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    Day 2 Outlook >
Aug 20, 2014 1300 UTC Day 1 Convective Outlook
Updated: Wed Aug 20 12:51:54 UTC 2014 (Print Version | 20140820 1300Z Day 1 shapefile | 20140820 1300Z Day 1 KML)
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 Forecast Discussion
   SPC AC 201251

   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0751 AM CDT WED AUG 20 2014

   VALID 201300Z - 211200Z

   ...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PORTIONS OF THE NORTHERN
   AND CENTRAL PLAINS...

   ...SUMMARY...
   SEVERE THUNDERSTORMS ARE EXPECTED ACROSS PORTIONS OF THE NORTHERN
   AND CENTRAL PLAINS TODAY. LARGE HAIL AND DAMAGING WINDS WILL BE THE
   MAIN THREATS WITH THESE STORMS...BUT A TORNADO OR TWO COULD OCCUR AS
   WELL. OTHER STRONG TO SEVERE STORMS WITH HAIL AND GUSTY WINDS WILL
   BE POSSIBLE FROM THE MISSISSIPPI VALLEY SOUTHEAST ACROSS THE
   MIDWEST/OHIO VALLEY TO EASTERN TENNESSEE. SCATTERED STORMS WITH
   GUSTY WINDS WILL ALSO BE POSSIBLE FROM PENNSYLVANIA SOUTHWARD ALONG
   AND EAST OF THE APPALACHIANS.

   ...SYNOPSIS...
   A TRANSITION TO A MORE ACTIVE SEVERE WEATHER REGIME WILL COMMENCE
   OVER THE NORTHERN AND CENTRAL PLAINS TODAY AS A COMPACT SHORTWAVE
   DISTURBANCE EJECTS NORTHEASTWARD FROM A LARGER-SCALE POSITIVE-TILT
   UPPER TROUGH ACROSS THE WEST. MEANWHILE...A BROAD ZONE OF WEAK TO
   MODEST CYCLONIC MID-LEVEL FLOW WILL PERSIST FROM THE UPPER
   MIDWEST/OH VALLEY ACROSS THE APPALACHIANS...TO THE SOUTH AND EAST OF
   A WEAKENING DEEP-LAYER CYCLONE CENTERED OVER THE GREAT LAKES.
   MID-LEVEL HEIGHT RISES ARE FORECAST FROM THE SOUTHERN PLAINS TO
   UPPER MS VALLEY AS SUB-TROPICAL ANTICYCLONE STRENGTHENS AND EXPANDS
   NORTHWARD FROM THE GULF OF MEXICO.

   ...NORTHERN/CENTRAL PLAINS...
   A WARM FRONT WAS ALIGNED WITH THE MO RIVER VALLEY THIS
   MORNING...FROM WESTERN IA NORTHWESTWARD TO A SURFACE LOW OVER
   EASTERN MT. A VERY WARM AND MOIST AIRMASS RESIDES ALONG AND SOUTH OF
   THE FRONT AND IS CAPPED BY AN EML PLUME WITH 700-500MB LAPSE RATES
   OF 7-8 C/KM. RELATIVELY VIGOROUS STORMS ASSOCIATED WITH SLOPED
   ASCENT ACROSS THE WARM FRONT...AND ON THE EDGE OF THE EML
   PLUME...INCREASED IN COVERAGE OVERNIGHT ACROSS IA WITH ISOLATED SVR
   HAIL BEING REPORTED FROM A FEW OF THE MORE ROBUST STORMS. A DECREASE
   IN STORM COVERAGE SHOULD OCCUR THROUGH LATE MORNING AS THE LOW-LEVEL
   JET WEAKENS AND CAP STRENGTHENS OVER THE CENTRAL PLAINS.
   HOWEVER...ONE OR TWO STORM CLUSTERS MAY PERSIST NEAR AND NORTH OF
   THE SURFACE WARM FRONT WHILE TRACKING SOUTHEAST FROM IA/MO AND IL
   THROUGH LATE MORNING/EARLY AFTERNOON. MODEST SHEAR AND A DIURNAL
   INCREASE IN INSTABILITY NEAR THIS CONVECTION WOULD SUPPORT AT LEAST
   AN ISOLATED RISK OF SVR HAIL/WIND INTO THE AFTERNOON IF CONVECTION
   PERSISTS.

   LARGE SCALE ASCENT ASSOCIATED WITH THE EJECTING SHORTWAVE TROUGH 
   AND RESULTANT QG-FORCING WAS ALREADY EVIDENT IN SATELLITE/RADAR
   IMAGERY ACROSS THE CORRIDOR OF WEAK TO MODEST INSTABILITY...FROM NEB
   NORTH ACROSS THE WESTERN/CENTRAL DAKOTAS EARLY TODAY. WHILE THIS
   EARLY CLOUD COVER AND CONVECTION MAY TEND TO LIMIT MORE SUBSTANTIAL
   BOUNDARY LAYER DESTABILIZATION OVER WESTERN AND CENTRAL PARTS OF THE
   DAKOTAS...SUFFICIENT DIURNAL DESTABILIZATION COINCIDENT WITH
   INCREASING DEEP-LAYER VERTICAL SHEAR AND FOCUSED FORCING FOR ASCENT
   SHOULD PROMOTE A GRADUAL INCREASE IN STORM COVERAGE AND ORGANIZATION
   BY AFTERNOON. SCATTERED SVR HAIL/WIND SEEM POSSIBLE WITH THIS
   ACTIVITY NORTH OF THE WARM FRONT IN SD/ND.

   A MORE SIGNIFICANT...BUT LIKELY QUITE FOCUSED...SVR WEATHER RISK IS
   FORECAST TO EVOLVE THROUGH LATE AFTERNOON FROM CENTRAL SD SOUTH AND
   EAST AS STRONG DESTABILIZATION OCCURS NEAR THE WARM FRONT/DRYLINE
   AND DEVELOPING SURFACE LOW. SURFACE TEMPERATURES WARMING TO AROUND
   90F IN THE PRESENCE OF FOCUSED MESOSCALE FORCING SHOULD ACT TO
   OVERCOME INHIBITION WHERE SBCAPE OF 2000-3000 J PER KG WILL RESIDE.
   ROBUST CONVECTION APPEARS POSSIBLE WHERE FLOW WILL BOTH STRENGTHEN
   AND VEER WITH HEIGHT. INITIAL SUPERCELL STORMS WILL BE CAPABLE OF
   LOCALLY DAMAGING WINDS AND VERY LARGE HAIL. ANY DISCRETE UPDRAFTS
   NEAR THE SURFACE LOW AND WARM FRONT ARE LIKELY TO ENCOUNTER
   SUFFICIENT SR-HELICITY TO PROMOTE STRONGER LOW LEVEL ROTATION AND
   POSSIBLY A COUPLE OF TORNADOES. STORMS MAY SPREAD AS FAR EAST AS MN
   AND WESTERN IA BY LATE EVENING AND...WHILE MORE SIGNIFICANT SVR RISK
   IS LIKELY TO DIMINISH AFTER DARK...SOME LIMITED POTENTIAL FOR
   HAIL/WIND MAY LINGER INTO THE OVERNIGHT HOURS AS LIFT PERSISTS NORTH
   OF SURFACE WARM FRONT.

   MORE ISOLATED ROBUST STORM INITIATION MAY ALSO OCCUR AS WARM SECTOR
   CAPPING IS OVERCOME BY HEATING/MIXING AND WEAK HEIGHT FALLS OVER
   NORTHERN AND CENTRAL NEB. LARGE HAIL AND HIGH WINDS SEEM LIKELY IF
   STORMS CAN INITIATE IN THESE AREAS THROUGH LATE AFTERNOON.
   HOWEVER...OVERALL STORM COVERAGE IS EXPECTED TO REMAIN QUITE
   LIMITED.

   ...EASTERN KY/TN...
   VERY STRONG DESTABILIZATION /SBCAPE TO 3000 J PER KG/ SEEMS LIKELY
   ACROSS THESE AREAS AHEAD OF WEAK CONVECTIVELY-INDUCED PERTURBATIONS
   SKIRTING THE SOUTHERN FLANK OF GREAT LAKES UPPER LOW. DESPITE
   GRADUAL TREND TOWARD LARGE SCALE HEIGHT RISES ACROSS THE
   REGION...LATEST GUIDANCE SUGGESTS SUFFICIENT BACKGROUND AND PERHAPS
   MESOSCALE FORCING FOR AT LEAST ISOLATED STORM INITIATION FROM LATE
   MORNING INTO THE AFTERNOON. A COUPLE OF STORMS COULD PRODUCE
   DAMAGING WINDS AND PERHAPS SVR HAIL GIVEN MAGNITUDE OF ANTECEDENT
   INSTABILITY AND MODEST 25-30KT WEST-NORTHWESTERLY MID-LEVEL FLOW.

   ...PA/APPALACHIANS...
   WARM/MOIST CONVEYOR BELT BENEATH DIFLUENT EXIT REGION OF MID/UPPER
   WIND MAX ROTATING AROUND GREAT LAKES UPPER LOW MAY CONTRIBUTE TO
   WEAK DESTABILIZATION AND AN INCREASE IN SHOWER/THUNDERSTORM
   DEVELOPMENT OVER PORTIONS OF THE EASTERN U.S. THROUGH THE AFTERNOON.
   A COUPLE OF BRIEFLY STRONGER STORMS COULD PRODUCE LOCAL GUSTY WINDS
   BUT LACK OF GREATER SHEAR/INSTABILITY SHOULD KEEP SVR RISK LIMITED.

   ..CARBIN/COHEN.. 08/20/2014

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Page last modified: August 20, 2014
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