SMA PLT GETS CAUGHT IN UNFORECASTED WX DURING SHORT FLT BTWN 2 ARPTS AND ENTERS AN ATA AND IFR CONDITIONS. HE RECOGNIZED HIS LOCATION AND EXITED THE AIRSPACE AND BROKE OUT IN VFR CONDITIONS.

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I HAD DECIDED TO FLY FROM ISO TO CLS FOR LUNCH AND TO FUEL; BUT WX HAD BECOME A PROB. I HAD A WX BRIEF ON FRIDAY AND AN AUTOMATIC BRIEF ON SATURDAY. I DEPARTED ISO APPROX XA00 AND LANDED 30 MINS LATER AT CLS (DISTANCE OF 38 MI). AS I FUELED; IT STARTED TO SNOW AND THE VISIBILITY STARTED TO DECREASE. I DECIDED TO TAKE OFF ASAP AS THE WX AT MY DEST AND MOST OF MY RTE WAS STILL GOOD. THE WX WAS APCHING FROM THE OPPOSITE DIRECTION. THE TERRAIN TO THE E WAS HIGHER THAN THE TERRAIN TO THE N; SO I DECIDED TO GO N AND TO FOLLOW INTERSTATE 5. THE WX CLOSED IN BEHIND ME AND ALSO ALONG THIS NORTHERLY RTE. MY ATL WAS ABOUT 700-1000 FT AGL. VISIBILITY WAS ABOUT 1 1/2 MI. I THOUGHT I WOULD BE IN VFR CONDITIONS IN A SHORT TIME; AS I HAD ONLY BEEN ON THE GND FOR ABOUT 15 MINS AND MY RTE HAD BEEN VFR ALL THE WAY TO CLS. I DID NOT REALIZE HOW FAR N I HAD FLOWN UNTIL I SAW THE DEPT STORE AT TUMWATER; WA. I KNEW THEN THAT THE OLYMPIA ARPT NEARBY WAS IFR AND THAT I WAS IN THE CTL ZONE. AT THIS TIME; IT WENT TO VFR AND I CLBED TO 2800 FT AND CONTINUED TO ISO. MY DECISION TO TRY TO OUTRUN THE WX WAS MADE IN HASTE AND MY PLANNING DID NOT MAKE ME AWARE OF THE RTE AND THE CTL ZONE ALONG IT. IN RETROSPECT; MORE TIME FLT PLANNING WOULD HAVE BEEN A GREAT BENEFIT. HASTE WAS THE PROB HERE. IT ALTERED THE DECISION TO GO -- AND TO GO NOT PROPERLY PREPARED.