SPC AC 190610
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0110 AM CDT WED AUG 19 2009
VALID 191200Z - 201200Z
...THERE IS A SLGT RISK OF SVR TSTMS FROM PORTIONS OF THE
CENTRAL/SRN PLAINS TO THE LOWER MO/MID MS VALLEYS...
...SYNOPSIS...
MID-UPPER LEVEL HEIGHT FALLS ARE EXPECTED ACROSS MUCH OF THE CENTRAL
U.S. THIS FORECAST PERIOD. THESE FALLS WILL BE ATTENDANT TO A
SHORTWAVE TROUGH...CURRENTLY TRACKING SEWD THROUGH SASKATCHEWAN PER
WATER VAPOR IMAGERY...AMPLIFYING SEWD ACROSS THE NRN PLAINS AND
EVOLVING INTO A CLOSED LOW OVER THE UPPER MIDWEST BY 12Z THURSDAY.
MEANWHILE...A LEAD IMPULSE WILL TRACK FROM ERN KS TO ERN IA/WRN
GREAT LAKES THROUGH EARLY THIS EVENING AND THEN WEAKEN AS IT REACHES
THE CENTRAL GREAT LAKES. A SECOND SHORT WAVE TROUGH PRECEEDING THE
AMPLIFYING NRN PLAINS TROUGH IS EXPECTED TO MOVE EWD ACROSS THE MID
MO TO UPPER MS VALLEYS BY EVENING BEFORE IT TOO WEAKENS OVER THE
GREAT LAKES.
IN THE LOW LEVELS...AT 12Z...A PRE-FRONTAL TROUGH/WIND SHIFT IS
EXPECTED TO EXTEND FROM THE ERN DAKOTAS SWD TO CENTRAL/ERN KS. THIS
BOUNDARY SHOULD MOVE SLOWLY EWD TODAY. A COLD FRONT ASSOCIATED WITH
AMPLIFYING UPPER TROUGH WILL SPREAD E/SEWD ACROSS THE NRN/CENTRAL
PLAINS AND SHOULD EXTEND FROM THE ERN DAKOTAS TO CENTRAL NEB BY LATE
THIS AFTERNOON. THE PRIMARY SYNOPTIC WARM FRONT SHOULD RETREAT NWD
FROM THE LOWER OH/MID MS VALLEYS THIS MORNING...AND EXTEND WWD FROM
THE SRN GREAT LAKES TO NRN IA.
...SRN PLAINS TO MID MO/MS VALLEYS...
MID LEVEL WINDS ARE EXPECTED TO STRENGTHEN FROM THE CENTRAL PLAINS
TO LOWER MO VALLEY AND INTO THE GREAT LAKES REGION THIS FORECAST
PERIOD...WITH BASE OF AMPLIFYING MID/UPPER LEVEL TROUGH. SRN EXTENT
OF A SWLY LLJ IS EXPECTED TO WEAKEN OVER THE SRN PLAINS THIS
MORNING...BUT PORTION OF THIS LLJ EXTENDING INTO LOWER MO VALLEY
WILL PERSIST THROUGH THE FORECAST PERIOD IN RESPONSE TO THE HEIGHT
FALLS.
DESPITE A MOIST WARM SECTOR /SURFACE DEWPOINTS MID-UPPER 60S/
BENEATH STEEP MID LEVEL LAPSE RATES...MORNING CLOUDS/PRECIPITATION
ASSOCIATED WITH MCS/S MOVING ACROSS THE MID/LOWER MO VALLEY WILL
LIKELY LIMIT SURFACE HEATING ACROSS MUCH OF NRN MO/IA TODAY.
STRONGER SURFACE HEATING SHOULD OCCUR ALONG EDGES OF MORNING CLOUD
SHIELD AND ALSO WWD OVER OTHER THE PLAINS STATES. MODELS SUGGEST
STRONGEST INSTABILITY SHOULD DEVELOP FROM THE CENTRAL PLAINS SWD TO
OK/SRN MO...WITH INCREASING INSTABILITY VALUES NWD INTO NRN MO AS
MORNING CLOUDS ERODE OR MOVE NWD AWAY FROM THAT AREA.
EFFECTIVE BULK SHEAR 35-50 KT IS EXPECTED ACROSS MUCH OF THE SLIGHT
RISK AREA ASSOCIATED WITH THE BAND OF STRONG MID LEVEL WINDS ACROSS
THIS REGION. THIS COMBINED WITH THE INSTABILITY WILL PROVIDE
FAVORABLE FOR STORMS TO BECOME ORGANIZED WITH SUPERCELLS LIKELY.
INITIAL TSTM DEVELOPMENT SHOULD OCCUR ALONG COLD FRONT IN CENTRAL
NEB AS IT ENCOUNTERS THE STRONGER INSTABILITY WITH ADDITIONAL
DEVELOPMENT POSSIBLE OVER ERN KS AND ALONG ERN PERIPHERY OF CLOUD
SHIELD IN THE MID MS VALLEY. HAIL AND STRONG WINDS GUSTS WILL BE
POSSIBLE WITH THIS ACTIVITY. STORMS SHOULD THEN DEVELOP SWWD ALONG
A BOUNDARY INTO WRN OK TO NW TX BY LATE AFTERNOON/EARLY
EVENING...WITH A POTENTIAL FOR TORNADOES ACROSS ERN KS INTO NRN
MO/ERN IA WHERE LARGE CURVED HODOGRAPHS ARE LIKELY IN VICINITY OF
LLJ. MODELS SUGGEST POTENTIAL FOR A QUASI-LINEAR MCS TO SPREAD
E/NEWD TO THE WRN GREAT LAKES AND MID MS VALLEY BY 12Z THURSDAY.
LIMITED DESTABILIZATION WITH E/NEWD EXTENT MAY TEND TO LIMIT THE
SEVERE THREAT INTO THE OVERNIGHT PERIOD...BUT STRENGTHENING LLJ MAY
SUPPORT A DAMAGING WIND THREAT. GIVEN UNCERTAINTY IN DEGREE OF
DESTABILIZATION...THIS OUTLOOK WILL NOT INTRODUCE HIGHER WIND
PROBABILITIES.
...UPPER MS VALLEY...
EXTENSIVE CLOUDINESS/PRECIPITATION IS EXPECTED ALONG AND E OF THE
PRE-FRONTAL TROUGH ACROSS MUCH OF MN/IA THIS MORNING INTO THE
AFTERNOON. GREATER SURFACE HEATING SHOULD OCCUR AHEAD OF THE COLD
FRONT ACROSS THE ERN DAKOTAS/FAR WRN MN BENEATH NRN EXTENT OF STEEP
MID LEVEL LAPSE RATES. THESE THERMODYNAMICS COMBINED WITH A NWD
RETURN OF LOW LEVEL MOISTURE SHOULD SUPPORT MLCAPE 1000-1500 J/KG.
FORCING FOR ASCENT AHEAD OF AMPLIFYING UPPER TROUGH AND INCREASING
LOW LEVEL CONVERGENCE AS COLD FRONT OVERTAKES NRN EXTENT OF
PRE-FRONTAL TROUGH WILL SUPPORT CONVECTIVE DEVELOPMENT OVER ERN
DAKOTAS/WRN MN AND SPREADING EWD. DESPITE MODERATE INSTABILITY...
EFFECTIVE SHEAR /UP TO 25 KT/ SUGGESTS MULTICELLS PRODUCING ISOLATED
HAIL/STRONGER WIND GUSTS SHOULD BE THE PRIMARY THREATS.
..PETERS/GRAMS.. 08/19/2009
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z
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