ACFT FLEW INTO SEVERE TURB AND DSNDED 600 FT BELOW CLRED ALT.

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ENRTE AT 9000 FT WITH BOTH RADAR AND STORMSCOPE OPERATIVE. THE RADAR (BLACK AND WHITE) WAS NOT SHOWING ANY CONTOURS AND STORMSCOPE WAS SHOWING WX ON EITHER SIDE OF COURSE ABOUT 20 MI OUT. I WAS FEELING PRETTY GOOD THAT WE WOULD NOT HAVE TO DIVERT AROUND TSTMS WHEN WE ENTERED MODERATE RAIN. THE RIDE WAS SMOOTH WHEN SUDDENLY LIGHTNING DEVELOPED AND THE STORMSCOPE LIT UP LIKE A CHRISTMAS TREE. AT THIS POINT; WE HAD BEEN IN THE RAIN FOR ABOUT 1 MIN SO I DECIDED THE BEST COURSE OF ACTION WAS TO MAKE A 180 DEG TURN. I INFORMED ATC OF MY INTENTIONS AND WAS CLRED TO 10000 FT. AS I ENTERED THE TURN; I ENCOUNTERED SEVERE TURB. ALTHOUGH THE ACFT WAS SET FOR CLB; I LOST 600 FT OF ALT. ATC QUESTIONED ALT LOSS AS I AGAIN STARTED MY CLB TO 10000 FT AND I EXPLAINED TO HIM THAT I WAS IN A TSTM AND COULD NOT MAINTAIN ALT. ENTIRE ENCOUNTER WAS OVER IN ABOUT 2 MINS. I ASSUME I ENTERED A DEVELOPING TSTM AND THAT IS WHY IT DID NOT SHOW UP ON THE STORMSCOPE. ALTHOUGH I TOOK WHAT I THOUGHT TO BE THE BEST COURSE OF ACTION; THE 180 DEG TURN; THE ENCOUNTER WITH TURB WAS UNAVOIDABLE AND THEREFORE THE LOSS OF ALT WAS UNAVOIDABLE. IF I WAS I THE SAME SIT AGAIN; I WOULD DO THE SAME THING.