SMA ENCOUNTERS SEVERE TURB. PRECAUTIONARY LNDG AT AF BASE.

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I WAS ENRTE FROM EL PASO; TX; TO MONTGOMERY FIELD IN SAN DIEGO IN A BRAND NEW SMA. AT APPROX :00 TO :30 PM; I ENCOUNTERED TSTMS ENRTE AND HAD TO MAKE AN UNAUTH LNDG AT THE GILA BEND AF AUX BASE IN GILA BEND; AZ. BEFORE DEPARTING EL PASO; TX; I RECEIVED A STANDARD WX BRIEFING WHICH INDICATED A CHANCE OF TSTMS AND ISOLATED RAIN SHOWERS DUE TO THE LOW PRESSURE AND UNSTABLE AIR MASS IN THE AREA. AFTER RECEIVING MY BRIEFING; I FILED AN IFR FLT PLAN THE RTE WAS DEFINED: EL PASO V16 TUCSON; V66 MISSION BAY; DIRECT MONTGOMERY FIELD. WHILE IN RTE I CLBED TO 12000 FT AND WAS TALKING WITH ALBUQUERQUE CENTER. AT ABOUT 25 MI E OF TUCSON ALBUQUERQUE CENTER CAME WITH A WX ADVISORY CONVECTIVE SIGMET WHICH EXTENDED 50 MI SW OF PHOENIX TO 10 MI ESE OF YUMA. A BAND OF TSTMS 30 MI WIDE WITH LITTLE OR NO MOVEMENT. AFTER CHKING WITH FLT WATCH; I DECIDED TO AMEND MY RTING TO: AFTER GILA BEND VOR; V94 BLYTHE VOR; V460 JULIAN VOR DIRECT MONTGOMERY FIELD. TALKING WITH ALBUQUERQUE CENTER THE CTLR SAID THAT AN SMA ABOUT 15 MINS AHEAD OF ME HAD GONE THROUGH THAT RTE AT ABOUT 10000 FT AND HAD EXPERIENCED ONLY LIGHT RAIN. ALSO AT THAT TIME CENTER WAS WORKING ANOTHER ACFT GOING EBOUND AT 7000 FT AND THIS ACFT HAD GND CONTACT AND A SMOOTH RIDE. AS SOON AS THE ACFT PASSED; I ASKED FOR LOWER AND WAS CLRED TO 9000 FT; BUT WAS STILL IN THE CLOUDS AND COULD NOT SEE. I ASKED CENTER FOR 7000 FT AND WAS CLRED TO 7000 FT. BY THIS TIME I WAS ABOUT 20 MI E OF GILA BEND VOR AND I HAD GND CONTACT. AT ABOUT 10 MI E OF THE VOR I SUDDENLY EXPERIENCED SEVERE TURB AND HVY RAIN. THE TURB WAS SEVERE ENOUGH THAT THE DOOR POPPED OPEN; THE TOW BAR IN THE BACK HIT CEILING OF THE ACFT SEVERAL TIMES; THE ACFT WAS ALMOST IMPOSSIBLE TO CTL AND VISIBILITY WAS REDUCED TO ABOUT A MI IN LESS THAN A MIN. IT WAS AT THIS TIME I CANCELLED IFR AND STARTED TO HEAD FOR THE GILA BEND ARPT BUT VISIBILITY WAS POOR SO I DECIDED TO LAND AT THE GILA BEND AIR FORCE BASE SINCE I HAD IT IN SIGHT. I STARTED TO LOOK FOR A FREQ TO GET IN TOUCH WITH THE AIR FORCE BASE BUT COULD NOT LOCATE ONE IN MY COMMERCIAL CHARTS OR ON THE LOW ENRTE CHARTS AT THE TIME. SO THE DECISION WAS MADE TO GO AHEAD AND LAND AT THE AIR FORCE BASE BY ANNOUNCING MY INTENTIONS OVER UNICOM FREQ.