####018004717####
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