EMB ERJ 145 ER/LR
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew Overcame Equipment Problem
Aircraft; Weather
A line of thunderstorms was approaching PNS from the west.  After discussions with ATC and Dispatch; we elected to hold at GCV VOR to try to let the storms pass PNS. After about [45 minutes] of holding; in IMC; at FL350; we decided to divert to ZZZ.  We notified ATC of our intentions; and they gave us a new heading and a lower altitude.  Upon commencing our descent; we got a SPS ADVANCE caution on the EICAS.  Immediately following the message; the First Officer (FO) informed me [that] my airspeed was low and trending lower.  We both realized very quickly that the Captain (CA) side airspeed was unreliable.  I continued the descent and turned off the autopilot while the FO ran the QRH.  The FOs airspeed seemed to be functioning properly; at first.  We informed ATC that we would need a lower altitude and had an emergency in progress.  The CA airspeed eventually went blank; and the FOs went as low as about 150KIAS.  The event continued for about 4 to 5 minutes; and both airspeeds returned to functioning normally in the mid 20's.  After descending to 11;000 feet; and verifying that all airspeed indicators were indicating correctly with our pitch/power tables; and verifying airspeed and altitude with ATC; and with the ground in sight; we cancelled the emergency and landed with no event.Threats: No airspeed indicationAvoid holding in IMC.  If possible.
An EMB-145 Captain reported losing reliable airspeed indications while operating in IMC.