Schweizer Sturmforum

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1141 PM CST MON FEB 16 2009

VALID 171200Z - 181200Z

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF FAR ERN
TX...NRN LA...AR...MS...TN AND NRN AL...

...LOWER MS AND TN VALLEYS...
A FLAT UPPER-LEVEL RIDGE OVER THE CNTRL U.S. WILL MOVE QUICKLY EWD
TODAY AS AN UPPER-LEVEL TROUGH REPLACES THE RIDGE OVER THE PLAINS
STATES. A 50 TO 60 KT LOW-LEVEL JET WILL STRENGTHEN AHEAD OF THE
DEVELOPING TROUGH OVER THE LOWER MS VALLEY. IN RESPONSE...LOW-LEVEL
MOISTURE WILL QUICKLY RETURN NWD TODAY ACROSS THE SABINE RIVER
VALLEY ADVECTING EWD INTO THE LOWER MS VALLEY TONIGHT. ALTHOUGH
MODEL FORECASTS DEVELOP THE GREATEST AMOUNT OF INSTABILITY OVER EAST
TX...A CAPPING INVERSION SHOULD KEEP THUNDERSTORMS FROM DEVELOPING
FOR MOST OF THE AFTERNOON INTO THE EARLY EVENING IN THE SRN PLAINS.
FURTHER EAST NORTHEAST ACROSS THE LOWER MS AND TN VALLEYS...THE GFS
AND NAM MODELS ARE IN AGREEMENT DEVELOPING A LARGE AREA OF
CONVECTION OVERNIGHT.

ALTHOUGH LARGE-SCALE ASCENT WILL NOT BE THAT STRONG ACROSS THE LOWER
MS AND TN VALLEYS...LIFT ASSOCIATED WITH THE LOW-LEVEL JET SHOULD
SUPPORT A LARGE AREA OF CONVECTION EXPANDING ENEWD WITH TIME
OVERNIGHT. A WARM FRONT IS FORECAST TO BE LOCATED ACROSS SW AR
EXTENDING SEWD ACROSS LA ALONG THE NERN GRADIENT OF THE LOW-LEVEL
MOIST AXIS. EAST NORTHEAST OF THE WARM FRONT...FORECAST SOUNDINGS
SUGGEST THE THUNDERSTORM ACTIVITY WILL BE ELEVATED IN NATURE WITH
PARCELS STARTING AT OR ABOUT 850 MB WITH MUCAPE VALUES GENERALLY
FROM 250 TO 750 J/KG. THIS WILL LIMIT THE WIND DAMAGE THREAT BUT
STRONG VERTICAL SHEAR PROFILES AND COLD TEMPS ALOFT /500 MB TEMPS OF
-14 TO -16C/ SHOULD BE SUFFICIENT FOR LARGE HAIL. THE LARGE HAIL
THREAT SHOULD EXPAND ENEWD WITH TIME AS CONVECTION EXPANDS IN
COVERAGE WITH THREAT REACHING MIDDLE TN AND NRN AL LATE IN THE
PERIOD.

CONCERNING THUNDERSTORM DEVELOPMENT FURTHER WEST SOUTHWEST TOWARD
CNTRL TX LATE THIS AFTERNOON...FORECAST SOUNDINGS SHOW A STOUT
CAPPING INVERSION IN PLACE. THIS ALONG WITH A LACK OF LARGE-SCALE
ASCENT SHOULD HAMPER UPDRAFT STRENGTH IN SPITE OF THE GREATER
INSTABILITY IN PLACE. FOR THIS REASON...IF THUNDERSTORMS CAN
INITIATE...ANY HAIL OR WIND DAMAGE THREAT SHOULD REMAIN MARGINAL
THROUGH EARLY EVENING. IN ERN TX...THERE EXISTS A POSSIBILITY FOR
SFC-BASED THUNDERSTORM DEVELOPMENT LATE IN THE PERIOD. FOR THIS
REASON...WILL MAINTAIN LOW PROBABILITIES FOR TORNADO AND WIND DAMAGE
IN THIS PART OF TX.

..BROYLES.. 02/17/2009

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