MESO: Mesoscale Discussion 1826

ACUS11 KWNS 281201
SWOMCD
SPC MCD 281201
FLZ000-ALZ000-MSZ000-LAZ000-281430-

MESOSCALE DISCUSSION 1826
NWS STORM PREDICTION CENTER NORMAN OK
0701 AM CDT TUE AUG 28 2012

AREAS AFFECTED...PORTIONS FL PANHANDLE...SRN AL...EXTREME SERN
MS...COASTAL EXTREME SERN LA.

CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE

VALID 281201Z - 281430Z

PROBABILITY OF WATCH ISSUANCE...40 PERCENT

SUMMARY...TC-RELATED MINI-SUPERCELLS AND RELATED TORNADO POTENTIAL
MAY BEGIN TO AFFECT GULF COASTAL AREAS...MAINLY BETWEEN
AAF-GPT...THROUGH LATE MORNING LOCAL TIME. SUBSEQUENTLY...RISK MAY
SHIFT INLAND IN STEP WITH BOTH SHOREWARD TRANSLATION OF ISAAC AND
PACE OF INLAND DIABATIC DESTABILIZATION. INITIAL THREAT AREA MAY BE
TOO NARROW WITH MRGL TORNADO THREAT...LENDING SOME UNCERTAINTY AS TO
NEED FOR WW. HOWEVER...THREAT MAY INCREASE INLAND DURING DAYLIGHT
HOURS.

DISCUSSION...ISAAC CONTINUES TO MOVE NWWD TOWARD SERN LA
LANDFALL...PER NHC FCSTS. AS THIS OCCURS...FAVORABLE LOW-LEVEL WIND
FIELDS AND BOUNDARY-LAYER SHEAR FOR SUPERCELLS WILL OVERSPREAD
PROGRESSIVELY LARGER PORTION OF THIS AREA. MAIN LIMITING FACTORS
HAVE BEEN...AND WILL REMAIN...TWOFOLD--
1. EXTENT AND MAGNITUDE OF SUPPORTIVE INSTABILITY.
2. CONVECTIVE MODE AND COVERAGE...WITH SUSTAINED/DISCRETE ACTIVITY
BEING MOST FAVORABLE FOR CELL-SCALE ROTATION.

UNTIL SUBSTANTIAL DIURNAL HEATING OCCURS INLAND...PRIMARY CONCERN
FOR TORNADOES WILL BE WITHIN 15-20 NM OF COAST. SFC MESOANALYSIS
SHOWS RELATIVELY SHARP LAND/SEA THERMODYNAMIC GRADIENT...WITH INLAND
TEMPS/DEW POINTS AROUND 77/73 RESPECTIVELY COMPARED TO AROUND 82/76
IN PRECONVECTIVE/NEARSHORE MARINE ENVIRONMENT. AS CENTER OF ISAAC
PROCEEDS NWWD...GRADUAL VEERING OF FLOW WITH TIME ALONG COAST SHOULD
ADVECT MARINE LAYER SLIGHTLY INLAND. OVERALL WEAKENING IS EXPECTED
FOR ANY ECHOES CROSSING THAT BOUNDARY...HOWEVER CONVERGENCE ALONG
IT...AND BACKED FLOW JUST TO ITS N...WILL SUPPORT POTENTIAL FOR
BRIEF INTENSIFICATION OF ANY PRE-EXISTING MESOCYCLONES. FARTHER
INLAND...SUBTLE SFC TROUGH AND CONFLUENCE LINE -- ALSO CORRESPONDING
TO DEW POINT GRADIENT THAT MARKED SRN EXTENT OF SW-NE SWATH OF
LOW-LEVEL DRYING -- WAS DRAWN NEAR BHM-GPT LINE...AND WAS
QUASISTATIONARY. THIS BOUNDARY ALSO IS EVIDENT IN AUTOMATED
MESOANALYSES OF SFC VORTICITY...EFFECTIVE SRH AND ALONG NWRN EDGE OF
WEAK MLCAPE PLUME. BY ABOUT 14-16Z...HEATING OF INLAND AIR
MASS...UNDER RELATIVELY CLOUD-DEPRIVED SLOT EVIDENT IN IR
IMAGERY...WILL MAKE INTERIOR BOUNDARY MORE IMPORTANT BY BOOSTING
MLCAPE AND REDUCING CINH ALONG AND TO ITS S.

A FEW SMALL/SHALLOW ECHOES HAVE EXHIBITED WEAK CYCLONIC SHEAR
OFFSHORE AND INVOF AAF. HOWEVER...AMIDST APPARENT WEAK LOW-LEVEL
LIFT...CONVECTION HAS BEEN RATHER SPARSE...LACKING BOTH INTENSITY
AND TIME CONTINUITY. GIVE ABOVE FACTORS...CONVECTIVE TRENDS
PROBABLY WILL BE MAIN DETERMINANT IN ANY WW DECISION THIS MORNING.

..EDWARDS/THOMPSON.. 08/28/2012

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...TAE...BMX...MOB...LIX...

LAT...LON 29798959 31068887 32178793 31408579 29828459 29678520
29678536 29848544 29908541 29748537 29718532 29888534
29958546 30328620 30378663 30308731 30228804 30208845
30198882 29728884 28978913 29798959

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