Wednesday, February 18, 2009

Moderate Risk - I 20 to the Gulf Coast


From the Storm Prediction Center
   DAY 1 CONVECTIVE OUTLOOK 
NWS STORM PREDICTION CENTER NORMAN OK
0656 AM CST WED FEB 18 2009

VALID 181300Z - 191200Z

...THERE IS A MDT RISK OF SVR TSTMS OVER PARTS OF LA...MS...AL...GA
AND THE FL PANHANDLE...

...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MDT RISK AREA
FROM LA INTO PARTS OF TN...GA...NRN FL AND THE S ATLANTIC CST...

...SYNOPSIS...
FAST WSW FLOW WILL PREVAIL OVER THE S CNTRL AND SE U.S. TODAY/
TONIGHT...ON SRN FRINGE OF NRN STREAM TROUGH AMPLIFYING SE ACROSS
THE UPR GRT LKS/MID MS VLY. S OF THE TROUGH...EXISTING SRN BRANCH
JET...NOW EXTENDING FROM NM TO AR...WILL DEVELOP ESEWD. THIS WILL
ENHANCE MID LVL FLOW OVER THE SERN STATES TODAY...WITH MORE
SUBSTANTIAL STRENGTHENING EXPECTED TONIGHT/EARLY THURSDAY.

AT THE SFC...LOW ASSOCIATED WITH NRN STREAM IMPULSE SHOULD TRACK NE
TO NEAR LK HURON THIS EVE AS TRAILING COLD FRONT CONSOLIDATES AND
ACCELERATES SE ACROSS THE OH...TN...AND LWR MS VLYS. AHEAD OF THE
FRONT...LATEST HAND AND STREAMLINE ANALYSES SUGGEST PRESENCE OF A
CONFLUENCE ZONE ATTM EXTENDING FROM NEAR MEM SW TO NEAR SHV.
SATELLITE PW AND VWP DATA SUGGEST THAT THIS FEATURE MAY MARK WRN
LIMIT OF POTENTIAL STORM DEVELOPMENT TODAY AS WINDS APPEARED TO HAVE
VEERED BEHIND IT...AND DRYING IS OCCURRING TO ITS W.

...TN VLY TO CNTRL GULF CST/GA/N FL...
POTENTIAL EXISTS FOR SIGNIFICANT SVR STORMS THIS AFTN THROUGH EARLY
THURSDAY AHEAD OF AFOREMENTIONED COLD FRONT AND CONFLUENCE ZONE.

LATEST SATELLITE PW DISPLAY SHOWS PW AXIS WITH VALUES AOA 1 INCH
EXTENDING FROM SE TX AND LA NE INTO MIDDLE TN. THIS MOISTURE AXIS
SHOULD CONTINUE TO PIVOT CLOCKWISE ACROSS THE CNTRL GULF CST STATES
TODAY AS HEIGHTS SLOWLY FALL OVER REGION ON SRN FRINGE OF AMPLIFYING
TROUGH.

ALTHOUGH THE INCREASING MOISTURE WILL ENHANCE LIKELIHOOD FOR
STORMS...SEVERAL FACTORS ARGUE AGAINST MORE THAN ISOLD ACTIVITY IN
THE SLGT AND MDT RISK AREAS UNTIL PERHAPS LATE THIS AFTN AND THIS
EVE. FIRST /1/ MORNING RAOBS AND MODEL FCSTS SHOW THAT MID LVL
LAPSE RATES ARE AND LIKELY WILL REMAIN ONLY MODESTLY STEEP...WITH A
WARM NOSE AROUND 750 MB. IN ADDITION.../2/ CLOUDS WITH S-DRIFTING
SUBTROPICAL JET WILL LIMIT SFC HEATING THROUGH AT LEAST PART OF THE
DAY NEAR THE GULF CST. THIRD /3/ DEEP FORCING FOR ASCENT WILL BE
WEAK AS UPLIFT LIKELY WILL BE CONCENTRATED ON CYCLONIC SIDE OF SRN
STREAM JET...I.E. OVER THE OH VLY/CNTRL APPALACHIANS.

NEVERTHELESS...COMBINATION OF SFC HEATING...ESPECIALLY IN THE TN
VLY...WITH PERSISTENT LOW LVL CONFLUENCE SHOULD BE SUFFICIENT TO
OVERCOME NEGATIVE FACTORS AND SUPPORT AT LEAST WIDELY SCTD STORMS
ALONG CONFLUENCE ZONE OVER PARTS OF LA...MS AND AL. A SEPARATE AREA
OF ISOLD TO WDLY SCTD STORMS ALSO MAY FORM ALONG COLD FRONT IN
TN/KY.

GIVEN INTENSITY AND EXPECTED INCREASE IN DEEP WSWLY FLOW /SPEEDS AOA
70 KTS AT 500 MB/...EXPECT ANY SUSTAINED UPDRAFTS THAT DO FORM WILL
QUICKLY BECOME SVR SUPERCELLS. DESPITE LARGELY UNIDIRECTIONAL
FLOW...LENGTH OF HODOGRAPHS /60-70 KT 0-6 KM SHEAR/ COULD YIELD
ISOLD STRONG TORNADOES...IN ADDITION TO BOWING SEGMENTS WITH HIGH
WIND.

ALTHOUGH THE STORMS ALONG THE COLD FRONT SHOULD DIMINISH UPON
ENCOUNTERING LESS FAVORABLE LOW LVL THERMODYNAMIC ENVIRONMENT IN THE
SRN/CNTRL APPALACHIANS...THOSE THAT FORM ALONG PRE-FRONTAL
CONFLUENCE BAND COULD STRENGTHEN THROUGH THE EVE AS /1/ MORE
SUBSTANTIAL HEIGHT FALLS OVERSPREAD REGION AND /2/ LOW LVL MOISTURE
INFLOW CONTINUES TO INCREASE ALONG THE CNTRL/ERN GULF CST. SOME OF
THE STORMS COULD MOVE/DEVELOP E ACROSS CNTRL/SRN GA AND N FL GIVEN
DEEP UNIDIRECTIONAL FLOW.

..CORFIDI.. 02/18/2009

No comments: