Narrative:

We passed through a heavy line of thunderstorms -- heavy rain and turbulence. At 35;000 ft we climbed to 37;000 ft. At approximately 36;000 ft the captain's airspeed began to gradually increase. As we leveled off at 37;000 ft the clacker [overspeed indicator] fell far below airspeed on captain's side (no other indications at this time) and began to sound. We began a descent to 33;000 ft -- mach trim caution message came on. I disconnected the autopilot because both airspeed and altitude were unreliable and I utilized the standby instruments at this time. At 33;000 ft; because of unreliable altitude and airspeed; decided to descend to lower altitude. I asked for and was given 27;000 ft. As we descended; IAS and altitudes messages appeared on both pilot's pilot flight display's. The mach trim caution message and other related caution messages came on repeatedly during the descent. As we descended below 30;000 ft the captain's airspeed began to fall (I believe that it fell below 100KIAS at one point). During the descent the first officer was working the checklist and speaking with maintenance - no solution to our particular problem was forthcoming. The intense noise of the airspeed clacker and bleed-over from the com 1 and com 2 made for a difficult work environment. We were approximately ten minutes out from an airport and I decided to declare an emergency and land there. The weather was good; and they have a station that is capable of handling passengers. Sometime around 25;000 ft the instruments appeared to return to normal; however there remained an approximate 800 ft discrepancy between the captain's altitude and the first officer's altimeter. Maintenance personnel noted that water was found in both #1 and #2 pitot drains. These drains were purged; the next morning we continued on without any further incident.

Related reports:

Google
 

Original NASA ASRS Text

Title: A CRJ 900 Captain reported losing flight instruments from water in the pitot system after encountering heavy rain enroute.

Narrative: We passed through a heavy line of thunderstorms -- heavy rain and turbulence. At 35;000 ft we climbed to 37;000 ft. At approximately 36;000 ft the Captain's Airspeed began to gradually increase. As we leveled off at 37;000 ft the clacker [overspeed indicator] fell far below airspeed on Captain's side (no other indications at this time) and began to sound. We began a descent to 33;000 ft -- MACH Trim caution message came on. I disconnected the autopilot because both airspeed and altitude were unreliable and I utilized the standby instruments at this time. At 33;000 ft; because of unreliable altitude and airspeed; decided to descend to lower altitude. I asked for and was given 27;000 ft. As we descended; IAS and ALTS messages appeared on both pilot's Pilot Flight Display's. The MACH Trim caution message and other related caution messages came on repeatedly during the descent. As we descended below 30;000 ft the Captain's airspeed began to fall (I believe that it fell below 100KIAS at one point). During the descent the First Officer was working the checklist and speaking with Maintenance - no solution to our particular problem was forthcoming. The intense noise of the airspeed clacker and bleed-over from the Com 1 and Com 2 made for a difficult work environment. We were approximately ten minutes out from an airport and I decided to declare an emergency and land there. The weather was good; and they have a station that is capable of handling passengers. Sometime around 25;000 ft the instruments appeared to return to normal; however there remained an approximate 800 ft discrepancy between the Captain's altitude and the First Officer's altimeter. Maintenance personnel noted that water was found in both #1 and #2 pitot drains. These drains were purged; the next morning we continued on without any further incident.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.