Narrative:

A large line of thunderstorms was running approximately north and south in west texas across our proposed route of flight. Our clearance was direct to sso and we entered IMC. One particularly large thunderstorm caused us to ask for and receive clearance to deviate south of our course by about 30 degrees. As we deviated and the radar sweep stabilized 3 very small separate red areas of convective activity appeared on our chosen track. I verbalized that I would have preferred to have turned hard right and gone between the large storm and the 3 very small ones; but I didn't think aircraft performance would allow. I turned left in an attempt to round the very southern end of the 3 small cells. In this attempt moderate turbulence was encountered for about 3 seconds; but up- and downdrafts caused the aircraft to gain about 300 ft and then lose about 600 ft all with the autoplt engaged in altitude hold mode. I attempted to accelerate the return to the assigned altitude by overriding the autoplt in nose up pitch; but it corrected as I made my input. First officer made an immediate call to ATC to report the ride and the altitude excursion. No injuries. My major concern is that we were in rvsm airspace and because of natural forces; were off our assigned altitude. When convective WX dictates opposite direction aircraft go through the same 'holes;' rvsm should be suspended and 2000 ft vertical separation re-instituted.

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Original NASA ASRS Text

Title: A B737-300 OVER FST DEVIATED 600 FT FROM ASSIGNED ALT BECAUSE OF TURB NEAR TSTMS.

Narrative: A LARGE LINE OF TSTMS WAS RUNNING APPROX N AND S IN WEST TEXAS ACROSS OUR PROPOSED RTE OF FLT. OUR CLRNC WAS DIRECT TO SSO AND WE ENTERED IMC. ONE PARTICULARLY LARGE TSTM CAUSED US TO ASK FOR AND RECEIVE CLRNC TO DEVIATE S OF OUR COURSE BY ABOUT 30 DEGS. AS WE DEVIATED AND THE RADAR SWEEP STABILIZED 3 VERY SMALL SEPARATE RED AREAS OF CONVECTIVE ACTIVITY APPEARED ON OUR CHOSEN TRACK. I VERBALIZED THAT I WOULD HAVE PREFERRED TO HAVE TURNED HARD R AND GONE BTWN THE LARGE STORM AND THE 3 VERY SMALL ONES; BUT I DIDN'T THINK ACFT PERFORMANCE WOULD ALLOW. I TURNED L IN AN ATTEMPT TO ROUND THE VERY SOUTHERN END OF THE 3 SMALL CELLS. IN THIS ATTEMPT MODERATE TURB WAS ENCOUNTERED FOR ABOUT 3 SECONDS; BUT UP- AND DOWNDRAFTS CAUSED THE ACFT TO GAIN ABOUT 300 FT AND THEN LOSE ABOUT 600 FT ALL WITH THE AUTOPLT ENGAGED IN ALT HOLD MODE. I ATTEMPTED TO ACCELERATE THE RETURN TO THE ASSIGNED ALT BY OVERRIDING THE AUTOPLT IN NOSE UP PITCH; BUT IT CORRECTED AS I MADE MY INPUT. FO MADE AN IMMEDIATE CALL TO ATC TO RPT THE RIDE AND THE ALT EXCURSION. NO INJURIES. MY MAJOR CONCERN IS THAT WE WERE IN RVSM AIRSPACE AND BECAUSE OF NATURAL FORCES; WERE OFF OUR ASSIGNED ALT. WHEN CONVECTIVE WX DICTATES OPPOSITE DIRECTION ACFT GO THROUGH THE SAME 'HOLES;' RVSM SHOULD BE SUSPENDED AND 2000 FT VERT SEPARATION RE-INSTITUTED.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.