SMA LANDS ON HWY WHEN CAUGHT BY IMC WX IN MOUNTAINOUS TERRAIN.

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AFTER RECEIVING A FAVORABLE WX BRIEF FOR A VFR FLT FROM ANP TO I77; I DEPARTED ANP WITH 6 HRS (AUX TANKS) OF FUEL. AFTER PASSING KESSEL VOR I NOTICED DETERIORATING WX CONDITIONS AND RAISED CLARKSBURG TWR TO OBTAIN LCL WX OBSERVATIONS (I WAS 32 KTS E OF CKB). WAS ADVISED CONDITIONS VFR AT ARPT; HOWEVER; RPTS OF LIGHT SCATTERED RAIN SHOWERS. CONDITIONS CONTINUED TO DETERIORATE WITH SEVERE TURB IN AREA. I INITIATED A 180 DEG TURN DUE E AND NOTICED WX DETERIORATING TO E AS WELL. IN LIGHT OF WX SIT AND SEVERE TURB (MOUNTAIN EFFECT) I THOUGHT IT PRUDENT TO LAND THE ACFT AS SOON AS PRACTICABLE. DETERMINED CLOSEST ARPT TO BE W99 (13 KTS 170 DEGS); HOWEVER; DID NOT FEEL COMFORTABLE WITH PRESENT WX. DISCOVERED APPROX 4000 FT STRAIGHT STRETCH OF HWY 93 (E-W) AND SAFELY LANDED. I CONTACTED ELKINS FLT SVC AND RPTED DIFFICULTIES. I DISCUSSED SIT WITH BALTIMORE FSDO AND DETERMINED THAT I WOULD DEPART HWY RTE 93 AND LAND W99 ONCE CONDITIONS IMPROVED. CONDITIONS IMPROVED AND I FLEW SAFELY FROM HWY TO W99 (SECURITY OFFICERS FROM WV ELECTRIC SECURED BOTH ENDS OF HWY TO PERMIT SAFE DEPS). RECOMMEND ADVISING PLTS TRANSITIONING OVER MOUNTAINOUS TERRAIN IN AREA OF LCL (ELKINS) OF 'MICRO-METEOROLOGICAL' CONDITIONS UNIQUE TO THIS AREA.