Hurricane Tammy Discussion Number  20
NWS National Hurricane Center Miami FL       AL202023
500 AM AST Mon Oct 23 2023

Tammy's structure overnight appears to have undergone an evolution. 
After maintaining a small compact presentation for several days, 
Tammy's convective structure has broadened out, with convective 
bands taking on a more fragmented appearance around the center. Some 
of this change could be related to some dry air that may have been 
entrained into the circulation earlier tonight. On the other hand, 
the upper-level outflow has expanded some to the south and west, 
suggesting a brief respite in the shear that has been affecting 
Tammy. Data from the last pass of the NOAA-P3 reconnaissance mission 
suggested the winds were perhaps a bit lower, while both subjective 
and objective satellite estimates were also somewhat lower than 
earlier. The initial intensity was nudged downward to 70 kt this 
advisory.

Tammy has made the long awaited turn northward, with the estimated 
motion now at 355/9 kt. A continued turn to the north-northeast and 
northeast is anticipated over the next 24-36 hours as the hurricane 
briefly becomes steered in between a mid-latitude trough passing by 
to its north, and an established mid-level ridge located to its 
east. The track guidance is at least in good agreement on this track 
evolution for the first 48-60 h. After that time, a piece of the 
aforementioned trough is expected to cutoff and dig southward to the 
west of Tammy. How the tropical cyclone reacts to this synoptic 
feature will likely be dependent on its vertical depth, and its 
location when the trough interacts with it. If anything, the track 
guidance spread is even larger than it was a few cycles ago, with a 
range of solutions from a quick pivot back west (the HAFS-A/B runs) 
after 60 h versus a continued northeastward track without much of a 
leftward turn at all (the latest GFS forecast). Overall the track 
consensus aids have shifted a bit more poleward before Tammy turns 
westward, and the NHC track was shifted a little further east and 
north compared to the prior one. However, this track forecast 
remains of low confidence and is highly uncertain, which is best 
exemplified by the 00 UTC ECMWF ensemble guidance, which shows more 
than a 1500 mile southwest-to-northeast spread in the various member 
solutions in 5 days.

Tammy's somewhat broader structure could delay some intensification 
in the immediate short-term, but most of the intensity guidance 
remains adamant about some strengthening occuring in the next 24-36 
h as the hurricane remains over 28-29 C sea-surface-temperatures and 
moderate wind shear between 15-20 kt. However after 48 hours, 
vertical wind shear is expected to rapidly increase out of the 
southwest which should begin to weaken the tropical cyclone. This 
forecast remains near or just a bit below the consensus aids HCCA 
and IVCN during this time frame. After 72 h, The cutoff trough 
digging in to the west of Tammy will likely impart some cool and 
very dry mid-latitude air into the storm's core, initiating 
extratropical transition as deep central convection is also stripped 
away by the increased shear. This process could be completed in 96 
h, especially if Tammy moves further north like the GFS and ECMWF 
models are forecasting. However, there also remains some uncertainty 
about this evolution, as a further south track before the storm 
pivots west could allow it to remain over warmer sea-surface 
temperatures and potentially prevent extratropical transition from 
completing, as suggested by the HAFS-A/B runs. 

Even as Tammy moves away from the Leeward islands, a prominent 
feeder band has formed over these islands, likely prolonging the 
heavy rain threat for these areas through this morning. 


KEY MESSAGES:

1.  Heavy rain from Tammy will remain possible over the British 
Virgin Islands, Leeward Islands, and northern Windward Islands 
through this morning.  This rainfall may produce isolated flash and 
urban flooding, along with isolated mudslides in areas of higher 
terrain.


FORECAST POSITIONS AND MAX WINDS

INIT  23/0900Z 21.4N  64.0W   70 KT  80 MPH
12H  23/1800Z 22.2N  63.7W   70 KT  80 MPH
24H  24/0600Z 23.2N  62.9W   75 KT  85 MPH
36H  24/1800Z 24.1N  61.7W   80 KT  90 MPH
48H  25/0600Z 25.1N  60.5W   80 KT  90 MPH
60H  25/1800Z 26.5N  59.3W   75 KT  85 MPH
72H  26/0600Z 28.0N  58.7W   65 KT  75 MPH
96H  27/0600Z 30.4N  59.4W   55 KT  65 MPH...POST-TROP/EXTRATROP
120H  28/0600Z 32.0N  62.9W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
Forecaster Papin