Convective Outlook- National Severe WX

By: SPC
By: SPC

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0759 AM CDT WED SEP 22 2010

VALID 221300Z - 231200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM PARTS OF THE CNTRL/NRN
PLNS INTO THE UPR MS VLY...

...THERE IS A SLGT RISK OF SVR TSTMS ...

...SYNOPSIS...
ELONGATED RIDGE WILL PERSIST FROM THE SRN PLNS INTO THE WRN ATLANTIC
WHILE A FAIRLY STRONG/AMPLIFIED TROUGH PROGRESSES FROM CA ACROSS THE
GRT BASIN TO THE CNTRL RCKYS. AT THE SFC...ERN PART OF FRONT THAT
CROSSED THE MID MS VLY/UPR GRT LKS YESTERDAY WILL CONTINUE S AS A
COLD FRONT ACROSS THE NERN STATES TODAY...WHILE WRN PART ADVANCES
NWD AS A WARM FRONT OVER THE CNTRL PLNS AND THE MID/UPR MS VLY.

...CNTRL/NRN PLNS TO UPR MS VLY THROUGH EARLY THU...
APPROACH OF WRN STATES TROUGH WILL INDUCE SFC CYCLOGENESIS OVER THE
CNTRL HI PLNS TODAY THROUGH EARLY THU...WITH ASSOCIATED
STRENGTHENING OF LOW LVL SLY FLOW INCREASING NWD MOISTURE TRANSPORT
OVER THE CNTRL PLAINS. COUPLED WITH DAYTIME HEATING... SUBSTANTIAL
MOISTURE RETURN /WITH PW AOA 1.50 INCHES/ WILL SUPPORT CONSIDERABLE
DESTABILIZATION ACROSS NEB...WRN IA...SRN SD AND SRN MN BY LATE
TODAY.

SCTD ELEVATED WAA TSTMS NOW PRESENT OVER NEB APPEAR TO BE TIED...IN
PART...TO A WEAK LEAD IMPULSE MOVING ENEWD FROM CO. THE STORMS
SHOULD SPREAD ENE ACROSS PARTS OF SD/IA/MN THROUGH THE DAY...AND MAY
SOMEWHAT RETARD THE NWD PROGRESS OF THE SFC WARM FRONT. BY LATE
AFTN...THE FRONT SHOULD EXTEND FROM W CNTRL NEB ENE INTO NW IA.

MID LVL WARMING/SUBSIDENCE IN WAKE OF LEAD UPR IMPULSE MAY
STRENGTHEN CINH/DELAY CONVECTIVE DEVELOPMENT FOR A TIME TODAY OVER
THE CNTRL PLNS. BUT SFC HEATING SHOULD EVENTUALLY SUPPORT SCTD
STORM DEVELOPMENT ALONG BOTH WARM FRONT AND LEE TROUGH IN NEB...AND
PERHAPS SERN SD. SBCAPE 2000-2500 J/KG AND 30-40 KT WSWLY DEEP
SHEAR COULD YIELD A FEW SUSTAINED STORMS/SUPERCELLS WITH SVR
HAIL/WIND. SUBSTANTIAL LOW LVL SHEAR...WITH 0-1 KM SRH OF 200-300
M2/S2...WILL EXIST INVOF WARM FRONT IN NE NEB/NW IA/SE
SD...SUGGESTING THE POTENTIAL FOR TORNADOES. HOWEVER... UNCERTAINTY
EXISTS REGARDING /1/ WHETHER STORMS IN THE REGION WILL BE
SFC-BASED...AND /2/ THE POSSIBILITY THAT ANY SUSTAINED ACTIVITY IN
THE REGION WILL AWAIT ARRIVAL OF HEIGHT FALLS/UVV AND ASSOCIATED
INCREASE IN THE LLJ AFTER DARK.

MORE WIDESPREAD STORM COVERAGE IS EXPECTED FROM NEB/SD INTO PARTS OF
IA AND MN TONIGHT THROUGH EARLY THU AS WRN STATES TROUGH APPROACHES
REGION AND PW INCREASES WITH THE ARRIVAL OF MID LVL TROPICAL
MOISTURE SURGE NOW OVER NM/SRN CO. SOME OF THIS ACTIVITY MAY PROVE
SVR...ESPECIALLY WHERE STRENGTHENING LLJ INTERSECTS WARM FRONT. BUT
WEAKENING LOW LVL INSTABILITY AND STORM COMPETITION SHOULD LIMIT
OVERALL DEGREE OF THREAT.

...NERN STATES/MID ATLANTIC/SRN NEW ENG AFTN/EARLY EVE...
SCTD TSTMS SHOULD FORM LATER THIS MORNING THROUGH THIS AFTN ALONG
COLD FRONT SETTLING S/SE OVER PARTS OF PA/NY/SRN NEW ENG. OTHER
STORMS MAY FORM A BIT ALONG LEE TROUGH IN ERN PA/MD. SATELLITE SHOWS
AN APPARENT WEAK UPR IMPULSE NOW OVER THE ERN OH VLY THAT WILL
PROVIDE SOME LARGE SCALE ASCENT OVER THE REGION. WHILE MOISTURE
WILL BE SOMEWHAT LIMITED BY LATE SUMMER ERN U.S. STANDARDS...IT
SHOULD BE SUFFICIENT FOR STRONG/SUSTAINED STORMS GIVEN RELATIVELY
STEEP LOW TO MID LVL LAPSE RATES PER 12Z RAOB DATA. MODERATE TO
STRONG UNIDIRECTIONAL WSWLY FLOW WILL FOSTER DEVELOPMENT OF BOWING
SEGMENTS WITH DMGG WIND AND PERHAPS MARGINALLY SVR HAIL THROUGH
EARLY EVE.

...ERN GRT BASIN/CNTRL RCKYS AFTN/EARLY EVE...
AS THE CA TROUGH MOVES E INTO THE GRT BASIN...AN AXIS OF 50-60 KNOT
SWLY MID LVL FLOW WILL OVERSPREAD PARTS OF UT/CO/WY. STEEP LOW TO
MID LVL LAPSE RATES AND LIMITED...BUT SUFFICIENT...LOW/MID LVL
MOISTURE WILL BE PRESENT FOR SCTD TSTMS. GIVEN STRENGTH OF THE
WIND FIELD...ANY SUSTAINED STORM WILL BE CAPABLE OF GUSTY/DMGG WIND
THIS AFTN AND EVE. SLIGHTLY GREATER MOISTURE CONTENT ALSO MAY
SUPPORT A RISK FOR HAIL IN CELLS OVER ERN UT/WRN CO.


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