Sunday, August 18, 2013

DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 182000
SWODY1
SPC AC 181957

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0257 PM CDT SUN AUG 18 2013

VALID 182000Z - 191200Z

...THERE IS A SLGT RISK OF SVR TSTMS INTO THIS EVENING FOR PARTS OF
THE NRN PLAINS...

...NRN PLAINS THIS AFTERNOON/EVENING...
WATER VAPOR IMAGERY SHOWED A PROGRESSIVE SHORTWAVE TROUGH MOVING
THROUGH ERN MT/FAR WRN ND EARLY THIS AFTERNOON. HEIGHT FALLS WITH
THIS FEATURE...ALTHOUGH WEAK...WILL SPREAD ACROSS CENTRAL/ERN
ND...NERN SD INTO MN THROUGH THIS EVENING. OBJECTIVE ANALYSES
INDICATED THE WARM SECTOR HAS BECOME MODERATELY UNSTABLE /MLCAPE
1500-2500 J PER KG/...ALONG AND E-S OF A FRONT WHICH EXTENDED FROM
NERN ND THROUGH SOUTH CENTRAL ND TO NWRN SD. CLOUDS FROM EARLIER
CONVECTION HAVE CONTINUED TO TEMPER SURFACE HEATING AND HAVE
PRECLUDED A GREATER REDUCTION IN INHIBITION. IT/S LIKELY THAT
ASCENT ATTENDANT TO THE SHORTWAVE TROUGH WILL BE NEEDED FOR STORMS
TO FORM ALONG THE FRONT LATE THIS AFTERNOON/EVENING. ONCE THAT
OCCURS...THEN THE AVAILABLE CAPE/SHEAR PARAMETER SPACE WILL SUPPORT
ORGANIZED STRONG TO SEVERE STORMS. THUS...NO CHANGES WILL BE MADE TO
THE SLIGHT RISK AND SEE TEXT AREA IN THE NRN PLAINS.

...ERN AL TO WRN/CENTRAL GA...
POTENTIAL EXISTS FOR STORMS TO PRODUCE AN ISOLATED THREAT FOR STRONG
WIND GUSTS INTO ERN AL AND WRN/CENTRAL GA. THE LOW SEVERE WIND
PROBABILITIES HAVE BEEN EXPANDED TO INCLUDE MORE OF ERN AL AND
WRN/CENTRAL GA. FOR ADDITIONAL MESOSCALE DETAILS...REFER TO SPC
MESOSCALE DISCUSSION 1736.

..PETERS.. 08/18/2013

.PREV DISCUSSION... /ISSUED 1126 AM CDT SUN AUG 18 2013/

...NRN PLAINS THIS AFTERNOON/EVENING...
MORNING WATER VAPOR IMAGERY AND UPPER AIR ANALYSES REVEAL A SERIES
OF MIDLEVEL SHORTWAVE TROUGHS MOVING EWD OVER THE NRN PLAINS. THE
PRIMARY WAVE IS NOW LOCATED OVER CENTRAL MT AND WILL CROSS ND BY
THIS EVENING...WHILE SMALLER-SCALE WAVES ARE MOVING EWD FROM NE WY
AND OVER SRN MANITOBA. EACH WAVE IS ASSOCIATED WITH ONGOING
CONVECTION...THE STRONGEST OF WHICH IS WITH THE MANITOBA TROUGH AND
COULD BRUSH NW MN THIS AFTERNOON. OTHERWISE...THE WEAK ELEVATED
CONVECTION MOVING OVER NW SD AND NE MT WILL LIKELY PERSIST FOR
SEVERAL MORE HOURS WHILE SPREADING EWD OVER NW ND...AND ALONG THE
WRN/CENTRAL BORDER OF THE DAKOTAS.

CLOUDS WITH THE ELEVATED CONVECTION WILL SLOW SURFACE HEATING IN
WHAT IS EXPECTED TO BE THE MOST UNSTABLE PORTION OF THE WARM SECTOR
FROM WRN SD INTO SRN ND. 12Z SOUNDINGS FROM THIS AREA ALSO REVEALED
A WARM ELEVATED MIXED LAYER WITH A STRONG CAP...SUCH THAT AFTERNOON
SURFACE TEMPERATURES WILL NEED TO REACH THE MID 90S TO LARGELY
REMOVE CONVECTIVE INHIBITION. THUS...THERE IS SOME UNCERTAINTY
REGARDING STORM COVERAGE THIS AFTERNOON ALONG THE NE-SW ORIENTED
SURFACE TROUGH FROM NW SD INTO SE ND. ASSUMING SURFACE HEATING AND
LARGER-SCALE ASCENT ARE SUFFICIENT FOR CONVECTIVE
INITIATION...INVERTED-V PROFILES WITH MODEST UNIDIRECTIONAL WLY
SHEAR WILL SUPPORT MULTICELL CLUSTERS CAPABLE OF PRODUCING DAMAGING
GUSTS. SOMEWHAT STRONGER BUOYANCY AND VERTICAL SHEAR ARE EXPECTED
INTO NW SD /IN THE WAKE OF THE MORNING CONVECTION/...SUCH THAT
PROFILES MAY SUPPORT A SUPERCELL OR TWO...WITH AN ATTENDANT RISK FOR
LARGE HAIL GIVEN STEEP MIDLEVEL LAPSE RATES. THE DAMAGING
WIND/LARGE HAIL THREAT WILL BEGIN TO DIMINISH BY LATE EVENING AS THE
BOUNDARY LAYER COOLS AND CONVECTIVE INHIBITION STRENGTHENS.

OTHERWISE...A FEW HIGH-BASED STORMS WILL BE POSSIBLE FARTHER S ALONG
THE SURFACE LEE TROUGH THIS AFTERNOON FROM THE NEB PANHANDLE INTO NE
CO. ISOLATED STRONG/DAMAGING OUTFLOW GUSTS AND MARGINALLY SEVERE
HAIL WILL BE POSSIBLE FOR A FEW HOURS LATE THIS AFTERNOON/EVENING.

...N CENTRAL/NE GULF COAST TODAY...
A WEAK SURFACE LOW...ASSOCIATED WITH THE MIDLEVEL TROUGH OVER THE MS
VALLEY AND A SURFACE BAROCLINIC ZONE FROM THE SW TO N CENTRAL GULF
OF MEXICO...WILL DRIFT NWD FROM SE LA TO SE MS. A FEW ROTATING
STORMS HAVE BEEN OBSERVED NEAR THE COAST AND OFFSHORE THROUGH THE
MORNING...THOUGH SUPERCELL STRUCTURES HAVE TENDED TO WEAKEN IN THE
PAST COUPLE OF HOURS. LIKEWISE...VWP/S FROM MOB AND EVX BOTH SHOW
GRADUAL WEAKENING OF THE LOW-LEVEL SHEAR WITH TIME...AND THIS TREND
WILL CONTINUE AS THE UNSTABLE WARM SECTOR MAKES ONLY SLOW INLAND
PROGRESS. OVERALL...THE RISK FOR A BRIEF TORNADO MAY HAVE ALREADY
PEAKED THIS MORNING...WHILE ISOLATED MARGINALLY DAMAGING GUSTS WILL
STILL BE POSSIBLE THIS AFTERNOON ACROSS SRN AL AND THE FL
PANHANDLE/SW GA.

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