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| #1157897 (Received by flhurricane at: 4:53 PM 18.Oct.2023) TCDAT5 Tropical Storm Tammy Discussion Number 1 NWS National Hurricane Center Miami FL AL202023 500 PM AST Wed Oct 18 2023 The tropical disturbance that NHC has been monitoring for many days as it traversed the tropical Atlantic (AL94) has finally become sufficiently organized, both convectively and circulation-wise, to be designated as a tropical cyclone. The circulation still appears a bit elongated in visible satellite images, but the convective activity suggests that a well-defined center has formed. Earlier scatterometer data showed winds of 30-35 kt to the northeast of the center, and TAFB provided a Dvorak estimate of T2.5/35 kt. Therefore, advisories are being initiated on Tropical Storm Tammy, with an initial intensity of 35 kt. The initial motion is westward (275 degrees) at a fast 20 kt, but there is uncertainty in this estimate given that the center has likely just recently formed. A strong mid-level ridge to the north is forecast to keep Tammy on a westward motion, but slower, over the next 24 hours or so. After that, a deep-layer trough moving across the eastern United States is expected to push the ridge eastward, allowing Tammy to turn toward the northwest and then north into the weekend. The track models agree on this general scenario, with Tammy moving over or near the Leeward Islands Friday and Saturday. However, there are some notable differences, with some of the stronger models (e.g., HWRF and GFS) showing a turn just before Tammy reaches the islands, while the weaker models (e.g., HAFS and HMON) move the storm farther west into the northeastern Caribbean Sea. The initial NHC track forecast is between these two periphery scenarios, and is close to the ECMWF, TVCA, and HCCA solutions. After passing the Leeward Islands, Tammy is expected to accelerate northeastward over the central Atlantic ahead of the trough. Global model fields and SHIPS model diagnoses suggest that Tammy may deal with some vertical shear, and possibly some dry air in the vicinity, over the next few days. On the other hand, the storm will be moving over very warm waters of 29-30 degrees Celsius. Therefore, gradual strengthening is forecast during the next couple of days, with the NHC intensity prediction very close to the IVCN and HCCA aids. While the forecast depicts a 55-kt tropical storm moving across the islands, there could be adjustments to this forecast once Tammy`s current intensity and structure become clearer, and users should be prepared for possible forecast changes. After Tammy passes the Leeward Islands, the intensity models suggest that some further intensification will be possible as it accelerates northeastward over the central Atlantic. KEY MESSAGES: 1. Tropical storm conditions are possible across portions of the Lesser Antilles beginning on Friday. Tropical storm watches are currently in effect for Barbados, Dominica, Martinique, and Guadeloupe, and additional watches or warnings will likely be required tonight or on Thursday. 2. Heavy rains from Tammy will begin to affect the northern Windward and Leeward 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 18/2100Z 13.0N 51.7W 35 KT 40 MPH 12H 19/0600Z 13.1N 53.7W 40 KT 45 MPH 24H 19/1800Z 13.5N 55.9W 50 KT 60 MPH 36H 20/0600Z 14.1N 58.2W 55 KT 65 MPH 48H 20/1800Z 15.0N 60.2W 55 KT 65 MPH 60H 21/0600Z 15.9N 61.7W 55 KT 65 MPH 72H 21/1800Z 17.5N 62.7W 55 KT 65 MPH 96H 22/1800Z 21.4N 63.4W 60 KT 70 MPH 120H 23/1800Z 26.1N 60.2W 60 KT 70 MPH $$ Forecaster Berg |
