MESO: Mesoscale Discussion 1813

ACUS11 KWNS 261106
SWOMCD
SPC MCD 261105
FLZ000-261330-

MESOSCALE DISCUSSION 1813
NWS STORM PREDICTION CENTER NORMAN OK
0605 AM CDT SUN AUG 26 2012

AREAS AFFECTED...PORTIONS SRN FL AND KEYS.

CONCERNING...SEVERE POTENTIAL...WATCH LIKELY

VALID 261105Z - 261330Z

PROBABILITY OF WATCH ISSUANCE...95 PERCENT

SUMMARY...TORNADO POTENTIAL WITH ISAAC IS FCST TO INCREASE THROUGH
REMAINDER MORNING INTO AFTN. THREAT SHOULD START AS MRGL DURING
NEXT 1-2 HOURS...BUT INCREASE WITH TIME. AS SUCH...TORNADO WW MAY
BE REQUIRED WITHIN NEXT 1-2 HOURS AND IS QUITE LIKELY SOMETIME THIS
MORNING. ALSO...IN UPCOMING 13Z DAY-1 OUTLOOK...TORNADO
PROBABILITIES WILL BE INCREASED FOR S FL...WITHIN BROADER
CATEGORICAL SLGT RISK.

DISCUSSION...TC TORNADO THREAT IS MRGL ATTM...BUT WILL RAMP UP AND
EXPAND NWWD ACROSS AREA TODAY AS THESE PROCESSES TAKE PLACE LARGELY
IN TANDEM...

1. CENTER OF ISAAC PASSES S OF MAINLAND...ACROSS LOWER KEYS INTO
EXTREME ERN GULF...BASED ON LATEST NHC TRACK FCST. THIS WILL
TRANSLATE FAVORABLE NERN SECTOR OF CYCLONE ACROSS DISCUSSION AREA.
2. STRENGTHENING/EXPANDING LOW-LEVEL WIND FIELDS...PER NHC FCST.
THAT...IN TURN...WILL RESULT IN INCREASING LOW-LEVEL
CONVERGENCE...AS WELL AS GREATER MAGNITUDE AND AREAL COVERAGE OF
ENHANCED LOW-LEVEL VERTICAL SHEAR FAVORABLE FOR TORNADIC SUPERCELLS.
SHAPE/LENGTH OF FCST HODOGRAPHS CONTRIBUTES TO EFFECTIVE SRH
INCREASING TO 300-500 J/KG...AND NEARLY 90-DEG CRITICAL ANGLES
SUGGESTING FAVORABLE STREAMWISE VORTICITY. THIS WILL ENCOURAGE
PROGRESSIVELY MORE POTENTIAL FOR SUSTAINED ROTATION IN ANY
RELATIVELY DISCRETE UPDRAFTS.
3. RELATIVELY LARGE-THETAE AIR MASS -- NOW EVIDENT OFFSHORE SE FL
OVER WATERS OF GULF STREAM WHERE OCEAN HEAT CONTENT IS HIGH. THIS
AIR MASS SHOULD BE ADVECTED INLAND AMIDST INCREASING BOUNDARY-LAYER
WINDS TODAY...HELPING TO OFFSET MINOR OUTFLOW EFFECTS OF PASSING
CONVECTION. RESULTING MODIFICATIONS TO FCST SOUNDINGS YIELD
500-1000 J/KG MLCAPE AND LITTLE OR NO SBCINH.
4. POCKETS OF DIABATIC SFC HEATING MAY BOOST BUOYANCY LOCALLY EVEN
FURTHER...IN AREAS OF RELATIVE CLOUD-COVER BREAKS.
5. MULTIPLE ROUNDS OF AT LEAST LOOSELY BANDED CONVECTIVE MODE ARE
POSSIBLE AS CONVERGENCE INCREASES. THIS IS SUGGESTED BY STG
CONSENSUS OF HIGH-RES/CONVECTION-ALLOWING MODELS...AND SUPPORTS
INCREASING SUPERCELL/TORNADO CHANCES.

..EDWARDS/THOMPSON.. 08/26/2012

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

ATTN...WFO...MFL...MLB...KEY...TBW...

LAT...LON 24588217 24608170 24678164 24758137 24718111 24928066
25028074 25058102 25198117 25348113 25538121 25798139
25908161 25818167 26168182 26398189 26458197 26418206
26588220 26848222 26998185 27158049 27128013 26738000
25668012 25278027 24958051 24748085 24648115 24638131
24568159 24528181 24518218 24588217

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