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| #1158072 (Received by flhurricane at: 11:02 PM 19.Oct.2023) TCDAT5 Tropical Storm Tammy Discussion Number 7 NWS National Hurricane Center Miami FL AL202023 1100 PM AST Thu Oct 19 2023 Tammy appears to be in the process of reorganizing. The low-level circulation we had been following appears to have slowed down tonight, as confirmed by the last few fixes from the NOAA-P3 reconnaissance aircraft that has been in the storm tonight. In addition, the Tail-Doppler Radar (TDR) on board the aircraft has shown that the tilt between the low-level (1 km) and mid-level (5 km) center has been reduced as a result of this slowdown. There is also evidence of convection beginning to wrap into the up-shear quadrant of the system on both GPM microwave imagery available at 2350 UTC and recent radar images from Barbados. However, this restructuring has not yet resulted in an increase in the maximum sustained winds, which remain about 50 kt for this advisory, in agreement with a blend of the subjective Dvorak data and the earlier TDR data in the northeast quadrant. Recon fixes indicate the Tammy is still moving generally west-northwestward but slower at 290/9 kt. There is not much new to report from the track reasoning this cycle. An enhanced mid-level ridge (anomalously strong for this time of year) is beginning to shift eastward as a sharp mid-latitude trough approaches from the west. This shift should allow Tammy to turn gradually to the northwest and then north-northwest over the next 2-3 days. However, the forward motion related to this turn is forecast to be rather slow, related to an upper-level cutoff low south of the mid-level ridge weakening its steering influence on Tammy. Compared to 24 hours ago, the global model guidance is in better agreement on this track solution, but continues to trend slower and a bit east of the prior cycle. Thus, the NHC track forecast has been adjusted quite a bit slower and a little further east of the prior one, blending the prior track with the consensus aids HCCA and TVCN. It should be noted that both the GFS and ECMWF remain on the east side of the guidance envelope, and further adjustments in that direction may be needed in subsequent cycles. Tammy is still forecast to recurve to the north-northeast by the end of the forecast period. However, there is a substantial amount of spread in the along-track direction in both the GFS and ECMWF ensembles, related to whether or not Tammy is fully picked up by the mid-latitude trough. Assuming Tammy is in the process of becoming better vertically aligned, the storm has an opportunity to intensify as shear remains light to moderate (10-20 kt) and sea-surface temperatures remain quite warm (near 30 C). The guidance this cycle shows a bit more short-term intensification than before, and the NHC intensity forecast follows suit, showing gradual intensification to hurricane intensity in 36 h and a bit more intensification through day 4. Increasing southwesterly vertical wind shear at the end of the of forecast period may begin to induce weakening as Tammy moves into the stronger mid-latitude flow ahead of the mid-latitude trough. The NHC intensity forecast is close to or just a shade under the latest consensus aids HCCA and IVCN. KEY MESSAGES: 1. Tropical storm conditions are expected to begin in portions of the Lesser Antilles beginning Friday, where a tropical storm warning is in effect. Tropical storm and hurricane conditions are possible elsewhere in the Leeward Islands where hurricane and tropical storm watches are currently in effect. Additional watches and warnings could be required on Friday. 2. Heavy rains from Tammy will begin to affect the Leeward and northern Windward 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 20/0300Z 13.8N 57.1W 50 KT 60 MPH 12H 20/1200Z 14.3N 58.2W 55 KT 65 MPH 24H 21/0000Z 15.3N 59.9W 60 KT 70 MPH 36H 21/1200Z 16.6N 61.3W 65 KT 75 MPH 48H 22/0000Z 18.0N 62.0W 70 KT 80 MPH 60H 22/1200Z 19.7N 62.5W 75 KT 85 MPH 72H 23/0000Z 21.5N 63.0W 80 KT 90 MPH 96H 24/0000Z 24.0N 62.0W 80 KT 90 MPH 120H 25/0000Z 28.0N 57.5W 75 KT 85 MPH $$ Forecaster Papin |
