Dec 21, 2013 0600 UTC Day 1 Convective Outlook

Dec 21, 2013 0600 UTC Day 1 Convective Outlook

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1157 PM CST FRI DEC 20 2013

VALID 211200Z – 221200Z

…THERE IS A MDT RISK OF SVR TSTMS FOR NRN/CNTRL LA/MS…ERN AR…WRN TN…SWRN KY…NWRN AL…MO BOOTHEEL…

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

…OUTBREAK OF SEVERE STORMS POSSIBLE TODAY INTO TONIGHT WITH POTENTIAL FOR WIDESPREAD DAMAGING WINDS AND SEVERAL TORNADOES…SOME OF WHICH SHOULD BE SIGNIFICANT…

…SYNOPSIS…
A VIGOROUS SHORTWAVE TROUGH INVOF THE AZ/NM/MEXICAN BORDER WILL MOVE EWD INTO WRN TX THIS MORNING BEFORE EJECTING NEWD AND DAMPENING SLIGHTLY AS IT TRACKS TOWARDS THE MID-MS VALLEY. A SWATH OF INTENSE LOW/MID-LEVEL FLOW WILL ACCOMPANY THIS IMPULSE FROM THE WRN/CNTRL GULF COAST STATES TO THE MIDWEST. MODEST CYCLOGENESIS WILL OCCUR ALONG AN INITIALLY QUASI-STATIONARY FRONTAL ZONE. PRIMARY SURFACE CYCLONE SHOULD TRACK FROM THE ARKLATEX TO THE MIDWEST. AS THIS OCCURS…TRAILING PORTION OF FRONT WILL ACCELERATE EWD AS A COLD FRONT.

…WRN/CNTRL GULF COAST TO THE OH VALLEY…
A BROAD WARM SECTOR OF 60S SURFACE DEW POINTS IS ALREADY ESTABLISHED AHEAD OF THE QUASI-STATIONARY FRONT FROM CNTRL TX TO THE MID-SOUTH. THIS MOISTURE PLUME WILL ADVANCE NEWD INTO THE OH VALLEY AND WILL LARGELY DELINEATE THE NRN/ERN EXTENT OF SURFACE-BASED INSTABILITY. ALTHOUGH ABUNDANT CLOUD COVERAGE WILL INHIBIT STEEPER LAPSE RATES…PROGRESSIVELY RICH PW VALUES TOWARDS THE GULF COAST SHOULD YIELD WEAK TO MODEST BUOYANCY WITH MLCAPE OF 500-1000 J/KG PROBABLE ACROSS THE LOWER MS VALLEY/DEEP SOUTH.

WIDESPREAD CONVECTION IS EXPECTED TO BE ONGOING AT 12Z/SAT…GENERALLY WITHIN AND N OF THE FRONTAL ZONE. AS STRONGER FORCING FOR ASCENT OVERSPREADS THE WARM SECTOR…00Z CONVECTION-ALLOWING WRF GUIDANCE IS CONSISTENT IN INDICATING PRE-FRONTAL BANDS OF CONVECTION FORMING ACROSS THE SABINE TO LOWER MS VALLEYS. ALTHOUGH GUIDANCE SUGGESTS SLIGHT VEER-BACK DEEP-LAYER WIND PROFILES SHOULD EXIST WITH SWRN EXTENT…LOW-LEVEL S/SWLYS AOA 50 KT WILL BE HIGHLY FAVORABLE FOR ROTATING UPDRAFTS. THE GREATEST RISK FOR SEMI-DISCRETE SUPERCELLS SHOULD EXIST ACROSS THIS LOWER MS VALLEY THIS AFTERNOON…WITH POTENTIAL FOR SEVERAL TORNADOES /SOME OF WHICH SHOULD BE STRONG/.

AS THE SHORTWAVE TROUGH PIVOTS NEWD…CONSOLIDATION OF FRONTAL/PRE-FRONTAL CONVECTIVE BANDS WILL LIKELY YIELD AN EXTENSIVE QLCS BY AFTERNOON. AMIDST 700 MB SWLYS AOA 60 KT AND VERY LARGE LOW-LEVEL HODOGRAPHS…FAST-MOVING BOW/LEWP STRUCTURES SHOULD EVOLVE E/NEWD WITH RISKS OF SCATTERED TO WIDESPREAD DAMAGING WINDS…ISOLATED SIGNIFICANT GUSTS AND SEVERAL TORNADOES. ALTHOUGH INSTABILITY WILL WANE WITH NERN EXTENT AND BECOME RATHER LIMITED…THE STRENGTH OF THE FLOW REGIME MAY MAINTAIN A VIGOROUS QLCS AS FAR NE AS THE OH VALLEY.

FARTHER S…ALTHOUGH LARGE-SCALE ASCENT WILL GRADUALLY WANE LATER IN THE PERIOD…RISKS FOR DAMAGING WINDS AND TORNADOES SHOULD PERSIST THROUGH SAT NIGHT ACROSS THE DEEP SOUTH AS STRONG LOW/MID-LEVEL FLOW IS MAINTAINED AMIDST MIDDLE TO UPPER 60S SURFACE DEW POINTS.