SWODY1
SPC AC 171609
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1009 AM CST TUE FEB 17 2009
VALID 171630Z - 181200Z
...THERE IS A SLGT RISK OF SVR TSTMS OVER THE LWR MS AND TN VLYS...
...SYNOPSIS...
FAIRLY LOW AMPLITUDE PATTERN WILL PREVAIL OVER MUCH OF THE CONUS
THIS PERIOD AS FLAT RIDGE NOW OVER THE OZARKS/SRN PLNS CONTINUES E
TO THE MID/S ATLANTIC CST...AND UPR LOW NOW NEAR SFO DRIFTS E AND
WEAKENS. SHORTWAVE IMPULSE NOW MOVING THRU CENTRAL ROCKIES EXPECTED
TO ACCELERATE E ACROSS THE CNTRL PLNS LATER TODAY/TONIGHT AHEAD OF
NRN STREAM DISTURBANCE AMPLIFYING SE FROM *SK*/MB.
AT LWR LVLS...A STRENGTHENING SW TO WSWLY 850 JET WILL PREVAIL FROM
E TX AND THE NW GULF OF MEXICO INTO THE TN AND LWR OH VLYS. THIS
JET WILL OVERRIDE A MORE S OR SSWLY FLOW OF MODIFIED POLAR AIR
RETURNING NWD ON BACKSIDE OF RETREATING SFC RIDGE ALONG THE S
ATLANTIC CST. DEVELOPING SURFACE LOW LEE OF ROCKIES MOVES/DEEPENS
EWD TO MID MS VALLEY BY 12Z WED.
...LWR MS/TN VLYS...
SATELLITE AND GPS PW DATA SHOW FAIRLY RICH MOISTURE /PW AOA 1 INCH/
IN PLACE OVER THE ERN THIRD OF TX AND WRN LA. THIS MOISTURE WILL
SPREAD ENE INTO THE LWR MS AND TN VLYS WITH 50+ KT LLJ LATER
TODAY INCREASING TO 60-65KT OVERNIGHT...ATOP SHALLOW LAYER OF
COOLER/DRIER AIR AT THE SFC.
ALTHOUGH MODERATE SFC-BASED INSTABILITY MAY DEVELOP WITH DAYTIME
HEATING TODAY OVER E TX...COMBINATION OF LOW CLOUDS AND CAPPING
INVERSION ON S SIDE OF SRN STREAM JET SHOULD PROHIBIT TSTM
DEVELOPMENT. WILL NEVERTHELESS MAINTAIN LOW PROBABILITIES FOR THE
UNLIKELY DEVELOPMENT OF AN ISOLD STORM THERE THIS AFTN. FARTHER
ENE...EXPECT AREA OF WEAK ELEVATED CONVECTION NOW OVER AR AND THE
LWR TN VLY TO BE FOLLOWED BY ADDITIONAL...STRONGER DEVELOPMENT
TONIGHT/EARLY WEDNESDAY AS RICHER ELEVATED MOISTURE INFLOW IS LIFTED
BY STRENGTHENING WAA.
FORCING FOR ASCENT ACROSS REGION WILL BE LIMITED TO 850-700 MB WAA
AS DPVA WITH AFOREMENTIONED CENTRAL ROCKIES IMPULSE REMAINS WELL TO
THE N AND W. BUT COMBINATION OF INCREASING MOISTURE
AVAILABILITY...FAIRLY STEEP MID LVL LAPSE RATES...AND 55-65 KT
UNIDIRECTIONAL WLY FLOW IN THE 700-500 MB LAYER WILL SUPPORT STORM
ORGANIZATION/ SUSTENANCE. SLIGHTLY ELEVATED ROTATING STRUCTURES AND
BOWS WILL POSE A THREAT FOR SVR HAIL...AND...ESPECIALLY TOWARD
12Z...POSSIBLY DMGG WIND.
IN FAR E TX...WRN/NRN LA AND SRN AR...THERE EXISTS A POSSIBILITY FOR
SFC-BASED STORMS LATE IN THE PERIOD AS RESIDUAL MODIFIED POLAR AIR
IS REPLACED BY MARITIME AIR AT THE SFC. FOR THIS REASON... WILL
MAINTAIN LOW PROBABILITIES FOR TORNADOES AND WIND DAMAGE IN THOSE
AREAS.
...CA...
COLD UPPER LOW JUST OFF NRN CA COAST FILLS TODAY BUT WILL MAINTAIN A
MOIST UNSTABLE WESTERLY FLOW CENTRAL CA UNTIL THIS EVENING. SURFACE
LOW JUST OFF COAST NW OF SFO IS MAINTAINING A BACKED S/SELY BOUNDARY
LAYER FLOW CENTRAL VALLEY. THIS FAVORABLE SHEAR PROFILE SUPPORTS
POTENTIAL FOR ROTATING LOW TOPPED CONVECTION PROVIDED AIR MASS ABLE
TO GENERATE SOME SURFACE INSTABILITY. CURRENT CLOUDINESS/SHOWERS
SHOULD BREAK UP SUFFICIENTLY BY EARLY AFTERNOON FOR SOME SURFACE
HEATING. WITH STEEP LAPSE RATES AND A COUPLE HUNDRED J/KG OF SBCAPE
THIS AFTERNOON...A FEW STORMS COULD PRODUCE HAIL AND A BRIEF FUNNEL
CLOUD OR WEAK TORNADO. BY EVENING ANY THREAT OF STRONG STORMS
SHOULD BE OVER AS OVERALL SHEAR/FLOW WEAKENS AND AIRMASS STABILIZES.
..HALES.. 02/17/2009
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