SWODY1
SPC AC 191242
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0742 AM CDT SAT SEP 19 2009
VALID 191300Z - 201200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
A DEEPENING UPPER TROUGH WILL SHIFT EWD THROUGH THE PACIFIC NW INTO
THE NRN ROCKIES...WHILE ANOTHER TROUGH EXITS NEW ENGLAND. A PACIFIC
COLD FRONT IS FORECAST TO MOVE EWD AHEAD OF THE NWRN U.S. UPPER
TROUGH AND STRETCH FROM WRN MT INTO NRN CA BY 20/00Z.
IN BETWEEN THESE TWO UPPER TROUGHS...A LONGWAVE RIDGE WILL REMAIN
SITUATED OVER MUCH OF THE CONUS. HOWEVER...WEAK LOW PRESSURE SYSTEM
ALOFT WAS TRAPPED BENEATH THE RIDGE IN THE KS/OK AREA. IN
ASSOCIATION WITH THIS UPPER LOW...WEAK SURFACE LOW PRESSURE/
INVERTED TROUGH SHOULD REMAIN QUASI-STATIONARY NEAR THE MS RIVER
FROM SERN MO SWD INTO SERN LA.
..SERN U.S./TN VALLEY...
COMBINATION OF PERSISTENT WARM ADVECTION AND WEAK SHORTWAVE TROUGHS
ROTATING AROUND UPPER LOW SHOULD PROVIDE FOCUS FOR SCATTERED
THUNDERSTORMS. ALTHOUGH SHEAR WILL BE WEAK TODAY...EFFECTIVE SHEAR
WILL INCREASE SOMEWHAT TONIGHT AS UPPER LOW HEIGHTS GRADUALLY LOWER
AND MID LEVEL WINDS INCREASE TO 20-25 KT OVER THE LOWER MS/TN
VALLEYS. HOWEVER...MARGINAL INSTABILITY...POOR MID LEVEL LAPSE RATES
AND HIGH PW VALUES SUGGEST THAT HEAVY RAINS ARE MORE LIKELY THAN
HAIL OR DAMAGING WINDS.
...ROCKIES...
STRONG SOLAR INSOLATION OVER HIGHER TERRAIN COMBINED WITH 500 MB
TEMPERATURES FROM -10 TO 12C WILL ONCE AGAIN PROVIDE SUFFICIENT
BUOYANCY FOR MOSTLY AFTERNOON/EVENING THUNDERSTORMS ACROSS THE
SRN/CENTRAL ROCKIES.
AS MID LEVEL HEIGHTS FALL AND LARGE SCALE ASCENT SPREADS IN FROM THE
WEST...SCATTERED THUNDERSTORMS SHOULD DEVELOP ALONG COLD FRONT AS IT
PROGRESSES EWD THROUGH MT/NRN WY LATE TODAY AND TONIGHT.
ALTHOUGH VERTICAL SHEAR WILL STRENGTHEN AS MID LEVEL WIND SPEEDS
INCREASE...VERY MARGINAL INSTABILITY SHOULD PRECLUDE A SEVERE
THREAT.
..IMY/JEWELL.. 09/19/2009
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