Flight Crew Private; Flight Crew Instrument
Flight Crew Last 90 Days 36; Flight Crew Total 3828; Flight Crew Type 2955
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Weather / Turbulence; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Regained Aircraft Control
Weather
Cruising at six thousand as assigned in IMC in relatively smooth air we encountered a large downdraft of building storm that was not on our NEXRAD as there was very little if any rain. ATC also did not advise of any condition such as this. Aircraft [was] put into 1;500 FPM dive and 50 degree bank. This upset the autopilot which we promptly disconnected. Upon recovery we were almost one thousand feet below assigned altitude. Advised ATC of the deviation and issued Pirep to ATC.
M20 pilot reported an upset from a building storm that resulted in a 50 degree bank and 1;500 FPM rate of descent. Aircraft control was recovered after a loss of 1;000 feet.