A319 FLT CREW ENCOUNTERS TURB AND DOWNDRAFT VICINITY OF TSTMS; HAS 400 FT ALTDEV.

Narrative

ATC APPROVED WX DEV BTWN TSTM CELLS. CRUISE FLT; FL370; AUTOPLT ON; SMOOTH AIR. SPLIT DIFFERENCE BTWN CELLS ON RADAR; APPROX 15 MI FROM EACH CELL; VARIABLE CLOUD CONDITIONS; ENCOUNTERED SOME LIGHT TURB; NO TFC IN IMMEDIATE AREA; EXITED TURB; AND WITH AUTOPLT ON; ENCOUNTERED DOWNDRAFT IN EXCESS OF 1500 FPM. QUICKLY LOST 400 FT. AS I WAS ABOUT TO CONTACT ATC AND ADVISE; ATC CONTACTED US AND ASKED WHAT OUR ALT WAS. I INFORMED ATC THAT WE HAD ENCOUNTERED A DOWNDRAFT AND HAD LOST 400 FT AND WERE NOW RESUMING LEVEL FLT AT FL370. NO FURTHER COMMENTS FROM ATC REGARDING ALT DEV. NO KNOWN TFC CONFLICTS; VERY LITTLE TURB OF ANY CONSEQUENCE AND AUTOPLT ENGAGED DURING ENTIRE EVENT. FLT PROCEEDED NORMALLY TO DEST; ON TIME. IT IS UNREALISTIC TO SAY NEVER FLY NEAR TSTMS. IN REALITY IT IS INEVITABLE. WE ALL TRY TO GIVE TSTMS A WIDE BERTH AND IN THIS CASE WE DID. WE HAD A 30 MI GAP TO FLY THROUGH; SPLIT THE DIFFERENCE; ENTERED SOME CLOUDS THAT WERE NOT SHOWING ANY RADAR RETURN. WE ENCOUNTERED SOME LIGHT TURB OF LITTLE CONSEQUENCE; GOT INTO A DOWNDRAFT THAT WE HAD NO WAY OF KNOWING ABOUT. THE ONLY ACTION I CAN ESPOUSE WOULD BE TO STAY HOME OR CARRY ENOUGH FUEL TO GIVE THESE SMALL TSTM LINES AT LEAST 100 MILES OF CLEARANCE. I'VE BEEN CIRCUMNAVIGATING TSTMS FOR OVER 30 YRS AND HAVE NOT YET BEEN ABLE TO MAKE A FOOLPROOF DETERMINATION AS TO PRECISELY HOW CLOSE IS TOO CLOSE. KEEP YOUR DISTANCE IS THE BEST ADVICE.