B737 flight crew reported airspeed and altitude indication errors in IMC cruise flight.

Attributes
ACN	1605757
Time
Date	201812
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZZ.ARTCC
State Reference	FO
Environment
Flight Conditions	IMC
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Ice/Rain Protection System
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Total 7250
Person 2
Function	Pilot Not Flying Captain
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Crossing Restriction Not Met Deviation - Speed All Types Inflight Event / Encounter Weather / Turbulence

Narrative:

We were deviating around weather southwest of zzzzz in ZZZZ's airspace at FL240. We entered a cloud layer (green on wx radar) with engine a/I (anti-ice) and probe heat on. Within a few minutes; on both pfd's; 'airspeed disagree' appeared followed by autopilot disconnect and erroneous airspeed and altitude indications on all instruments. Auto throttles also were affected and so I disconnected the a/T (auto-throttles) to manually fly the aircraft and maintain control. The captain side asi's were roughly within 10-15knots with airspeed increasing. My asi was about 30 knots slower and decreasing rapidly. Captain called out GPS ground speed was increasing and indicated increasing airspeed. We tried to figure out which if any of our airspeed indications were valid. We appeared to be descending but were not positive as we were receiving different altitude indications. As we exited the clouds into clear day VMC; our airspeed indicators and altimeters all self-corrected as the effects of the icing dissipated. We realized that we had descended below FL240 and climbed back to altitude. All systems were now normal and after leveling and re-trimming; full automation was returned. We reviewed all items applicable and continued on to our destination.when possible; avoid clouds/wx over the warm moist gulf air.

Original NASA ASRS Text

Title: B737 flight crew reported airspeed and altitude indication errors in IMC cruise flight.

Narrative: We were deviating around weather southwest of ZZZZZ in ZZZZ's airspace at FL240. We entered a cloud layer (green on wx radar) with engine A/I (Anti-Ice) and probe heat on. Within a few minutes; on both PFD's; 'airspeed disagree' appeared followed by autopilot disconnect and erroneous airspeed and altitude indications on all instruments. Auto throttles also were affected and so I disconnected the A/T (Auto-Throttles) to manually fly the aircraft and maintain control. The Captain side ASI's were roughly within 10-15knots with airspeed increasing. My ASI was about 30 knots slower and decreasing rapidly. Captain called out GPS ground speed was increasing and indicated increasing airspeed. We tried to figure out which if any of our airspeed indications were valid. We appeared to be descending but were not positive as we were receiving different altitude indications. As we exited the clouds into clear day VMC; our airspeed indicators and altimeters all self-corrected as the effects of the icing dissipated. We realized that we had descended below FL240 and climbed back to altitude. All systems were now normal and after leveling and re-trimming; full automation was returned. We reviewed all items applicable and continued on to our destination.When possible; avoid clouds/wx over the warm moist gulf air.

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.