Date: 201812
Local time of day: 1201-1800
Location: ZZZZ.ARTCC, FO
Flight conditions: IMC
Weather elements / visibility: Cloudy; Icing
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZZ
Aircraft component: Ice/Rain Protection System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Confusion
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Crossing Restriction Not Met; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Became Reoriented
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

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.


Second narrative: Deviating around weather approximately 15 NM South of ZZZZZ waypoint FL 240 we entered the bottom of an overhang/cloud layer. Engine Anti-Ice was turned on prior to entering IMC conditions; a few minutes later on both CA/FO (Captain/ First Officer] PFD'S an 'Airspeed Disagreement' appeared followed by Auto-pilot disconnect. Co-Pilot disconnected Auto-throttles and began hand flying the A/C. Captain's and Stand-by Airspeed indications were within approximately 10 knots of each other and increasing. FO's airspeed indication was approximately 25 knots slower and rapidly decreasing. I cross checked our GPS ground speed and it indicated an increasing ground speed. We began trying to determine if any of our airspeed indications were valid.The Amber Master Caution light 'ENG' illuminated next and the FO and I agreed that one or all of our Pitot-tubes must have been frozen and/or blocked. At this time it appeared that we had descended but were not exactly sure how much because of different altitude indications. We exited IMC conditions and shortly thereafter our Airspeed/Altimetry systems began giving us valid information.We realized that we had descended below FL 240 and climbed back to altitude. We reviewed all applicable QRH procedures and continued on to [our destination.]Avoid lower level overhang/layer.


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