FLC OF MLG REMAINS AT LOW ALT DUE TO SEVERE TURB HIGHER. THEY STILL ENCOUNTER UPDRAFTS AND DOWNDRAFTS AND REQUEST STILL LOWER. ATC CLRS LOWER AND TURNS THEM FOR TFC.

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INCIDENT TOOK PLACE DURING A FLT FROM ACY TO CLT. UPON ARR AT THE ARPT FOR MY FLT FROM DTW; MY RELEASE AND WX PACKAGE AGREED WITH MY PRIOR INFO. PIREPS BY B757; B737; DC9; ETC; RPTED SEVERE TURB ABOVE FL180 FOR MY AREA OF FLT THAT THE FLT WOULD PASS. AS WE APCHED HARRISBURG ENRTE TO ACY; WE WERE SLOWED TO 250 KTS DUE TO TFC SATURATION INTO PHL. I QUERIED ATC AND HE STATED THAT DUE TO THE ROUGH RIDES ABOVE FL180 EVERYONE WAS DOWN LOW; INCLUDING TFC NORMALLY AT FL290 AND ABOVE. AGAIN; MORE ADVANCE KNOWLEDGE OF WX WE WOULD ENCOUNTER FROM ACY TO CLT. FLT PUSHED AND DEPARTED FOR CLT. WE WERE FILED FOR FL240 AND CLBED TO 16000 FT JUST N OF BWI. I ASKED MY FO TO QUERY WHEN WE MIGHT GET HIGHER. THE APCH CTLR AT DCA SAID HE ASSUMED WE WOULD NOT WANT TO GO HIGHER DUE TO ALL THE ENCOUNTERS WITH SEVERE TURB BY OTHER ACFT. SINCE FLT PLAN FUEL BURN WAS 8340 LBS AT FL240 AND WE WERE CARRYING 24600 LBS; I CHOSE TO TAKE HIS ADVICE. ABOUT 45 MINS OUT OF ACY; SOMEWHERE NEAR OR S OF GVE VOR; I NOTICED MY AIRSPD GO FROM 310 KIAS TO 340 KIAS IN ABOUT 5-10 SECONDS. SINCE 340 KIAS WAS 'RED' LINE I REDUCED PWR SO AS NOT TO OVERSPD; AT THE SAME TIME TRYING TO FIGURE OUT WHAT WAS HAPPENING AS THE FLT WAS IN SMOOTH AIR. AS SOON AS MY AIRSPD BEGAN TO SETTLE DOWN IT JUST AS ABRUPTLY LOST 30 KTS AND WAS DROPPING TO 280 KIAS. AT THAT POINT I ADDED PWR AND WE ENTERED AN AREA OF SEVERE TURB; INITIALLY CAUSING THE ACFT TO CLB +600 FT TO 14600 FT; AT WHICH POINT THE AUTOPLT DISENGAGED. WITH HANDS READY; I ASSUMED CTL OF THE ACFT AND REDUCED PWR; PUSHING THE NOSE OVER TO DSND; SIMULTANEOUSLY I CALLED ATC (EITHER DCA OR ATL). 'CTR XYZ.' ATC CAME BACK; 'CHK YOUR ALT.' 'CTR XYZ IS IN SEVERE TURB AND WE WANT AN IMMEDIATE DSCNT TO 14000 FT.' ATC; 'DSND NOW TO 14000 FT; TURN R TO 270 DEGS FOR YOUR DSCNT.' AT THAT POINT I ASKED THE FO TO MAKE AN ANNOUNCEMENT AND GET THE FLT ATTENDANTS SEATED. AS I STARTED MY TURN TO 270 DEGS I ATTEMPTED TO DSND BUT FOUND IT DIFFICULT TO DO; EVEN AT 1.1 EPR DUE TO WHAT MUST HAVE BEEN A WAVE ASSOCIATED WITH THE JETSTREAM. AFTER ABOUT 3 MINS WE WERE LEVEL AT 14000 FT AND THE AIR WAS AGAIN SMOOTH. OUR ENCOUNTER WITH THE TURB LASTED ABOUT 30 SECONDS CAUSING THE ALT TO CHANGE +/-600 FT FROM OUR ASSIGNED ALT OF 16000 FT. APPROX 3-5 MINS BEHIND US WAS ANOTHER AIRLINER AT 16000 FT FOLLOWING US S ON THE SAME ATC FREQ AS OUR FLT. THEY CAME ON THE AIR AND SAID THEY TOO HAD ENCOUNTERED THE SAME TURB AND TOO WANTED TO DSND TO 14000 FT.