** Regional lender First Interstate BancSystem's FIBK.O shares rise 4% to $33 premarket

** FIBK will join the S&P SmallCap 600 index prior to market open on December 2, S&P Dow Jones Indices announced late Monday

** S&P SmallCap 600 measures the small-cap segment of the US equity market

** Index inclusion typically boosts a company's stock price as passive funds and exchange-traded funds tracking the benchmark are forced to buy in, creating a demand surge

** Passive funds will need to purchase 12.5 million FIBK shares, which implies about 14 days of buying demand - brokerage Stephens

** Four of eight brokerages rate the stock "buy" or higher, three "hold" and one "sell"; median PT $36 - data compiled by LSEG

** As of last close, FIBK stock down 2.3% YTD

(Reporting by Arasu Kannagi Basil in Bengaluru)

((ArasuKannagi.Basil@thomsonreuters.com;))