SWODY1
SPC AC 271939
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0239 PM CDT FRI MAR 27 2009
VALID 272000Z - 281200Z
...THERE IS A MDT RISK OF SVR TSTMS THIS EVE AND OVERNIGHT ACROSS
PARTS OF SRN AR/NRN LA THRU NRN AND CNTRL MS....
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTN/TONIGHT ACROSS
PARTS OF THE SERN PLAINS THRU THE LWR MS VALLEY AND GULF STATES....
...20Z UPDATE...
VARIABILITY DOES EXIST AMONG THE MODELS CONCERNING SOME DETAILS OF
SURFACE CYCLOGENESIS NOW UNDERWAY ACROSS THE SOUTH CENTRAL PLAINS.
HOWEVER...CONSIDERABLE GUIDANCE...INCLUDING THE 27/12Z ECMWF AND
027/09Z NCEP SREF...CONTINUES TO INDICATE SECONDARY SURFACE LOW
DEVELOPMENT NEAR THE CENTRAL LOUISIANA/ARKANSAS BORDER AREA THROUGH
SOUTHEASTERN ARKANSAS DURING THE 28/03-06Z TIME FRAME THIS EVENING
...BEFORE IT CONSOLIDATES WITH THE MAIN CYCLONE ACROSS NORTHWEST
ARKANSAS OVERNIGHT. THIS FEATURE APPEARS OF PERHAPS GREATEST
SIGNIFICANCE TO THE SEVERE WEATHER POTENTIAL FOR THIS FORECAST
PERIOD...AS IT PROVIDES A FOCUS FOR INTENSE UPWARD VERTICAL
MOTION...RAPID BOUNDARY LAYER BASED DESTABILIZATION AND
STRENGTHENING LOW-LEVEL FLOW FIELDS.
SURFACE DEW POINTS ARE ALREADY RISING THROUGH THE MID/UPPER 60S
ACROSS PARTS OF NORTHERN AND CENTRAL LOUISIANA. THIS SHOULD BE ABLE
TO ADVECT NORTHWARD TO THE CENTER OF THE DEVELOPING SURFACE
LOW...NORTHWEST OF THE STALLED CONVECTIVELY ENHANCED/GENERATED
BOUNDARY NOW EXTENDING ACROSS SOUTHEAST LOUISIANA INTO THE
NORTHEASTERN GULF OF MEXICO. ADDITIONALLY...SOUTHERLY 850 MB FLOW
TO THE IMMEDIATE EAST OF THE LOW IS PROGGED TO STRENGTHEN TO AROUND
40 KTS LATE THIS EVENING INTO THE OVERNIGHT HOURS. THIS WILL
CONTRIBUTE TO SIZABLE CLOCKWISE CURVED LOW-LEVEL HODOGRAPHS...
SUFFICIENT FOR THE EVOLUTION OF LONG LIVED LOW-LEVEL MESOSCALE
CYCLONES. AND...THERE IS SIGNIFICANT CONCERN THAT THIS ENVIRONMENT
WILL BECOME CONDUCIVE TO THE DEVELOPMENT OF A FEW STRONG TORNADOES
...PARTICULARLY ALONG A SWATH FROM NORTHEAST LOUISIANA/SOUTHEAST
ARKANSAS THROUGH NORTHWEST MISSISSIPPI.
IF CONVECTION NEAR THE CENTRAL GULF COAST IS ABLE TO PERSIST AND
MAINTAIN THE SURFACE BOUNDARY ACROSS SOUTHEAST LOUISIANA/EXTREME
SOUTHERN MISSISSIPPI...THIS FEATURE COULD ALSO BECOME A FOCUS FOR
TORNADIC DEVELOPMENT. OTHERWISE...IT IS POSSIBLE THAT THE THREAT
FOR TORNADIC SUPERCELLS COULD INCREASE SOUTH OF THE SURFACE CYCLONE
THROUGH MISSISSIPPI OVERNIGHT...AHEAD OF AN INCREASINGLY NEGATIVELY
TILTED UPSTREAM UPPER TROUGH AXIS.
..KERR.. 03/27/2009
.PREV DISCUSSION... /ISSUED 1130 AM CDT FRI MAR 27 2009/
...TX/OK TO THE LOWER MS VALLEY AND NE GULF COAST...
A CLOSED MID LEVEL LOW NOW OVER ERN NM WILL MOVE EWD OVER NW TX THIS
EVENING...AND THEN BEGIN TO PIVOT ENEWD OVER SE OK BY THE END OF THE
PERIOD. AN INITIAL SURFACE CYCLONE BETWEEN ABI-SEP WILL GRADUALLY
DEEPEN NEWD INTO SE OK AS THE LARGE SCALE SYSTEM OCCLUDES
TONIGHT...THEN THE LOW SHOULD THEN DEVELOP EWD ACROSS AR OVERNIGHT
WITH THE TRIPLE POINT.
A LARGE ONGOING MCS ACROSS THE NE GULF COAST WILL LIKELY POSE A
THREAT FOR DAMAGING WINDS AND A COUPLE OF TORNADOES INTO EARLY
AFTERNOON...BEFORE FINALLY MOVING AWAY FROM THE RICHER MOISTURE AND
STRONGER INSTABILITY FEED FARTHER W IN THE GULF BASIN. AN OUTFLOW
BOUNDARY IN THE WAKE OF THIS THUNDERSTORM COMPLEX IS BEGINNING TO
LIFT NWD ACROSS SE TX AND SW LA...WITH 68-70 F BOUNDARY LAYER
DEWPOINTS S OF THE OUTFLOW. A SECOND BAROCLINIC ZONE EXTENDS ACROSS
NRN MS/NRN LA/NE TX...E OF THE DEVELOPING SURFACE CYCLONE. THIS
FRONT WILL PROBABLY MOVE ONLY SLOWLY NWD INVOF I-20...UNTIL TONIGHT
WHEN THE PRIMARY CYCLONE DEVELOPS EWD ACROSS AR.
THE DEVELOPING THUNDERSTORMS NEAR AUS APPEAR TO BE ASSOCIATED WITH
THE SRN EXTENT OF A BAND OF LOW-MID LEVEL ASCENT PRECEDING THE COLD
FRONT AND MID LEVEL TROUGH TO THE W. THIS CONVECTION SHOULD EXPAND
IN COVERAGE AND INTENSIFY THROUGH THE AFTERNOON ACROSS E/SE TX ALONG
THE RETREATING OUTFLOW BOUNDARY AND POTENTIALLY AS FAR N AS THE
FRONT NEAR I-20. THE EARLY STORM DEVELOPMENT AND OBSERVED WEAK
LOW-LEVEL FLOW/SHEAR IN REGIONAL PROFILERS COMPLICATES THE FORECAST
FOR THIS AFTERNOON ACROSS E/NE TX...LA...AND SRN AR. THE DEVELOPING
STORMS WILL HAVE THE POTENTIAL TO DISRUPT THE NWD SPREAD OF THE
RICHEST MOISTURE...AND KEEP THE STRONGER INSTABILITY DISPLACED FROM
THE GRADUALLY INCREASING LOW-LEVEL FLOW/SHEAR FARTHER N NEAR TXK
LATER THIS AFTERNOON. THUS...EVEN AS OF LATE MORNING...THE
SUPERCELL/TORNADO THREAT IS STILL UNCLEAR LATER THIS
AFTERNOON/EVENING ACROSS NE TX/NRN LA/SRN AR.
MEANWHILE...SOME PORTION OF THE DEVELOPING CENTRAL TX CONVECTION
COULD EVENTUALLY PROGRESS EWD ACROSS SE TX AND LA ALONG THE REMNANT
OUTFLOW BOUNDARY. DEEP LAYER SHEAR WILL BE SUFFICIENT FOR AN
ORGANIZED CLUSTER OF STORMS WITH SOME EMBEDDED SUPERCELLS AND/OR
BOWING SEGMENTS. ASIDE FROM THE PRESENCE OF THE OUTFLOW BOUNDARY
AND ANY LINGERING INFLUENCE ON THE LOW-LEVEL SHEAR ACROSS
LA...DAMAGING WINDS AND SOME HAIL WOULD APPEAR TO BE THE MAIN THREAT
WITH THIS ACTIVITY.
OVERNIGHT...VERTICAL SHEAR AND HODOGRAPH STRUCTURES ARE FORECAST TO
BECOME MORE FAVORABLE FOR TORNADIC SUPERCELLS FROM SE AR INTO
WRN/CENTRAL MS...IMMEDIATELY SE OF THE SURFACE CYCLONE. THE PRIMARY
UNCERTAINTY IN THIS THREAT WILL BE THE IMPACTS OF POTENTIALLY
WIDESPREAD CONVECTION SPREADING EWD OUT OF TX...AND ESPECIALLY THE
EXTENT TO WHICH CONVECTION LIMITS THE NWD RECOVERY OF THE LOW-LEVEL
MOISTURE INVOF THE MS RIVER. THERE IS STILL ENOUGH POTENTIAL FOR
RECOVERY TO MAINTAIN THE MDT RISK AREA...GIVEN THE POSSIBILITY OF A
COUPLE OF SIGNIFICANT TORNADOES AND DAMAGING WINDS OVERNIGHT.
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