The SPC is forecasting ...a significant severe weather episode is expected over parts of the lower ohio valley...tennessee valley into lower mississippi valley this afternoon and tonight....   Please read the latest public statement about this event.  View What is a Watch? clip.

   SPC AC 201213

   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0613 AM CST THU FEB 20 2014

   VALID 201300Z - 211200Z

   ...THERE IS A MDT RISK OF SVR TSTMS LATE THIS AFTERNOON INTO TONIGHT
   FOR A SMALL PART OF SRN IND...WRN AND CNTRL KY...WRN AND MIDDLE
   TN...NRN MS AND FAR NWRN AL...

   ...THERE IS A SLGT RISK OF SVR TSTMS FROM THE OH VALLEY TO CNTRL
   GULF COAST...

   --POTENTIALLY WIDESPREAD DAMAGING WIND EVENT OVER PARTS OF THE OH
   AND TN VALLEYS INTO THE LOWER MS VALLEY AND CNTRL GULF STATES LATE
   THIS AFTERNOON THROUGH TONIGHT--

   ...SYNOPSIS...

   AN INITIALLY NEUTRAL-TILTED SHORT-WAVE TROUGH OVER THE HIGH PLAINS
   AS OF 12Z WILL ASSUME AN INCREASING NEGATIVE TILT AS A 100+ KT MID
   LEVEL JET STREAK OVER THE SRN PLAINS ROUNDS THE TROUGH BASE AND
   SUBSEQUENTLY DEVELOPS NEWD INTO THE GREAT LAKES AND OH VALLEY BY
   21/12Z. THIS PROCESS WILL PROMOTE SUBSTANTIAL INTENSIFICATION OF THE
   OVERALL SYNOPTIC SYSTEM...LEADING TO A CLOSED MID LEVEL CIRCULATION
   OVER THE UPPER MIDWEST AND A CORRIDOR OF 12-HR HEIGHT FALLS
   EXCEEDING 100-200 M AT 500 MB ALONG THE SYSTEM TRACK.

   AT THE SURFACE...LOW PRESSURE OVER CNTRL KS WILL UNDERGO SIGNIFICANT
   DEEPENING LATER TODAY THROUGH TONIGHT WHILE DEVELOPING NEWD TO THE
   VICINITY OF THE WI-IL BORDER BY 21/00Z AND THEN LAKE SUPERIOR BY
   21/12Z WHERE IT WILL HAVE BECOME DEEPLY OCCLUDED. MEANWHILE...THE
   TRAILING COLD FRONT IS EXPECTED TO STRENGTHEN MARKEDLY LATER TODAY
   FROM THE MID MS VALLEY TO ARKLATEX BEFORE SWEEPING EWD THROUGH THE
   OH AND TN VALLEYS...AS WELL AS THE WRN/CNTRL GULF COAST STATES. THE
   MIGRATORY LOW AND COLD FRONT WILL BE PRECEDED BY A W-E-ORIENTED WARM
   FRONT WHICH WILL DEVELOP NWD THROUGH THE OH VALLEY. THE POLEWARD
   ADVANCEMENT OF THIS BOUNDARY WILL LIKELY BE SLOWED OWING TO THE
   INFLUENCE OF SIGNIFICANT SNOW COVER TO THE NORTH OF THE I-70
   CORRIDOR.  

   ...MID/LOWER MS VALLEY EWD THROUGH THE OH/TN VALLEYS AND CNTRL GULF
   STATES THIS AFTERNOON THROUGH TONIGHT...

   THE CYCLOGENETIC CHARACTER OF THE SYNOPTIC SYSTEM WILL PROMOTE A
   MARKED INCREASE IN THE BREADTH AND INTENSITY OF WIND MAXIMA...BOTH
   IN THE LOW LEVELS AND ALOFT. THIS IN TURN WILL HASTEN THE POLEWARD
   TRANSPORT OF A PARTIALLY MODIFIED AIR MASS BENEATH A RESIDUAL STEEP
   MIDLEVEL LAPSE RATE PLUME...AS WELL AS ENHANCE DEEP-LAYER SHEAR
   ACROSS THE EXPANDING WARM SECTOR.

   LATEST MODEL GUIDANCE REMAINS CONSISTENT WITH EARLIER DATA IN
   SUGGESTING THE GRADUAL DEEPENING OF A PRE-FRONTAL CONFLUENCE BAND OF
   CONVECTION INTO TSTMS BY MID TO LATE AFTERNOON FROM NEAR THE
   OH-MS-RIVER CONFLUENCE SWWD TO AT LEAST N-CNTRL LA. AT THE SAME
   TIME...LOW-TOPPED TSTMS ARE EXPECTED TO EVOLVE UPSTREAM ALONG THE
   COLD FRONT FROM E-CNTRL MO/W-CNTRL IL TO NWRN LA...AS THE
   FRONTOGENETICALLY FORCED CIRCULATION IN THE LOW-LEVELS IS COUPLED
   WITH THE MID TO UPPER-LEVEL HEIGHT FALLS/DCVA PRECEDING THE MIDLEVEL
   TROUGH. 

   CONSENSUS OF CONVECTION-ALLOWING MODEL DATA INDICATE A RAPID
   TRANSITION TO A FAST-MOVING SQUALL LINE/QLCS AS THE TWO INCIPIENT
   CONVECTIVE BANDS MERGE. GIVEN A MOIST AND AT LEAST MODESTLY UNSTABLE
   AIR MASS /I.E. MLCAPE OF 500-750 J PER KG/ AND STRONG ENVIRONMENTAL
   SHEAR /I.E. EFFECTIVE BULK SHEAR OF 50-70 KT AND EFFECTIVE SRH OF
   200-400 M2 PER S2/...THE POTENTIAL WILL EXIST FOR EMBEDDED
   BOWING/LEWP STRUCTURES CAPABLE OF WIDESPREAD DAMAGING WINDS AND A
   FEW MESOVORTEX-RELATED TORNADOES. THIS IS ESPECIALLY THE CASE FROM
   WRN/CNTRL KY INTO NRN MS/NWRN AL WHERE THE MOST FAVORABLE OVERLAP OF
   FORCING FOR ASCENT...INSTABILITY AND VERTICAL SHEAR WILL EXIST.

   EXPECT THE SQUALL LINE /AND ASSOCIATED SEVERE RISK/ TO RAPIDLY
   ADVANCE THROUGH THE OH/TN VALLEYS TONIGHT WITH THE CONVECTIVE BAND
   GRADUALLY WEAKENING AS IT REACHES THE CNTRL APPALACHIANS BY 21/12Z.
   FARTHER SOUTH ACROSS MS/AL/SERN LA...THE STORMS WILL PROGRESS MORE
   SLOWLY WITH THE LINE POSSIBLY BREAKING INTO SEGMENTS OR MORE
   DISCRETE CELLS. GIVEN THE GRADUAL INFLUX OF A MORE MOIST BOUNDARY
   LAYER INTO THIS REGION...AN INCREASE IN SUPERCELLULAR TORNADO
   POTENTIAL IS POSSIBLE AFTER 03Z OVER ERN MS INTO WRN AL. AS
   SUCH...FIVE PERCENT TORNADO PROBABILITIES HAVE BEEN ADJUSTED EWD TO
   ACCOMMODATE THIS RISK.

   ...LOWER MO/MID MS VALLEYS TODAY...

   A CLUSTER OF STRONG TO SEVERE ELEVATED TSTMS IS IN PROGRESS AS OF
   12Z OVER NWRN MO...TO THE NORTH OF THE SURFACE WARM FRONT WHERE
   LOW-LEVEL WAA AND MOISTURE TRANSPORT ARE SURMOUNTED BY DCVA AND THE
   DIVERGENT POLE OF A CYCLONIC MID TO UPPER-LEVEL JET STREAK OVER THE
   SRN PLAINS. GIVEN THE STEEP MIDLEVEL LAPSE RATES...MUCAPE UP TO 1000
   J/KG AND STRONG CLOUD-BEARING SHEAR...THE POTENTIAL WILL EXIST FOR
   SUPERCELLS CAPABLE OF SEVERE HAIL. THESE SAME PROCESSES ARE EXPECTED
   TO MAINTAIN A LOW-PROBABILITY HAIL RISK EWD/NEWD ACROSS NRN MO INTO
   CNTRL/ERN IA...AND PERHAPS EVENTUALLY NWRN IL.

   ..MEAD/SMITH.. 02/20/2014

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   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z