Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Aircraft Equipment Problem Dissipated; Flight Crew Landed As Precaution; General Declared Emergency; General Maintenance Action
Aircraft; Weather
While cruising at 37;000 feet we observed what looked like windshield condensation on the front windows that turned out to be ice.  The ice detector did not detect ice so we manually selected the anti ice to 'ALL.'  We then received an amber IAS comparator alert.  Checking my instrument with the standby and Captain's; we deemed mine to be inaccurate.  Shortly after; we got an altitude comparator alert with SPS advanced and the AUTOPILOT FAIL. We decided this could be severe icing and requested an immediate descent.  ATC was unable to accommodate us due to traffic so we declared an emergency.  When we began our descent; my airspeed showed slowing while the Captain's accelerating leading me to believe that my pitot tube had been blocked. We were about 100 miles from ZZZ; it was our nearest suitable airport as we had to work hard to get it down in time to do the ILS. Aircraft handling was reported normal by the captain (PF) and as we descended airspeed and altimeter readings appeared to return to normal indications.  We requested ATC to give us frequent airspeed and altitude read outs to back us up in case we were experiencing any failures that we were unaware of.  When we landed all EICAS messages were cleared and all indications in the cockpit were normal. The Captain contacted Dispatch and Maintenance while I did my post flight inspection so they could have Maintenance check ADC and ice detector issues.  No defects were noted during post flight and the aircraft was found with no ice upon landing.
Ice detector did not detect ice and we lost valuable minutes until ice was visible to flight crew. Suspected unforecast severe ice encounter.  Perhaps better detection of severe icing if that is possible (we were just completing a long deviation around level 5 thunderstorms). Better maintenance of ADC instruments and/or ice detector.
EMB145 Flight Crew experiences the loss of the First officer's airspeed indication at FL370.  An emergency is declared and the flight continues to destination for a normal laneding.  During descent all pitot static systems return to normal.