Narrative:

After takeoff from ZZZ on runway xxr; we received an EFIS comp mon message for altitude and airspeed. After comparing between the captain (ca) pfd; standby instruments; and my pfd; we determined the cas was the bad one. We then transferred the autopilot to my side and switched the transponder to my side. Cas side airspeed and altitude was off by as much as 50 knots airspeed and 300-400 feet on altitude. After switching autopilot and transponder to my side; I took the controls and radios. We believed this to be an ADC1 failure; and ran the appropriate QRH procedures which led to us selecting ADC2 for the ca. We continued on to [destination] until we heard from maintenance and dispatch on what they wanted to do. After contacting them via ZZZ radio; they determined they wanted us to return to ZZZ. We informed ATC that we were not an emergency aircraft; but we needed to return to ZZZ due to a maintenance issue. After starting towards ZZZ; we determined we were about 1;500-2;000 pounds over max landing weight with our current fuel. This was complicated because we couldn't view our normal indications on the mfd that tell us what our landing fuel is and times. We asked for delay vectors from approach and they gave us multiple delay vectors at 7;000 feet while we tried to burn fuel. We eventually got down to max landing weight and safely landed on runway xxr and taxied in. During the flight; we believed it to be an ADC1 failure. Once we were on the ground and maintenance looked it over; their preliminary hypothesis was that it was water trapped in the cas side pitot tube and inside the avionics bay. I don't know if that was the final result; because we left to take another plane and continue on with the flight.

Related reports:

Google
 

Original NASA ASRS Text

Title: CRJ-200 flight crew reported returning to departure airport after noting EFIS COMP MON anomalies that Maintenance later said were related to water in the Captain's pitot/static system.

Narrative: After takeoff from ZZZ on Runway XXR; we received an EFIS COMP MON message for ALT and Airspeed. After comparing between the Captain (CA) PFD; Standby Instruments; and my PFD; we determined the CAs was the bad one. We then transferred the autopilot to my side and switched the transponder to my side. CAs side Airspeed and Altitude was off by as much as 50 knots airspeed and 300-400 feet on altitude. After switching autopilot and transponder to my side; I took the controls and radios. We believed this to be an ADC1 failure; and ran the appropriate QRH procedures which led to us selecting ADC2 for the CA. We continued on to [destination] until we heard from Maintenance and Dispatch on what they wanted to do. After contacting them via ZZZ Radio; they determined they wanted us to return to ZZZ. We informed ATC that we were not an emergency aircraft; but we needed to return to ZZZ due to a maintenance issue. After starting towards ZZZ; we determined we were about 1;500-2;000 LBS over max landing weight with our current fuel. This was complicated because we couldn't view our normal indications on the MFD that tell us what our landing fuel is and times. We asked for delay vectors from approach and they gave us multiple delay vectors at 7;000 feet while we tried to burn fuel. We eventually got down to max landing weight and safely landed on Runway XXR and taxied in. During the flight; we believed it to be an ADC1 failure. Once we were on the ground and Maintenance looked it over; their preliminary hypothesis was that it was water trapped in the CAs side pitot tube and inside the avionics bay. I don't know if that was the final result; because we left to take another plane and continue on with the flight.

Data retrieved from NASA's ASRS site 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.