SWODY2
SPC AC 200600
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0100 AM CDT TUE SEP 20 2011
VALID 211200Z - 221200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
INCREASINGLY AMPLIFIED FLOW FIELD ALOFT IS PROGGED ACROSS NOAM THIS
PERIOD...WITH A LARGE ALEUTIAN LOW AND ASSOCIATED TROUGH EXPECTED TO
SLOWLY IMPINGE ON THE NW NOAM COAST...AND THEN DOWNSTREAM RIDGING
FROM THE WRN CONUS NNEWD INTO THE CANADIAN ROCKIES AND PRAIRIE.
ANOTHER LARGE/EXPANDING TROUGH IS EXPECTED TO SHIFT SLOWLY SEWD
ACROSS ONTARIO/THE CENTRAL CONUS...AND THEN DOWNSTREAM RIDGING IS
PROGGED OVER THE E COAST OF THE CONUS.
AT THE SURFACE...LOW PRESSURE INITIALLY INVOF THE UPPER MS VALLEY IS
EXPECTED TO SHIFT NEWD INTO ONTARIO...WHILE A TRAILING COLD FRONT
CROSSES THE GREAT LAKES/OH VALLEY/MID SOUTH/LOWER MS VALLEY REGION
THROUGH THE PERIOD. PROGRESS/LOCATION OF THIS FRONT IS NOT BEING
HANDLED WELL BY THE MODELS -- WITH THE GFS ROUGHLY 12 HOURS FASTER
THAN THE NAM WITH RESPECT TO FRONTAL PROGRESS ACROSS THE MIDWEST.
THE UNCERTAINTY REGARDING FRONTAL POSITION COMPLICATES THE
CONVECTIVE FORECAST THIS PERIOD...AS VERY WEAK INSTABILITY EXPECTED
FROM THE MID MS VALLEY NWD SUGGESTS THAT ANY STORMS WILL BE FOCUSED
IN A NARROW AXIS ALONG THE IMMEDIATE FRONTAL ZONE. WHILE SHEAR
WOULD BE SUFFICIENT TO SUPPORT POTENTIAL FOR SOME ORGANIZATION AND
ISOLATED SEVERE THREAT...THE LACK OF INSTABILITY AND UNCERTAINTY
REGARDING FRONTAL -- AND THUS CONVECTIVE -- LOCATION PRECLUDES
INTRODUCTION OF SEVERE PROBABILITIES ATTM.
OTHER STORMS WILL LIKELY OCCUR ACROSS THE SOUTHEAST AND ACROSS THE
APPALACHIANS INTO THE ERN STATES...WITHIN BROADER ZONE OF CYCLONIC
FLOW AROUND THE MAIN TROUGH. WITH A MORE MOIST/SUFFICIENTLY
UNSTABLE AIRMASS EXPECTED IN THESE AREAS...EMBEDDED DISTURBANCES
WITHIN THE LARGER-SCALE FLOW SHOULD AID IN AFTERNOON SHOWER/STORM
DEVELOPMENT. THOUGH SHEAR SHOULD REMAIN MARGINAL FOR APPRECIABLE
ORGANIZATION...A FEW STRONGER/MULTICELL CLUSTERS COULD
EVOLVE...ALONG WITH AN ISOLATED THREAT FOR HAIL/WIND. AGAIN
HOWEVER...GIVEN THE OVERALL LIMITED NATURE OF THE THREAT AND THE
BROAD GEOGRAPHIC REGION AFFECTED...WILL OPT NOT TO INCLUDE EVEN LOW
SEVERE PROBABILITIES THIS FORECAST.
..GOSS.. 09/20/2011
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