Tropical Storm Isaac Forecast Discussion

000
WTNT45 KNHC 261437
TCDAT5

Tropical Storm Isaac Discussion Number   3
NWS National Hurricane Center Miami FL       AL102024
1100 AM AST Thu Sep 26 2024

Isaac's structure has remained mostly steady state during the past 
6 hours. Cloud tops associated with the tropical storm are still 
not particularly cold, but as noted in previous discussions, this 
is not unexpected for its synoptic environment. A blend of 
subjective Dvorak estimates from TAFB and SAB plus objective 
estimates from UW-CIMSS still supports an intensity of 45 kt for 
this advisory.

For the next 48-60 h, the track forecast appears to be mostly 
straightforward. All reliable guidance indicates that Isaac will 
continue moving eastward to east-northeastward with a mid-latitude 
trough during that period. However, there is a significant 
bifurcation in the dynamical models after that. A majority of the 
guidance, including most of the ECMWF ensemble, indicate that Isaac 
will turn back eastward, continuing with the mid-latitude flow 
associated with another deep-layer trough.  However, a few members 
of the global ensembles, plus the deterministic GFS, indicate Isaac 
could move slightly slower, which could cause the tropical storm to 
interact with a different trough, currently located over eastern 
Canada. This may cause the tropical cyclone to move northward after 
60 h, on a very different track. The NHC forecast treats the GFS as 
an outlier, and is instead near the consensus of the remaining 
guidance. It is therefore very similar to the previous forecast. 
However, confidence in this forecast is particularly low, and we 
can't rule out needing to make an unusually large change to later 
forecasts.

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 
could help sustain the tropical cyclone for a while, even as it 
moves over colder SSTs in the 24-25 deg C. By the end of the 
forecast period, much colder SSTs and a dramatic increase in shear 
should cause Isaac to become post-tropical. No changes of note were 
made to the NHC forecast, which still shows Isaac reaching 
hurricane strength during the next couple of days.


FORECAST POSITIONS AND MAX WINDS

INIT  26/1500Z 37.3N  51.7W   45 KT  50 MPH
 12H  27/0000Z 37.4N  49.8W   50 KT  60 MPH
 24H  27/1200Z 37.9N  46.9W   55 KT  65 MPH
 36H  28/0000Z 38.7N  43.5W   60 KT  70 MPH
 48H  28/1200Z 39.9N  39.9W   65 KT  75 MPH
 60H  29/0000Z 41.2N  36.7W   65 KT  75 MPH
 72H  29/1200Z 42.3N  33.3W   60 KT  70 MPH
 96H  30/1200Z 43.5N  25.5W   55 KT  65 MPH
120H  01/1200Z 43.5N  18.0W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster D. Zelinsky