B737 FLC OVERSHOT CLRED ALT WHEN SEVERE TURB ENCOUNTERED AND CTL OF ACFT LOST.

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FLT DEPARTED RWY 18R IN TPA; CLBING TO 12000 FT ON A HDG OF 180 DEGS. COPLT WAS HAND FLYING ACFT AND I WAS WORKING ATC AND RADAR. APCHING 10000 FT; WE ENCOUNTERED MODERATE TURB; WHICH RAPIDLY ESCALATED TO SEVERE TURB ACCOMPANIED BY VERY HVY PRECIP. COPLT ASKED FOR HELP CTLING THE ACFT AND BOTH OF US WERE UNABLE TO LEVEL IT OFF AT 12000 FT. THE ACFT WAS BOUNCING SO HARD THAT THE INSTS WERE IMPOSSIBLE TO READ SO I TOLD THE COPLT TO CONTINUE ON AND I ATTEMPTED TO DECLARE AN EMER. WAS UNABLE TO GET THROUGH INITIALLY; DUE TO OTHER RADIO TFC; AND WHEN I FINALLY GOT THROUGH AND DECLARED THE EMER; WE WERE AT APPROX 13500 FT. I WAS IMMEDIATELY SWITCHED OVER TO ZMA; RE-DECLARED THE EMER WITH THEM AND WAS CLRED TO FL230. 1 MIN OR 2 LATER; WE BROKE OUT OF THE WORST OF THE WX; AND WAS ASKED AGAIN ABOUT THE EMER BY ZMA. SINCE WE APPEARED TO BE CLR OF THE WORST OF IT AND THE COPLT WAS ONCE MORE ABLE TO CTL THE ACFT BY HERSELF; I TOLD ZMA TO CANCEL THE EMER. THE REMAINDER OF THE FLT WAS UNEVENTFUL AND POSTFLT INSPECTION OF THE ACFT BY THE COPLT REVEALED NO PERCEPTIBLE DAMAGE. SUPPLEMENTAL INFO FROM ACN 569382: PASSING THROUGH 10000 FT; WE ENCOUNTERED SEVERE TURB WITH HVY HAIL. PASSING THROUGH FL180; THIS TURB SUBSIDED. THE CAPT ENTERED THE EVENT INTO THE MAINT LOGBOOK.