Tropical Storm Isaac Forecast Discussion

000
WTNT45 KNHC 260840
TCDAT5

Tropical Storm Isaac Discussion Number   2
NWS National Hurricane Center Miami FL       AL102024
500 AM AST Thu Sep 26 2024

Isaac's structure on satellite looks fairly healthy for a 
high-latitude tropical cyclone. There are plenty of curved banding 
features rotating around the estimated center on IR satellite, 
matched by a recent AMSR2 microwave pass also showing convective 
banding. While the cloud top temperatures are not that cold, only 
-50 to -60 C, that is rather typical for a tropical cyclone along 
the axis of an upper-level trough with a depressed tropopause. Based 
on the earlier scatterometer wind data, plus a DMINT estimate from 
the AMSR2 pass of 46 kt, the initial intensity is held at 45 kt 
this advisory. 

The tropical storm is continuing eastward this morning at 90/10 kt. 
This general motion, albeit with a gradual turn more 
east-northeastward is expected over the next several days as Isaac 
slowly accelerates under the influence of mid-level ridging to the 
southeast of the storm. The track guidance is in pretty good 
agreement in the short-term. However, differences primarily in the 
along-track direction begin to be evident beyond 48 h, which could 
ultimately play a role in more notable across-track differences 
shown between the GFS and ECMWF solutions by early next week. For 
now, the NHC track forecast is very similar to the previous track 
forecast, and favors a solution very similar to the HFIP corrected 
consensus approach (HCCA) and ECMWF track. 

Given the storm's healthy current structure, there is an opportunity 
for it to intensify over the next couple of days. During this time 
span, sea-surface temperatures (SSTs) hover between 26-27 C, colder 
than normal upper-level temperatures should aid tropospheric 
instability, and vertical wind shear is forecast to remain in the 
15-20 kt range, in the same direction as the storm motion. The 
simulated IR imagery from the latest ECMWF and GFS runs also show 
Isaac attempting to clear out an eye occasionally in the 36-60 h 
period. These signals suggest that Isaac could become a hurricane, 
and that is now explicitly reflected in the latest NHC forecast, 
which is very close to the latest HCCA consensus aid. After 60 h, 
shear increases markedly as SSTs also decrease, which should lead to 
weakening, and Isaac is forecast to become post-tropical sometime 
near the end of the forecast as it passes by to the north of the 
Azores. 


FORECAST POSITIONS AND MAX WINDS

INIT  26/0900Z 37.1N  52.8W   45 KT  50 MPH
 12H  26/1800Z 37.2N  50.9W   50 KT  60 MPH
 24H  27/0600Z 37.6N  48.1W   55 KT  65 MPH
 36H  27/1800Z 38.3N  44.9W   60 KT  70 MPH
 48H  28/0600Z 39.4N  41.2W   65 KT  75 MPH
 60H  28/1800Z 40.5N  37.9W   65 KT  75 MPH
 72H  29/0600Z 41.8N  34.0W   60 KT  70 MPH
 96H  30/0600Z 43.0N  27.2W   55 KT  65 MPH
120H  01/0600Z 43.4N  19.4W   50 KT  60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Papin