SPC AC 031229
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0729 AM CDT SUN MAY 03 2009
VALID 031300Z – 041200Z
…THERE IS A MDT RISK OF SVR TSTMS FROM PARTS OF CENTRAL/NRN LA
ACROSS CENTRAL MS INTO CENTRAL AL…
…THERE IS A SLGT RISK OF SVR TSTMS FROM ERN TX INTO THE SOUTHERN
APPALACHIANS…
…THERE IS A SLGT RISK OF SVR TSTMS ACROSS SERN VA/NRN NC…
…ERN TX/LOWER MS RIVER VALLEY INTO THE SRN APPALACHIANS…
WELL DEFINED SHORTWAVE TROUGH NOW OVER TX WILL CONTINUE EJECTING
ENEWD ACROSS THE LOWER MS RIVER VALLEY/MID SOUTH TODAY. AT THE
SURFACE…COMPOSITE OUTFLOW BOUNDARY/FRONT FROM THE LOWER TN RIVER
VALLEY INTO CENTRAL LA AT 12Z WILL LIKELY REMAIN STALLED OR SAG
SLOWLY SSEWD THROUGH THE AFTERNOON. SURFACE DEW POINTS IN THE UPPER
60S/LOWER 70S F WILL PERSIST AHEAD OF THE FRONT AND SUPPORT MODERATE
TO STRONG INSTABILITY ACROSS THE WARM SECTOR WITH AFTERNOON HEATING.
THIS WILL ALSO ALL BUT ELIMINATE CAPPING BY THE EARLY TO MID
AFTERNOON NEAR THE FRONT.
ONGOING CLUSTERS OF ELEVATED STORMS MAY CONTINUE THROUGH THE MORNING
FROM NRN LA INTO NRN/CENTRAL MS. SMALL LEADING LINE OF STORMS OVER
CENTRAL MS THIS MORNING MAY CONTINUE TO BE FED BY RESERVOIR OF
MODERATE INSTABILITY IN PLACE ACROSS WARM SECTOR. ISOLATED SEVERE
MAY ACCOMPANY THIS CLUSTER OF STORMS INTO WRN AL THIS MORNING.
HOWEVER…PRIMARY SEVERE THREAT IS EXPECTED TO EVOLVE AS A POSSIBLE
DERECHO MCS FROM ACTIVITY MOVING OUT OF ERN TX AT 12Z. PRIMARY AXIS
OF WIND DAMAGE THREAT SHOULD EXTEND ENEWD ACROSS THE LOWER MS RIVER
VALLEY AND INTO PORTIONS OF AL THROUGH THE AFTERNOON INVOF EFFECTIVE
SURFACE FRONT. AT SOME POINT THIS MCS MAY RACE ENEWD ACROSS LA
FASTER THAN PRECEEDING AIR MASS CAN DESTABILIZE. HOWEVER…WITH
STRONG/DEEP ASCENT AND CONTINUED DESTABILIZATION THIS AFTERNOON
ACTIVITY SHOULD CYCLE UPWARDS OR REGENERATE NEARER THE UPPER SYSTEM
BY THE EARLY AFTERNOON. IN ADDITION TO POSSIBLE WIDESPREAD WIND
DAMAGE…LOW LEVEL SHEAR WILL REMAIN QUITE STRONG UNDER 40-50 KT
WSWLY H85 FLOW SUPPORTING A THREAT OF TORNADOES BOTH FROM LEADING
SUPERCELLS AND FROM EMBEDDED SUPERCELLS WITHIN THE DERECHO MCS.
SECONDARY BOWING MCS/CLUSTER OF SUPERCELLS MAY EVOLVE SW OF THE
LEADING SYSTEM DURING THE AFTERNOON AS CAP WEAKENS/BREAKS ALONG
SURFACE COLD FRONT ADVANCING SEWD ACROSS THE LOWER MS RIVER VALLEY.
THIS ACTIVITY WOULD THEN TRACK QUICKLY ENEWD POSSIBLY INTO NRN GA
BEFORE WEAKENING THIS EVENING. MID/UPPER TROUGH AXIS IS FORECAST TO
REMAIN ACROSS THE LOWER MS RIVER VALLEY/CENTRAL GULF COAST STATES
OVERNIGHT…SUGGESTING SUFFICIENT LIFT/SHEAR WILL REMAIN IN PLACE
ACROSS THE SRN APPALACHIANS FOR CONTINUED SEVERE THREAT INTO MONDAY
MORNING…DEPENDANT ON QUALITY OF INSTABILITY LATE IN THE PERIOD.
…NRN NC/SRN VA…
AT THE SFC…A LOW WILL MOVE QUICKLY EWD ACROSS TN VALLEY INTO THE
SRN APPALACHIANS AS A WARM FRONT MOVES SLOWLY NWD ACROSS NRN NC INTO
SRN VA BY MID-AFTERNOON. THUNDERSTORMS SHOULD DEVELOP ALONG THE
BOUNDARY IN THE HIGHER TERRAIN OF NRN NC AND INVOF THE FRONT DURING
THE MID TO LATE AFTERNOON AS CAP WEAKENS AND AIR MASS BECOMES
MODERATELY UNSTABLE. MODEST DEEP LAYER SHEAR WILL REMAIN IN PLACE
ACROSS THIS REGION UNDER 35-45 KT SWLY H5 FLOW. THIS SHOULD BE
SUFFICIENT FOR ORGANIZED CLUSTERS AND POSSIBLY ISOLATED SUPERCELLS
AS INSTABILITY IS MAXIMIZED THIS AFTERNOON. THE MOST FAVORABLE
SEVERE THREAT MAY BE DAMAGING WIND GUSTS DUE TO UNIDIRECTIONAL WIND
PROFILES AND STRONG LOW-LEVEL FLOW. IF ENOUGH INSTABILITY CAN
MATERIALIZE…THEN A CLUSTER OF STORMS WITH HAIL POTENTIAL WOULD
ALSO BE POSSIBLE.
…SRN ID/NRN UT/SW WY…
ZONAL FLOW WILL BE IN PLACE TODAY ACROSS THE INTERMOUNTAIN WEST AS A
MID-LEVEL JET MOVES INLAND ACROSS THE NRN COAST OF CA. AS SFC TEMPS
WARM…THUNDERSTORMS SHOULD INITIATE THIS AFTERNOON ACROSS PARTS OF
THE INTERMOUNTAIN WEST. THE EXIT REGION OF THE MID-LEVEL JET IS
FORECAST TO SPREAD EWD ACROSS SRN ID AND NRN UT LATE THIS AFTERNOON
RESULTING IN STRONG VERTICAL SHEAR PROFILES. THIS COMBINED WITH
STEEP MID-LEVEL LAPSE RATES SHOULD BE ENOUGH FOR A MARGINAL SEVERE
THREAT WITH THE STRONGER CELLS.
..EVANS/GARNER.. 05/03/2009
