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Feb 20, 2014 1300 UTC Day 1 Convective Outlook

February 20, 2014

Feb 20, 2014 1300 UTC Day 1 Convective Outlook

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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.

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