Tuesday, September 14, 2010

DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 141622
SWODY1
SPC AC 141621

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1121 AM CDT TUE SEP 14 2010

VALID 141630Z - 151200Z

...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON INTO TONIGHT FOR
PARTS OF THE CENTRAL PLAINS...

...SYNOPSIS...
A LARGELY ZONAL FLOW REGIME EXISTS OVER THE NRN TWO THIRDS OF THE
CONUS...WITH SEVERAL EMBEDDED SHORTWAVE TROUGHS TRAVERSING THE GREAT
LAKES/NEW ENGLAND...AND A WEAK SPLIT FLOW PATTERN OVER THE PAC COAST
AND ROCKIES. WITHIN THE SRN BRANCH...ONE SHORTWAVE TROUGH OVER NV
THIS MORNING WILL PROGRESS ENEWD TO THE CENTRAL HIGH PLAINS
OVERNIGHT...AND A DOWNSTREAM WAVE OVER MO WILL MOVE ESEWD AND
WEAKEN. THE PRIMARY SEVERE STORM THREAT ACROSS THE CONUS LATER
TODAY INTO TONIGHT WILL BE FOCUSED ACROSS THE CENTRAL PLAINS IN
ADVANCE OF THE EJECTING GREAT BASIN TROUGH.

...CENTRAL PLAINS REGION THROUGH TONIGHT...
THE REMNANTS OF OVERNIGHT CONVECTION CONTINUE TO DEVELOP SWD/SEWD
OVER ERN OK ALONG THE ERN EDGE OF THE LOW-LEVEL MOISTURE AND
INSTABILITY CORRIDOR. THIS CONVECTION HAS BEEN ASSOCIATED WITH A
SHORTWAVE TROUGH OVER MO AND WAA WITH A SWLY NOCTURNAL LLJ. SINCE
THE MO TROUGH WILL MOVE ESEWD AWAY FROM THE INSTABILITY CORRIDOR
TODAY AND THE NOCTURNAL LLJ WILL WEAKEN THROUGH MIDDAY...IT APPEARS
THAT THE ONLY FOCUS FOR CONTINUED THUNDERSTORM DEVELOPMENT ACROSS OK
WILL BE LOW-LEVEL ASCENT ALONG THE OUTFLOW BOUNDARY ACROSS CENTRAL
AND E/SE OK. GIVEN MODERATE INSTABILITY...WEAKENING LOW-LEVEL SHEAR
WITH TIME AND WEAKENING DEEP-LAYER SHEAR WITH SWD EXTENT...ONLY
ISOLATED DAMAGING WINDS AND MARGINALLY SEVERE HAIL ARE EXPECTED WITH
THE OK STORMS THIS AFTERNOON.

A ZONE OF MID 60S BOUNDARY LAYER DEWPOINTS EXTENDS NNWWD ACROSS
CENTRAL KS/NEB TO THE W OF THE OUTFLOW WITH THE ONGOING OK
STORMS...AND S OF WEAK LOW IN S CENTRAL SD. LEE CYCLOGENESIS IS
EXPECTED THIS AFTERNOON FARTHER TO THE W ACROSS THE CENTRAL HIGH
PLAINS /IN ADVANCE OF THE GREAT BASIN MIDLEVEL TROUGH/. BASED ON
MODIFIED 12Z REGIONAL SOUNDINGS...SURFACE TEMPERATURES WILL NEED TO
WARM INTO THE 90S ACROSS WRN KS/NEB TO REMOVE CONVECTIVE INHIBITION
AND ALLOW THUNDERSTORM DEVELOPMENT ALONG THE LEE TROUGH THIS
AFTERNOON. THE WARMER EML AND STRONGER CAP RESIDES OVER
KS/NEB...WITH A WEAKER CAP AND RICHER LOW-LEVEL MOISTURE ACROSS WRN
OK...WHERE ISOLATED THUNDERSTORM DEVELOPMENT WILL ALSO BE POSSIBLE
THIS AFTERNOON. THE STRENGTH OF THE CAP AND LACK OF LARGE SCALE
SUPPORT FOR ASCENT SUGGESTS THAT THUNDERSTORM COVERAGE WILL REMAIN
RATHER SPARSE ACROSS THE HIGH PLAINS THIS AFTERNOON/EVENING. WHAT
STORMS DO FORM COULD BECOME SUPERCELLS CAPABLE OF PRODUCING LARGE
HAIL AND DAMAGING WINDS...GIVEN STEEP MIDLEVEL LAPSE RATES AND
EFFECTIVE BULK SHEAR OF 40-50 KT.

STRONGER LOW-LEVEL SHEAR WILL BE PRESENT ALONG THE STALLED FRONT
NEAR THE SD/NEB BORDER THIS AFTERNOON/EVENING...WHERE AT LEAST WEAK
LOW-LEVEL ASCENT WILL BE PRESENT. HOWEVER...THE WARM UPSTREAM EML
MAY ALSO SERVE TO INHIBIT SURFACE-BASED THUNDERSTORM DEVELOPMENT
ALONG THIS BOUNDARY DURING THE AFTERNOON. IF STORMS
FORM...SUPERCELLS WILL CERTAINLY BE POSSIBLE. OTHERWISE...THE MORE
PROBABLE SCENARIO APPEARS TO BE FOR CONVECTION TO INCREASE TONIGHT
ACROSS NEB/SD AS LOW-LEVEL WAA INCREASES IN CONJUNCTION WITH A 35-45
KT LLJ...AND WITH THE ARRIVAL OF THE GREAT BASIN MIDLEVEL TROUGH.
ONE OR MORE THUNDERSTORM CLUSTERS SHOULD EVOLVE FROM THE OVERNIGHT
CONVECTION ACROSS SD/NEB...WITH THE POTENTIAL FOR A FEW DAMAGING
WIND AND LARGE HAIL EVENTS.

..THOMPSON/ROGERS.. 09/14/2010

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