SWODY1
SPC AC 200505
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1205 AM CDT SUN APR 20 2008
VALID 201200Z - 211200Z
...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS VA/NC TIDEWATER AND
COASTAL PLAIN...
...SYNOPSIS...
IN MID-UPPER LEVELS...CONUS PORTION OF LARGE SCALE PATTERN WILL
CONTINUE TO FEATURE MEAN TROUGH OVER WRN STATES AND ERN MEAN RIDGE.
DEEP-LAYER CYCLONE...WITH MIDLEVEL CENTER INITIALLY ANALYZED OVER
ERN KY -- IS FCST TO PENETRATE PORTIONS OF ERN RIDGING IN EARLY
STAGES OF REX BLOCK EVOLUTION. IN DOING SO...LOW WILL REDEVELOP
GRADUALLY ESEWD-SEWD THROUGH PERIOD...EFFECTIVELY DRIFTING TO
POSITION OVER NRN OR ERN NC BY 21/12Z. SREF GUIDANCE AND
DETERMINISTIC/OPERATIONAL MODELS NOW PROVIDE STG CONSENSUS ON THIS
SCENARIO.
MEANWHILE...SERIES OF VORTICITY LOBES AND SMALL SCALE CIRCULATIONS
COMPRISE LARGE CYCLONIC GYRE NOW CENTERED OVER WRN WA. AS SMALLER
PERTURBATIONS MOVE THROUGH STG FLOW BELT AROUND THIS SYSTEM -- I.E.
90-100 KT 250 MB JET ACROSS NRN CA AND NRN GREAT BASIN -- EXPECT
HEIGHT FALLS ACROSS MOST OF NRN ROCKIES...MAINLY ASSOCIATED WITH EWD
MOTION OF SHORTWAVE TROUGH NOW OVER WRN ORE. LATTER PERTURBATION
SHOULD REACH NRN HIGH PLAINS BY 21/12Z. AT SFC...LEE-SIDE
CYCLOGENESIS ALREADY HAS OCCURRED OVER SERN MT. THIS LOW IS FCST TO
BIFURCATE NEAR BEGINNING OF PERIOD...NRN PORTION LIFTING INTO SERN
SASK AND SRN PORTION FOLLOWING SEGMENT OF SFC COLD FRONTAL ZONE SEWD
TO SWRN SD/NWRN NEB BY 21/00Z. FRONT SHOULD CONTINUE EWD ACROSS
MOST OF DAKOTAS BY 21/12Z...EXTENDING SSWWD ACROSS WRN KS AND TX
PANHANDLE.
...CAROLINAS/VA...
SCATTERED TO LOCALLY NUMEROUS TSTMS ARE EXPECTED TO DEVELOP MAINLY
IN CURVED BANDS...WITH GREATEST COVERAGE/INTENSITY DURING DIURNAL
WARM PERIOD OF MIDDAY THROUGH EARLY EVENING. HAIL PROBABILITIES
SHOULD BE RELATIVELY MAXIMIZED CLOSER TO COLD CORE REGION OF UPPER
LOW...LIMITING FACTOR BEING LIKELIHOOD OF WEAKER LOW LEVEL
CONTRIBUTION TO BUOYANCY BENEATH CLOUD/PRECIP AREAS. HAIL THREAT
APPEARS TOO CONDITIONAL/MRGL TO JUSTIFY CATEGORICAL OUTLOOK ON ITS
OWN.
BY CONTRAST...TORNADO POTENTIAL IS DISPLACED FARTHER E OVER AREA
GENERALLY ALONG/E OF I-95...IN ZONE OF MORE FAVORABLE BOUNDARY LAYER
THETAE. SEVERAL HOURS OF STG SFC HEATING MAY OCCUR OVER PORTIONS
ERN CAROLINAS AND PERHAPS SERN VA/SRN MD...AMIDST SELY SFC FLOW AND
BENEATH DEEP LAYER OF SLYS ALOFT. THIS KINEMATIC GEOMETRY WILL
DISPLACE LOW LEVEL HODOGRAPH LEFTWARD AND CURVE IT...RESULTING IN
FAVORABLE SRH VALUES FOR SUPERCELLS. PRIOR/ONGOING ACTIVITY MAY
LEAD TO NNE-SSW ALIGNED BOUNDARY WHICH WOULD SERVE BOTH AS
DIFFERENTIAL HEATING/DESTABILIZATION ZONE...AND AS POTENTIAL SOURCE
OF LOW LEVEL VORTICITY AND DEVIANT/RIGHTWARD STORM MOTION.
LONGEVITY OF DISCRETE TSTMS MAY BE LIMITED BY RELATIVELY WEAK
STORM-RELATIVE FLOW IN UPPER LEVELS AND LIKELY RELATED IMPACTS ON
STORM-SCALE PRECIP DISTRIBUTION. HOWEVER...MOIST LOW LEVEL PROFILES
AND LOW LCL ARE EXPECTED. THOUGH WITH COOLER TEMPS BOTH SFC AND
ALOFT...OVERALL SHAPE AND COMBINATION OF FCST
THERMODYNAMIC/KINEMATIC PROFILES RESEMBLES THAT OF DISSIPATING
TROPICAL CYCLONE ENVIRONMENTS IN THIS REGION. THEREFORE WILL
INTRODUCE SMALL AREA OF MRGL CATEGORICAL CRITERIA TORNADO
PROBABILITIES. UNCERTAINTIES REGARDING LONGEVITY OF FAVORABLE STORM
MODES...AND SMALL GEOGRAPHIC AREA AFFECTED...EACH PRECLUDES GREATER
PROBABILITIES ATTM.
...NRN PLAINS...
STRENGTHENING FRONTOGENETIC FORCING/LIFT FROM LATE AFTERNOON THROUGH
EVENING WILL RESULT IN INCREASING TSTM POTENTIAL OVER PORTIONS WRN
DAKOTAS. THIS ACTIVITY MAY BEGIN AS ELEVATED CONVECTION BEHIND
FRONT...OVERTAKING BAROCLINIC ZONE AT OR JUST ABOVE SFC THEN
INTENSIFYING FOR SEVERAL HOURS AFTER DARK. LIKELIHOOD OF
SIGNIFICANT NOCTURNAL COOLING/STABILIZATION OF NEAR SFC AIR WILL
LIMIT WIND THREAT SOMEWHAT AFTER ABOUT 03Z...THOUGH RESIDUAL/DRY
MIXED LAYER ABOVE SFC MAY COUNTERBALANCE THAT EFFECT FOR A FEW HOURS
IN SUPPORT OF GUST POTENTIAL. ISOLATED HAIL ALSO MAY OCCUR.
SIMILAR PATTERN TYPICALLY RESULTS IN CONSIDERABLY GREATER SVR
POTENTIAL IN ANOTHER MONTH OR TWO...WHEN MORE ACCESS TO BOTH
REGIONALLY GENERATED EVAPOTRANSPIRATIVE MOISTURE AND TRAJECTORIES OF
GULF ORIGIN MOISTURE WOULD BE LIKELY. BUT THAT LACK OF MOISTURE
WILL CONSTITUTE PRIMARY LIMITING FACTOR FOR SVR POTENTIAL ACROSS
THIS AREA TODAY AND TONIGHT. ANOTHER DETERRENT TO DIURNAL
CONVECTION WILL BE STG CAPPING...THOUGH SFC HEATING AND CONVERGENCE
MAY BE SUFFICIENT TO FORM ISOLATED ACTIVITY BEFORE DARK INVOF SFC
LOW.
..EDWARDS/GRAMS.. 04/20/2008
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