AN ACR LGT LOST AIRSPD AND ALT IN A DOWNDRAFT ASSOCIATED WITH A SEVERE WX SYS.

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FL390. PRIOR TO OCCURRENCE; AUTO THROTTLE VARYING PLUS OR MINUS 10 KTS AT CRUISE. NOTICED FAST SLOW INDICATOR ADI PEGGED SLOW AND AIRSPD DECAYING. WITHIN 60 SECONDS OR LESS; IAS WAS BELOW 200 IAS. NO PITOT HEAT OR AOA MALFUNCTION INDICATED. AUTO THROTTLES DID NOT RESPOND TO COMMAND BUG SPD; YET ACFT WAS HOLDING ALT WITH NO PITCH INCREASE. LIGHT BUFFET FELT. REQUESTED FL370 AND INITIATED 1500 FPM DSCNT. AUTO THROTTLE OFF AND AUTOPLT OFF AND THROTTLES MAX FORWARD. APCHING FL370 AIRSPD STILL BELOW 200 KIAS. FL350 REQUESTED AND DSCNT CONTINUED. AT FL360 FELT AS THOUGH ACFT JUST RELEASED AND EVERYTHING TURNED TO NORMAL. STAYED AT FL350 FOR 10 MINS; THEN CLBED TO FL370 AND THEN FL390. ALL OPS NORMAL. AREA OF WX WE ENTERED WAS A LINE 50 PLUS MI WIDE EXTENDING FROM CANADA TO TX. PRIOR TO DEPARTING IAD AND 20 MINS FROM AREA; DISPATCH CONFIRMED TRACK WAS GOOD. ATC APPROVED MINOR DEVS AND PUT LARGE CELL 20 PLUS NM N OF TRACK AND SMALLER CELL S OF TRACK. CONTINUOUS IGNITION AND ENG ANTI-ICE 'ON' BEFORE ENTERING STRATUS/CIRRUS CLOUDS. AS WE ENTERED; INTERMITTENT VERY LIGHT CHOP ENCOUNTERED; THEN IAS BEGAN DROPPING AND LIGHT BUFFET FELT. ACFT FELT AS THOUGH WE HAD A WT INCREASE OF 2000-3000 LBS. PAX COMMENT AFTER LNDG SFO; 'WE MUST HAVE BEEN IN AN AIR POCKET.' ENTIRE TIME FRAME 4-5 MINS; YET IT FELT LIKE AN ETERNITY. TORNADO SIGHTED OVER DECATUR; IL; JUST PRIOR TO OCCURRENCE. ACR DISPATCH HAD NO WX PHENOMENON AT TIME IN EXACT AREA. ACFT HAD 1/8 INCH ICE ON WIPER ARMS; NONE ON WING; NACELLES OR WINDSHIELD. RADAR WAS ADJUSTED TO INDICATE SIGNIFICANT WX AT OUR ALT. I USED 2-3 DEGS NOSE DOWN TO CONFIRM EXTENSIVE RAIN N AND S OF TRACK. 26 YRS AIRLINE EXPERIENCE: SOUTH AMERICA; SOUTH PACIFIC; ETC; NEVER HAD THIS TYPE OCCURRENCE.