Flight Crew Instrument; Flight Crew Private
Flight Crew Last 90 Days 12; Flight Crew Total 1748; Flight Crew Type 1650
Training / Qualification
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Human Factors; Weather; Aircraft
I was flying through an area of developing isolated precipitation. I had been authorized by ATC to deviate multiple times due to precip and build-ups but at the time was on my flight plan direct route. At the time my Nexrad weather (Sirius XM through a Garmin display) showed no precip directly in front of me. Upon entering a cloud (cumulus with base at about 6;000; top estimated at 13;000) I immediately encountered heavy precipitation and severe turbulence with increases (300+ FT) and decreases (200 FT) in altitude. Upward turbulence caused me to exceed 300 FT (by about 50 FT) from my assigned 10;000 FT altitude on one occasion. I had switched off my autopilot and was hand flying the aircraft as soon as I encountered turbulence.  As I was recovering and was back at 10;000; Memphis Center advised me that he showed me more than 300 FT over my assigned altitude. He then gave me the local altimeter setting. I advised him of the brief encounter with precip and turbulence that caused my changes in altitude. Center advised that he showed no precip on my route at that time. He authorized me to deviate as needed to avoid further build-ups.  My Nexrad had shown no precip either. Less than a minute after the incident my Nexrad showed an area of precip immediately behind me. I continued to my destination without further incident. In retrospect I should have asked to deviate around the cloud in front of me. I was in VMC prior to entering the cloud and saw no indication I would encounter precip. I could see the cloud base and saw no precip and since I saw none on my Nexrad I chose to stay on my direct route rather than ask for a deviation around. The fact that I was in an area of rapidly developing isolated precip should have told me that conditions were such that there was a likelihood of encountering what I encountered; regardless. In such conditions requesting deviation off course would be a much better plan.
PA-32 pilot reports entering a rather benign looking cumulus cloud; that has no returns showing on XM Nexrad; to find a rough ride with 300 foot altitude deviations. Once clear of the cloud the Nexrad does show some returns.