007.3 million as of September 30, 2021, from $15,565.7 million as of June 30, 2021 and increased $2,124.9 million, or 15.3%, from $13,882.4 million as of September 30, 2020, primarily related to increases in non-interest-bearing business deposits and interest-bearing demand and savings deposits. These increases were partially offset by decreases in interest-bearing time deposits.

Securities sold under repurchase agreements decreased $31.2 million, or 3.0%, to $1,007.5 million as of September 30, 2021, from $1,038.7 million as of June 30, 2021, and increased $187.2 million, or 22.8%, as of September 30, 2021, from $820.3 million as of September 30, 2020. Fluctuations in repurchase agreement balances correspond with fluctuations in the liquidity of the Company’s clients.

Other liabilities increased $7.4 million, or 4.5%, to $173.2 million as of September 30, 2021, from $165.8 million as of June 30, 2021, primarily due to timing of accrued expenses and payables offset by a decrease in the deferred tax liability. Year-over-year, other liabilities decreased $16.6 million, or 8.7%, as of September 30, 2021, from $189.8 million as of September 30, 2020, primarily due to a decrease of $12.4 million in the deferred tax liability.

The loans held for investment to deposit ratio decreased to 60.1%, as of September 30, 2021, compared to 63.2% and 73.1% as of June 30, 2021 and September 30, 2020, respectively.

The Company is considered to be “well-capitalized” as of September 30, 2021, having exceeded all regulatory capital adequacy requirements. During the third quarter of 2021, the Company paid regular common stock dividends of approximately $25.4 million, or $0.41 per share, and did not make any share repurchases of common stock pursuant to its previously announced stock repurchase program.

CREDIT QUALITY

As of September 30, 2021, non-performing assets decreased $0.2 million, or 0.5%, to $37.4 million, compared to $37.6 million as of June 30, 2021, primarily due to a decrease in non-accrual loans of $0.5 million, or 1.6%, offset by an increase in other real estate owned of $0.3 million, or 15.0%.

Criticized loans decreased $22.2 million, or 8.1%, to $251.2 million as of September 30, 2021, from $273.4 million as of June 30, 2021, driven primarily by upgrades and loan payoffs in the commercial loan and commercial real estate loan portfolios. As of September 30, 2021, criticized loans decreased $128.1 million from $379.3 million as of September 30, 2020.

Net loan charge-offs decreased $0.5 million, or 45.5%, to $0.6 million during the third quarter of 2021 as compared to $1.1 million during the second quarter of 2021. The net loan charge-offs in the third quarter of 2021 were composed of charge-offs of $2.1 million and recoveries of $1.5 million. Net loan charge-offs during the third quarter of 2020 were $4.6 million.

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NON-GAAP FINANCIAL MEASURES

In addition to results presented in accordance with accounting principles generally accepted in the United States of America, or GAAP, this press release contains the following non-GAAP financial measures that management uses to evaluate our performance relative to our capital adequacy standards: (i) tangible common stockholders’ equity; (ii) tangible assets; (iii) tangible book value per common share; (iv) tangible common stockholders’ equity to tangible assets; (v) average tangible common stockholders’ equity; and (vi) return on average tangible common stockholders’ equity. Tangible common stockholders’ equity is calculated as total common stockholders’ equity less goodwill and other intangible assets (excluding mortgage servicing rights). Tangible assets are calculated as total assets less goodwill and other intangible assets (excluding mortgage servicing rights). Tangible book value per common share is calculated as tangible common stockholders’ equity divided by common shares outstanding. Tangible common stockholders’ equity to tangible assets is calculated as tangible common stockholders’ equity divided by tangible assets. Average tangible common stockholders’ equity is calculated as average stockholders’ equity less average goodwill and other intangible assets (excluding mortgage servicing rights). Return on average tangible common stockholders’ equity is calculated as net income available to common shareholders divided by average tangible common stockholders’ equity. These non-GAAP financial measures may not be comparable to similarly titled measures reported by other companies because other companies may not calculate these non-GAAP measures in the same manner. They also should not be considered in isolation or as a substitute for measures prepared in accordance with GAAP.

The Company adjusts the foregoing capital adequacy measures to exclude goodwill and other intangible assets (except mortgage servicing rights), adjusts its non-interest expense to exclude acquisition related expenses, and adjusts its net interest margin ratio to exclude the impact of the recovery of charged-off interest and interest accretion on acquired loans. Management believes these non-GAAP financial measures, which are intended to complement the capital ratios defined by banking regulators and to present on a consistent basis our and our acquired companies’ organic continuing operations without regard to the acquisition costs and adjustments that we consider to be unpredictable and dependent on a significant number of factors that are outside our control, are useful to investors in evaluating the Company’s performance because, as a general matter, they either do not represent an actual cash expense and are inconsistent in amount and frequency depending upon the timing and size of our acquisitions (including the size, complexity and/or volume of past acquisitions, which may drive the magnitude of acquisition related costs, but may not be indicative of the size, complexity and/or volume of future acquisitions or related costs), or they cannot be anticipated or estimated in a particular period (in particular as it relates to unexpected recovery amounts). This impacts the ratios that are important to analysts and allows investors to compare certain aspects of the Company’s capitalization to other companies.

See the Non-GAAP Financial Measures table included herein and the textual discussion for a reconciliation of the above described non-GAAP financial measures to their most directly comparable GAAP financial measures.

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Cautionary Note Regarding Forward-Looking Statements and Factors that Could Affect Future Results

This press release contains “forward-looking statements” within the meaning of Section 27A of the Securities Act of 1933, as amended, and Rule 175 promulgated thereunder, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act, and Rule 3b-6 promulgated thereunder, that involve inherent risks and uncertainties. Any statements about our, Great Western’s or the combined company’s plans, objectives, expectations, strategies, beliefs, or future performance or events constitute forward-looking statements. Such statements are identified by words or phrases such as “believes,” “expects,” “anticipates,” “plans,” “trends,” “objectives,” “continues” or similar expressions, or future or conditional verbs such as “will,” “would,” “should,” “could,” “might,” “may,” or similar expressions. Forward-looking statements involve known and unknown risks, uncertainties, assumptions, estimates and other important factors that change over time and could cause actual results to differ materially from any results, performance or events expressed or implied by such forward-looking statements. Such forward-looking statements include statements about the business combination transaction between FIBK and Great Western (the “Transaction”), including future financial and operating results, the combined company’s plans, objectives, expectations and intentions, and other statements that are not historical facts. Furthermore, the following factors, among others, may cause actual results to differ materially from current expectations in the forward-looking statements, including those set forth in this press release:

•new, or changes in, governmental regulations or policies;

•tax legislative initiatives or assessments;

•more stringent capital requirements, to the extent they may become applicable to us;

•heightened regulatory requirements resulting from our total assets exceeding $10 billion;

•changes in accounting standards;

•any failure to comply with applicable laws and regulations, including the Community Reinvestment Act and fair lending laws, the USA PATRIOT ACT, Office of Foreign Asset Control guidelines and requirements, the Bank Secrecy Act, and the related Financial Crimes Enforcement Network and Federal Financial Institutions Examination Council’s guidelines and regulations;

•lending risks and risks associated with loan sector concentrations;

•supply-chain disruptions, labor shortages, and any other decline in economic conditions that could reduce demand for our products and services and negatively impact the credit quality of loans;

•loan credit losses exceeding estimates;

•the soundness of other financial institutions;

•changes in oil and gas prices, and declining demand for coal could negatively impact the demand and credit quality of loans;

•the availability of financing sources for working capital and other needs;

•a loss of deposits or a change in product mix that increases the Company’s funding costs;

•changes in interest rates;

•changes in inflationary pressures;

•changes to United States trade policies, including the imposition of tariffs and retaliatory tariffs;

•competition from new or existing competitors;

•variable interest rates tied to London Interbank Offered Rate that may no longer be available or may become unreliable;

•cyber-security risks, including “denial-of-service attacks,” “hacking,” and “identity theft” that could result in the disclosure of confidential information;

•privacy, information security, and data protection laws, rules, and regulations that affect or limit how we collect and use personal information;

•the potential impairment of our goodwill;

•exposure to losses in collateralized loan obligation securities;

•our reliance on other companies that provide key components of our business infrastructure;

•events that may tarnish our reputation;

•the loss of the services of our management team and directors;

•our ability to attract and retain qualified employees to operate our business;

•costs associated with repossessed properties, including environmental remediation;

•the effectiveness of our systems of internal operating controls;

•our ability to implement new technology-driven products and services or be successful in marketing these products and services to our clients;

•our ability to execute on our intended expansion plans;

•difficulties we may face in combining the operations of acquired entities or assets with our own operations or assessing the effectiveness of businesses in which we make strategic investments or with which we enter into strategic contractual relationships;

•our status as a “controlled company” under NASDAQ Marketplace Rules;

•the volatility in the price and trading volume of our Class A common stock;

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•“anti-takeover” provisions and the regulations, which may make it more difficult for a third party to acquire control of us even in circumstances that could be deemed beneficial to stockholders;

•changes in our dividend policy or our ability to pay dividends;

•our Class A common stock not being an insured deposit;

•the holders of the Class B common stock having voting control of the Company and the ability to determine virtually all matters submitted to stockholders, including potential change in control transactions, and our dual class common stock structure putting downward pressure on our common stock price;

•the potential dilutive effect of future equity issuances;

•the subordination of our Class A common stock to our existing and future indebtedness;

•the COVID-19 pandemic and the U.S. government’s response to the pandemic; and

•the effect of global conditions, earthquakes, tsunamis, floods, fires, and other natural catastrophic events.

These factors are not necessarily all the factors that could cause our, Great Western’s or the combined company’s actual results, performance or achievements to differ materially from those expressed in or implied by any of our forward-looking statements. Other unknown or unpredictable factors also could harm our, Great Western’s or the combined company’s results, including the following factors relating to the proposed Transaction: the occurrence of any event, change, or other circumstance that could give rise to the right of one or both of the parties to terminate the merger agreement related to the Transaction; the outcome of any legal proceedings that may be instituted against FIBK or Great Western; the possibility that the Transaction does not close when expected or at all because required regulatory, shareholder, or other approvals and other conditions to closing are not received or satisfied on a timely basis or at all (and the risk that such approvals may result in the imposition of conditions that could adversely affect the combined company or the expected benefits of the Transaction); the risk that the benefits from the Transaction may not be fully realized or may take longer to realize than expected, including as a result of changes in, or problems arising from, general economic and market conditions, interest and exchange rates, monetary policy, laws and regulations and their enforcement, and the degree of competition in the geographic and business areas in which FIBK and Great Western operate; the ability to promptly and effectively integrate the businesses of FIBK and Great Western; the possibility that the Transaction may be more expensive to complete than anticipated, including as a result of unexpected factors or events; reputational risk and potential adverse reactions of FIBK’s or Great Western’s customers, employees or other business partners, including those resulting from the announcement or completion of the Transaction; the dilution caused by FIBK’s issuance of additional shares of its capital stock in connection with the Transaction; the diversion of management’s attention and time from ongoing business operations and opportunities on merger-related matters; and the impact of the global COVID-19 pandemic on FIBK’s or Great Western’s businesses, the ability to complete the Transaction or any of the other foregoing risks.

All forward-looking statements attributable to us or persons acting on our behalf are expressly qualified in their entirety by the cautionary statements set forth above and included and described in more detail in our periodic reports filed with the Securities and Exchange Commission, or SEC, under the Securities Exchange Act of 1934, as amended, under the caption “Risk Factors.” Interested parties are urged to read in their entirety such risk factors prior to making any investment decision with respect to the Company. Forward-looking statements speak only as of the date they are made and we do not undertake or assume any obligation to update publicly any of these statements to reflect actual results, new information or future events, changes in assumptions or changes in other factors affecting forward-looking statements, except to the extent required by applicable laws. If we update one or more forward-looking statements, no inference should be drawn that we will make additional updates with respect to those or other forward-looking statements.

Third Quarter 2021 Conference Call for Investors

First Interstate BancSystem, Inc. will host a conference call to discuss the results for the third quarter of 2021 at 11 a.m. Eastern Time (9 a.m. Mountain Time) on Wednesday, October 27, 2021. The conference call will be accessible by telephone and through the Internet. Participants may join the call by dialing 1-844-200-6205; the access code is 814314. To participate via the Internet, visit https://events.q4inc.com/attendee/115918852. The call will be recorded and made available for replay after 1 p.m. Eastern Time (11 a.m. Mountain Time) on October 27, 2021 through 9 a.m. Eastern Time (7 a.m. Mountain Time) on November 26, 2021, by dialing 1-866-813-9403. The replay access code is 420744. The call will also be archived on our website, www.FIBK.com, for one year.

About First Interstate BancSystem, Inc.

First Interstate BancSystem, Inc. is a financial and bank holding company focused on community banking. Incorporated in 1971 and headquartered in Billings, Montana, the Company operates banking offices, including detached drive-up facilities, in communities across Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming, in addition to offering online and mobile banking services. Through our bank subsidiary, First Interstate Bank, the Company delivers a comprehensive range of banking products and services to individuals, businesses, municipalities, and others throughout the Company’s market areas.

8

Additional Information about the Transaction and Where to Find It

In connection with the Transaction, FIBK will file with the SEC a Registration Statement on Form S-4 to register the shares of FIBK capital stock to be issued in connection with the Transaction. The Registration Statement will include a joint proxy statement of FIBK and Great Western that also constitutes a prospectus of FIBK. The definitive joint proxy statement/prospectus will be sent to the shareholders of FIBK and stockholders of Great Western seeking their approval of the Transaction and other related matters.

INVESTORS AND SECURITY HOLDERS ARE URGED TO READ THE REGISTRATION STATEMENT ON FORM S-4 AND THE JOINT PROXY STATEMENT/PROSPECTUS INCLUDED WITHIN THE REGISTRATION STATEMENT ON FORM S-4 WHEN THEY BECOME AVAILABLE, AS WELL AS ANY OTHER RELEVANT DOCUMENTS FILED WITH THE SEC IN CONNECTION WITH THE TRANSACTION OR INCORPORATED BY REFERENCE INTO THE JOINT PROXY STATEMENT/PROSPECTUS, BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION REGARDING FIBK, GREAT WESTERN, THE TRANSACTION AND RELATED MATTERS.

Investors and security holders may obtain free copies of these documents and other documents filed with the SEC by FIBK or Great Western through the website maintained by the SEC at http://www.sec.gov or from FIBK at its website, www.fibk.com, or from Great Western at its website, www.greatwesternbank.com. Documents filed with the SEC by FIBK will be available free of charge by accessing the “SEC Filings” page of FIBK’s website at www.fibk.com/sec-filings, or alternatively by directing a request by mail or telephone to First Interstate BancSystem, Inc., 401 N. 31st Street, Billings, Montana, 59116, Attention: John Stewart, Deputy Chief Financial Officer, telephone: 406-255-5311, and documents filed with the SEC by Great Western will be available free of charge by accessing Great Western’s website at www.greatwesternbank.com under the tab “Investor Relations” and then under the heading “Financial Info – Documents” or, alternatively, by directing a request by telephone or mail to Great Western Bancorp Inc., 225 South Main Avenue, Sioux Falls, South Dakota 57104, (605) 988-9253.

Participants in the Solicitation

FIBK, Great Western, and certain of their respective directors and executive officers may be deemed to be participants in the solicitation of proxies from the shareholders of FIBK and stockholders of Great Western in connection with the Transaction under the rules of the SEC. Information about the interests of the directors and executive officers of FIBK and Great Western and other persons who may be deemed to be participants in the solicitation of shareholders of FIBK and stockholders of Great Western in connection with the Transaction and a description of their direct and indirect interests, by security holdings or otherwise, will be included in the joint proxy statement/prospectus related to the Transaction, which will be filed with the SEC. Additional information about FIBK, the directors and executive officers of FIBK and their ownership of FIBK common stock is also set forth in the definitive proxy statement for FIBK’s 2021 Annual Meeting of Shareholders, as filed with the SEC on Schedule 14A on April 14, 2021, and other documents subsequently filed by FIBK with the SEC. Additional information about Great Western, the directors and executive officers of Great Western and their ownership of Great Western common stock can also be found in Great Western’s definitive proxy statement in connection with its 2021 Annual Meeting of Stockholders, as filed with the SEC on December 23, 2020, and other documents subsequently filed by Great Western with the SEC. Free copies of these documents may be obtained as described above.

Contact:	John R. Stewart	NASDAQ: FIBK
	Deputy Chief Financial Officer First Interstate BancSystem, Inc. (406) 255-5311 john.stewart@fib.com	www.FIBK.com (FIBK-ER)

9

FIRST INTERSTATE BANCSYSTEM, INC. AND SUBSIDIARIES

Consolidated Statements of Income

(Unaudited)

	Quarter Ended										% Change
(In millions, except % and per share data)	Sep 30, 2021		Jun 30, 2021		Mar 31, 2021		Dec 31, 2020		Sep 30, 2020		3Q21 vs 2Q21		3Q21 vs 3Q20
Net interest income	$	126.9	$	118.8	$	120.7	$	128.4	$	123.0	6.8	%	3.2	%
Net interest income on a fully-taxable equivalent ("FTE") basis	127.5		119.2		121.4		128.9		123.5		7.0		3.2
Provision for (reduction in) credit losses	—		—		(5.1)		3.2		5.2		—		(100.0)
Non-interest income:
Payment services revenues	12.2		11.4		10.2		11.1		10.5		7.0		16.2
Mortgage banking revenues	11.6		9.6		11.6		7.9		14.3		20.8		(18.9)
Wealth management revenues	6.5		6.3		6.3		6.3		5.9		3.2		10.2
Service charges on deposit accounts	4.4		3.9		3.8		4.3		4.3		12.8		2.3
Other service charges, commissions, and fees	1.4		1.6		2.1		2.1		5.0		(12.5)		(72.0)
Total fee-based revenues	36.1		32.8		34.0		31.7		40.0		10.1		(9.8)
Investment securities (loss) gain	0.3		(0.1)		—		0.2		0.1		NM		NM
Other income	3.3		2.6		4.1		2.0		4.6		26.9		(28.3)
Total non-interest income	39.7		35.3		38.1		33.9		44.7		12.5		(11.2)
Non-interest expense:
Salaries and wages	42.0		41.6		39.0		43.6		46.0		1.0		(8.7)
Employee benefits	12.9		14.7		16.1		13.0		11.8		(12.2)		9.3
Occupancy and equipment	11.8		11.2		11.7		11.6		11.3		5.4		4.4
Core deposit intangible amortization	2.4		2.5		2.5		2.6		2.7		(4.0)		(11.1)
Other expenses	30.2		29.0		29.2		26.7		27.7		4.1		9.0
Other real estate owned (income) expense	—		—		(0.1)		(0.1)		—		—		—
Acquisition related expenses	6.6		—		—		—		—		100.0		100.0
Total non-interest expense	105.9		99.0		98.4		97.4		99.5		7.0		6.4
Income before taxes	60.7		55.1		65.5		61.7		63.0		10.2		(3.7)
Income taxes	13.6		12.6		14.1		14.8		14.7		7.9		(7.5)
Net income	$	47.1	$	42.5	$	51.4	$	46.9	$	48.3	10.8	%	(2.5)	%
Weighted-average basic shares outstanding	61,674		61,658		61,592		61,906		63,764		—	%	(3.3)	%
Weighted-average diluted shares outstanding	61,748		61,728		61,714		62,059		63,861		—		(3.3)
Earnings per share - basic	$	0.76	$	0.69	$	0.83	$	0.76	$	0.76	10.1		NM
Earnings per share - diluted	0.76		0.69		0.83		0.76		0.76		10.1		NM
NM - not meaningful

10

FIRST INTERSTATE BANCSYSTEM, INC. AND SUBSIDIARIES

Consolidated Balance Sheets

(Unaudited)

											% Change
(In millions, except % and per share data)	Sep 30, 2021		Jun 30, 2021		Mar 31, 2021		Dec 31, 2020		Sep 30, 2020		3Q21 vs 2Q21		3Q21 vs 3Q20
Assets:
Cash and due from banks	$	227.6	$	238.8	$	254.0	$	261.4	$	291.4	(4.7)	%	(21.9)	%
Interest bearing deposits in banks	2,005.8		1,709.5		1,941.9		2,015.3		1,569.1		17.3		27.8
Federal funds sold	0.1		0.1		0.1		0.1		0.1		—		—
Cash and cash equivalents	2,233.5		1,948.4		2,196.0		2,276.8		1,860.6		14.6		20.0
Investment securities	6,021.7		5,643.3		4,886.4		4,060.3		3,508.5		6.7		71.6
Mortgage loans held for sale, at fair value	42.5		48.8		57.2		74.0		102.0		(12.9)		(58.3)
Loans held for investment	9,622.5		9,834.7		9,863.2		9,807.5		10,152.2		(2.2)		(5.2)
Allowance for credit losses	135.1		135.5		136.6		144.3		145.5		(0.3)		(7.1)
Net loans held for investment	9,487.4		9,699.2		9,726.6		9,663.2		10,006.7		(2.2)		(5.2)
Goodwill and intangible assets (excluding mortgage servicing rights)	693.3		695.7		698.2		700.8		703.4		(0.3)		(1.4)
Company owned life insurance	300.5		299.0		297.6		296.4		294.9		0.5		1.9
Premises and equipment	297.3		299.1		305.5		312.3		307.8		(0.6)		(3.4)
Other real estate owned	2.3		2.0		2.2		2.5		5.7		15.0		(59.6)
Mortgage servicing rights	27.0		27.4		28.0		24.0		24.1		(1.5)		12.0
Other assets*	266.7		277.6		270.5		238.4		255.8		(3.9)		4.3
Total assets	$	19,372.2	$	18,940.5	$	18,468.2	$	17,648.7	$	17,069.5	2.3	%	13.5	%
Liabilities and stockholders' equity:
Deposits	$	16,007.3	$	15,565.7	$	15,094.0	$	14,217.0	$	13,882.4	2.8	%	15.3	%
Securities sold under repurchase agreements	1,007.5		1,038.7		1,052.6		1,091.4		820.3		(3.0)		22.8
Long-term debt	112.4		112.4		112.4		112.4		112.4		—		—
Subordinated debentures held by subsidiary trusts	87.0		87.0		87.0		87.0		87.0		—		—
Other liabilities*	173.2		165.8		188.8		181.1		189.8		4.5		(8.7)
Total liabilities	17,387.4		16,969.6		16,534.8		15,688.9		15,091.9		2.5		15.2
Stockholders' equity:
Common stock	943.6		941.6		938.5		941.1		976.8		0.2		(3.4)
Retained earnings	1,026.9		1,005.2		988.2		962.1		938.9		2.2		9.4
Accumulated other comprehensive income	14.3		24.1		6.7		56.6		61.9		(40.7)		(76.9)
Total stockholders' equity	1,984.8		1,970.9		1,933.4		1,959.8		1,977.6		0.7		0.4
Total liabilities and stockholders' equity	$	19,372.2	$	18,940.5	$	18,468.2	$	17,648.7	$	17,069.5	2.3	%	13.5	%
Common shares outstanding at period end	62,231		62,240		62,230		62,096		63,115		—	%	(1.4)	%
Book value per common share at period end	$	31.89	$	31.67	$	31.07	$	31.56	$	31.33	0.7		1.8
Tangible book value per common share at period end**	20.75		20.49		19.85		20.28		20.19		1.3		2.8
*Certain reclassifications were made to the March 31, 2021 other assets and other liabilities to conform to the June 30 and September 30, 2021 periods.
**Non-GAAP financial measure - see Non-GAAP Financial Measures included herein for a reconciliation of book value per common share at period end (GAAP) to tangible book value per common share at period end (non-GAAP).

11

FIRST INTERSTATE BANCSYSTEM, INC. AND SUBSIDIARIES

Loans and Deposits

(Unaudited)

											% Change
(In millions, except %)	Sep 30, 2021		Jun 30, 2021		Mar 31, 2021		Dec 31, 2020		Sep 30, 2020		3Q21 vs 2Q21		3Q21 vs 3Q20
Loans:
Real Estate:
Commercial real estate	$	3,883.2	$	3,753.4	$	3,718.7	$	3,743.2	$	3,690.9	3.5	%	5.2	%
Construction:
Land acquisition and development	260.2		261.1		263.2		265.0		274.8		(0.3)		(5.3)
Residential	268.4		263.5		242.1		250.9		227.9		1.9		17.8
Commercial	610.2		632.0		581.0		523.5		530.8		(3.4)		15.0
Total construction	1,138.8		1,156.6		1,086.3		1,039.4		1,033.5		(1.5)		10.2
Residential real estate	1,554.9		1,577.7		1,488.8		1,396.3		1,311.2		(1.4)		18.6
Agricultural real estate	229.9		223.5		218.8		220.6		227.7		2.9		1.0
Total real estate	6,806.8		6,711.2		6,512.6		6,399.5		6,263.3		1.4		8.7
Consumer:
Indirect	756.8		773.7		782.9		805.1		812.8		(2.2)		(6.9)
Direct	132.9		134.8		139.7		150.6		162.1		(1.4)		(18.0)
Credit card	64.1		64.4		64.6		70.2		69.9		(0.5)		(8.3)
Total consumer	953.8		972.9		987.2		1,025.9		1,044.8		(2.0)		(8.7)
Commercial	1,668.7		1,959.4		2,181.1		2,153.9		2,599.6		(14.8)		(35.8)
Agricultural	212.4		217.7		214.7		247.6		274.7		(2.4)		(22.7)
Other	1.3		6.0		1.5		1.6		4.2		(78.3)		(69.0)
Deferred loan fees and costs	(20.5)		(32.5)		(33.9)		(21.0)		(34.4)		(36.9)		(40.4)
Loans held for investment	$	9,622.5	$	9,834.7	$	9,863.2	$	9,807.5	$	10,152.2	(2.2)	%	(5.2)	%
Deposits:
Non-interest bearing	$	5,617.9	$	5,416.8	$	5,004.0	$	4,633.5	$	4,798.2	3.7	%	17.1	%
Interest bearing:
Demand	4,496.5		4,389.0		4,327.0		4,118.9		3,814.1		2.4		17.9
Savings	4,904.9		4,748.4		4,726.7		4,405.9		4,158.0		3.3		18.0
Time, $250 and over	186.3		185.8		185.5		192.9		186.6		0.3		(0.2)
Time, other	801.7		825.7		850.8		865.8		925.5		(2.9)		(13.4)
Total interest bearing	10,389.4		10,148.9		10,090.0		9,583.5		9,084.2		2.4		14.4
Total deposits	$	16,007.3	$	15,565.7	$	15,094.0	$	14,217.0	$	13,882.4	2.8	%	15.3	%
Total core deposits (1)	$	15,821.0	$	15,379.9	$	14,908.5	$	14,024.1	$	13,695.8	2.9	%	15.5	%
(1) Core deposits are defined as total deposits less time deposits, $250 and over, and brokered deposits.

12

FIRST INTERSTATE BANCSYSTEM, INC. AND SUBSIDIARIES

Credit Quality

(Unaudited)

																% Change
(In millions, except %)	Sep 30, 2021			Jun 30, 2021			Mar 31, 2021			Dec 31, 2020			Sep 30, 2020			3Q21 vs 2Q21		3Q21 vs 3Q20
Allowance for Credit Losses:
Allowance for credit losses	$	135.1		$	135.5		$	136.6		$	144.3		$	145.5		(0.3)	%	(7.1)	%
As a percentage of loans held for investment	1.40		%	1.38		%	1.38		%	1.47		%	1.43		%
As a percentage of non-accrual loans	451.84			445.72			369.19			365.32			324.78
Net charge-offs during quarter	$	0.6		$	1.1		$	2.9		$	4.2		$	4.6		(45.5)	%	(87.0)	%
Annualized as a percentage of average loans	0.02		%	0.04		%	0.12		%	0.16		%	0.18		%
Non-Performing Assets:
Non-accrual loans	$	29.9		$	30.4		$	37.0		$	39.5		$	44.8		(1.6)	%	(33.3)	%
Accruing loans past due 90 days or more	5.2			5.2			4.4			8.5			9.6			—		(45.8)
Total non-performing loans	35.1			35.6			41.4			48.0			54.4			(1.4)		(35.5)
Other real estate owned	2.3			2.0			2.2			2.5			5.7			15.0		(59.6)
Total non-performing assets	$	37.4		$	37.6		$	43.6		$	50.5		$	60.1		(0.5)	%	(37.8)	%
Non-performing assets as a percentage of:
Loans held for investment and OREO	0.39		%	0.38		%	0.44		%	0.51		%	0.59		%
Total assets	0.19			0.20			0.24			0.29			0.35
Non-accrual loans to loans held for investment	0.31			0.31			0.38			0.40			0.44
Accruing Loans 30-89 Days Past Due	$	27.3		$	22.1		$	26.3		$	54.2		$	36.1		23.5	%	(24.4)	%
Accruing troubled debt restructurings (TDRs)	2.1			2.2			3.1			3.2			3.2			(4.5)		(34.4)
Criticized Loans:
Special Mention	$	99.1		$	129.1		$	152.0		$	150.3		$	157.1		(23.2)	%	(36.9)	%
Substandard	149.7			141.2			157.4			187.0			209.8			6.0		(28.6)
Doubtful	2.4			3.1			2.0			4.8			12.4			(22.6)		(80.6)
Total	$	251.2		$	273.4		$	311.4		$	342.1		$	379.3		(8.1)	%	(33.8)	%

13

FIRST INTERSTATE BANCSYSTEM, INC. AND SUBSIDIARIES

Selected Ratios - Annualized

(Unaudited)

	Sep 30, 2021				Jun 30, 2021			Mar 31, 2021			Dec 31, 2020			Sep 30, 2020
Annualized Financial Ratios (GAAP)
Return on average assets	0.98		%		0.91		%	1.17		%	1.07		%	1.15		%
Return on average common stockholders' equity	9.41				8.77			10.60			9.48			9.57
Yield on average earning assets	3.00				2.93			3.15			3.39			3.44
Cost of average interest-bearing liabilities	0.14				0.16			0.17			0.20			0.24
Interest rate spread	2.86				2.77			2.98			3.19			3.20
Net interest margin ratio	2.91				2.82			3.04			3.25			3.29
Efficiency ratio	62.12				62.62			60.39			58.41			57.72
Loans held for investment to deposit ratio	60.11				63.18			65.35			68.98			73.13
Annualized Financial Ratios - Operating** (Non-GAAP)
Tangible book value per common share	$	20.75			$	20.49		$	19.85		$	20.28		$	20.19
Tangible common stockholders' equity to tangible assets	6.91		%		6.99		%	6.95		%	7.43		%	7.79		%
Return on average tangible common stockholders' equity	14.48				13.67			16.46			14.74			14.74
Consolidated Capital Ratios:
Total risk-based capital to total risk-weighted assets	14.00		%	*	13.89		%	14.15		%	14.19		%	14.45		%
Tier 1 risk-based capital to total risk-weighted assets	12.30			*	12.17			12.37			12.33			12.56
Tier 1 common capital to total risk-weighted assets	11.59			*	11.45			11.60			11.57			11.79
Leverage Ratio	7.81			*	7.84			8.12			8.16			8.62
*Preliminary estimate - may be subject to change. Additionally, the regulatory capital ratios presented above include the assumption of the transitional method relative to recent legislation by Congress to provide relief for the economy and financial institutions in the United States from the COVID‑19 pandemic. The referenced relief allows a total five-year phase-in of the CECL impact on capital and relief over the next two years for the impact on the allowance for credit losses resulting from COVID‑19.
**Non-GAAP financial measures - see Non-GAAP Financial Measures included herein for a reconciliation of book value per common share to tangible book value per common share, return on average common stockholders’ equity (GAAP) to return on average tangible common stockholders’ equity, and tangible common stockholders’ equity to tangible assets (non-GAAP).

14

FIRST INTERSTATE BANCSYSTEM, INC. AND SUBSIDIARIES
Average Balance Sheets
(Unaudited)
	Three Months Ended
	September 30, 2021						June 30, 2021						September 30, 2020
(In millions, except %)	Average Balance		Interest		Average Rate		Average Balance		Interest		Average Rate		Average Balance		Interest		Average Rate
Interest earning assets:
Loans (1) (2)	$	9,805.2	$	112.2	4.54	%	$	9,969.2	$	105.6	4.25	%	$	10,219.2	$	112.6	4.38	%
Investment securities (2)	5,875.0		18.8		1.27		5,105.2		17.4		1.37		3,435.1		16.3		1.89
Interest bearing deposits in banks	1,712.2		0.7		0.16		1,883.9		0.7		0.15		1,295.1		0.5		0.15
Federal funds sold	0.1		—		—		0.1		—		—		0.1		—		—
Total interest earning assets	$	17,392.5	$	131.7	3.00	%	$	16,958.4	$	123.7	2.93	%	$	14,949.5	$	129.4	3.44	%
Non-earning assets	1,688.7						1,706.8						1,739.9
Total assets	$	19,081.2					$	18,665.2					$	16,689.4
Interest-bearing liabilities:
Demand deposits	$	4,474.0	$	0.4	0.04	%	$	4,392.3	$	0.5	0.05	%	$	3,708.8	$	0.4	0.04	%
Savings deposits	4,842.9		0.4		0.03		4,752.7		0.4		0.03		4,081.4		0.3		0.03
Time deposits	996.9		1.1		0.44		1,025.2		1.3		0.51		1,167.3		2.7		0.92
Repurchase agreements	993.5		0.1		0.04		1,002.0		0.1		0.04		781.2		0.1		0.05
Long-term debt	112.4		1.5		5.29		112.4		1.5		5.35		112.3		1.7		6.02
Subordinated debentures held by subsidiary trusts	87.0		0.7		3.19		87.0		0.7		3.23		86.9		0.7		3.20
Total interest-bearing liabilities	$	11,506.7	$	4.2	0.14	%	$	11,371.6	$	4.5	0.16	%	$	9,937.9	$	5.9	0.24	%
Non-interest-bearing deposits	5,416.5						5,160.8						4,517.3
Other non-interest-bearing liabilities	172.7						188.5						226.0
Stockholders’ equity	1,985.3						1,944.3						2,008.2
Total liabilities and stockholders’ equity	$	19,081.2					$	18,665.2					$	16,689.4
Net FTE interest income			$	127.5					$	119.2					$	123.5
Less FTE adjustments (2)			(0.6)						(0.4)						(0.5)
Net interest income from consolidated statements of income			$	126.9					$	118.8					$	123.0
Interest rate spread					2.86	%					2.77	%					3.20	%
Net FTE interest margin (3)					2.91	%					2.82	%					3.29	%
Cost of funds, including non-interest-bearing demand deposits (4)					0.10	%					0.11	%					0.16	%
(1) Average loan balances include mortgage loans held for sale and non-accrual loans. Interest income on loans includes amortization of deferred loan fees net of deferred loan costs of $13.6 million, $6.4 million, and $8.6 million at September 30, 2021, June 30, 2021, and September 30, 2020, respectively.
(2) Interest income and average rates for tax exempt loans and securities are presented on an FTE basis.
(3) Net FTE interest margin during the period equals (i) the difference between annualized interest income on interest earning assets and the annualized interest expense on interest bearing liabilities, divided by (ii) average interest earning assets for the period.
(4) Calculated by dividing total annualized interest on interest bearing liabilities by the sum of total interest-bearing liabilities plus non-interest-bearing deposits.

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FIRST INTERSTATE BANCSYSTEM, INC. AND SUBSIDIARIES
Non-GAAP Financial Measures
(Unaudited)
		As of or For the Quarter Ended
(In millions, except % and per share data)		Sep 30, 2021			Jun 30, 2021			Mar 31, 2021			Dec 31, 2020			Sep 30, 2020
Total common stockholders' equity (GAAP)	(A)	$	1,984.8		$	1,970.9		$	1,933.4		$	1,959.8		$	1,977.6
Less goodwill and other intangible assets (excluding mortgage servicing rights)		693.3			695.7			698.2			700.8			703.4
Tangible common stockholders' equity (Non-GAAP)	(B)	$	1,291.5		$	1,275.2		$	1,235.2		$	1,259.0		$	1,274.2
Total assets (GAAP)		$	19,372.2		$	18,940.5		$	18,468.2		$	17,648.7		$	17,069.5
Less goodwill and other intangible assets (excluding mortgage servicing rights)		693.3			695.7			698.2			700.8			703.4
Tangible assets (Non-GAAP)	(C)	$	18,678.9		$	18,244.8		$	17,770.0		$	16,947.9		$	16,366.1
Average Balances:
Total common stockholders' equity (GAAP)	(D)	$	1,985.3		$	1,944.3		$	1,966.5		$	1,968.0		$	2,008.2
Less goodwill and other intangible assets (excluding mortgage servicing rights)		694.5			696.9			699.5			702.0			704.8
Average tangible common stockholders' equity (Non-GAAP)	(E)	$	1,290.8		$	1,247.4		$	1,267.0		$	1,266.0		$	1,303.4
Total quarterly average assets	(F)	$	19,081.2		$	18,665.2		$	17,891.3		$	17,473.5		$	16,689.4
Annualized net income available to common shareholders	(G)	186.9			170.5			208.5			186.6			192.2
Common shares outstanding	(H)	62,231			62,240			62,230			62,096			63,115
Return on average assets (GAAP)	(G)/(F)	0.98		%	0.91		%	1.17		%	1.07		%	1.15		%
Return on average common stockholders' equity (GAAP)	(G)/(D)	9.41			8.77			10.60			9.48			9.57
Average common stockholders' equity to average assets (GAAP)	(D)/(F)	10.40			10.42			10.99			11.26			12.03
Book value per common share (GAAP)	(A)/(H)	$	31.89		$	31.67		$	31.07		$	31.56		$	31.33
Tangible book value per common share (Non-GAAP)	(B)/(H)	20.75			20.49			19.85			20.28			20.19
Tangible common stockholders' equity to tangible assets (Non-GAAP)	(B)/(C)	6.91		%	6.99		%	6.95		%	7.43		%	7.79		%
Return on average tangible common stockholders' equity (Non-GAAP)	(G)/(E)	14.48			13.67			16.46			14.74			14.74

First Interstate BancSystem, Inc.

P.O. Box 30918     Billings, Montana 59116     (406) 255-5390

www.FIBK.com

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