ACUS01 KWNS 182001
SWODY1
SPC AC 181959
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0259 PM CDT SUN SEP 18 2011
VALID 182000Z - 191200Z
...THERE IS A SLGT RISK OF SVR TSTMS FROM CENTRAL TX NEWD INTO THE
WRN OZARKS...
...CENTRAL TX TO PARTS OF SRN MO THIS AFTERNOON/EVENING...
MINOR ADJUSTMENTS HAVE BEEN MADE TO THE SLIGHT RISK AND SEVERE
WEATHER PROBABILITIES. GIVEN THE CONTINUED ESEWD MOVEMENT OF THE
COLD FRONT...A 10-30 MILE WIDE ZONE HAS BEEN TRIMMED FROM THE WRN
EXTENT OF THE SLIGHT. MEANWHILE...STRONGER INSTABILITY AND
INCREASING DEEP LAYER SHEAR ACROSS NERN TX WARRANTS THE INCLUSION OF
THIS AREA TO THE SLIGHT RISK.
OTHERWISE...THE FORECAST REMAINS CONSISTENT WITH EARLIER OUTLOOK
ISSUANCES GIVEN OBJECTIVE ANALYSES SHOWING THE AIR MASS IN ADVANCE
OF THE COLD FRONT HAS BECOME MODERATELY UNSTABLE /MLCAPE 1500-2500 J
PER KG/. IN ADDITION TO THE DESTABILIZATION...AN INCREASE IN
MIDLEVEL WINDS ATTENDANT TO THE APPROACH OF A SHORTWAVE TROUGH...NOW
ENTERING NWRN OK...WILL SUPPORT A FURTHER INCREASE IN DEEP LAYER
SHEAR. FORCING FOR ASCENT WITH THIS MIDLEVEL FEATURE SHOULD PROVE
SUFFICIENT...GIVEN THE STRONG INSTABILITY...FOR ADDITIONAL TSTM
DEVELOPMENT ALONG AND AHEAD OF THE COLD FRONT. SHORT TERM MESOSCALE
DETAILS FOR ERN OK/WRN AR INTO SWRN MO ARE AVAILABLE IN A RECENTLY
ISSUED SPC MESOSCALE DISCUSSION /#2188/. THE 30 PERCENT WIND/HAIL
PROBABILITIES HAVE BEEN ADJUSTED TO INDICATE THE GREATEST
CONCENTRATION OF SEVERE WEATHER LATER THIS AFTERNOON/EVENING.
THE 2 PERCENT TORNADO PROBABILITY HAS BEEN EXTENDED NEWD INTO NERN
OK/NWRN AR AND SWRN MO GIVEN THAT A TORNADO CANNOT BE RULED OUT FROM
NERN TX TO SWRN MO WHERE LOW LEVEL SHEAR IS ENHANCED SOME DUE TO
BACKED SURFACE WINDS. HOWEVER...THE LACK OF A FOCUSED SURFACE LOW
OVER ERN OK SUPPORTING A MORE PERSISTENT BACKING OF LOW LEVEL WINDS
ISALLOBARICALLY AND POSITIVE ORIENTATION OF MID/UPPER LEVEL TROUGH
SHOULD PRECLUDE A GREATER TORNADO THREAT.
..PETERS.. 09/18/2011
.PREV DISCUSSION... /ISSUED 1130 AM CDT SUN SEP 18 2011/
...SYNOPSIS...
BROAD MULTI-STREAM TROUGH OVER THE PLNS STATES WILL CONTINUE E INTO
THE MS VLY EARLY MON AS HEIGHTS RISE OVER THE RCKYS/GRT BASIN. TWO
IDENTIFIABLE VORT LOBES IN THE SRN PART OF THE TROUGH...ONE NOW OVER
NW OK/S CNTRL KS AND THE OTHER NOW NEAR FWD...LIKELY WILL AFFECT
TSTM DEVELOPMENT LATER TODAY OVER PARTS OF THE S CNTRL U.S.
AT THE SFC...PATTERN WILL BE DOMINATED BY SLOWLY PROGRESSIVE COLD
FRONT THAT ATTM EXTENDS FROM E CNTRL KS SSW INTO W CNTRL TX.
CONVERGENCE ALONG THE FRONT WILL...HOWEVER...BE COMPROMISED BY THE
CONTINUING PRESENCE OF SEVERAL WEAK...SIMILARLY-ORIENTED WIND SHIFT
LINES...CONFLUENCE ZONES...AND CONVECTIVE OUTFLOW BOUNDARIES AHEAD
OF THE FRONT AS FAR E AS THE ARKLATEX.
...CNTRL TX TO WRN OZARKS THIS AFTN/EVE...
SCTD BANDS/SMALL CLUSTERS OF STORMS EXPECTED TO PERSIST IN LOW LVL
WAA ZONE WELL AHEAD OF COLD FRONT FROM CNTRL MO SSW TO THE ARKLATEX
TODAY. THE STORMS SHOULD MOVE/DEVELOP ENE INTO THE MID/LWR MS VLY
THROUGH THE AFTN AND SHOULD REMAIN LARGELY SUB-SVR /GIVEN
COMPARATIVELY WEAK INSTABILITY AND SHEAR/...DESPITE SOME LARGE SCALE
FORCING FOR ASCENT OVER SRN SECTIONS PROVIDED BY THE N TX UPR
IMPULSE.
FARTHER W...BOUNDARY LAYER WILL REMAIN FAVORABLY MOIST AHEAD OF COLD
FRONT...WITH PW AROUND 1.50 INCHES AND DEW POINTS IN THE LOW-MID 60S
F FROM CNTRL TX NE INTO SE KS/FAR SW MO...WHERE MODERATE TO STRONG
HEATING WILL OCCUR. COUPLED WITH 500 MB TEMPS OF MINUS 9 TO MINUS
11 C IN CORE OF UPR TROUGH...EXPECT SBCAPE TO RANGE FROM 1500 TO
2500 J/KG BY MID AFTN FROM SW MO TO CNTRL TX.
FORCING FOR ASCENT WILL INCREASE THIS AFTN ALONG AN ARC EXTENDING
FROM SE KS/SW MO THROUGH ERN OK INTO N CNTRL TX AS OK/KS VORT LOBE
CONTINUES ESEWD. COUPLED WITH SFC HEATING...THIS SHOULD SUPPORT A
BAND OF FAIRLY VIGOROUS STORM DEVELOPMENT FROM SW MO THROUGH ERN OK
INTO CNTRL AND NE TX BY MID TO LATE AFTN.
GIVEN DEGREE OF INSTABILITY AND RICH LOW LVL MOISTURE...RELATIVELY
MODEST WIND FIELD /20-25 KT SW FLOW AT 850 MB VEERING TO 30 KT WLY
WINDS AT 500 MB/ SHOULD BE SUFFICIENT FOR SUSTAINED MULTICELL
CLUSTERS AND MARGINAL SUPERCELLS...WITH LARGE HAIL AND LOCALLY DMGG
WINDS THE MAIN SVR THREAT. SOME OF THE STORMS COULD MERGE INTO ONE
OR TWO LARGER BANDS WITH EMBEDDED BOWING SEGMENTS...ESPECIALLY IN
ERN OK/NERN TX.
THE OK STORMS SHOULD MOVE E INTO THE OZARKS AND ARKLATEX THIS
EVE...WHERE COMPARATIVELY WEAKER THERMODYNAMICS WILL EXIST. FARTHER
SW...THE ACTIVITY OVER CNTRL/N TX...MEANWHILE...SHOULD ULTIMATELY BE
UNDERCUT BY THE COLD FRONT. TOGETHER WITH THE LOSS OF SFC
HEATING...THIS SHOULD RESULT IN AN OVERALL WEAKENING OF THE SVR
THREAT BY MID TO LATE EVE.
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