Thursday, April 23, 2009

DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 230553
SWODY1
SPC AC 230551

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1251 AM CDT THU APR 23 2009

VALID 231200Z - 241200Z

...NO SVR TSTM AREAS FORECAST...

...SYNOPSIS...
HEIGHTS ALOFT ARE FCST TO RISE OVER MOST OF ERN CONUS IN WAKE OF
DEPARTING/DEAMPLIFYING NERN TROUGH...AND FALL ACROSS LARGE PART OF
GREAT BASIN AND INTER-MOUNTAIN W REGION. SRN LOBE OF
POSITIVELY-TILTED/HIGH-AMPLITUDE SHORTWAVE TROUGH WILL DIG SEWD
ACROSS NV BY END OF PERIOD...WITH REMAINDER OF TROUGH EXTENDING NEWD
OVER NRN HIGH PLAINS.

RELATED SFC COLD FRONT -- ALSO ALIGNED SW-NE -- WILL SAG SEWD ACROSS
NRN HIGH PLAINS AND MUCH OF DAKOTAS BY 24/12Z. WEAK SFC
CYCLOGENESIS IS POSSIBLE ALONG THIS FRONT...OVER WRN/CENTRAL SD
DURING AFTERNOON...WITH LOW SHIFTING/RE-DEVELOPING EWD OVER MN
OVERNIGHT. LEE TROUGHING WILL EXTEND FROM WRN SD SWD ACROSS TX
PANHANDLE AND BIG BEND REGION BY 24/00Z. WARM FRONT SHOULD MOVE
SLOWLY NWD ACROSS CENTRAL PLAINS...OZARKS...TN VALLEY REGION AND GA
DURING PERIOD.

...UPPER MIDWEST...
WIDELY SCATTERED TO SCATTERED TSTMS SHOULD DEVELOP ACROSS THIS
REGION DURING AFTERNOON/EVENING HOURS...A FEW OF WHICH MAY PRODUCE
HAIL AND STG-SVR GUSTS. FCST SOUNDINGS SUGGEST STG SFC HEATING MAY
WEAKEN CAP SUFFICIENTLY - IN COMBINATION WITH FRONTAL LIFT -- TO
SUPPORT POTENTIAL FOR SFC-BASED TSTM DEVELOPMENT DURING LATE
AFTERNOON AND EARLY EVENING. 45-55 DEG F SFC DEW POINTS WILL BE
MRGL...BUT SHOULD SUPPORT MLCAPE APCHG 1000 J/KG BENEATH 8.5-9 DEG
C/KM LOW-MIDLEVEL LAPSE RATES. SRN BELT OF STRONGEST MID-UPPER FLOW
-- E.G. 40-50 KT AT 500 MB -- WILL HELP TO ENHANCE DEEP-LAYER SHEAR
IN SUPPORT OF POTENTIAL FOR SOME STORM ORGANIZATION. ACTIVITY MAY
PERSIST INTO LATE EVENING ALONG OR EVEN SLIGHTLY BEHIND SFC COLD
FRONT...WIND POTENTIAL DIMINISHING BUT WITH HAIL STILL POSSIBLE.

...SW TX...
CONDITIONAL SVR THREAT EXISTS FOR SMALL PART OF RIO GRANDE VALLEY
REGION THIS AFTERNOON -- JUST AHEAD OF SRN STREAM SHORTWAVE
TROUGH...AMD DOWNSHEAR FROM LIKELY OROGRAPHIC TSTM GENESIS REGION OF
SERRANIAS DEL BURRO RANGE IN COAHUILA. MODIFIED NAM/ETA-KF FCST
SOUNDINGS SUGGEST CAPPING WILL REMAIN ROBUST AROUND DRT...AT BASE OF
STG ELEVATED MIXED LAYER THAT WILL ADVECT EWD FROM HIGHER TERRAIN TO
W. HOWEVER...COMBINATION OF UPSLOPE/ELY BOUNDARY LAYER WIND
COMPONENT MOISTENING WITH TIME...AND STG HEATING OF THAT HIGHER MEX
TERRAIN...SHOULD RESULT IN ISOLATED TO WIDELY SCATTERED TSTMS
FORMING DURING MID-LATE AFTERNOON. SOME OF THIS ACTIVITY MAY REACH
BORDERLANDS BEFORE DISSIPATING. FCST STORM MOTIONS ONLY 10-15 KT
ARE BASED ON SUPERCELL MOTION ALGORITHMS...HOWEVER
OUTFLOW-AIDED/DRIVEN STORMS MAY FORWARD-PROPAGATE FASTER. STG
VEERING WITH HEIGHT AND 35-45 KT EFFECTIVE SHEAR INDICATES SOME
SUPERCELL AND/OR BOW POTENTIAL DESPITE PRONOUNCED WEAKNESSES IN
LOWER MIDLEVEL FLOW -- I.E. AROUND 700 MB.

STG-SVR HAIL AND GUSTS ALSO MAY ACCOMPANY ANY TSTMS INITIATING INVOF
DIFFUSE DRYLINE N OF NRN BEND OF RIO GRANDE. ALTHOUGH LACK OF MORE
ROBUST MOISTURE WILL LIMIT COVERAGE/DURATION OF ANY SUCH
CONVECTION...HOT/WELL-MIXED BOUNDARY LAYER AND RELATED POTENTIAL FOR
SUBCLOUD EVAPORATIONAL COOLING MAY SUPPORT SURVIVAL OF
HAIL/DOWNBURSTS TO SFC FOR A 2-3 HOUR WINDOW DURING LATE AFTERNOON.

...PORTIONS TN VALLEY TO SRN PIEDMONT REGION...
CONDITIONAL AND MRGL SVR POTENTIAL EXISTS OVER THIS
REGION...PRIMARILY DURING AFTERNOON. STG SFC HEATING...AT LEAST
MRGL MOISTURE AND SUBTLE ASCENT INVOF SFC FRONTAL ZONE WILL
CONTRIBUTE TO POTENTIAL FOR CAP TO BREAK...WITH STG-SVR
MULTICELLULAR/CLUSTERED MODES POSSIBLE. IN ADDITION TO WARM
FRONT...SMALLER/MESOBETA SCALE BOUNDARIES MAY ACT AS MORE SPECIFIC
FOCI...INCLUDING OUTFLOWS AND/OR DIFFERENTIAL HEATING/MOISTENING
ZONES RELATED TO PRIOR/OVERNIGHT ACTIVITY NOW UNDERWAY. ALTHOUGH
WINDS WILL VEER WITH HEIGHT...ESPECIALLY WITHIN WARM FRONTAL
ZONE...WEAK MID-UPPER WINDS WILL MINIMIZE DEEP-LAYER SHEAR.
HOWEVER...ERN TONGUE OF ELEVATED MIXED LAYER AIR WILL PROVIDE STEEP
MIDLEVEL LAPSE RATES...COMBINING WITH SFC DEW POINTS MID 50S TO LOW
60S F TO CONTRIBUTING TO MLCAPE 1500-2000 J/KG...ATOP WELL-MIXED
SUBCLOUD LAYER.

..EDWARDS.. 04/23/2009

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