SWODY1
SPC AC 172000
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0300 PM CDT FRI SEP 17 2010
VALID 172000Z - 181200Z
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTERNOON INTO
TONIGHT OVER PARTS OF THE CENTRAL PLAINS AND MID/LOWER MO VALLEY...
...CENTRAL PLAINS/MID AND LOWER MO VALLEY...
EARLY-MID AFTERNOON SURFACE ANALYSES SHOWED THE PORTION OF A FRONT
EXTENDING FROM NWRN IA INTO ERN/SRN NEB TO WRN KS HAS LIKELY
STALLED...WHILE A SECOND COLD FRONT APPROACHES THIS REGION FROM THE
NRN PLAINS. 19Z SPECIAL SOUNDING AT OMA INDICATED SOME BOUNDARY
LAYER MOISTENING AND STRENGTHENING OF THE CAP PER LOW-MID LEVEL WAA.
GIVEN THE STRENGTH OF THE CAP AND LACK OF A DISCERNIBLE MIDLEVEL
IMPULSE FOR SUFFICIENT FORCING TO SUPPORT CONVECTIVE
DEVELOPMENT...CURRENT THINKING SUGGESTS TSTMS MAY NOT DEVELOP UNTIL
LOW LEVEL CONVERGENCE INCREASES WHEN THE SECONDARY FRONT OVERTAKES
THE INITIAL. AND...THE INCREASE OF LOW LEVEL WAA AS THE LLJ
STRENGTHENS THIS EVENING SHOULD SUPPORT CONVECTIVE DEVELOPMENT.
SINCE STORMS MAY NOT DEVELOP UNTIL EARLY EVENING...THE 30 PERCENT
SEVERE WIND AND HAIL PROBABILITIES HAVE BEEN REMOVED...AND THREAT
FOR SIGNIFICANT HAIL HAS BEEN REMOVED AS THE GREATEST STORM COVERAGE
SHOULD OCCUR AFTER PEAK HEATING/TIME OF GREATEST AVAILABLE
INSTABILITY.
...LOWER-MIDDLE TX COASTAL PLAINS...
A VERY MOIST AIR MASS /SURFACE DEWPOINTS IN THE MIDDLE 70S AND
PRECIPITABLE WATER VALUES 2-2.5 INCHES/ RESIDES ACROSS THIS
REGION...AND COMBINED WITH SURFACE HEATING HAS RESULTED IN MODERATE
INSTABILITY /MLCAPE 2000-2500 J PER KG/. ADDITIONAL CONVECTIVE
BANDS ATTENDANT TO A MID-UPPER LEVEL TROUGH EXTENDING NNEWD FROM
HURRICANE KARL...WHICH WAS CENTERED WELL S OF THIS REGION...WILL
CONTINUE TO SPREAD INLAND FROM THE WRN GULF. GIVEN THE
AFOREMENTIONED RICH MOISTURE AND WEAK DEEP LAYER SHEAR...HEAVY
RAINFALL WILL BE THE PRIMARY THREAT WITH THE TSTMS OVER AND INLAND
OF THE LOWER-MIDDLE TX COAST. HOWEVER...MODEST LOW LEVEL SHEAR PER
OBJECTIVE ANALYSES /SFC-1 KM SHEAR 10-15 KT AND EFFECTIVE SRH AROUND
50 M2 PER S2/ SUGGESTS A NON-ZERO TORNADO THREAT ENVIRONMENT
EXISTS...AND THUS THIS OUTLOOK HAS INTRODUCED A 2 PERCENT TORNADO
PROBABILITY.
..PETERS.. 09/17/2010
.PREV DISCUSSION... /ISSUED 1109 AM CDT FRI SEP 17 2010/
...CNTRL PLAINS/MID AND LOWER MO VALLEY...
12Z OBSERVED SOUNDINGS AND SATELLITE-DERIVED PW IMAGERY SHOW MODEST
MOISTURE CONTENT /I.E. DEW POINTS IN THE 50S TO LOW 60S AND PW
VALUES OF 1.0-1.3 INCHES/ PRESENT OVER THE REGION...WITH THE TRUE
MARITIME TROPICAL AIR MASS DISPLACED WELL TO THE S...AHEAD OF
INITIAL FRONTAL SURGE OVER ERN TX INTO THE LOWER MS VALLEY.
NONETHELESS...THE PRESENCE OF A STEEP LAPSE RATE ENVIRONMENT AND
DAYTIME HEATING WILL YIELD A MODERATELY UNSTABLE AIR MASS BY
AFTERNOON WITH MLCAPE VALUES APPROACHING 2000-2500 J/KG.
AS MENTIONED IN PREVIOUS DAY ONE DISCUSSIONS...LARGE-SCALE FORCING
FOR ASCENT WILL REMAIN NEGLIGIBLE THROUGH MUCH OF THE DIURNAL
HEATING CYCLE. HOWEVER...12Z MODEL GUIDANCE AND MORNING WATER VAPOR
IMAGERY SUGGEST THAT REGION MAY BE GLANCED BY A LOW AMPLITUDE
IMPULSE TRANSLATING ESEWD FROM THE NRN HIGH PLAINS. WHEN COUPLED
WITH A DEEPENING/STRENGTHENING CIRCULATION ATTENDANT TO
FRONTOGENETIC COLD FRONT...SETUP SHOULD ALLOW FOR THE DEVELOPMENT OF
ISOLATED TO SCATTERED TSTMS LATE THIS AFTERNOON INTO EVENING ACROSS
PARTS OF IA/NEB.
REGION WILL RESIDE ALONG THE SRN FRINGE OF STRONGER MID AND
HIGH-LEVEL CYCLONIC FLOW ATTENDANT TO UPPER LOW OVER CNTRL CANADA
WITH SUFFICIENT /30-40 KT/ DEEP WLY SHEAR TO SUPPORT ORGANIZED STORM
MODES INCLUDING SUPERCELLS. WHILE A TORNADO OR TWO WILL BE POSSIBLE
WITHIN THE FIRST HOUR OR TWO AFTER STORM INITIATION...A GENERAL
ANAFRONTAL AIR FLOW REGIME WILL LIKELY RESULT IN STORMS
TRANSITIONING TO MORE OUTFLOW DOMINANT WITH THE PRIMARY HAZARDS
BEING LARGE HAIL AND DAMAGING WINDS. THIS WIND/HAIL THREAT MAY
PERSIST INTO TONIGHT AS LOW-LEVEL INFLOW IS ENHANCED BY A
STRENGTHENING NOCTURNAL LLJ.
...CAROLINAS/SERN VA...
SHORT WAVE TROUGH CURRENTLY MOVING THROUGH THE CUMBERLAND PLATEAU
WILL PROGRESS EWD INTO THE PIEDMONT AND COASTAL PLAIN LATER TODAY.
AT THE SAME TIME...A COLD FRONT WILL CONTINUE TO PUSH SEWD THROUGH
THE REGION...PROVIDING THE FOCUS FOR DIURNAL STORM DEVELOPMENT. AIR
MASS AHEAD OF THE FRONT IS RATHER MOIST WITH DEW POINTS IN THE 60S.
AND WHILE MIDLEVEL LAPSE RATES ARE NOT PARTICULARLY STEEP...THE
PRESENCE OF STRONG DIABATIC HEATING SHOULD SUPPORT MLCAPE INCREASING
TO 1000-1500 J/KG THIS AFTERNOON...ALONG WITH STEEPENING LOW-LEVEL
LAPSE RATES. AS SUCH...SOME RISK FOR ISOLATED STRONG WIND GUSTS
WILL EXIST WITH ANY STRONGER STORMS THAT MAY DEVELOP.
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