SWODY1
SPC AC 301951
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0251 PM CDT MON MAY 30 2011
VALID 302000Z - 311200Z
...THERE IS A MDT RISK OF SVR TSTMS LATE THIS AFTN AND EVE ACROSS
PARTS OF CENTRAL AND EASTERN NEBRASKA...ERN SOUTH DAKOTA...NORTHWEST
IOWA...WESTERN MINNESOTA AND A PARTS OF SOUTHEASTERN NORTH DAKOTA...
...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MODERATE RISK
AREA...FROM THE CENTRAL PLAINS INTO THE UPPER MISSISSIPPI
VALLEY/GREAT LAKES REGION...
...20Z UPDATE...
ADJUSTMENTS TO THE CATEGORICAL AND PROBABILISTIC LINES HAVE BEEN
MADE ...MOSTLY TO ACCOUNT FOR THE CURRENT POSITION OF THE SURFACE
FRONT AND THE PRE-FRONTAL CORRIDOR OF STRONGER SURFACE HEATING AND
BOUNDARY LAYER DESTABILIZATION. THIS HAS GENERALLY RESULTED IN A
SOUTHEASTWARD DISPLACEMENT OF HIGHEST SEVERE PROBABILITIES RELATIVE
TO THE 1630Z OUTLOOK.
OTHERWISE...FORECAST REASONING REMAINS GENERALLY UNCHANGED FROM THAT
INDICATED IN THE PRIOR DISCUSSION AND APPENDED BELOW. AS A
SIGNIFICANT UPPER TROUGH AND EMBEDDED CLOSED LOW CONTINUE TO
GRADUALLY LIFT NORTH NORTHEAST OF THE ROCKIES LATE THIS AFTERNOON
...ASSOCIATED FORCING FOR UPWARD VERTICAL MOTION APPEARS LIKELY TO
WEAKEN INHIBITION IN SUCH A MANNER AS TO ALLOW RAPID WIDESPREAD
STORM FORMATION WITHIN THE DEVELOPING INSTABILITY AXIS.
IF STORMS MAINTAIN A DISCRETE NATURE AS THE SOUTHERLY PRE-FRONTAL
LOW-LEVEL JET INTENSIFIES THIS EVENING /GUIDANCE INDICATES UP TO
AROUND 70+ KT AT 850 MB/...VERY LARGE CLOCKWISE-CURVED LOW-LEVEL
HODOGRAPHS WILL SUPPORT A RISK FOR STRONG TORNADOES. HOWEVER... IT
STILL APPEARS THAT THE WINDOW OF OPPORTUNITY FOR THIS MAY BE FAIRLY
SMALL...WITH LARGE-SCALE FORCING LIKELY TO CONTRIBUTE TO A QUICK
UPSCALE CONVECTIVE GROWTH INTO ONE OR MORE ORGANIZED STORM CLUSTERS
CAPABLE OF PRODUCING LOCALLY STRONG TO EXTREME SURFACE
GUSTS...PARTICULARLY ACROSS EASTERN PORTIONS OF THE MODERATE RISK
AREA.
..KERR.. 05/30/2011
.PREV DISCUSSION... /ISSUED 1121 AM CDT MON MAY 30 2011/
A BROAD UPPER TROUGH IS PRESENT TODAY OVER THE WESTERN STATES...WITH
A SHORTWAVE TROUGH EVIDENT IN WATER VAPOR IMAGERY ROUNDING THE BASE
OF THE TROUGH OVER NM. THIS FEATURE WILL LIFT NORTHWARD ACROSS THE
HIGH PLAINS THIS AFTERNOON AND EVENING...LEADING TO AN ACTIVE ZONE
OF SEVERE THUNDERSTORMS FROM CENTRAL KS INTO THE DAKOTAS AND WESTERN
MN.
...NEB/DAKOTAS/MN...
A WEAK SURFACE COLD FRONT CURRENTLY EXTENDS FROM SOUTHEAST ND INTO
NORTH CENTRAL NEB. THIS FRONT WILL MOVE ONLY SLOWLY SOUTHEASTWARD
TODAY...AND WILL LIKELY RETREAT NORTHWARD LATER TODAY OVER ND/MN AS
CYCLOGENESIS OCCURS OVER THE REGION. THE FRONT WILL SERVE AS THE
PRIMARY FOCUS FOR THUNDERSTORM INITIATION THIS AFTERNOON AS FULL
SUNSHINE OCCURS IN THE WARM SECTOR. AFTERNOON MLCAPE VALUES OF
3000+ J/KG ARE EXPECTED...ALONG WITH A NARROW AXIS OF WEAK CINH
ALONG THE FRONT.
PRESENT INDICATIONS ARE THAT STORMS WILL FORM ON THE FRONT AROUND
21Z FROM THE TRIPLE POINT OVER CENTRAL NEB NORTHWARD INTO THE
EASTERN DAKOTAS. STRONG WINDS THROUGH THE TROPOSPHERE WILL PROMOTE
SUPERCELL STORM STRUCTURES. HOWEVER...ORIENTATION OF FRONT TO UPPER
WINDS MAKE STORM-MODE FORECASTS COMPLEX. INITIAL STORMS MAY POSE A
RISK OF TORNADOES. BUT A RATHER QUICK EVOLUTION TO A SQUALL LINE
APPEARS LIKELY WITH EMBEDDED BOWS POSSIBLE. THROUGH THE
EVENING...LARGE HAIL AND DAMAGING WINDS ARE EXPECTED ALONG THE
SQUALL LINE AS IT MOVES INTO WESTERN NEB AND NORTHWEST IA.
...KS SOUTHWARD ACROSS THE HIGH PLAINS...
THE DRYLINE WILL EXTEND FROM CENTRAL NEB SOUTHWARD INTO WESTERN OK
AND WEST TX. THE POTENTIAL FOR CONVECTIVE INITIATION THIS AFTERNOON
AND EVENING IS HIGHEST NEAR THE TRIPLE POINT OVER NEB/NORTHERN
KS...AND DECREASES WITH SOUTHWARD EXTENT. STORMS THAT FORM OVER
NEB/KS WILL POSE A RISK OF LARGE HAIL...DAMAGING WINDS...AND PERHAPS
A TORNADO. HOWEVER...THERE REMAINS CONSIDERABLE UNCERTAINTY
REGARDING COVERAGE AND LONGEVITY OF STORMS. FROM WESTERN OK INTO
WEST TX...ONLY VERY ISOLATED DRYLINE STORMS ARE EXPECTED.
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