SWODY1
SPC AC 230558
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1258 AM CDT TUE AUG 23 2011
VALID 231200Z - 241200Z
...THERE IS A SLGT RISK OF SVR TSTMS FROM THE UPPER MS VALLEY INTO
THE MIDWEST AND CENTRAL GREAT LAKES...
...SYNOPSIS...
A SHORTWAVE TROUGH...CURRENTLY MOVING ESEWD ACROSS THE DAKOTAS PER
WATER VAPOR IMAGERY...WILL AMPLIFY TODAY AS IT MAINTAINS AN ESEWD
TRACK THROUGH THE WRN GREAT LAKES BY 24/00Z...AND REACH THE LOWER
GREAT LAKES/UPPER OH VALLEY REGION BY 12Z WEDNESDAY. MEANWHILE...AN
UPSTREAM SHORTWAVE TROUGH...CURRENTLY MOVING EWD INTO BRITISH
COLUMBIA...WILL BEGIN TO AMPLIFY IN LEE OF THE CANADIAN ROCKIES
TODAY...AND UNDERGO FURTHER DEEPENING/AMPLIFICATION TUESDAY NIGHT
REACHING NWRN ONTARIO AND NRN MN BY THE END OF THE PERIOD.
OTHERWISE...A STRONG RIDGE WILL REMAIN ANCHORED ACROSS THE FOUR
CORNERS STATES EXTENDING EWD TO THE LOWER MS VALLEY/CENTRAL GULF
COAST STATES.
IN THE LOW LEVELS...A NNW-SSE ORIENTED WARM FRONT WILL ADVANCE EWD
ACROSS MN...IA...AND MO TODAY REACHING THE MN/WI BORDER TO THE MID
MS VALLEY BY MID-LATE THIS AFTERNOON. BY THIS TIME...A DRY LINE
SHIFTING ESEWD SHOULD INTERSECT THE WARM FRONT OVER EAST
CENTRAL-SERN MN AND EXTEND SWWD INTO CENTRAL NEB. MEANWHILE...A
COLD FRONT ATTENDANT TO THE UPSTREAM AMPLIFYING CANADIAN TROUGH WILL
MOVE ESEWD ACROSS THE NRN PLAINS TODAY...REACHING THE UPPER MS
VALLEY TO CENTRAL NEB BY 12Z WEDNESDAY.
...UPPER AND MID MS VALLEY TO CENTRAL GREAT LAKES...
AT 12Z TODAY...MODELS ARE IN GENERAL AGREEMENT SUGGESTING AT LEAST
ONE MCS SHOULD BE ONGOING OVER PARTS OF SRN MN/NRN IA AND
POTENTIALLY INTO WRN/SWRN WI. MODERATE INSTABILITY AND EFFECTIVE
BULK SHEAR AROUND 35 KT IN THIS REGION INDICATE POTENTIAL FOR SOME
ONGOING SEVERE WEATHER THREAT WITH THIS MCS. ALTHOUGH LOW LEVEL WAA
SHOULD PERSISTENT THROUGH THE MORNING...WEAKENING OF THE SWLY LLJ
INTO ERN IA/SRN WI SUGGESTS THE MCS MAY UNDERGO AT LEAST TEMPORARY
WEAKENING BETWEEN 12-18Z AS THE LLJ WEAKENS. A CONVECTIVE OUTFLOW
BOUNDARY ATTENDANT TO THIS MCS SHOULD EXTEND WWD THROUGH NRN IL AND
THEN NWWD THROUGH NERN IA TO EAST CENTRAL-SERN MN BY MID-LATE
AFTERNOON WHERE IT WILL INTERSECT AND/OR MERGE WITH THE WARM FRONT.
THE ERN EXTENT OF AN EML/STEEP MIDLEVEL LAPSE RATES /OBSERVED WITH
THE NRN PLAINS 23/00Z SOUNDINGS/ SHOULD BE LOCATED ACROSS THE UPPER
MS VALLEY BY 12Z TODAY...AND ADVANCE EWD TO THE WRN GREAT
LAKES/MIDWEST INTO THE AFTERNOON AND SEWD TO THE MID MS VALLEY LATE
TUESDAY NIGHT. THESE LAPSE RATES WILL BE SPREADING ATOP A QUITE
MOIST WARM SECTOR /SURFACE DEWPOINTS IN THE LOWER 70S AND
PRECIPITABLE WATER VALUES 1.5+ INCH/ BUILDING EWD TODAY...ALONG AND
W THROUGH S OF THE WARM FRONT AND ANY ONGOING EARLY MORNING MCS IN
THE UPPER MS VALLEY. THESE THERMODYNAMICS COMBINED WITH SURFACE
HEATING INVOF AND W-S OF THE AFOREMENTIONED WARM FRONT/CONVECTIVE
OUTFLOW WILL RESULT IN THE STRONGEST INSTABILITY /MLCAPE 3000-4000 J
PER KG/ EXTENDING FROM EAST CENTRAL MN/WRN WI SWD INTO IA AND
ADJACENT NWRN-WRN IL. SIMILAR INSTABILITY VALUES WILL ALSO EXTEND
SWWD INTO THE MID MO VALLEY...THOUGH FORECAST SOUNDINGS INDICATE THE
AIR MASS OVER WRN IA/ERN NEB SHOULD REMAIN CAPPED LIMITING
CONVECTIVE DEVELOPMENT.
FORCING FOR ASCENT ATTENDANT TO THE DAKOTAS SHORTWAVE TROUGH AND
INCREASING LOW LEVEL WAA INVOF OF A RE-STRENGTHENING SWLY LLJ
EXTENDING FROM NRN MO TO SRN WI ARE EXPECTED TO SUPPORT NEW TSTM
DEVELOPMENT ALONG THE WARM FRONT IN EAST CENTRAL-SERN MN INTO ERN IA
AND ALONG THE OUTFLOW BOUNDARY IN NWRN IL. THIS STORM INITIATION
SHOULD OCCUR BY MID-LATE AFTERNOON. SUPERCELLS WILL BE LIKELY WITH
THE FIRST STORMS...GIVEN VERY STRONG INSTABILITY...STEEP MIDLEVEL
LAPSE RATES...EFFECTIVE BULK SHEAR UP TO 40 KT AND ENHANCED LOW
LEVEL SHEAR INVOF THE WARM FRONT/MORNING OUTFLOW BOUNDARY. VERY
LARGE HAIL...SOME EXCEEDING 2 INCH IN DIAMETER...WILL BE POSSIBLE
WITH THE INITIAL ACTIVITY ALONG WITH DAMAGING WINDS. WHILE A
TORNADO THREAT WILL ALSO BE POSSIBLE...THIS THREAT SHOULD INCREASE
INTO THE EARLY EVENING FROM SERN MN/NERN IA TO SRN WI AND NWRN IL AS
A STRENGTHENING LLJ INCREASES THE SIZE OF HODOGRAPH CURVATURE.
AFTER DARK...MODELS SUGGEST STORMS SHOULD GROW UPSCALE INTO ONE OR
TWO MCS SPREADING ESEWD TOWARD LOWER MI AND THE MIDWEST...INCLUDING
THE CHICAGO METRO AREA. LOW LEVEL WAA ALONG THE SWLY LLJ EXTENDING
THROUGH THESE REGIONS TUESDAY NIGHT AND ASCENT ATTENDANT TO THE
SHORTWAVE TROUGH SUPPORT THIS SCENARIO WITH A THE PRIMARY THREATS
BEING HAIL AND STRONG WIND GUSTS.
..PETERS/SMITH.. 08/23/2011
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