Hurricane Isaac Discussion Number   6
NWS National Hurricane Center Miami FL       AL102024
500 AM AST Fri Sep 27 2024

Isaac has continued to become better organized since the prior 
advisory. The earlier warm spot noted on IR satellite imagery has 
persisted and warmed further, with sufficently cold -55 to -60 C 
convective cloud tops encircling the feature. A helpful AMSR2 pass 
that became available after the prior advisory also showed an eye 
feature on both the 89-GHz and 37-GHz channels, the latter showing a 
cyan ring. These features suggest that Isaac has become a hurricane, 
and the initial intensity has been raised to 65 kt for this 
advisory.

Isaac still is moving generally eastward this morning, estimated at
090/10 kt. This motion with a gradual turn to the east-northeast
should occur over the next couple of days as the hurricane remains
steered mostly by deep-layer flow along the northern periphery of a
mid-level ridge centered to the southeast of Isaac. Once again, how
quick this forward motion ends up being will be critical for its
ultimate track down the road, with a track bifurcation continuing
between the GFS and ECMWF solutions occuring beyond 60 h as to how
much the upstream trough located to its east is able to interact
and pick up the cyclone. Despite the spread in the guidance, the
consensus aids are not that much altered from the prior cycle, and
the NHC track forecast is quite similar to the prior advisory.

Additional intensification is expected while the vertical wind shear
remains only low to moderate and Isaac continues to be over
sufficently warm sea-surface temperatures (SSTs) with instability
being aided by cold upper-level temperatures. After 36 h, SSTs drop
to 24 C and lower as shear increases markedly, which should induce
a gradual weakening trend. Both the GFS and ECMWF suggest the
cyclone should lose its deep convection sometime in the 72-96 h
period, marking its transition to post-tropical in that time frame.
The NHC intensity forecast is on the high side of the guidance, but
is generally in line with the latest GFS-based SHIPS and LGEM
guidance.


FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 37.0N  48.5W   65 KT  75 MPH
12H  27/1800Z 37.5N  45.9W   70 KT  80 MPH
24H  28/0600Z 38.8N  42.4W   75 KT  85 MPH
36H  28/1800Z 40.4N  39.3W   75 KT  85 MPH
48H  29/0600Z 41.8N  37.3W   70 KT  80 MPH
60H  29/1800Z 43.1N  35.9W   60 KT  70 MPH
72H  30/0600Z 44.0N  34.3W   55 KT  65 MPH
96H  01/0600Z 46.9N  30.4W   45 KT  50 MPH...POST-TROP/EXTRATROP
120H  02/0600Z 50.4N  26.2W   35 KT  40 MPH...POST-TROP/EXTRATROP

$$
Forecaster Papin