SMA PLT ON IFR FLT ENCOUNTERS UNFORECAST SEVERE ICING AND BEGINS UNCTLED DSCNT ON TOP OF OTHER IFR IMC TFC. ARTCC CTLR APPARENTLY DOES NOT HAVE RADAR AND ISSUES A TURN TO THE RPTR FOR CONFLICT AVOIDANCE.

Narrative

ENTERED A CLOUD LAYER AT 8000 FT. AIR TEMP WAS PLUS 1 DEG C. FIRST FEW MINS; ENCOUNTERED RAIN. THIS IMMEDIATELY TURNED TO ICE-MODERATE TO SEVERE MIXED ICE. LOST 200 FT. TOLD ATC OF PROBLEM. ASKED FOR LOWER AS AN SMA CAN'T CLB. WAS DENIED DUE TO PLANE OVERTAKING ME AT 7000 FT. (RADAR SVC WAS NOT WORKING AT THIS TIME). IMMEDIATELY; MY PLANE WENT INTO A RAPID DSCNT AND AIRSPD STARTED DECREASING. CHKED TO SEE IF PITOT HEAT WAS ON. IT WAS. NOW; I'M TELLING CTR; I'M IN A RAPID DSCNT. THEY ASKED MY ALT. I TOLD THEM 6500 FT (NOTE: I INFORMED CTR TWICE THAT I COULD NOT HOLD ALT). THEY TOLD ME TO MAKE AN IMMEDIATE R TURN. I COMPLIED. I THEN BROKE INTO VMC AT 6300 FT WITH A 20 MI VISIBILITY BELOW. I THEN LANDED AT UKIAH; AND; AT ATC REQUEST; CALLED THEM ON THE PHONE TO GIVE THEM AN EXPLANATION OF WHAT HAPPENED. THE PROBLEM; AS I DESCRIBED; HAPPENED OVER A PERIOD OF 30 SECS. I HAVE OVER 150 HRS OF ACTUAL IFR. BUT HAVE NEVER ENCOUNTERED SO MUCH ICE SO FAST. THE FORECAST WAS FOR SCATTERED TO BROKEN ALL ALONG THE RTE WITH NO FORECAST OR PIREPS OF ICING AT THAT ALT. IN ANALYZING THIS; I FEEL THAT I ACTED IN THE CORRECT MANNER AND CONTINUOUSLY KEPT ATC ADVISED OF THE SITUATION. THE LNDG AT UKIAH WAS MY DECISION AS A PRECAUTIONARY MEASURE. I WAS ENRTE FROM CONCORD TO EUREKA. I FELT ONCE ON THE GND I COULD CHK ON MY EQUIP. CHK ON WX AND GIVE A PIREP TO FSS.