Tropical Storm Isaac Discussion Number   4
NWS National Hurricane Center Miami FL       AL102024
500 PM AST Thu Sep 26 2024

Little change with the structure of Isaac has been noted this 
afternoon. Episodic bursts of deep convection have been observed 
over the western semi-circle throughout the day, but cloud tops have 
largely failed to break -60C. There remains a relative void of 
thunderstorms over the eastern half of Isaac's circulation as it 
traverses across 26C water. ASCAT passes, combined with a blend of 
subjective Dvorak estimates from TAFB and SAB, plus objective 
estimates from UW-CIMSS collectively support maintaining an initial 
intensity of 45 kt for this advisory.

For the next couple of days, the track guidance is tightly clustered 
as Isaac continues on a general east to east-northeast heading, 
remaining embedded within a well established mid-latitude trough. 
Beyond Saturday, there has been a large change in the dynamic model 
suite. A vast majority of the the guidance, including the 
deterministic ECMWF and several of its ensemble members, have shown 
a rather dramatic poleward shift, similar to previous runs of the 
GFS. This is ultimately induced by a slight decrease in forward 
speed, which in turn allows Isaac to be steered by a strong closed 
mid-latitude trough digging out of eastern Canada into the north 
Atlantic. This would force Isaac to the northeast and eventually 
northward. Isaac's medium range track confidence is quite low, owing 
to lack of model consistency during the past several runs. To avoid 
making too large of a change, the forecast track after Day 3 has 
shifted to the left for this advisory. However, it falls well to the 
east of the guidance envelope and further large changes may be 
needed.

Despite the longer range track uncertainty, Isaac appears to be in 
an environment that should be generally conducive for some 
strengthening, especially during the next day or two.  After that, 
unusually cold upper-tropospheric temperatures should help sustain 
the tropical cyclone for a while, even as it moves over colder SSTs 
in the 24-25 deg C range. By the end of the forecast period, much 
colder SSTs and a dramatic increase in shear should cause Isaac to 
become post-tropical. This transition could occur more quickly than 
forecast if the system takes a more northerly turn early next week. 
No changes of note were made to the official intensity forecast, 
which still shows Isaac reaching hurricane strength during the next 
couple of days.


FORECAST POSITIONS AND MAX WINDS

INIT  26/2100Z 37.5N  50.6W   45 KT  50 MPH
12H  27/0600Z 37.6N  48.7W   50 KT  60 MPH
24H  27/1800Z 38.2N  45.5W   60 KT  70 MPH
36H  28/0600Z 39.2N  42.0W   65 KT  75 MPH
48H  28/1800Z 40.5N  38.7W   65 KT  75 MPH
60H  29/0600Z 41.9N  35.9W   60 KT  70 MPH
72H  29/1800Z 43.0N  33.8W   60 KT  70 MPH
96H  30/1800Z 44.2N  27.8W   50 KT  60 MPH...POST-TROP/EXTRATROP
120H  01/1800Z 45.5N  22.5W   40 KT  45 MPH...POST-TROP/EXTRATROP

$$
Forecaster D. Zelinsky/Bookbinder