SPC AC 281939
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0239 PM CDT THU MAY 28 2009
VALID 282000Z - 291200Z
...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTN AND EVE ACROSS PARTS
OF THE MID SOUTH/ERN GULF STATES INTO THE CNTRL/SRN APPALACHIANS....
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTN AND TONIGHT
ACROSS PARTS OF SW TX AND THE RIO GRANDE VALLEY....
...20Z UPDATE...
CHANGES TO THE 1630Z OUTLOOK ARE GENERALLY MINOR.
...MS VALLEY INTO ATLANTIC SEABOARD...
MODESTLY STEEP LOW-LEVEL LAPSE RATES...AHEAD OF A COLD FRONT NOW
ADVANCING TOWARD THE OHIO RIVER AND EAST OF THE MID/LOWER
MISSISSIPPI VALLEY...INTO THE MID/SOUTH ATLANTIC SEABOARD...ARE
CONTRIBUTING TO SUFFICIENT DESTABILIZATION TO SUPPORT A SEVERE
THREAT WITH MOST DEVELOPING STORMS THIS AFTERNOON. STEEPER
MID-LEVEL LAPSE RATES ASSOCIATED WITH ELEVATED MIXED LAYERS
EMANATING FROM THE PLATEAU REGION ARE GENERALLY CONFINED TO AREAS
WEST OF THE APPALACHIANS. THESE CAPPING LAYERS ARE SLOWING
CONVECTIVE DEVELOPMENT RELATIVE TO AREAS FARTHER EAST...BUT
CONTRIBUTING TO A BIT STRONGER INSTABILITY...AS A BROAD WEAK
MID/UPPER TROUGH CONTINUES TO SLOWLY SHIFT EAST OF THE MISSISSIPPI
VALLEY.
WEAK LOWER/MID TROPOSPHERIC WIND FIELDS REMAIN THE PRIMARY LIMITING
FACTOR TO A MORE SUBSTANTIAL SEVERE THREAT LATE THIS AFTERNOON AND
EVENING. BUT...SPEED MAXIMA EMBEDDED WITHIN THE BROADER SCALE
CYCLONIC FLOW COULD ENHANCE THE RISK FOR LARGE HAIL/DAMAGING WIND
GUSTS AT LEAST IN POCKETS ACROSS THE MID SOUTH AND EASTERN GULF
STATES INTO THE CENTRAL/SOUTHERN APPALACHIANS. ONE OF THESE
FEATURES WILL CONTINUE TO SLOWLY NOSE INTO AND ACROSS
KENTUCKY/TENNESSEE LATE THIS AFTERNOON THROUGH TONIGHT.
THOUGH A BIT MORE UNCERTAIN...MODELS CONTINUE TO BE SUGGESTIVE THAT
FORCING ASSOCIATED WITH ANOTHER SPEED MAXIMUM COULD ENHANCE
CONVECTIVE POTENTIAL BY EARLY EVENING AS IT OVERSPREADS EAST CENTRAL
ALABAMA INTO WEST CENTRAL GEORGIA. THIS IS WHERE SURFACE DEW POINTS
NEAR 70F APPEAR TO BE MAXIMIZING POTENTIAL INSTABILITY ...WITH MIXED
LAYER CAPE ON THE ORDER OF 2000 J/KG.
**FOR MORE SPECIFIC DETAILS CONCERNING ONGOING OR MORE IMMINENT
CONVECTIVE HAZARDS...PLEASE REFER TO THE LATEST SPC WATCHES AND
MESOSCALE DISCUSSIONS.
..KERR.. 05/28/2009
.PREV DISCUSSION... /ISSUED 1119 AM CDT THU MAY 28 2009/
...SYNOPSIS...
THE MAIN FEATURE OF CONVECTIVE SIGNIFICANCE THIS AFTERNOON/EVENING
WILL BE A MID LEVEL TROUGH SHIFTING SLOWLY EWD ACROSS THE OH/TN
VALLEYS SWD INTO THE GULF COASTAL STATES. MORNING WV IMAGERY SHOWED
SHORTWAVE TROUGHS EMBEDDED WITHIN THIS TROUGH OVER KY AND NRN AR/MO.
A HIGH LEVEL SPEED MAX AT 80-100 KT WAS LOCATED ON THE SOUTH SIDE OF
THE AR/MO SYSTEM...STRETCHING FROM CENTRAL OK INTO WRN TN. AT THE
SURFACE...WEAK SURFACE LOW WAS LOCATED IN FAR SRN IL...WITH A COLD
FRONT EXTENDING NEWD FROM THE LOW INTO NWRN OH...AND SWD ACROSS FAR
NWRN AL/ERN MS.
...CENTRAL APPALACHIANS SWWD INTO AL/GA...
SUNSHINE THROUGH SCT-BROKEN CLOUDS HAS WARMED TEMPERATURES INTO THE
MID/UPPER 70S FROM AL NEWD INTO SRN KY. AS READINGS WARM INTO THE
80S...MID/UPPER 60S DEWPOINTS WILL CONTRIBUTE TO MLCAPE VALUES NEAR
2000 J/KG. THOUGH WINDS THROUGH THE LOWER HALF OF THE TROPOSPHERE
ARE GENERALLY WLY...STRENGTHENING WINDS ALOFT... ESPECIALLY BETWEEN
400 AND 200 MB...ARE EXPECTED TO PROVIDE SUFFICIENT DEEP LAYER SHEAR
FOR ORGANIZED STRONG/SEVERE LINE SEGMENTS DURING THE AFTERNOON/EARLY
EVENING HOURS. THE DEGREE OF INSTABILITY AND UNDIRECTIONAL WLY WINDS
COMBINED WITH MID LEVEL DRYING...AS INDICATED ON 12Z LIT/JAN/SHV
SOUNDINGS...SHOULD FAVOR WIND DAMAGE WITH LINE SEGMENTS.
HOWEVER...500 MB TEMPERATURES AROUND -10C AND 500-700 MB LAPSE RATES
AT 7C/KM MAY ALSO SUPPORT HAIL.
FARTHER NEWD ACROSS OH AND WRN PA/NY...MORE PERSISTENT CLOUDS WILL
LIMIT INSTABILITY COMPARED TO AREAS FARTHER SOUTH. HOWEVER...
FORCING ASSOCIATED WITH UPPER TROUGH AND TEMPERATURES/DEWPOINTS IN
THE 70S/60S MAY YIELD A FEW STORMS WITH SEVERE HAIL AND STRONG
WINDS. FARTHER E ACROSS THE MID/SRN ATLANTIC STATES...ISOLATED
STRONG STORMS ARE ALSO POSSIBLE...BUT WEAKER SHEAR SHOULD RESULT IN
LESS ORGANIZED/PERSISTENT STORMS THAN IN THE SLIGHT RISK AREA.
...SWRN TX AND THE RIO GRANDE VALLEY...
LINGERING LOW-LEVEL MOISTURE N OF STALLED FRONT IN MEXICO...PLUS
COMBINATION OF WEAK ASCENT ASSOCIATED WITH SRN JET STREAM AND
DAYTIME HEATING SHOULD SUPPORT THUNDERSTORM DEVELOPMENT ACROSS SWRN
TX BY LATE AFTERNOON. VEERING WIND PROFILES IN THE LOWER 3 KM AND
MLCAPE VALUES OF 1000-2000 J/KG WOULD SUPPORT A FEW STORMS WITH HAIL
AND STRONG OUTFLOW WINDS.
...FL...
STORMS HAVE ALREADY DEVELOPED OVER THE WRN COAST OF THE PENINSULA
AND THE WLY WINDS ALOFT WILL CARRY THESE STORMS TOWARD THE EAST
COAST. THOUGH DEEP LAYER SHEAR IS WEAK...MLCAPES AOA 2000 J/KG WILL
SUPPORT SHORT LIVED WIND/HAIL EVENTS.
...ORE/NERN CA...
DAYTIME HEATING AND LINGERING MID LEVEL MOISTURE AROUND THE NWRN
PERIPHERY OF THE MID LEVEL RIDGE IN NV ARE EXPECTED TO SUPPORT
WIDELY SCATTERED THUNDERSTORM DEVELOPMENT OVER THE HIGHER TERRAIN OF
SRN ORE AND NERN CA LATE TODAY. DESPITE WEAK INSTABILITY...MID LEVEL
LAPSE RATES IN EXCESS OF 8 C/KM...AND INVERTED-V SOUNDINGS SHOULD
YIELD STORMS WITH STRONG GUSTY WINDS.
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