A Mooney M20 pilot reported encountering ice in cruise flight despite a forecast that indicated ice at his altitude was unlikely. His airspeed began to decay and he got a descent clearance from ATC.

Narrative

I was in cruise at 11;000. An AIRMET was in effect for the area for moderate ice between 11;000 and 16;000; along with convective activity (showing on my Stormscope) ahead and moving in the same direction I was. I entered IMC at 11;000 FT with no ice; requested and received climb to 13;000 FT in order to stay VMC for better weather avoidance. Temperatures at 11;000 FT were plus 1 degrees C. I reentered IMC at 13;000 FT with no turbulence and; initially; no ice; temperature at minus 1 degree C. After a few minutes; I began picking up a trace of rime ice; but with minimal accumulation. Pre-flight review of the weather icing forecasts; using the websites flight path tool; indicated freezing level along my route of 11;000 and a minimal chance of trace ice at that altitude; with a slight chance of moderate ice at 13;000 FT. Despite the AIRMET; the combination of the more specific weather forecast and the existence of good escape routes (freezing level 1;000 FT below; bases running around 6;000-8;000 FT; and MEAs at or below 4;000 FT) led me to be willing to remain at 13;000 FT despite the trace of ice.About 5 minutes later; I started to pick up moderate rime ice. Within about 30 seconds; the airspeed had decayed about 5-8 KTS. With significant congestion on the frequency; it took me another 45 seconds or so to request descent to 11;000 FT. ATC told me to stand by because of crossing jet traffic descending from 13;000 FT behind me. I acknowledged; voicing my hope that the jet 'hustle on down!' After another 30 seconds (and a further 3 or so knot airspeed decay); ATC cleared me to 11;000 FT; and I began an immediate and expeditious descent. I felt that in another 30-60 seconds I would have had to begin descending with or without the ATC clearance; even if that meant some kind of certificate action. (I felt that I would rather face certificate action for an altitude deviation than face the potential consequences of remaining in moderate icing conditions much longer!) In retrospect; I should have requested descent as soon as the trace of ice accumulated at 13;000 FT. Temperatures around minus 1 degree C in visible moisture are pretty ripe for ice and; with ATC as busy as they were; I could have expected a delay in making my request. Moreover; the reason for being at 13;000 FT instead of 11;000 FT was gone (though the 20 KTS ground speed increase at 13;000 FT was very nice to have!). Despite the encounter; I think that the internet websites advanced forecast products were very valuable for planning this flight.