AN EMB-145LR (ERJ-145LR) ACFT EXPERIENCED AN APPARENT SUDDEN AIRSPEED REDUCTION. CAPTAIN'S AIR DATA COMPUTER AIRSPEED FAILED; FOLLOWED SHORTLY BY CO-PILOT'S ADC FAILURE; ALLEGEDLY DUE TO WATER IN THE PITOT STATIC SYSTEM. STANDBY AIRSPD INDICATOR APPARENTLY READ CORRECTLY AT 310 KTS.

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CAPT FLYING PILOT -- LEVEL 37000 FT; IMC -- THUNDERSTORMS -- WE RECEIVED A SUDDEN AIRSPEED REDUCTION. I; THE FLYING PILOT REACTED WITH FULL THRUST WITH NO IMPROVEMENT AND CONTINUOUS SPEED REDUCTION. I PUT THE PLANE IN A DESCENT TO GAIN AIRSPEED MEANWHILE NOTIFYING ATC WE NEEDED AT LEAST DOWN TO 33000 FT TO TRY AND GAIN AIRSPEED. AT THIS POINT I REALIZED MY AIR DATA COMPUTER FAILED. WE THEN LEVELED AT THE ASSIGNED ALTITUDE AND RAN THE QRH AND CONTACTED MAINT. MEANWHILE THE CREW NOTICED THE AIRCRAFT SLOWING ON THE NUMBER TWO ADC 210 KTS; BUT THE AIRSPEED ON THE STAND-BY STEAM INDICATOR [AIRSPD] WAS READING 310 KTS. I THEN ASSUMED THE NUMBER 2 AIR DATA COMPUTER AIRSPEED FAILED. WE THEN DECLARED AN EMERGENCY AND REQUESTED TO GET OUT OF IMC CONDITIONS. WE BEGAN OUR DESCENT IMMEDIATELY AND LEVELED AT 23000 FT. THE NUMBER 2 AIRSPEED CAME BACK OVER TIME AND MATCH THE STAND-BY. WE CONTINUED DOWN AND LANDED AT OUR EXPECTED DESTINATION. FIRST DESCENT FROM 37000 FT TO 34000 FT WAS DUE TO MYSELF TRYING TO CORRECT FOR LACK OF AIRSPEED (THINKING A DECREASE IN PERFORMANCE FROM THE WEATHER) AND WITH THEN NO CHOICE; BUT TO DESCEND. I WAS NOT MORE CONCERNED WITH NOT STALLING. AFTER STARTING THE DESCENT I THEN HAD THE OPPORTUNITY TO SEE ADC FAILURE. SECOND DESCENT WAS DUE TO TWO AIRSPEED INDICATION FAILURES AND BEING IN ICING AND IMC. THERE IS ONLY A QRH FOR SINGLE ADC FAILURE. THERE IS NO QRH FOR DUAL AIRSPEED OR ALT (ETC).CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED AN AMBER INDICATION NOTING THE #1 AIR DATA COMPUTER AIRSPEED FAILURE HAD COME ON HIS PFD DURING THE AIRSPEED INDICATION BLEED-OFF; BUT; NO RED 'X' DISPLAYED THRU HIS AIRSPEED. THE EICAS AMBER INDICATION DISPLAYED; BUT NO MESSAGE. ALSO; HIS CO-PILOT'S PFD DID NOT DISPLAY ANY AMBER WARNING EVEN THOUGH THE AIRSPEED INDICATION HAD BLED DOWN TO 210 KTS. REPORTER STATED WHEN HE NOTICED THE STAND-BY AIRSPEED INDICATOR SHOWING 310 KTS; HE FELT THAT WAS FAIRLY CLOSE TO THE SPEED HE THOUGHT THEY SHOULD HAVE BEEN AT. THE ENGINE THROTTLES DID RESPOND APPROPRIATELY. HE BELIEVES THE STAND-BY AIRSPEED INDICATOR IS DIRECT READING FROM THE #3 PITOT ON THE RIGHT SIDE OF THE FWD NOSE AREA. ALTHOUGH THEY WERE IN THUNDERSTORM WEATHER WHICH WAS REACHING 50000 FT; REPORTER STATED HE DOES NOT BELIEVE ANY RAIN HAD FALLEN DURING THE PREVIOUS NIGHT AT THE AIRPORT THEY HAD DEPARTED. EVEN THOUGH THEY HAD DECLARED AN EMERGENCY; THEY DID NOT HAVE TO DIVERT. UPON ARRIVAL; ONE OF THE MAINT PERSONNEL MENTIONED HE THOUGHT HE KNEW WHAT THE PROBLEM WAS. MECHANIC RETURNED SHORTLY SAYING THERE WAS WATER IN THE PITOT STATIC LINE AND THIS WAS A FAMILIAR PROBLEM WITH THIS PITOT SYSTEM.