FIRST INTERSTATE BANCSYSTEM INC

To the Shareholders and the Board of Directors of First Interstate BancSystem, Inc.

Opinion on the Financial Statements

We have audited the accompanying consolidated balance sheets of First Interstate BancSystem, Inc. and its subsidiaries (the Company) as of December 31, 2023 and 2022, the related consolidated statements of income, comprehensive income, stockholders' equity and cash flows for each of the three years in the period ended December 31, 2023, and the related notes to the consolidated financial statements (collectively, the financial statements). In our opinion, the financial statements present fairly, in all material respects, the financial position of the Company as of December 31, 2023 and 2022, and the results of its operations and its cash flows for each of the three years in the period ended December 31, 2023, in conformity with accounting principles generally accepted in the United States of America.

We have also audited, in accordance with the standards of the Public Company Accounting Oversight Board (United States) (PCAOB), the Company's internal control over financial reporting as of December 31, 2023, based on criteria established in Internal Control — Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission in 2013. Our report dated February 29, 2024 expressed an opinion that the Company had not maintained effective internal control over financial reporting as of December 31, 2023, based on criteria established in Internal Control – Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission in 2023.

Basis for Opinion

These financial statements are the responsibility of the Company's management. Our responsibility is to express an opinion on the Company's financial statements based on our audits. We are a public accounting firm registered with the PCAOB and are required to be independent with respect to the Company in accordance with U.S. federal securities laws and the applicable rules and regulations of the Securities and Exchange Commission and the PCAOB.

We conducted our audits in accordance with the standards of the PCAOB. Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement, whether due to error or fraud. Our audits included performing procedures to assess the risks of material misstatement of the financial statements, whether due to error or fraud, and performing procedures that respond to those risks. Such procedures included examining, on a test basis, evidence regarding the amounts and disclosures in the financial statements. Our audits also included evaluating the accounting principles used and significant estimates made by management, as well as evaluating the overall presentation of the financial statements. We believe that our audits provide a reasonable basis for our opinion.

Critical Audit Matters

The critical audit matters communicated below are matters arising from the current period audit of the financial statements that were communicated or required to be communicated to the audit committee and that: (1) relate to accounts or disclosures that are material to the financial statements and (2) involved our especially challenging, subjective or complex judgments. The communication of critical audit matters does not alter in any way our opinion on the financial statements, taken as a whole, and we are not, by communicating the critical audit matters below, providing separate opinions on the critical audit matters or on the accounts or disclosures to which they relate.

Allowance for Credit Losses - Loans Held for Investment

The Company’s loans held for investment portfolio totaled $18,279.6 million as of December 31, 2023, and the associated allowance for credit losses on loans held for investment was $227.7 million. As described in Notes 1 and 6 to the financial statements, the allowance for credit losses on loans held for investment is a valuation account that is deducted from the Company’s amortized cost basis of loans held for investment to present the net amount of loans held for investment expected to be collected. Expected future principal and interest cash flows are calculated using contractual repayment terms while considering prepayment, utilization, interest rate, and probability of default assumptions. Macroeconomic models calculate segment-specific multipliers using third party forecast data. The multipliers adjust the annual loss rates to the level expected under the economic conditions over the 2-year forecast period, followed by a 1-year straight-line reversion to the unadjusted historical average loss rates. The unadjusted loss rates then apply for the remaining life of the loan. The Company’s allowance for credit losses on loans held for investment consists of three elements: (1) specific valuation allowances associated with collateral-dependent loans; (2) historical valuation allowances based on loan loss experience for similar loans with similar characteristics and trends and segment-specific multipliers using third party forecast data; and (3) adjustments to historical loss information for differences in current loan-specific risk characteristics such as differences in underwriting standards, portfolio mix and trends, asset quality, or term as well as for changes in environmental and economic conditions, such as changes in unemployment rates, property values, or other relevant factors.

We identified the adjustments to historical loss information component of the allowance for credit losses on loans held for investment, both as it relates to current conditions and economic forecast selection, as a critical audit matter, because auditing this component of the allowance for credit losses on loans held for investment required significant auditor judgement related to estimates determined by management which are highly subjective and are highly sensitive to change in significant assumptions.

Our audit procedures related to the Company’s adjustments to historical loss information component of the allowance for credit losses on loans held for investment included the following, among others:

•We obtained an understanding of the relevant controls related to the allowance for credit losses on loans held for investment and tested such controls for design and operating effectiveness, including controls relating to management’s review and approval of the allowance for credit losses on loans held for investment calculation, management’s approval of economic forecast selection used to derive segment-specific multipliers within its macroeconomic model, management’s assessment and review of the adjustments to historical loss information component of the allowance for credit losses on loans held for investment for current conditions, and management’s validation of underlying source data.

•We tested management’s calculation of adjustments to historical loss information within the allowance for credit losses on loans held for investment calculation by agreeing calculation inputs to the Company’s internal and external source data, verifying the mathematical accuracy of the calculation of adjustments to historical loss information, and evaluating whether adjustments to historical loss information within the allowance for credit losses on loans held for investment, or lack thereof, were reasonable and consistent with Company provided internal data and external independent data.

•We evaluated management’s selected economic forecast, used to derive segment-specific multipliers within its macroeconomic model, for reasonableness against alternative industry sources.

•We agreed management’s calculated adjustments to historical loss information to the allowance for credit losses on loans held for investment calculation.

The Company’s goodwill totaled $1,100.9 million as of December 31, 2023. As described in Notes 1 and 3 to the financial statements, the Company evaluates goodwill for impairment at least annually and on an interim basis if an event or circumstance indicates that it is more likely than not that an impairment has occurred. As a result of the ongoing financial services economic conditions, including changes to the interest rate environment, continued volatility in the Company’s common share price, and volatility in the Company’s overall market capitalization during the third and beginning of the fourth quarter of 2023, the Company performed a quantitative interim goodwill impairment assessment for its sole reporting unit as of October 31, 2023 by weighting income and market-based approaches. The income approach measures the fair value of an interest in a business by discounting expected future cash flows to present value. The market approaches take into consideration fair values of comparable companies operating in similar lines of business that are potentially subject to similar economic and environmental factors and could be considered reasonable investment alternatives. The result of the income and market approaches were weighted in determining the fair value of the reporting unit.

We identified management’s estimation of the fair value of its reporting unit as a critical audit matter, because the estimation of fair value required management to make significant assumptions and estimates around forecasted net income, the selection of discount rates, the selection of control premiums utilized in the valuation and the weighting of valuation methods used in determining the fair value of the reporting unit. In addition, our audit procedures to evaluate the reasonableness of management’s net income forecasts, selection of discount rates, selection of control premiums and weighting of valuation methods utilized in the valuation required a higher degree of auditor judgement as well as an increased level of audit effort, including the involvement of valuation specialists related to valuation inputs determined by management which are highly subjective and are highly sensitive to change in significant assumptions.

Our audit procedures related to our evaluation of the reasonableness of the Company’s net income forecasts, selection of discount rates, selection of the control premium utilized and weighting of the methods in the valuation included the following, among others:

•We obtained an understanding of the relevant controls related to net income forecasts and the selection of the discount rate, control premium and the weighting of valuation methods, and tested such controls for design and operating effectiveness, including controls relating to management’s review and approval of the net income forecasts, the selection of the discount rate and control premium and the weighting of valuation methods.

•We evaluated management’s ability to accurately forecast net income by comparing actual results to management’s historical forecasts.

•We evaluated the assumptions and estimates included in management’s net income forecasts by: 1) comparing to industry forecasts and information included in industry analyst reports; 2) considering the impact of changes in the competitive and regulatory environment on management’s forecast; and 3) assessing the reasonableness of strategic plans incorporated by the Company into the forecasts.

•We utilized internal valuation specialists to assist in evaluating the discount rate, the control premium selected by management and the weighting of valuation methods used in determining the fair value of the reporting unit. The evaluation included evaluating the reasonableness of market-based source information underlying the selection of the discount rate and control premium, testing the reasonableness of management’s method, developing independent expectations, and the mathematical accuracy of the discount rate, control premium and weighting of valuation methods calculations

/s/RSM US LLP

We have served as the Company’s auditor since 2004.

Des Moines, Iowa

February 29, 2024