SWODY1
SPC AC 211957
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0257 PM CDT TUE JUN 21 2011
VALID 212000Z - 221200Z
...THERE IS A SLGT RISK OF SVR TSTMS FROM TX TO THE MID/UPPER MS
VALLEY AND EWD TO THE APPALACHIANS...
...MID/UPPER MS VALLEY THIS AFTERNOON/EVENING...
A BROKEN ARC OF LOW-TOPPED CONVECTION WILL PERSIST AND SPREAD
NWD/NEWD THROUGH THE NARROW WARM SECTOR ACROSS ERN MN INTO WI THIS
AFTERNOON/EVENING. THOUGH INSTABILITY IS NOT PARTICULARLY
STRONG...EFFECTIVE BULK SHEAR OF 40-50 KT AND EFFECTIVE SRH AOA
250-350 M2/S2 WILL SUPPORT LOW-TOPPED SUPERCELLS CAPABLE OF
PRODUCING A COUPLE OF TORNADOES. FARTHER S...INSTABILITY IS
STRONGER IN THE WARM SECTOR WITH MLCAPE AOA 2000 J/KG ACROSS
CENTRAL/SRN IL...WHERE DEEP-LAYER VERTICAL SHEAR ALSO FAVORS
SUPERCELLS. SCATTERED CELLS/CLUSTERS WILL CONTINUE TO POSE A RISK
FOR A COUPLE OF TORNADOES...IN ADDITION TO DAMAGING WINDS AND LARGE
HAIL. LATER THIS AFTERNOON/EVENING...THE STORMS WILL MERGE AND
SPREAD NWD/NEWD TOWARD INDIANA/SW LOWER MI WHERE STRONG INSTABILITY
/AND SOMEWHAT WEAKER VERTICAL SHEAR/ COULD SUPPORT ONE OR MORE
CLUSTERS WITH DAMAGING WINDS AND LARGE HAIL.
...LOWER MS VALLEY TO SRN APPALACHIANS THIS AFTERNOON...
MAIN CHANGES WERE TO MODIFY SLGT RISK AREA TO ACCOUNT FOR CONVECTIVE
OUTFLOW AND TO REMOVE AREAS INVOF THE ARKLAMISS STABILIZED BY
EARLIER CONVECTION. OTHERWISE...EXPECT MULTICELL CLUSTERS TO
PERSIST WITH A RISK OF AT LEAST ISOLATED DAMAGING DOWNBURSTS...AND
MARGINALLY SEVERE HAIL.
...NRN/WRN AR INTO TX THIS AFTERNOON THROUGH EARLY TONIGHT...
PRIOR CONVECTION IN AR/LA HAS LEFT AN OUTFLOW BOUNDARY FROM NE TX
INTO WRN/NRN AR. ISOLATED SEVERE STORMS WILL BE POSSIBLE ALONG THIS
BOUNDARY...ESPECIALLY NE TX WHERE THE STRONGER INSTABILITY IS
PRESENT. FARTHER W...A FEW STORMS SHOULD DEVELOP BY THIS EVENING
ALONG THE SLOW-MOVING FRONT ACROSS NRN/CENTRAL TX. HERE...MLCAPE
NEAR 3000 J/KG AND EFFECTIVE BULK SHEAR NEAR 30 KT ON THE SRN
PERIPHERY OF THE IA/MN CLOSED LOW WILL SUPPORT MULTICELL CLUSTERS
AND MARGINAL SUPERCELLS. LARGE HAIL AND DAMAGING WINDS WILL BE
POSSIBLE WITH THE INITIAL STORMS THIS AFTERNOON...AND THE THREAT MAY
CONTINUE INTO EARLY TONIGHT IF STORMS CAN GROW UPSCALE INTO A
CLUSTER OR TWO AND MOVE SEWD.
..THOMPSON.. 06/21/2011
.PREV DISCUSSION... /ISSUED 1111 AM CDT TUE JUN 21 2011/
...SYNOPSIS...
THE STRONG LOW THAT ENTERED THE PLAINS ON MONDAY HAS BECOME
BAROTROPIC OVER NERN NEB THIS MORNING. THRU THIS FORECAST PERIOD IT
WILL GRADUALLY WEAKEN AND DRIFT EWD ACROSS NRN IA WHILE THE BAND OF
STRONGER FLOW ON S SIDE OF THE LOW SPREADS EWD ACROSS MID MS VALLEY
INTO OH/TN VALLEYS.
A WEAKENING COLD FRONT EXTENDS FROM CENTRAL IA SWWD THRU MO INTO
SERN OK THEN TO SWRN TX. SWLY FLOW IN ADVANCE OF THE FRONT HAS
SPREAD A VERY MOIST GULF AIR MASS NEWD SUCH THAT CONDITIONS ARE
FAVORABLE FOR WIDESPREAD PRIMARILY DIURNALLY DRIVEN THUNDERSTORMS
ALONG AND TO E OF FRONTAL BOUNDARY OVER MUCH OF ERN US EXCLUDING NEW
ENGLAND.
WHILE A LARGE AREA WILL HAVE POTENTIAL FOR STRONG THUNDERSTORMS THIS
AFTERNOON GIVEN THE AVAILABLE MOISTURE AND INSTABILITY...A FOCUSED
SEVERE THREAT GIVEN THE RELATIVELY WARM AIR ALOFT AND MODEST SHEAR
IS DIFFICULT TO DETERMINE.
...MN/WI/IL...
SOUTHEASTERLY LOW LEVEL WINDS TO E OF SURFACE LOW WILL
TRANSPORT RICH LOW LEVEL MOISTURE BACK INTO PARTS OF
SOUTHERN/CENTRAL WI AND SOUTHEAST MN. HEATING CURRENTLY ERN IA INTO
SRN WI SHOULD LEAD TO MODERATE INSTABILITY AS MLCAPES CLIMB TO AOA
1500 J/KG. THUNDERSTORMS WILL DEVELOP LATER TODAY FROM SOUTHEAST MN
INTO SOUTHERN WI...TRACKING NORTHEASTWARD THROUGH THE EARLY EVENING.
FORECAST SOUNDINGS IN THIS REGION SUGGEST SUFFICIENT DEEP-LAYER
EFFECTIVE SHEAR TO PROMOTE SUPERCELL STORMS. IF DISCRETE CONVECTIVE
MODES CAN BE ATTAINED...ISOLATED TORNADOES WILL BE POSSIBLE.
OTHERWISE...LARGE HAIL AND DAMAGING WINDS WOULD BE THE MAIN THREATS.
..IL/MO INTO ARKLATEX...
AHEAD OF THE COLD FRONT IT DOES APPEAR LIKELY THAT SCATTERED
THUNDERSTORMS WILL REDEVELOP THIS AFTERNOON ALONG THIS BROAD FRONTAL
ZONE FROM IL INTO TX/LA. SHEAR PROFILES INDICATE THAT MULTICELL
STORMS CAPABLE OF HAIL AND GUSTY WINDS ARE THE MAIN THREAT. GIVEN
THE LACK OF STEEP MID LEVEL LAPSE RATES AND GENERALLY UNIDIRECTIONAL
SHEAR OF 30KT OR SO...AN ORGANIZED SEVERE THREAT IS HARD TO PIN DOWN
ATTM.
...OH/TN VALLEYS SWD INTO NRN MS/AL...
STRENGTHENING SOUTHWESTERLY MID LEVEL WINDS ARE FORECAST THROUGHOUT
MUCH OF THE OH/TN VALLEYS TODAY AS THE MAIN UPPER LOW DRIFTS
EASTWARD. ALSO...RICH LOW LEVEL MOISTURE IS IN PLACE ACROSS THE
SOUTHEAST STATES...WHICH WILL LIKELY RESULT IN NUMEROUS SHOWERS AND
THUNDERSTORMS ACROSS THIS AREA. MODERATE CAPE VALUES AND MODEST
STRENGTH OF WINDS ALOFT SUGGEST A RISK OF SCATTERED ORGANIZED
MULTICELL STORMS AND BOWING SEGMENTS CAPABLE OF DAMAGING WINDS AND
HAIL.
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