[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 230850
TCDAT5

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
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