Hurricane Isaac Forecast Discussion

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WTNT45 KNHC 280315
TCDAT5

Hurricane Isaac Discussion Number   9
NWS National Hurricane Center Miami FL       AL102024
300 AM GMT Sat Sep 28 2024

Isaac has maintained a well-defined eye on both visible and infrared 
imagery for the past 12 hours or so.  The latest subjective Dvorak 
classifications are a consensus T-4.5/77 kt from TAFB and SAB.  The 
latest objective intensity estimates from UW-CIMSS range from 72 to 
92 kt.  The initial intensity is adjusted upward to 80 kt based on a 
blend of the subjective and objective estimates.

The hurricane is moving east-northeastward, or 075/14 kt.  Isaac is 
currently moving in the zonal flow with the mid-latitude westerlies. 
A powerful deep-layer trough over Atlantic Canada will approach 
Isaac by Saturday night as a narrow mid-level ridge builds to the 
north of Isaac.  These steering features will cause Isaac to bend a 
bit more toward the northeast while slowing down slightly.  On 
Monday, after Isaac becomes post-tropical, a turn more toward the 
north-northeast is expected, as the deep-layer trough moves 
eastward and the ridge moves northward.  No significant changes 
have been made to the official forecast track forecast through the 
first 60-72 h.  After that time, a sizable westward adjustment is 
made to the official forecast track, but the new NHC forecast track 
is well east of the latest consensus models beyond 72 h.

It is possible Isaac could strengthen a little more tonight, 
although the hurricane is likely near its peak intensity.  The 
cyclone is crossing the 26 degree C isotherm now, but is forecast to 
remain over 24-26 degree C sea-surface temperatures for another 36 h 
or so.  The cold upper-level temperatures that Isaac is forecast to 
traverse should help maintain enough instability such that only slow 
weakening is forecast through 36 h, and Isaac is expected to 
maintain hurricane strength through that time.  Also, although 
moderate southwesterly shear is forecast to increase slightly, the 
shear direction is the same as the direction of Isaac's motion, so 
this should not be too unfavorable.  Beyond 48 h, sea-surface 
temperatures decrease significantly and wind shear is forecast to 
become very strong.  This should cause the cyclone to lose its deep 
convection by Monday and then become extratropical. Little to no 
change was made to the previous official intensity forecast, while 
lies near the middle of the guidance envelope.


FORECAST POSITIONS AND MAX WINDS

INIT  28/0300Z 38.2N  43.4W   80 KT  90 MPH
 12H  28/1200Z 39.5N  40.9W   80 KT  90 MPH
 24H  29/0000Z 41.0N  38.6W   75 KT  85 MPH
 36H  29/1200Z 42.5N  37.0W   70 KT  80 MPH
 48H  30/0000Z 43.8N  35.4W   60 KT  70 MPH
 60H  30/1200Z 45.0N  33.7W   55 KT  65 MPH...POST-TROPICAL
 72H  01/0000Z 46.9N  32.4W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  02/0000Z 50.9N  31.0W   40 KT  45 MPH...POST-TROP/EXTRATROP
120H  03/0000Z 53.8N  29.5W   35 KT  40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Hagen