The SPC is forecasting ...severe thunderstorms expected over parts of the southern plains and lower mississippi river valley through tonight.... Please read the latest public statement about this event.
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SPC AC 272209 DAY 1 CONVECTIVE OUTLOOK AMEND 1 NWS STORM PREDICTION CENTER NORMAN OK 0509 PM CDT FRI MAR 27 2009 VALID 272210Z - 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.... ...THERE IS A SLGT RISK OF SVR TSTMS OVER PARTS OF EASTERN NC... AMENDED TO ADD SLGT RISK TO EASTERN NC A SMALL AREA OF LOW-TOPPED SUPERCELLS HAS RECENTLY DEVELOPED ALONG WARM FRONT OVER EASTERN NC. THESE STORMS HAVE PRODUCED ISOLATED TORNADOES...AND MAY CONTINUE TO POSE A RISK FOR ANOTHER FEW HOURS. PLEASE REFER TO SWOMCD NUMBER 301 FOR FURTHER DETAILS. ...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. ..HART.. 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. CLICK TO GET WUUS01 PTSDY1 PRODUCT NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z