[Tropical] Tropical Cyclone Discussion

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WTNT45 KNHC 220856
TCDAT5

Hurricane Tammy Discussion Number  16
NWS National Hurricane Center Miami FL       AL202023
500 AM AST Sun Oct 22 2023

Tammy is beginning to pull away form the northernmost Leeward
islands this morning. The final fix of the prior Air Force
reconnaissance mission suggested the pressure was holding steady
with an eyewall that remained open to the southwest. This structure
was also consistent with the last few radar images out of Guadeloupe
as the Tammy moved out of its range. On satellite, the storm
continues to produce a compact central dense overcast (CDO) of deep
convection with overshooting cloud tops as cold as -85 to -90 C.
However moderate southwesterly vertical wind shear still appears to
be keeping the small hurricane in check with a sharp edge to its CDO
on the west side. The initial intensity is held at 75 kt this
advisory, which is a little under the subjective Dvorak
classifications form TAFB and SAB given the earlier aircraft data.
Another set of Air Force Reserve C-130 and NOAA-P3 aircraft are set
to sample the hurricane again later this morning.

The last center fix from the overnight Air Force mission suggested
that, after a brief west-northwest jog, Tammy had resumed a more
north-northwest motion, at 335/9 kt. Tammy should maintain this
motion or turn more northward during the remainder of today as it
moves along the western periphery of a large subtropical ridge. A
mid-latitude trough passing to the north of Tammy should then
position this ridge a bit southeast of the hurricane, and the
guidance shows Tammy responding by moving slowly north-northeastward
on Monday and Tuesday. However, most of the mid-latitude trough is
forecast to bypass the system, and this is where the guidance shows
dramatically increasing track spread.  Most of the deterministic
models now show a deep-layer cutoff low breaking off from the
mid-latitude trough and dropping down to the west of Tammy, and the
vertical depth of the tropical cyclone will likely dictate how much
influence the cutoff low has on it. Much of the deterministic
guidance shows Tammy swinging around this cutoff and pivoting back
to the north or northwest by the end of the forecast period. The NHC
track forecast has been shifted a bit east and north of the prior
one, towards the consensus aids TVCN and HCCA. However the leftward
turn at the end of the forecast happens at a variety of different
latitudes in the GFS and ECMWF ensemble guidance. Thus, the track
forecast towards the end of the period is highly uncertain and of
low confidence.

As has been the case for the last couple of days, Tammy remains in
an environment of moderate 15-20 kt of southwesterly shear but
remains over warm 29-30 C sea-surface temperatures. That has not
prevented gradual intensification of the hurricane thus far, and so
some additional intensification is forecast over the next day or so.
Afterwards as the shear begins to gradually increase, there will
likely be some inner core fluctuations and the NHC intensity
forecast shows a leveling off of Tammy during this period. After 72
h, the shear is likely to become prohibitively strong and weakening
is forecast to begin by that time, with Tammy likely dropping under
hurricane intensity sometime in the day 4-5 period. The significant
shear forecast however may be offset by some baroclinic forcing of
the upper-level trough interacting with Tammy at the end of the
period. However, like the track forecast, there is a range of
intensity solutions at these latter time periods, with the NHC
intensity forecast most closely following the consensus aids HCCA
and IVCN.


KEY MESSAGES:

1.  Hurricane conditions and coastal flooding in portions of the
northern Leeward Islands within the warning area should continue
for the next few hours.

2.  The heaviest rains from Tammy will continue over the Leeward
Islands through today. This rainfall may produce isolated flash and
urban flooding, along with isolated mudslides in areas of higher
terrain.


FORECAST POSITIONS AND MAX WINDS

INIT  22/0900Z 18.5N  62.3W   75 KT  85 MPH
 12H  22/1800Z 19.5N  62.9W   80 KT  90 MPH
 24H  23/0600Z 20.8N  63.5W   85 KT 100 MPH
 36H  23/1800Z 21.9N  63.5W   85 KT 100 MPH
 48H  24/0600Z 22.7N  63.1W   80 KT  90 MPH
 60H  24/1800Z 23.2N  62.5W   80 KT  90 MPH
 72H  25/0600Z 23.8N  61.9W   80 KT  90 MPH
 96H  26/0600Z 26.0N  61.4W   70 KT  80 MPH
120H  27/0600Z 29.0N  62.5W   60 KT  70 MPH

$$
Forecaster Papin

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