SWODY1
SPC AC 210527
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1227 AM CDT MON MAY 21 2007
VALID 211200Z - 221200Z
..THERE IS A SLGT RISK OF SVR TSTMS OVER THE WRN GREAT PLAINS...
..SYNOPSIS...
LONG WAVE TROUGH CURRENTLY EVOLVING OVER WRN CANADA INTO THE PACIFIC
NW IS FORECAST TO UNDERGO AMPLIFICATION WHILE PROGRESSING EWD
THROUGH THE NRN/CNTRL ROCKIES AS 100-120 KT UPSTREAM JET STREAK DIGS
SEWD INTO THE GREAT BASIN. WITHIN THIS LARGER-SCALE PATTERN...UPPER
LOW NOW OVER WRN WA WILL SHIFT ESEWD THROUGH THE NRN ROCKIES AND
INTO THE HIGH PLAINS BY TUESDAY MORNING...IMMEDIATELY DOWNSTREAM
FROM DIGGING JET STREAK. THOUGH LESS ENERGETIC...SEVERAL IMPULSES
EMBEDDED IN SUBTROPICAL BRANCH OF WESTERLIES ARE FORECAST TO
TRANSLATE EWD FROM THE SRN ROCKIES AND NRN MEXICO INTO THE SRN
PLAINS.
IN THE LOW-LEVELS...SURFACE LOW INITIALLY OVER WRN SD IS EXPECTED TO
DEEPEN SLIGHTLY OVER THE SAME GENERAL AREA...PRIOR TO DEVELOPING
NEWD INTO N-CNTRL ND BY THE END OF THE PERIOD. TRAILING COLD FRONT
WILL PUSH EWD THROUGH THE NRN HIGH PLAINS...EVENTUALLY REACHING THE
CNTRL DAKOTAS THIS EVENING. MEANWHILE...INCREASING MID-LEVEL FLOW
ACROSS THE ROCKIES WILL CONTRIBUTE TO THE INTENSIFICATION OF LEE
TROUGH/DRYLINE TODAY WITH THIS BOUNDARY STRETCHING FROM SURFACE LOW
OVER WRN SD SWD THROUGH WRN NEB...FAR ERN CO TO NEAR THE TX/NM
BORDER.
..NRN/CNTRL PLAINS...
21/00Z SOUNDINGS ACROSS THE GREAT PLAINS SHOWED THAT RICHER BOUNDARY
LAYER MOISTURE STILL REMAINS CONFINED TO S TX WITH MEAN MIXING
RATIOS OF 9-10 G/KG OBSERVED ACROSS THE REGION. THEREFORE...LITTLE
CHANGE IN LOW-LEVEL MOISTURE PROFILES ARE ANTICIPATED TODAY EXCEPT
WHERE LOCAL EVAPOTRANSPIRATION CAN CAUSE AN ADDITIVE INCREASE.
NONETHELESS...DAYTIME HEATING AND DEWPOINTS IN THE MID 50 TO LOW 60S
WILL CONTRIBUTE TO A MODERATELY UNSTABLE AIR MASS BY AFTERNOON WITH
MLCAPES RANGING FROM 2000-2500 J/KG OVER THE CNTRL DAKOTAS...TO
1000-2000 J/KG FROM CNTRL NEB SWD INTO WRN KS.
TSTMS WILL BECOME INCREASINGLY LIKELY BY MID/LATE AFTERNOON ALONG
COLD FRONT FROM WRN ND SWWD INTO NERN WY...AND ALONG LEE
TROUGH/DRYLINE FROM WRN SD SWD INTO WRN KS AS DAYTIME HEATING AND
SUSTAINED LOW-LEVEL CONVERGENCE WEAKEN CAP. WHILE THE STRONGEST MID
AND HIGH-LEVEL WIND FIELDS ASSOCIATED WITH EVOLVING TROUGH ARE
FORECAST TO REMAIN TO THE W OF LOW-LEVEL INSTABILITY AXIS...IT
APPEARS THAT AT LEAST MODEST DEEP LAYER VERTICAL SHEAR /I.E.
EFFECTIVE BULK SHEAR OF 30-40 KT/ WILL DEVELOP ALONG FRONTAL ZONE
OVER THE WRN DAKOTAS. GIVEN THE MODERATE INSTABILITY...POTENTIAL
WILL EXIST FOR A FEW SUPERCELLS CAPABLE OF LARGE HAIL AND LOCALLY
STRONG WIND GUSTS. LINEAR FORCING AND NEARLY PARALLEL ORIENTATION
OF DEEP LAYER SHEAR VECTOR TO LOW-LEVEL INITIATING BOUNDARY SUGGEST
THAT CONVECTION SHOULD RAPIDLY EVOLVE INTO A LINE WITH THE WIND AND
HAIL THREAT SPREADING EWD ACROSS THE DAKOTAS TONIGHT.
FARTHER TO THE S ACROSS NEB AND KS...VERTICAL SHEAR WILL REMAIN
QUITE MARGINAL WITH THE PRIMARY CONVECTIVE MODE LIKELY BEING
MULTICELL CLUSTERS. STILL...GIVEN THE STEEP LAPSE RATES AND
RESULTANT INSTABILITY...POTENTIAL WILL EXIST FOR A FEW SEVERE STORMS
CAPABLE OF LARGE HAIL AND PERHAPS LOCALLY DAMAGING WINDS.
..SRN PLAINS...
IT APPEARS THAT THE RICHEST BOUNDARY LAYER MOISTURE WILL REMAIN
REMOVED TO THE E OF DEVELOPING DRYLINE...HOWEVER DEWPOINTS IN THE
MID 50S TO LOWER 60S COUPLED WITH STEEP LOW/MID-LEVEL LAPSE RATES
WILL CONTRIBUTE TO AFTERNOON MLCAPES APPROACHING 1500-2500 J/KG.
LATEST SHORT TERM MODEL GUIDANCE INDICATES THAT A MID-LEVEL IMPULSE
SHIFTING EWD THROUGH CNTRL/ERN CO/NM WILL CONTRIBUTE TO AFTERNOON
TSTM INITIATION ALONG DRYLINE. PROXIMITY OF PERSISTENT SUBTROPICAL
JET BRANCH OVER THE NRN MEXICO AND SRN TX WILL RESULT IN SLIGHTLY
STRONGER MID AND HIGH-LEVEL FLOW IN COMPARISON TO NEB/KS WITH
EFFECTIVE BULK SHEARS OF AROUND 30 KT ACROSS DRYLINE LATE THIS
AFTERNOON AND EVENING.
THUS...POTENTIAL WILL EXIST FOR ORGANIZED MULTICELL CLUSTERS AND
PERHAPS A COUPLE OF SUPERCELLS CAPABLE OF LARGE HAIL AND STRONG WIND
GUSTS. GENERAL WEAKNESS IN MID AND HIGH-LEVEL WINDS INDICATES THE
LIKELIHOOD OF HIGH PRECIPITATION/OUTFLOW DOMINANT STORMS.
HOWEVER...LOW-LEVEL SHEAR IS FORECAST TO BECOME RELATIVELY STRONG
/I.E. 0-1 KM SRH OF 200-300 M2 PER S2/ OVER THE ERN TX PNHDL INTO
WRN OK BETWEEN 22/00Z-22/03Z AND POTENTIAL WILL EXIST FOR A TORNADO
OR TWO WITH ANY MATURE SUPERCELLS.
.MEAD/TAYLOR.. 05/21/2007
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