SWODY1
SPC AC 121259
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0659 AM CST SAT DEC 12 2009
VALID 121300Z - 131200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
TWO-STREAM ZONAL FLOW WILL PREVAIL OVER THE LWR 48 THROUGH EARLY
SUN. THE SRN BRANCH JET...EXTENDING FROM OFF THE CA CST ESE ACROSS
THE SWRN STATES...THE SRN RCKYS...AND THE SRN PLNS...WILL REMAIN THE
JET OF INTEREST AS FAR AS DEEP CONVECTION IS CONCERNED. TWO
PERTURBATIONS IN THE FLOW...ONE NOW OVER N CNTRL MEXICO/SW TX AND
THE OTHER OVER THE PACIFIC NEAR 130W...WILL BE THE MAIN FEATURES
AFFECTING TSTM DEVELOPMENT.
AT LWR LVLS...SFC LOW NOW OFF THE MIDDLE TX GULF CST WILL MOVE
SLOWLY NE THROUGH THE PERIOD...REACHING THE BPT AREA THIS EVE AND
CNTRL MS BY 12Z SUN. ASSOCIATED WARM/STNRY FRONT EXTENDING ESE FROM
THE LOW WILL ALSO MOVE NEWD...AND SHOULD MOVE ASHORE OVER SERN
LA/CSTL MS LATER TODAY/TONIGHT. BUT OVERALL NEWD MOTION OF FRONT
LIKELY WILL BE SLOWER THAN THAT OF LOW AS SHALLOW LAYER OF RESIDUAL
COLD AIR ONLY GRUDGINGLY GIVES AWAY OVER INTERIOR PARTS OF THE
CNTRL/ERN GULF CST STATES.
...CNTRL GULF CST...
LOW-LVL WAA/MOISTURE INFLOW WILL SPREAD NE ACROSS THE CNTRL GULF CST
REGION THROUGH EARLY SUN...ELONGATING ALONG NE-SW AXIS N AND NE OF
SFC WARM FRONT. THIS WILL ALLOW ELEVATED CONVECTION NOW OVER E TX
AND LA TO SPREAD FARTHER E/NE WITH TIME AS NRN MEXICO/SW TX UPR VORT
CONTINUES ENEWD.
AS SFC FRONT EDGES INLAND ACROSS SRN LA...LOW-LVL SHEAR
WILL BE SUPPORTIVE OF SUSTAINED STORMS/SUPERCELLS ALONG THE CST AND
INLAND TO AREAS JUST N OF FRONT. FCST HODOGRAPHS SUGGEST 0-1 KM SRH
OF 100-200 J/KG...WITH SFC-BASED CAPE LIMITED TO AOB 1000 J/KG BY
WEAK MID LVL LAPSE RATES. THE STRONGEST LARGE SCALE FORCING FOR
ASCENT LIKELY WILL REMAIN N OF FRONT...AND CLOUDS WILL LIMIT SFC
HEATING. THUS...PROSPECTS FOR TRULY SFC-BASED STORMS DO NOT APPEAR
GREAT. FURTHERMORE...ANY STORMS THAT DO FORM IN THE WARM SECTOR
WILL HAVE WEAKER LOW LVL SHEAR GIVEN VEERED FLOW S OF FRONT.
NONETHELESS...GIVEN INCREASING LOW LVL MOISTURE INFLOW...MODERATE
SHEAR...AND AT LEAST SOME POTENTIAL FOR SFC AND/OR NEAR-SFC-BASED
STORMS...A CONDITIONAL THREAT FOR LOCALLY DMGG WINDS OR A TORNADO
WILL EXIST LATER TODAY THROUGH EARLY SUN.
...CNTRL/NRN CA...
E PACIFIC UPR IMPULSE APPEARS ELONGATED IN SATELLITE IMAGERY
ATTM...WITH ASSOCIATED JET STREAK EXTENDING UPSTREAM TOWARD
40N/140W. THIS SUGGESTS THAT SYSTEM MAY SOMEWHAT AMPLIFY AS IT
CONTINUES ESEWD...BUT THAT STRONGEST FORCING FOR ASCENT WILL REACH
CA THIS EVE/TONIGHT...WELL AFTER MAX DIURNAL HEATING.
NEVERTHELESS...THE DISTURBANCE WILL BE ACCOMPANIED BY CONSIDERABLE
LOW-LVL AND DEEP SHEAR. HODOGRAPHS LIKELY WILL BECOME FAVORABLE FOR
SUPERCELLS AND/OR SMALL BOWS BY LATE IN THE DAY...ESPECIALLY OVER
THE SACRAMENTO VLY...WHERE NEAR-SFC FLOW WILL REMAIN BACKED BY
TERRAIN. GIVEN THE LIKELIHOOD FOR EXTENSIVE CLOUD COVER THIS
AFTN...UNCERTAINTY EXISTS REGARDING DEGREE OF LOW LVL
DESTABILIZATION FOR SUSTAINED SFC-BASED UPDRAFTS PRIOR TO SUNDOWN.
ATTM THREAT REMAINS TOO LOW/ CONDITIONAL TO JUSTIFY A SVR OTLK. BUT
AT LEAST LOW SVR PROBABILITIES MAY BE NEEDED IN LATER OUTLOOKS IF
THE PROSPECTS FOR AFTN HEATING INCREASE.
..CORFIDI.. 12/12/2009
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