SWODY1
SPC AC 012149
DAY 1 CONVECTIVE OUTLOOK CORR 1
NWS STORM PREDICTION CENTER NORMAN OK
0449 PM CDT FRI MAY 01 2009
VALID 012000Z - 021200Z
...THERE IS A MDT RISK OF SVR TSTMS LATE THIS AFTN/EVE ACROSS
WESTERN NORTH TEXAS AND PARTS OF SRN OK...
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTN INTO TONIGHT
FROM PARTS OF THE SRN PLAINS THRU THE OZARK PLATEAU/UPR PORTIONS OF
THE LWR MS VLY...
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTN ACROSS PARTS OF
EXTREME ERN VA AND THE DELMARVA...
CORRECTED CATEGORICAL OTLK TO MATCH PROBABILITIES IN NRN AL
...20Z UPDATE...
...SOUTHERN PLAINS...
HIGHEST SEVERE PROBABILITIES STILL APPEAR TO EXIST ACROSS WESTERN
NORTH TEXAS INTO EXTREME SOUTHWESTERN/SOUTH CENTRAL OKLAHOMA.
MID/UPPER FORCING REMAINS WEAK/UNCERTAIN...BUT LOW-LEVEL CONVERGENCE
APPEARS TO BE COMING FOCUSED ON THE NOSE OF A PRE-FRONTAL LOW-LEVEL
THERMAL RIDGE EMANATING FROM THE HIGH PLAINS ...NEAR THE DRY
LINE/FRONTAL INTERSECTION. IN THE PRESENCE OF MODERATE TO STRONG
INSTABILITY...ASSOCIATED WITH STEEP LAPSE RATES AND MIXED LAYER CAPE
ALREADY IN EXCESS OF 3000 J/KG...VEERING WIND FIELDS WITH HEIGHT
SEEM TO PROVIDE SUFFICIENT SHEAR FOR SUPERCELLS. DISCRETE INITIAL
STORM DEVELOPMENT PROBABLY WILL GROW UPSCALE INTO A SIZABLE STORM
CLUSTER PRIOR TO NIGHTFALL.
...OZARK PLATEAU INTO THE NORTHEAST...
STABILIZING INFLUENCE OF ONGOING STORM CLUSTERS ACROSS THE OZARK
PLATEAU INTO THE TENNESSEE VALLEY APPEAR TO HAVE REDUCED THE SEVERE
POTENTIAL ACROSS THE LOWER OHIO VALLEY THIS AFTERNOON. AT THE SAME
TIME...BREAKS IN CLOUD COVER ARE ALLOWING FOR INCREASING INSOLATION
WITHIN A MOISTENING PRE-FRONTAL AIR MASS ACROSS THE UPPER OHIO
VALLEY INTO UPSTATE NEW YORK. GIVEN A FAVORABLY SHEARED AND STRONG
DEEP LAYER WESTERLY MEAN FLOW FIELD TO THE EAST OF THE LOWER GREAT
LAKES REGION...FURTHER DESTABILIZATION COULD YIELD ISOLATED TO
WIDELY SCATTERED STORMS...SOME OF WHICH COULD BE ACCOMPANIED BY
HAIL/LOCALIZED GUSTS APPROACHING OR BRIEFLY EXCEEDING SEVERE LIMITS.
...MID ATLANTIC STATES...
APPRECIABLE SEVERE POTENTIAL APPEARS TO BE GENERALLY BECOMING
CONFINED TO EXTREME EASTERN VIRGINIA THRU THE DELMARVA. THIS IS
WHERE WARM ADVECTION ALONG AND EAST OF AN AXIS OF STRONGER SURFACE
HEATING...AHEAD OF A REMNANT SOUTHERN STREAM IMPULSE...MAY ENHANCE
LIFT LATE THIS AFTERNOON. OBJECTIVE ANALYSIS INDICATES MIXED LAYER
CAPE IS INCREASING IN EXCESS OF 1000 J/KG ACROSS THIS
REGION...SUPPORTIVE OF VIGOROUS UPDRAFTS IN THE PRESENCE OF AT LEAST
A MODERATELY SHEARED 30-40 KT SOUTHWESTERLY DEEP LAYER MEAN FLOW.
ACTIVITY IS EXPECTED TO BE MOSTLY DISCRETE IN NATURE...WITH AN
ISOLATED SUPERCELL NOT OUT OF THE QUESTION.
**FOR MORE SPECIFIC DETAILS ON ONGOING OR IMMINENT CONVECTIVE
THREATS...PLEASE REFER TO THE LATEST SPC MESOSCALE DISCUSSIONS AND
WATCHES.
..KERR.. 05/01/2009
.PREV DISCUSSION... /ISSUED 1200 PM CDT FRI MAY 01 2009/
CORRECTED FOR SPELLING/WORDING.
...SWRN OK INTO NRN TX...
MODERATE TO STRONG INSTABILITY AND /SBCAPE TO 4000 J PER KG/ AND
DEEP LAYER SHEAR 30-40 KT /SUFFICIENT FOR SUPERCELLS/ SHOULD
CONTINUE TO EVOLVE AHEAD OF THE ADVANCING COLD FRONT AND DRYLINE
FROM SWRN OK ACROSS THE RED RIVER VALLEY THIS AFTERNOON. THERE IS
NEARLY UNANIMOUS AGREEMENT ACROSS A BROAD RANGE OF AVAILABLE MODEL
GUIDANCE THAT SURFACE-BASED CONVECTION WILL INITIATE BY EARLY
EVENING NEAR FRONT/OUTFLOW/DRYLINE TRIPLE POINT AROUND SPS.
WHILE MID LEVEL SYNOPTIC SCALE FORCING FOR ASCENT ACROSS THIS AREA
OF INTEREST IS EXPECTED TO BE WEAK...WITH RELATIVELY FLAT MID-LEVEL
HEIGHT PATTERN AND WLY 500MB WINDS AOB 30KT...CONDITIONS WILL NOT
NECESSARILY BE HOSTILE TO STORM DEVELOPMENT. THIS IS ALSO SUPPORTED
BY LATEST NAM/GFS QPF GUIDANCE. THE EXISTENCE OF STRONG LOW LEVEL
FORCING ON THE MESOSCALE APPEARS TO AID CONVECTIVE INITIATION IN A
STRONGLY UNSTABLE AND SUFFICIENT SHEARED ENVIRONMENT FOR STORMS TO
QUICKLY ACQUIRE SUPERCELLULAR STRUCTURE. MAGNITUDE OF CAPE WILL
SUPPORT ROBUST UPDRAFTS AND MAY COMPENSATE SOMEWHAT FOR LACK OF
STRONGER SYNOPTIC ASCENT WITH STORM MERGERS AND COLD POOL
DEVELOPMENT AIDING UPSCALE ORGANIZATION ACROSS NRN TX THROUGH THE
EVENING.
GIVEN THE SIGNAL FOR STORM DEVELOPMENT...AND DEGREE OF
DESTABILIZATION EXPECTED...A SMALL HIGHER PROBABILITY HAIL AREA HAS
BEEN ADDED TO THIS FORECAST. BASEBALL OR LARGER HAIL APPEARS QUITE
POSSIBLE WITH THESE STORMS...ESPECIALLY WITHIN A FEW HOURS OF
INITIATION. LACK OF STRONGER DEEP SHEAR AND WEAK LARGER SCALE
FORCING MAY LIMIT STORM COVERAGE. THIS FACTOR...AND GENERALLY
MARGINAL FORECAST HODOGRAPH STRUCTURE...INDICATE TORNADO POTENTIAL
MAY BE LIMITED IN TIME/SPACE. HOWEVER...MOIST BOUNDARY LAYER AND
LIKELIHOOD FOR SOME STORM-SCALE AND MESOSCALE ENHANCEMENT TO THE LOW
LEVEL WIND FIELD...MAY PLAY A ROLE IN FAVORING A TORNADIC SUPERCELL
OR TWO IN THIS REGIME.
EVENTUALLY...STRONG INSTABILITY AND RESULTANT COLD POOLS SHOULD
RESULT IN A SMALL MCS ACROSS THE RED RIVER/NWRN TX. THE POTENTIAL
FOR BOWING LINE SEGMENTS/SUPERCELLS WITH SEVERE HAIL AND WIND COULD
SHIFT ESEWD ACROSS FAR NRN TX/DFW THROUGH EVENING/LATE EVENING.
...NERN OK TO AR AND ACROSS LWR/MID MS VALLEY...
LEADING PORTION OF SYNOPTIC FRONTAL ZONE ACROSS NW AND NCNTRL OK IS
BEING REINFORCED BY ONGOING MCS NOW SPREADING ESEWD ACROSS NERN OK
AND THE OZARKS. MODEST AND CONTINUING INFLUX OF LOW LEVEL MOISTURE
AND MASS CONVERGENCE INTO THE MCS OUTFLOW/COLD POOL WILL MAINTAIN
SOME POTENTIAL FOR SEVERE HAIL AND WIND FROM NERN OK ACROSS AR THIS
AFTERNOON. POSSIBLE MCV EVOLVING FROM THIS ONGOING SYSTEM MAY
SUPPORT THE DEVELOPMENT AND PERSISTENCE OF ADDITIONAL STORMS ALONG
EWD THE STALLING COLD FRONT...ALONG A LIT-MEM-BNA LINE...THROUGH
LATE TONIGHT. GENERALLY WEAKER DEEP LAYER FLOW ACROSS THIS ZONE
SUGGESTS THAT PRIMARY THREAT MAY BECOME EXCESSIVE RAIN. HOWEVER...A
FEW SEVERE STORMS ALSO APPEAR POSSIBLE IN THIS CORRIDOR THROUGH
EARLY SAT.
...LOWER/MID OH VALLEY...
A MARGINALLY UNSTABLE AIR MASS AHEAD OF THE SLOWLY ADVANCING COLD
FRONT ACROSS THIS AREA SHOULD LEAD TO AN INCREASE IN SHOWERS AND
THUNDERSTORMS THROUGH THIS AFTERNOON. DESPITE WEAK LAPSE
RATES...MODEST TO STRONGER DEEP LAYER WLY FLOW MAY SUPPORT SOME
ORGANIZATION TO THIS ACTIVITY INTO THE EVENING HOURS WITH WIND AND
MARGINALLY SEVERE HAIL BEING THE PRIMARY HAZARDS.
...MID-ATLANTIC...
LOW AMPLITUDE SHORT WAVE TROUGH AND DIURNAL DESTABILIZATION WILL
CONTRIBUTE TO AN INCREASING CHANCE FOR THUNDERSTORM DEVELOPMENT
ALONG ADVANCING LEE/SYNOPTIC TROUGH THIS AFTERNOON. WARMING SURFACE
TEMPERATURES INTO THE 70S SHOULD YIELD SBCAPE AROUND 500-1000 J/KG.
THIS INSTABILITY...ALONG WITH UNIDIRECTIONAL WLY WINDS AT 30-40 KT
BETWEEN 850-300 MB...SHOULD BE SUFFICIENT FOR SPORADIC WIND DAMAGE
WITH THE SQUALL LINE...BEFORE THE LINE PASSES OFF THE ATLANTIC
SEABOARD THIS EVENING.
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