SWODY3
SPC AC 200730
DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0130 AM CST THU DEC 20 2007
VALID 221200Z - 231200Z
..NO SVR TSTM AREAS FORECAST...
..SYNOPSIS...
SEVERAL SHORTWAVE PERTURBATIONS NOW DISTRIBUTED FROM OFFSHORE NRN CA
TO NWRN BC ATTM SHOULD DIG SEWD AND LEAD TO SYNOPTIC SCALE TROUGHING
BY DAY-2 OVER WRN CONUS. EMBEDDED/NRN LOW IS EXPECTED TO DEVELOP
AND MOVE SEWD ACROSS NRN PLAINS. SRN WAVE IS EXPECTED TO PIVOT
CYCLONICALLY THROUGH BASE OF SYNOPTIC TROUGH LATE DAY-2 AND EARLY
DAY-3 BEFORE EJECTING NEWD TOWARD GREAT LAKES...HOWEVER CONSIDERABLE
DIFFERENCES ARE EVIDENT IN TIMING/GEOMETRY OF THIS PROCESS BY
22/18Z-23/00Z TIME FRAME. OPERATIONAL NAM-WRF...AS WELL AS MOST
ETA-BASED SREF MEMBERS...INDICATE MORE SWD TRACK OF NRN STREAM
LOW...LEADING TO FASTER...MORE COMPACT AND MORE NEGATIVELY TILTED
SRN PERTURBATION LIFTING NEWD FROM ARKLATEX IN 22/18Z-23/00Z TIME
FRAME.
BY CONTRAST...SPECTRAL/UKMET/ECMWF AND MOST RSM/WRF ENSEMBLE MEMBERS
ARE IN CLOSE AGREEMENT WITH MORE POSITIVE TILT AND SLOWER EJECTION
OF SRN WAVE BY 23/00Z. ALTHOUGH WE ARE LEANING TOWARD LATTER
SOLUTION GIVEN MORE UNRELIABLE HISTORY OF HANDLING SHORTWAVE PHASING
PROBLEMS IN NAM...THIS DISPARITY LOWERS CONFIDENCE IN SHORTWAVE
SCENARIO...AND IN RESULTING LOW LEVEL FEATURES...ESPECIALLY FROM
ABOUT 22/18Z ONWARD. IN GENERAL...STRONG SFC COLD FRONT SHOULD MOVE
EWD ACROSS MS VALLEY AND SEWD INTO NWRN GULF DURING THIS PERIOD.
CONSIDERABLE TIMING DIFFERENCES ARE EVIDENT WITH FROPA. MORE
PROGRESSIVE/WELL-PHASED MODELS PREDICT FASTER FRONTAL MOVEMENT...AND
MORE NUMEROUS/SLOWER PROGS WITH SRN MIDLEVEL WAVE ACCORDINGLY
RESPOND WITH SLOWER FRONT AND BETTER DEFINED CYCLONE LIFTING NEWD
FROM ARKLATEX TO MID MS VALLEY.
..WRN GULF COASTAL PLAIN TO MS DELTA REGION...
THOUGH ACHIEVED IN SOMEWHAT DIFFERENT WAYS...STRONG CONSENSUS AMONG
SREF AND OPERATIONAL GUIDANCE IS FOR FAVORABLE VERTICAL SHEAR TO
OVERSPREAD MUCH OF PREFRONTAL SECTOR DURING THIS PERIOD.
HOWEVER...THERMODYNAMIC UNCERTAINTIES -- BOTH SFC AND ALOFT --
PRECLUDE MORE THAN MRGL UNCONDITIONAL SVR PROBABILITIES ATTM.
DIFFERENT POSSIBILITIES FOR PHASING OF NRN/SRN WAVES CASCADES
DOWNSCALE THROUGH STRENGTH/GEOMETRY OF SFC ISOBARIC/ISALLOBARIC
PATTERNS...BOUNDARY LAYER KINEMATIC FIELDS/TRAJECTORIES...AND
THEREFORE...DISTRIBUTION OF FAVORABLE LOW LEVEL MOISTURE INLAND.
WITHOUT STRONG FROPA FOLLOWING CURRENT GULF COAST SYSTEM...RICHEST
SFC MOISTURE SHOULD REMAIN OVER W-CENTRAL OPEN GULF. HOWEVER...ELY
COMPONENT OF TRAJECTORIES OVER SERN CONUS...FL AND NERN GULF MAY
PERSIST LONG ENOUGH TO LIMIT INLAND EXTENT OF RICH MOISTURE RETURN.
MOST BULLISH MOISTENING SCENARIO ACTUALLY APPEARS IN SPECTRAL
INSTEAD OF NAM...WHICH IS RATHER ANOMALOUS. EVEN THEN...STRONGEST
ASCENT ALOFT REMAINS HORIZONTALLY DISPLACED FROM MOST FAVORABLE
MOISTURE. WHILE TSTMS ARE EXPECTED ALONG/AHEAD OF FRONT...CONCERNS
REMAIN ABOUT DISTRIBUTION OF SFC-BASED VERSUS ELEVATED CONVECTION IN
THIS REGIME...AND IMPACT OF INLAND CLOUDS/PRECIP ON DIURNAL
DESTABILIZATION AND MOISTURE RETURN.
.EDWARDS.. 12/20/2007
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