SWODY1
SPC AC 211947
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0247 PM CDT TUE SEP 21 2010
VALID 212000Z - 221200Z
...THERE IS A SLGT RISK OF SVR TSTMS UPPER GREAT LAKES TO CNTRL
PLAINS...
...UPPER GREAT LAKES TO CNTRL PLAINS...
HAVE OPTED TO MAINTAIN HIGHEST PROBABILITIES OF SEVERE THUNDERSTORMS
ACROSS THE MID MS VALLEY INTO NRN IL AHEAD OF DEVELOPING
MCS...SUPPORTIVE OF EARLIER 1630Z OUTLOOK.
APPARENT CONVECTIVE VORT IS LOCATED OVER ECNTRL IA WITHIN EXPANDING
PRECIP SHIELD. THIS FEATURE IS EJECTING ENEWD TOWARD NRN IL AND
APPEARS TO BE AIDING SQUALL LINE INTENSIFICATION NEAR THE LEADING
EDGE OF AFOREMENTIONED CONVECTION. THUNDERSTORMS ARE INCREASING
FROM NEAR THE WI-IA-IL BORDER...SWWD INTO NWRN MO WHERE STRONG SFC
HEATING HAS CONTRIBUTED TO STEEP LOW LEVEL LAPSE RATES. ALTHOUGH
DEEP LAYER SHEAR IS NOT PARTICULARLY STRONG ACROSS THE WARM SECTOR
SUFFICIENT MID LEVEL WSWLY FLOW WILL ENHANCE THE PROSPECT FOR AN
UPWARD EVOLVING MCS AS COMPLEX OF STORMS SPREADS INTO NRN IL-SERN
IA-NRN MO. DAMAGING WINDS ARE THE MAIN THREAT WITH THIS ACTIVITY.
OTHER MINOR CHANGES HAVE BEEN MADE TO DECREASE THE PROBABILITY FOR
SEVERE HAIL LATE IN THE PERIOD ACROSS NEB. INHIBITION SHOULD
INCREASE ACROSS THIS REGION PREVENTING ROBUST ELEVATED UPDRAFTS
WHERE BUOYANCY WOULD PROVE SUFFICIENT FOR HAIL PRODUCTION.
..DARROW.. 09/21/2010
.PREV DISCUSSION... /ISSUED 1110 AM CDT TUE SEP 21 2010/
...UPPER GREAT LAKES INTO CNTRL PLAINS...
AS OF MID MORNING...MESOANALYSIS INDICATED THAT THE PRIMARY SYNOPTIC
COLD FRONT EXTENDED FROM LOW PRESSURE OVER CNTRL ONTARIO SWWD
THROUGH CNTRL PARTS OF U.P. OF MI/WI/IA...SERN NEB INTO W-CNTRL KS.
AIR MASS IN ADVANCE OF THIS FRONT IS RELATIVELY MOIST WITH DEW
POINTS IN THE 60S AND PW VALUES OF 1.0-1.5 INCHES. THIS MOISTURE
COUPLED WITH STRONG DIABATIC HEATING ALONG SRN/SERN FRINGE OF
FRONTAL CLOUD SHIELD SHOULD YIELD A MODERATELY UNSTABLE AIR MASS BY
AFTERNOON WITH MLCAPE VALUES INCREASING TO 1000-2000 J/KG.
BASED ON CURRENT WATER VAPOR IMAGERY AND 12Z MODEL GUIDANCE IT
APPEARS THAT A COUPLE OF CORRIDORS OF ENHANCED SEVERE WEATHER
POTENTIAL WILL DEVELOP LATER TODAY INTO TONIGHT. THE FIRST AREA OF
INTEREST IS OVER NRN/CNTRL LOWER MI IN ADVANCE OF A STRONG MIDLEVEL
IMPULSE NOW OVER NERN MN. 1Z APX SOUNDING SHOWED THE PRESENCE OF A
RATHER STRONG UNIDIRECTIONAL WIND FIELD WITH 50+ KT SWLY FLOW
OBSERVED BELOW 1 KM AGL. WHILE LOW-LEVEL GRADIENT /I.E. LOWEST 1-2
KM AGL/ IS FORECAST TO RELAX GRADUALLY THROUGH THE DAY...WIND FIELD
ABOVE THIS LEVEL SHOULD REMAIN RATHER STRONG. MOREOVER...REGION
SHOULD EXPERIENCE RELATIVELY STRONG DIABATIC HEATING WHICH WILL
EFFECTIVELY STEEPEN LOW-LEVEL LAPSE RATES...WHICH IN TURN WILL SERVE
TO ENHANCE THE DOWNWARD TRANSFER OF THE HIGHER MOMENTUM FLOW. AS
SUCH...DAMAGING WIND GUSTS APPEAR TO BE THE PRIMARY HAZARD WITH
STORMS WHICH SHOULD DEVELOP AND INTENSIFY AHEAD OF THE COLD FRONT
TODAY. HOWEVER...SOME HAIL AND PERHAPS A TORNADO WILL ALSO BE
POSSIBLE.
THE OTHER AREA OF POTENTIALLY GREATER SEVERE WEATHER RISK WILL BE
FROM PARTS OF ERN IA/NERN MO INTO SRN WI/NRN IL IN ADVANCE OF A
LOWER LATITUDE IMPULSE APPROACHING MID/LOWER MO VALLEY. TSTMS ARE
ALREADY INCREASING IN COVERAGE AND INTENSITY THIS MORNING IN ADVANCE
OF THESE FEATURES FROM E-CNTRL NEB EWD ACROSS CNTRL IA. HERE
TOO...MORNING VISIBLE SATELLITE IMAGERY SUGGESTS THAT POCKETS OF
STRONGER DIABATIC HEATING WILL DEVELOP AHEAD OF THE ONGOING STORMS
AND SURFACE COLD FRONT...SUPPORTING THE STEEPENING OF LOW-LEVEL
LAPSE RATES. WHILE WIND FIELD WILL BE COMPARATIVELY WEAKER THAN
POINTS TO THE NE...SUFFICIENT DEEP WLY/SWLY SHEAR WILL EXIST TO
SUPPORT UPSCALE GROWTH INTO ONE OR MORE CONVECTIVE SYSTEMS WITH THE
THREAT FOR DAMAGING WINDS AND HAIL.
SOME THREAT FOR ELEVATED...STRONG TO MARGINALLY SEVERE STORMS WILL
EXIST ACROSS WRN AND CNTRL NEB OVERNIGHT WITHIN STRENGTHENING
LOW-LEVEL WAA REGIME IN EXIT REGION OF NOCTURNAL LLJ. HAIL WILL BE
THE PRIMARY HAZARD WITH THE STRONGEST STORMS.
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