B737-800
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Total 15000
Confusion; Distraction; Troubleshooting; Workload
Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft; Weather
Descending from FL 300 to FL 270. Voice/Aural 'Airspeed Low' warning; accompanied by IAS Disagree and ALT disagree cautions. Captain airspeed tape shot down to approx. 170 KIAS; Captain altitude indication climbed rapidly; autopilot commenced nose down pitch and auto throttles increased to gain airspeed. Autopilot/autothrottles disconnected; crosscheck with flight path vector; AOA indication and standby instruments quickly confirmed cautions.  [Advised ATC]; QRC procedure and follow-up QRH procedure performed; cleared by ATC to descend and proceed direct ZZZ. Captain gave aircraft to First Officer once stable and insured all procedures completed and correct. First Officer indications returned to normal much more quickly (passing approximately FL210. Captain indications returned to normal more slowly; but were correct approaching approx. 15;000 [feet]. Entire evolution took place in solid IMC; with all pitot heat and engine anti-ice switches in correct places and appearing to be functioning normally. Opted to remain [a priority] aircraft due to uncertainty of actual malfunction; and because of intermittently poor ceiling/visibility conditions at ZZZ. Uneventful descent/approach/landing in ZZZ. Logbook write-up completed; Avionics Tech personally debriefed post flight in ZZZ.  Suspect some form of pitot tube ice up or blockage; despite pitot heat on and appearing to be functioning. Ice crystals maybe? Just an unlucky combination of OAT; moisture content; winds aloft; etc?
B737-800 pilot reported encountering erroneous inflight airspeed indications; possibly due to ice contamination of the pitot-static system.