SWODY2
SPC AC 201729
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1229 PM CDT SAT SEP 20 2008
VALID 211200Z - 221200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE NORTH CENTRAL HIGH
PLAINS...
...SYNOPSIS...
A CONSIDERABLE UPPER TROUGH/ASSOCIATED JET STREAK WILL DEVELOP
EASTWARD ACROSS MUCH OF THE WESTERN STATES/INTERMOUNTAIN REGION ON
SUNDAY...RESULTING IN INCREASING TSTM COVERAGE ACROSS MUCH OF THE
ROCKIES AND NORTHERN/CENTRAL PLAINS. DOWNSTREAM...SHORTWAVE RIDGING
WILL DEVELOP NORTHEASTWARD OVER THE UPPER MIDWEST/UPPER GREAT LAKES
REGION...WHILE A RESIDUAL UPPER LOW CONTINUES TO WEAKEN OVER THE
MIDDLE MISSISSIPPI VALLEY.
...HIGH PLAINS...
AS THE UPPER TROUGH DEVELOPS EASTWARD OVER THE INTERMOUNTAIN
WEST...LATEST 12Z-BASED GUIDANCE SUGGESTS A LEAD MID LEVEL IMPULSE
WILL BE CROSSING THE NORTHERN HIGH PLAINS SUNDAY MORNING. AS
SUCH...SCATTERED TSTMS WILL LIKELY INCREASE EARLY IN THE DAY ACROSS
WY/MT INTO THE WESTERN DAKOTAS/WESTERN NEB OWING TO AMPLE LARGE
SCALE FORCING FOR ASCENT. THIS POTENTIAL FOR EARLY DAY CLOUD
COVER/TSTMS COMPLICATES THE FORECAST DETAILS. BUT AT LEAST A
MARGINAL SEVERE RISK WILL DEVELOP/INCREASE THROUGH THE AFTERNOON
WITH THIS INITIAL ACTIVITY AND/OR ADDITIONAL DEVELOPMENT FARTHER
WEST IN VICINITY OF A LEE TROUGH LATE SUNDAY AFTERNOON ACROSS
WESTERN PORTIONS OF SD/NEB AND NORTHEAST CO/FAR NORTHWEST KS.
PROVIDED SUFFICIENT CLOUD BREAKS/HEATING...STEEPENING MID LEVEL
LAPSE RATES AND A MODESTLY MOIST BOUNDARY LAYER /MAINLY UPPER 40S TO
MIDDLE 50S F SURFACE DEWPOINTS/ WILL SUPPORT MLCAPES TO AS HIGH AS
1000-1250 J/KG. INCREASING WIND FIELDS ALOFT/AMPLE VEERING WOULD
SUPPORT BOTH WELL-ORGANIZED MULTICELLS/SOME SUPERCELLS...WITH SEVERE
HAIL AS THE PRIMARY RISK OVERALL SUNDAY AFTERNOON/EARLY EVENING. AN
INCREASING SOUTHERLY LOW LEVEL JET/WARM ADVECTION REGIME WILL HELP
TO MAINTAIN AN EASTWARD-SPREAD OF TSTMS ACROSS THE DAKOTAS/NEB
THROUGH THE NIGHT...BUT THE SEVERE HAIL THREAT SHOULD TEND TO WANE
BY MID/LATE EVENING.
...LOWER/MIDDLE MISSISSIPPI VALLEY TO EASTERN GREAT LAKES...
VERY SLOW MOVING UPPER LOW WILL CONTINUE TO WEAKEN OVER THE MIDDLE
MISSISSIPPI VALLEY/LOWER OHIO VALLEY. SIMILAR TO PREVIOUS
DAYS...SCATTERED TSTMS WILL REMAIN COMMON IN A GENERAL NORTH-SOUTH
CORRIDOR ACROSS PORTIONS OF THE LOWER/MIDDLE MISSISSIPPI
VALLEY/MID-SOUTH REGION ON SUNDAY...AIDED BY A RESIDUAL COOL POCKET
ALOFT AND A MOIST/UNSTABLE BOUNDARY LAYER. WEAK VERTICAL SHEAR WILL
CONTINUE TO KEEP ANY SEVERE THREAT VERY LIMITED/ISOLATED IN NATURE.
FARTHER NORTHEAST...OTHER MORE ISOLATED TSTMS WILL BE POSSIBLE
ACROSS THE EASTERN GREAT LAKES REGION IN VICINITY OF A SOUTH-SAGGING
COLD FRONT.
..GUYER.. 09/20/2008
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