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| #1158389 (Received by flhurricane at: 5:05 AM 22.Oct.2023) 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 |
