SEVER WEATHER OUTLOOK ..THERE IS A SLGT RISK OF SVR TSTMS OVER PARTS OF MN AND THE UPPER MS VALLEY... ...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE MIDWEST/SOUTHERN GREAT LAKES... ...SUMMARY... SEVERE THUNDERSTORM GUSTS AND HAIL ARE MOST PROBABLE TODAY OVER PORTIONS OF MINNESOTA AND THE EASTERN DAKOTAS...AS WELL AS FROM NORTHERN MISSOURI ACROSS CENTRAL AND NORTHERN ILLINOIS TO SOUTHERN LOWER MICHIGAN. A MARGINAL THREAT OF SEVERE WEATHER EXISTS ACROSS NEVADA...NORTHWESTERN ARIZONA AND WESTERN UTAH. ...EASTERN DAKOTAS ACROSS MN THIS AFTERNOON AND EVENING... A POTENT AND COMPACT SHORTWAVE TROUGH EVIDENT IN SATELLITE IMAGERY AND SHORT-TERM MODEL DATA WILL TRACK QUICKLY SOUTHEAST FROM ND AND ACROSS THE UPPER MS VALLEY THROUGH TONIGHT. LATEST INDICATIONS SUPPORT AT LEAST AN 80KT 500MB JET AIDING IN THE RAPID SOUTHEASTWARD TRANSLATION OF THIS IMPULSE ACROSS MN THIS AFTERNOON AND EVENING. STRONGLY FORCED DEEP-LAYER ASCENT WITHIN/BENEATH LEFT EXIT REGION OF THE MID/UPPER JET WILL BE REALIZED ALONG A SECONDARY COLD FRONT/WIND-SHIFT THAT IS FORECAST TO TRACK EAST-SOUTHEAST FROM ND/MN BORDER TO EXTREME SOUTHEAST MN AND WESTERN WI THROUGH EVENING. DESPITE LIMITED BOUNDARY LAYER MOISTURE AHEAD OF THE FRONTAL SURGE WITH SURFACE DEWPOINTS GENERALLY IN THE LOW-MID 50S F...THE MAGNITUDE OF FORCING/LIFT WILL COINCIDE PREFERENTIALLY WITH THE DIURNAL HEATING CYCLE TO SUPPORT DEVELOPMENT OF STRONG LOW-TOPPED LINEAR CONVECTION...AND POSSIBLY EMBEDDED SUPERCELL STRUCTURES ACROSS WESTERN MN BY EARLY AFTERNOON. GIVEN INTENSITY OF MESO-BETA SCALE FORCING...HIGH RES MODELS ARE QUITE CONSISTENT IN THE EVOLUTION OF A LINEAR MCS MOVING RAPIDLY SOUTHEAST ACROSS CENTRAL AND SOUTHEAST MN THROUGH THE DAYLIGHT HOURS. WITH LATEST VISIBLE SATELLITE IMAGERY INDICATING NEARLY CLOUDLESS CONDITIONS FROM EASTERN DAKOTAS ACROSS THE SOUTHERN HALF OF MN...CONFIDENCE IS INCREASING THAT SUFFICIENT HEATING/INSTABILITY WILL DEVELOP TO CONTRIBUTE TO A CORRIDOR OF STRONG TO DAMAGING THUNDERSTORM WIND GUSTS AS STORMS MOVE OVER THESE AREAS THROUGH THE AFTERNOON. HAIL UP TO AN INCH ALSO APPEARS LIKELY TO OCCUR WITH A FEW OF THE MORE ROBUST UPDRAFTS OR EMBEDDED SUPERCELLS WITH THIS CONVECTIVE SYSTEM. ...NORTHEAST MO/EASTERN IA TO SOUTHERN WI/NRN IL/IND AND LOWER MI... SOMEWHAT LOWER CONFIDENCE EXISTS REGARDING THUNDERSTORM DEVELOPMENT/EVOLUTION ACROSS THESE AREAS COMPARED TO FARTHER NORTH. ISOLATED...MOSTLY WEAK...CONVECTION HAS PERSISTED NEAR THE LEADING FRONTAL WIND-SHIFT ACROSS PARTS OF IA AND EXTREME SOUTHEAST NEB THIS MORNING. CI/DEBRIS CLOUDS AHEAD OF THIS ACTIVITY WERE THINNING AND THIS SHOULD CONTRIBUTE TO SURFACE HEATING/DESTABILIZATION THROUGH THE AFTERNOON. AS WEAK INHIBITION IS OVERCOME EXPECT MORE ROBUST STORM DEVELOPMENT TO OCCUR ALONG SOUTHWEST-NORTHEAST ORIENTED WEAK CONFLUENCE ZONES ASSOCIATED WITH 1) LEADING FRONT...AND 2) PRE-FRONTAL TROUGH/RESIDUAL OUTFLOW BOUNDARY...FROM NORTHERN MO TO SOUTHERN WI/NORTHERN IL. MODEST TO LOCALLY STRONG INSTABILITY /SBCAPE 2000-2500 J PER KG/ IS FORECAST TO EXIST NEAR/AHEAD OF THE INCREASE CONVECTION OVER MO/IL...WITH A GENERAL DECREASE IN CAPE TO UNDER 1000 J PER KG WITH EAST-NORTHEASTWARD EXTENT INTO LOWER MI. GREATEST SUPPORTIVE JUXTAPOSITION OF INSTABILITY AND STRENGTHENING DEEP-LAYER VERTICAL SHEAR FOR SEVERE STORMS THROUGH EARLY EVENING IS LIKELY TO EVOLVE FROM NEAR THE SOUTHERN TIP OF LAKE MICHIGAN SOUTHWESTWARD TO NEAR THE KS/MO BORDER. THIS ACTIVITY WILL HAVE A CHANCE TO PRODUCE DAMAGING GUSTS AND SPORADIC LARGE HAIL. A COUPLE OF SUPERCELLS MAY DEVELOP A BRIEF OPPORTUNITY FOR A TORNADO IF STORMS CAN RETAIN DISCRETE CHARACTER AND ENCOUNTER LOCALLY ENHANCED LOW LEVEL SHEAR NEAR ANY LAKE BREEZES...OR RESIDUAL BOUNDARIES. SEVERE THREAT SHOULD DIMINISH FROM LATE EVENING INTO OVERNIGHT HOURS AS AIRMASS BECOMES MORE STABLE. ...GREAT BASIN... WIDELY SCATTERED AFTERNOON AND EVENING STORMS ARE EXPECTED TO TRACK NORTHWARD/NORTHEASTWARD OVER NV THROUGH TONIGHT. ACTIVITY WILL BE SPURRED/SUSTAINED BY A SLOW INCREASE IN BOTH MID-UPPER LEVEL WINDS AND LARGE-SCALE ASCENT AS DEEP-LAYER CLOSED LOW DEVELOPS EASTWARD FROM CA. MOISTURE WILL REMAIN MARGINAL FOR MORE ROBUST STORMS BUT DEEPENING/WELL-MIXED SUBCLOUD LAYER WILL DEVELOP WITH NEARLY DRY-ADIABATIC LAPSE RATES EVOLVING COINCIDENT WITH GENERALLY WEAK MLCAPE AOB 500 J PER KG. ALTHOUGH DEEP-LAYER SHEAR VALUES ARE MODEST FOR MORE ORGANIZED STORMS...SUFFICIENT AND STRENGTHENING FLOW THROUGH THE CLOUD-BEARING LAYER...IN COMBINATION WITH THE INCREASE IN LARGE SCALE FORCING FOR ASCENT....SHOULD CONTRIBUTE TO MODEST STORM ORGANIZATION INTO THIS EVENING WITH LOCALIZED SEVERE WIND AND HAIL POSSIBLE.
Posted on: Sat, 20 Sep 2014 20:00:21 +0000
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