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Tropical Storm Tammy Discussion Number   2
NWS National Hurricane Center Miami FL       AL202023
1100 PM AST Wed Oct 18 2023
 
Tammy's center remains difficult to pinpoint this evening. While a 
persistent area of deep convection continues to be present near and 
to the south of the estimated center, an SSMIS microwave pass that 
came in after the prior advisory suggested the tropical storm still 
has poor structural organization with some vertical tilt. It is 
possible the center remains somewhat elongated or it could be 
attempting to redevelop further into the deep convection. The 
initial intensity will remain 35 kt for this advisory, in best 
agreement with the TAFB subjective intensity estimate at 00 UTC. 
Data from NOAA-P3 and Air Force reconnaissance missions, set to 
take off tomorrow, will be helpful to better diagnose Tammy's 
structure. 

The motion continues to be more uncertain than usual given the 
current structure. The tropical storm still appears to be moving 
westward, but is starting to slow down at 270/15 kt. A 
well-established mid-level ridge is steering Tammy westward 
currently, but it is expected to gradually erode as a sharp 
mid-latitude trough swings off the Eastern U.S. seaboard in the next 
couple of days. The net result of this pattern change is that Tammy 
is forecast to slow down and begin turning gradually poleward toward 
the west-northwest and northwest in the next 2-3 days. Beyond that 
time, Tammy is forecast to move northward and may begin to recurve 
into the open Atlantic by the end of the forecast. As mentioned 
previously, the guidance generally agrees on this scenario, but 
there are significant track details related to the model-depicted 
vertical depth of Tammy and how sharp of a turn northward the storm 
makes. There are also along-track differences too, with the ECMWF 
much faster than the latest GFS forecast. Compared to the previous 
cycle, the guidance has shifted eastward and is a bit slower too, 
and the NHC track was shifted in that direction, roughly in between 
the consensus aids HCCA and TVCN. 

The intensity forecast is somewhat tricky. SHIPS diagnosed shear 
from the GFS and ECMWF is currently light to moderate, between 10-20 
kt out of the northwest. The GFS-based SHIPS suggests this shear 
could even lower some over the next 24-48 h. The lower shear, 
combined with very warm 29-30 C sea-surface temperatures could 
potentially promote significant intensification. However, the 
potential for any intensification is dependent on the storm 
structure, which remains more uncertain than usual given the lack of 
recent high resolution microwave or scatterometer data. To add to 
the uncertainty, the most recent HAFS-A/B runs show little to no 
intensification, and in fact appear to lose the vortex in their 
inner-nest beyond 24 hours. The NHC intensity forecast continues to 
show intensification to around 55 kt over the next 36 h, but only 
more gradual intensification thereafter, assuming that moderate 
vertical wind shear will keep the storm in check. This forecast is 
near or just above the consensus aids. However, this could end up on 
the conservative side if Tammy is able to become more vertically 
aligned in the short-term, as suggested by the latest GFS and HWRF 
forecasts.
 
 
KEY MESSAGES:
 
1.  Tropical storm conditions are possible across portions of the 
Lesser Antilles beginning on Friday.  Tropical storm watches are 
currently in effect for Barbados, Dominica, Martinique, and 
Guadeloupe, and additional watches or warnings will likely be 
required on Thursday.
 
2.  Heavy rains from Tammy will begin to affect the northern
Windward and Leeward Islands on Friday, spreading into the British
and U.S. Virgin Islands and Puerto Rico over the weekend.  This
rainfall may produce isolated flash and urban flooding, along with
isolated mudslides in areas of higher terrain.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  19/0300Z 13.0N  53.1W   35 KT  40 MPH
 12H  19/1200Z 13.2N  55.0W   40 KT  45 MPH
 24H  20/0000Z 13.7N  56.8W   50 KT  60 MPH
 36H  20/1200Z 14.3N  58.6W   55 KT  65 MPH
 48H  21/0000Z 15.4N  60.3W   55 KT  65 MPH
 60H  21/1200Z 16.8N  61.6W   60 KT  70 MPH
 72H  22/0000Z 18.4N  62.5W   60 KT  70 MPH
 96H  23/0000Z 22.0N  63.4W   70 KT  80 MPH
120H  24/0000Z 27.5N  60.0W   70 KT  80 MPH
 
$$
Forecaster Papin
 
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