The SPC is forecasting ...severe thunderstorms expected over parts of the midwest this afternoon and tonight.... Please read the latest public statement about this event.
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SPC AC 181237 DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0737 AM CDT WED JUL 18 2007 VALID 181300Z - 191200Z ...THERE IS A MDT RISK OF SVR TSTMS THIS AFTERNOON AND TONIGHT ACROSS PARTS OF EASTERN SD...NORTHEAST NEB...MUCH OF IA...AND NORTHERN IL... ...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE NORTHERN/CENTRAL PLAINS AND INTO THE UPPER MS AND OH VALLEYS... ...THERE IS A SLGT RISK OF SVR TSTMS OVER PORTIONS OF THE MID ATLANTIC REGION... ...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF WA/ID/MT... MORNING WATER VAPOR IMAGERY SHOWS MID LEVEL SHORTWAVE TROUGH TOPPING THE RIDGE OVER THE ROCKIES ACROSS WY. THIS FEATURE WILL TRACK EASTWARD TODAY AND BECOME EMBEDDED IN FASTER WEST-NORTHWESTERLY FLOW. IMPULSE IS FORECAST TO TRACK DOWN SURFACE BOUNDARY THAT CURRENTLY EXTENDS FROM WESTERN SD INTO IL...AND RESULT IN A POTENTIALLY ACTIVE SEVERE WEATHER EVENT THIS AFTERNOON AND EVENING. ...ERN SD INTO IL/IND... 12Z SURFACE ANALYSIS SHOWS WEAK LOW OVER SOUTH CENTRAL SD...WITH BOUNDARY EXTENDING EASTWARD ACROSS NORTHERN IA AND INTO IL. THIS BOUNDARY IS EXPECTED TO REMAIN NEARLY STATIONARY TODAY AS UPPER TROUGH APPROACHES FROM THE WEST. STRONG DAYTIME HEATING...STEEP LOW AND MID LEVEL LAPSE RATES...AND SURFACE DEWPOINTS IN THE 60S TO NEAR 70F WILL RESULT IN A LARGE AREA OF EXTREME INSTABILITY THIS AFTERNOON FROM SERN SD/NERN NEB ACROSS MUCH OF IA INTO NRN IL. WEAK CAPPING SHOULD LIMIT CONVECTIVE DEVELOPMENT ALONG THIS AXIS UNTIL AT LEAST MID AFTERNOON...WHEN INCREASING LARGE SCALE FORCING FOR ASCENT WILL SPREAD INTO EASTERN SD/NEB. ISOLATED SUPERCELL THUNDERSTORMS MAY DEVELOP IN THIS AREA WITH A RISK OF VERY LARGE HAIL AND DAMAGING WINDS. STORMS ARE LIKELY TO INCREASE IN COVERAGE THROUGH THE EVENING AND ORGANIZE INTO A FAST-MOVING MCS ACROSS IA AND INTO IL. POTENTIAL EXISTS FOR A DERECHO EVENT ALONG THIS AXIS WITH WIDESPREAD DAMAGING WINDS. EASTWARD EXTENT OF SEVERE THREAT DEPENDS ON ORGANIZATION OF MCS...BUT MAY BE AS FAR EAST AS PARTS OF OH BEFORE 19/12Z. ...WY/WRN SD/WRN NEB... NORTHWEST FLOW ALOFT AND EASTERLY COMPONENT OF LOW LEVEL WINDS WILL RESULT IN A FAVORABLE VERTICAL SHEAR PROFILE FOR SUPERCELL THUNDERSTORMS THIS AFTERNOON FROM THE BIGHORN MTNS AND BLACK HILLS INTO PARTS OF CENTRAL SD AND THE NEB PANHANDLE. UPPER TROUGH NOW MOVING ACROSS WY MAY HELP TO INITIATE SEVERE STORMS IN THIS AREA RELATIVELY EARLY /18-21Z/ WITH A RISK OF LARGE HAIL AND DAMAGING WINDS IN STRONGER CELLS. ...WA/ID/MT... SEVERAL SHORTWAVE TROUGHS ARE EVIDENT IN WATER VAPOR IMAGERY ROTATING NORTHWARD ACROSS CA AND INTO ORE/WA. A DRY LOW LEVEL AIR MASS WILL LIMIT CONVECTIVE POTENTIAL OVER MUCH OF NV AND SOUTHERN ID TODAY. HOWEVER...CONSIDERABLE MOISTURE REMAINS IN THE MOUNTAINS OF NORTHERN ID AND EASTERN WA. THIS AREA WILL LIKELY SEE SCATTERED AFTERNOON THUNDERSTORM DEVELOPMENT. FORECAST SOUNDINGS SHOW FAVORABLE VERTICAL SHEAR PROFILES TO SUPPORT A RISK OF A FEW SEVERE THUNDERSTORMS CAPABLE OF LARGE HAIL AND DAMAGING WINDS. ...MID ATLANTIC REGION... A WEAK SHORTWAVE TROUGH IS CURRENTLY MOVING ACROSS PARTS OF NY/PA AND NORTHERN WV. THIS FEATURE WILL AFFECT THE MID ATLANTIC REGION AND SOUTHERN NEW ENGLAND LATER TODAY. MORNING VISIBLE SATELLITE IMAGERY SUGGESTS POCKETS OF DAYTIME HEATING WILL OCCUR FROM NJ SOUTHWARD...WITH DEWPOINTS NEAR 70F AND MLCAPE VALUES EXPECTED TO BE AROUND 1000 J/KG. SCATTERED THUNDERSTORMS ARE POSSIBLE BY MID AFTERNOON...SPREADING OFFSHORE AFTER DARK. SUFFICIENT WESTERLY FLOW ALOFT AND LARGE SCALE FORCING SUGGEST A RISK OF ISOLATED SEVERE STORMS. DAMAGING WINDS ARE EXPECTED TO BE THE MAIN THREAT. ..HART/JEWELL.. 07/18/2007 CLICK TO GET WUUS01 PTSDY1 PRODUCT NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z