Wednesday, September 5, 2012 5:41 am

by Rob Lightbown Crown Weather

HURRICANE LESLIE DISCUSSION NUMBER  26
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122012
500 PM AST WED SEP 05 2012

THE CIRRUS OUTFLOW OF LESLIE HAS EXPANDED BY MORE THAN 100 NMI TO
THE WEST SINCE THE PREVIOUS ADVISORY...INDICATING THAT THE VERTICAL
WIND SHEAR AFFECTING THE CYCLONE HAS DECREASED. A BANDING EYE
FEATURE HAS BEEN EVIDENT IN VISIBLE SATELLITE IMAGERY SINCE 1700
UTC...BUT A RECENT BURST OF EYEWALL CONVECTION HAS OBSCURED THE
EYE. SSMIS AND TRMM MICROWAVE DATA INDICATE A WELL-DEFINED
MID-LEVEL EYE HAS DEVELOPED AND A CONCENTRIC EYEWALL CYCLE COULD
BE UNDERWAY. THE INITIAL INTENSITY OF 65 KT IS BASED ON SATELLITE
INTENSITY ESTIMATES OF T4.0/65 KT FROM TAFB AND SAB...AND AN
AVERAGE ADT ESTIMATE OF T4.3/72 KT OVER THE PAST 6 HOURS.

THE INITIAL MOTION ESTIMATE REMAINS 360/02 KT. LESLIE HAS WOBBLED
SLIGHTLY TO THE NORTHEAST SINCE THE PREVIOUS ADVISORY...BUT THAT
ERRATIC MOTION IS LIKELY JUST DUE TO REFORMATION OF THE EYE. THE
LATEST GFS FORECAST TRACK HAS SHIFTED A LITTLE TO THE WEST...WHILE
THE ECMWF FORECAST HAS SHIFTED TO THE EAST. SINCE THESE TWO MODELS
ARE NOW CONVERGING CLOSER TO THE PREVIOUS FORECAST TRACK...THE NEW
NHC FORECAST IS JUST AN UPDATE OF THE PREVIOUS ADVISORY...WHICH
LIES A LITTLE TO THE EAST OF THE CONSENSUS MODELS TVCA AND TV15.

CONDITIONS ARE EXPECTED TO BE FAVORABLE FOR AT LEAST GRADUAL
STRENGTHENING TO OCCUR FOR THE NEXT 96 HOURS. COLD WATER UPWELLING
BENEATH THE CYCLONE DUE TO ITS VERY SLOW FORWARD SPEED AND THE
PRESENCE OF DRY MID-LEVEL AIR LURKING JUST TO THE WEST OF THE
CYCLONE MAY LIMIT THE RATE OF INTENSIFICATION. THE GFS-BASED SHIPS
AND LGEM MODELS CONTINUE TO HAVE NEGATIVE FEEDBACK ISSUES AS A
RESULT OF THE GFS MODEL CREATING A VERY ROBUST HURRICANE...WHICH
FURTHER GENERATES SIGNIFICANT 200 MB WARMING AFTER 72 HOURS. THE
ECMWF... UKMET...AND HWRF MODELS ALSO CONTINUE TO PRODUCE A STRONG
HURRICANE THROUGHOUT THE FORECAST PERIOD. THE OFFICIAL INTENSITY
FORECAST IS VERY SIMILAR TO THE PREVIOUS ADVISORY AND REMAINS
REMAINS HIGHER THAN MOST OF THE INTENSITY GUIDANCE AFTER 72 HOURS.

LESLIE IS A LARGE TROPICAL CYCLONE THAT IS FORECAST TO GROW IN SIZE
OVER THE NEXT SEVERAL DAYS. THE WIND FIELD OF THE CYCLONE...
COMBINED WITH ITS EXPECTED SLOW MOTION...WILL HELP TO GENERATE
LARGE LONG-PERIOD SWELLS THAT WILL PROPAGATE AWAY FROM THE TROPICAL
CYCLONE TOWARD BERMUDA AND MUCH OF THE U.S. EAST COAST FOR THE
REMAINDER OF THIS WEEK.

Northeast Gulf Of Mexico Tropical Development Prospects (Invest 90L):
I am closely monitoring an area of low pressure that is located early this morning over southern Alabama and the western Florida Panhandle. This low pressure system is forecast to track southward into the northeastern Gulf of Mexico to the south of the Alabama coastline during the day today and has the potential to spin up into a tropical storm as early as Thursday or Friday. Whatever becomes of this low pressure system, it will bring heavy rainfall with squally weather to

coastal sections of Mississippi, Alabama and the Florida Panhandle today through Friday. Additional rainfall amounts of 2 to 4 inches are expected across southernmost Alabama and across the Florida Panhandle today.

Let me explain what caused this low pressure system to develop over southern Alabama and the western Florida Panhandle: The mid-level circulation from Isaac split off a couple of days ago and tracked southward. This mid-level circulation is now interacting with an upper level low pressure system that is currently located in the eastern Gulf of Mexico. This interaction caused a brand new low pressure system to develop yesterday morning over northern Mississippi and northern Alabama. This low pressure system slowly tracked south-southeastward across Alabama yesterday afternoon into last night and is currently located over southern Alabama and the western Florida Panhandle this morning. So, since this is a brand new low pressure system, if it spins up into a tropical storm, it will get a new name which would be Nadine.

The forecast guidance are all in pretty good agreement that this low pressure system will track southward over the next couple of days getting as far south as 27.5 North Latitude on Friday before a trough of low pressure lifts it east-northeastward across northern Florida by Saturday night and Sunday and then out into the open Atlantic by early next week.

So, here are my thoughts on this potential system: I think we will see this low pressure system gradually track into the northernmost Gulf of Mexico just offshore of Mobile Bay by late this morning and then track south or south-southeast across the northeastern Gulf of Mexico tonight through Thursday and into Friday. I used the high resolution mesoscale model guidance rather than the global models for this forecast as its higher grid resolution may be able to pick out minute details of how this system may develop. Based on my analysis of the high resolution forecast guidance, it seems possible that this low pressure system will be located near 29 North Latitude and 87.5 West Longitude by early Thursday morning; in fact, the 4 km NAM model shows 40 to 45 mph maximum winds by tomorrow morning associated with this system. I think this may be a little aggressive, however, development into a tropical depression is quite possible during Thursday with strengthening into a tropical storm in the realm of possibility by Friday. I do agree that a trough of low pressure will pull this potential tropical storm across northern Florida by Saturday night and Sunday.

Heavy rainfall will be the main threat from this system with additional rainfall amounts of 2 to 4 inches likely across southern Alabama and across the Florida Panhandle today. For Thursday, additional rainfall amounts of 1 to 2 inches are possible across southern Mississippi, southern Alabama and across the Florida Panhandle. For Friday through Saturday, additional rainfall amounts of 2 to 4 inches are possible from southern Alabama eastward across the Florida Panhandle, much of northern Florida and southern Georgia.

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