[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 190255
TCDAT5

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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