MESO: Mesoscale Discussion 1832

ACUS11 KWNS 291649
SWOMCD
SPC MCD 291648
ALZ000-FLZ000-MSZ000-LAZ000-291845-

MESOSCALE DISCUSSION 1832
NWS STORM PREDICTION CENTER NORMAN OK
1148 AM CDT WED AUG 29 2012

AREAS AFFECTED...PORTIONS OF SWRN AL...SRN MS AND SERN LA

CONCERNING...TORNADO WATCH 594...

VALID 291648Z - 291845Z

THE SEVERE WEATHER THREAT FOR TORNADO WATCH 594 CONTINUES.

SUMMARY...TORNADO THREAT WILL CONTINUE THROUGH THE AFTERNOON IN THE
CURVED CONVECTIVE RAINBANDS WITHIN THE ERN SEMICIRCLE OF HURRICANE
ISAAC...AS EMBEDDED...SHALLOW TRANSIENT SUPERCELLS TRACK NNWWD
THROUGH WW 594. A SLOW NW TRACK OF HURRICANE ISAAC...AS INDICATED BY
NHC...SUGGESTS STRONGER WIND FIELDS SHOULD SPREAD A LITTLE FARTHER
NWD ACROSS PARTS OF CENTRAL LA/MS. THE INCREASE IN LOW LEVEL WINDS
WAS ALREADY INDICATED BY THE WSR-88D VAD AT WFO JAN. THUS...TRENDS
WILL BE MONITORED FOR ANY NWD SHIFT IN THE OUTER CONVECTIVE BAND AND
SUBSEQUENT INCREASE IN A TORNADO THREAT/NEW WW ISSUANCE POTENTIAL.

DISCUSSION...AT 15Z...NHC POSITION OF ISAAC SHOWED IT CENTERED OVER
SERN LA...NEAR HOUMA...WITH THE ERN SEMICIRCLE CONVECTIVE RAINBANDS
EXTENDING OUTWARD THROUGH SERN LA...SRN MS AND FAR SWRN AL. GIVEN
THE OFFICIAL NHC FORECAST MOVEMENT BEING SLOWLY TOWARD THE NW...SOME
NWD EXPANSION OF STRONGER WIND FIELDS ARE POSSIBLE...THOUGH IN THE
SHORT TERM AREAL COVERAGE OF WW 594 APPEARS TO MAINTAIN THE CURRENT
TORNADO THREAT. TIME-LAPSE OF LIX/MOB WSR-88DS INDICATED SHALLOW
SUPERCELLS WITH LOW LEVEL ROTATIONS EMBEDDED IN THE CONVECTIVE
RAINBANDS. THESE TRANSIENT ROTATIONS HAVE GENERALLY BEEN DETECTED
THUS FAR FROM OFFSHORE SERN LA THROUGH SERN LA...SERN MS AND FAR
SWRN AL.

ANALYSIS OF THE AFOREMENTIONED WSR-88D VADS INDICATED RECENT WINDS
AROUND 65 KT AT 0.5-1 KM AGL AT LIX AND 55-60 KT VALUES IN THE SAME
LEVELS AT MOB. GIVEN THESE WINDS SPEEDS AND ACCOUNTING FOR CURRENT
SURFACE WINDS AND GENERALLY 50 KT STORM MOTION IN THE OUTER BAND AND
60 KT MOTION IN THE MIDDLE BAND...0-1 KM SRH IS RANGING FROM AROUND
400 M2/S2 IN SERN LA AND ADJACENT MS COUNTIES TO AROUND 200 M2/S2 IN
FAR SERN MS/FAR SWRN AL. OBJECTIVE ANALYSES INDICATED GRADUAL
DESTABILIZATION ACROSS WW 594...WITH THIS PRIMARILY ATTRIBUTED TO
THE TRANSPORT/ADVECTION OF HIGH-THETAE MARINE AIR INTO THE NERN
QUADRANT OF ISAAC...GIVEN THE LACK OF DIABATIC HEATING. GIVEN THESE
KINEMATIC AND THERMODYNAMIC FACTORS...A TORNADO THREAT PERSISTS
ACROSS WW 594.

..PETERS.. 08/29/2012

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

ATTN...WFO...MOB...JAN...LIX...LCH...

LAT...LON 28929131 29859132 30719187 31239153 31629082 31888848
31548798 31168764 30038721 29738847 28738876 28448925
28688983 28609063 28719126 28929131

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