The Sensex at the Bombay Stock Exchange surged by 171 points in late morning trade on increased buying amid covering-up of short positions by speculators ahead of tomorrow's monthly expiry. The 30-share barometer which gained 66 points in its opening session, firmed up by 171 points to 19,426 when reports last came in. The wide-based National Stock Exchange index Nifty moved up by 49 points to 5,904 a short while ago.