C152 INSTRUCTOR AND STUDENT GOT TRAPPED BY A SNOW STORM AND WERE REQUIRED TO GET AN IMC CLRNC IN AN ACFT NOT APPROVED FOR IMC FLT.

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ON DEC/TUE/01; WHILE ON A ROUTINE TRAINING FLT IN A CESSNA 152 I NOTICED THAT A FAST MOVING SNOWSTORM WAS MOVING TOWARDS THE PULLMAN ARPT; OUR HOME BASE. THE WX WAS NOT PREDICTED. I CUT THE FLT SHORT; AND WE HEADED FOR THE ARPT; WHICH WAS APPROX 8 MI AWAY. JUST PRIOR TO ENTERING THE CLASS E AIRSPACE WE WERE OVERTAKEN BY THE STORM. VISIBILITY DROPPED TO APPROX 1 MI; THE GND WAS NOT OBSCURED. I TOOK CTL OF THE ACFT; AND TURNED AROUND 180 DEGS IN AN EFFORT TO GET OUT OF THE STORM; HOWEVER; WE WERE NOT ABLE TO GET INTO BETTER METEOROLOGICAL CONDITIONS. AT A LOW ALT WITH TERRAIN RISING AND CONDITIONS NOT IMPROVING; I ELECTED TO GET A POP UP IFR CLRNC; AND DIVERT TO THE LEWISTON ARPT; WHICH WAS APPROX 35 MI AWAY. THE ACFT IS NOT IFR APPROVED. WHILE; I HAVE NO INTENTION OF VIOLATING THE FAR'S; I FEEL THAT OBTAINING AN IFR CLRNC AT THAT TIME WAS THE ONLY WAY TO SAFELY CONCLUDE THE FLT. AFTER APPROX 25 MINS WITH ZSE VFR METEOROLOGICAL CONDITIONS WERE ENCOUNTERED; AND THE FLT WAS CONTINUED UNDER VFR UNTIL ITS SAFE TERMINATION AT THE LEWISTON ARPT.