SPC AC 250833
DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0233 AM CST WED FEB 25 2009
VALID 271200Z - 281200Z
...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS MID-SOUTH TO SRN
APPALACHIANS...
...SYNOPSIS...
UPPER AIR PATTERN FCST TO EVOLVE FROM NEARLY ZONAL FLOW PATTERN TO
BROADLY CYCLONIC OVER ERN 2/3 CONUS...AS SERIES OF NRN STREAM
SHORTWAVE TROUGHS CONTRIBUTES TO HEIGHT FALLS FROM CENTRAL/NRN
ROCKIES TO MID/UPPER MS VALLEY. STRONGEST OF THESE PERTURBATIONS --
NOW EVIDENT IN MOISTURE CHANNEL IMAGERY FROM COASTAL BC OVER CENTRAL
BC -- SHOULD PIVOT SLOWLY SWD THEN EWD AS CLOSED LOW...ACROSS
PACIFIC NW DAYS 1-2. THIS FEATURE THEN SHOULD ACCELERATE ESEWD
ACROSS PORTIONS NRN/CENTRAL PLAINS DAY-3...WHEREUPON CONSIDERABLE
SPREAD BEGINS TO APPEAR IN GUIDANCE REGARDING AMPLITUDE OF SYSTEM.
DOWNSTREAM/WEAKER TROUGH...NOW EVIDENT OFFSHORE NRN CA -- IS FCST TO
PHASE WITH NRN STREAM PERTURBATION THROUGH DAY-2. THIS FEATURE THEN
SHOULD MOVE FROM VICINITY LM EWD TO INLAND PORTIONS MID-ATLANTIC
STATES THIS PERIOD...ATTAINING MORE POSITIVE TILT WITH TIME.
ASSOCIATED SFC CYCLONE SHOULD EJECT FROM SRN ONT POSITION AROUND
27/12Z EWD TO ENEWD TOWARD NRN NEW ENGLAND AND LOWER ST LAWRENCE
RIVER AREA BY 28/12Z. ACCOMPANYING COLD FRONT SHOULD MOVE SEWD INTO
CENTRAL/SRN APPALACHIANS EARLY IN PERIOD. TRAILING SEGMENT OF FRONT
SHOULD STALL ACROSS TN VALLEY REGION...PERHAPS WITH WEAK/ATTACHED
CYCLOGENESIS IN ADVANCE OF SHORTWAVE TROUGH APCHG FROM CENTRAL/NRN
PLAINS. CONSIDERABLE UNCERTAINTY LINGERS REGARDING POSITIONING OF
FRONT AND STRENGTH/LOCATION OF WAVE LOW DEVELOPMENT OVER THIS
REGION.
...MID-SOUTH TO SRN APPALACHIANS...
PRIOR/OVERNIGHT CONVECTION DISCUSSED IN DAY-2 OUTLOOK SHOULD
CONTINUE INTO EARLY THIS PERIOD ALONG/AHEAD OF SFC COLD FRONT. LOW
LEVEL WAA AND MOISTURE TRANSPORT WILL CONTRIBUTE TO SUFFICIENTLY
STEEP LAPSE RATES AND LOW LEVEL THETAE TO SUPPORT POSSIBLE
EARLY-PERIOD MCS MOVING EWD OR ESEWD ACROSS REGION. FCST DEEP-LAYER
WIND PROFILES WILL SUPPORT 40-50 KT EFFECTIVE SHEAR FOR ELEVATED
PARCELS AND POTENTIAL FOR ORGANIZED SVR THREAT...WITH HAIL AND
PERHAPS WIND DEPENDING ON DEPTH OF NEAR-SFC STABLE AIR.
DURING DAY...INTENSIFICATION OF MORNING ACTIVITY AND/OR ADDITIONAL
DEVELOPMENT ALONG ITS OUTFLOW BOUNDARY IS POSSIBLE...WITH SFC-BASED
SVR POSSIBLE. RESIDUAL/ELEVATED MIXED LAYER WILL CONTRIBUTE TO
CONSIDERABLE SBCINH DURING THIS PERIOD AS WELL...WITH FCST SOUNDINGS
SUGGESTING SHARP SRN CUT-OFF TO CONVECTION. SFC HEATING IS EXPECTED
TO WEAKEN CAPPING ALONG SRN RIM OF CLOUD/PRECIP AREA. STRENGTH AND
LOCATION OF MAX LOW LEVEL CONVERGENCE WILL DEPEND ON AFOREMENTIONED
CYCLOGENESIS AS WELL AS MESOSCALE PROCESSES ALONG FRONT AND/OR
BOUNDARIES FROM OUTFLOW AND DIFFERENTIAL HEATING. HOWEVER...MOST
PROGS REASONABLY INDICATE SUFFICIENT CONCENTRATION OF CONVECTION
OVER THIS REGION...AMIDST FAVORABLE DEEP LAYER SHEAR...TO SUPPORT
SVR OUTLOOK.
SOME BACKBUILDING MAY OCCUR ALONG FRONT...ACROSS PORTIONS
AR/LA...LATER IN PERIOD...HOWEVER SVR POTENTIAL IS TOO CONDITIONAL
FOR CATEGORICAL OUTLOOK ATTM.
..EDWARDS.. 02/25/2009
CLICK TO GET WUUS03 PTSDY3 PRODUCT
NOTE: THE NEXT DAY 3 OUTLOOK IS SCHEDULED BY 0830Z
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