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| #1158525 (Received by flhurricane at: 5:04 AM 23.Oct.2023) TCDAT5 Hurricane Tammy Discussion Number 20 NWS National Hurricane Center Miami FL AL202023 500 AM AST Mon Oct 23 2023 Tammy`s structure overnight appears to have undergone an evolution. After maintaining a small compact presentation for several days, Tammy`s convective structure has broadened out, with convective bands taking on a more fragmented appearance around the center. Some of this change could be related to some dry air that may have been entrained into the circulation earlier tonight. On the other hand, the upper-level outflow has expanded some to the south and west, suggesting a brief respite in the shear that has been affecting Tammy. Data from the last pass of the NOAA-P3 reconnaissance mission suggested the winds were perhaps a bit lower, while both subjective and objective satellite estimates were also somewhat lower than earlier. The initial intensity was nudged downward to 70 kt this advisory. Tammy has made the long awaited turn northward, with the estimated motion now at 355/9 kt. A continued turn to the north-northeast and northeast is anticipated over the next 24-36 hours as the hurricane briefly becomes steered in between a mid-latitude trough passing by to its north, and an established mid-level ridge located to its east. The track guidance is at least in good agreement on this track evolution for the first 48-60 h. After that time, a piece of the aforementioned trough is expected to cutoff and dig southward to the west of Tammy. How the tropical cyclone reacts to this synoptic feature will likely be dependent on its vertical depth, and its location when the trough interacts with it. If anything, the track guidance spread is even larger than it was a few cycles ago, with a range of solutions from a quick pivot back west (the HAFS-A/B runs) after 60 h versus a continued northeastward track without much of a leftward turn at all (the latest GFS forecast). Overall the track consensus aids have shifted a bit more poleward before Tammy turns westward, and the NHC track was shifted a little further east and north compared to the prior one. However, this track forecast remains of low confidence and is highly uncertain, which is best exemplified by the 00 UTC ECMWF ensemble guidance, which shows more than a 1500 mile southwest-to-northeast spread in the various member solutions in 5 days. Tammy`s somewhat broader structure could delay some intensification in the immediate short-term, but most of the intensity guidance remains adamant about some strengthening occuring in the next 24-36 h as the hurricane remains over 28-29 C sea-surface-temperatures and moderate wind shear between 15-20 kt. However after 48 hours, vertical wind shear is expected to rapidly increase out of the southwest which should begin to weaken the tropical cyclone. This forecast remains near or just a bit below the consensus aids HCCA and IVCN during this time frame. After 72 h, The cutoff trough digging in to the west of Tammy will likely impart some cool and very dry mid-latitude air into the storm`s core, initiating extratropical transition as deep central convection is also stripped away by the increased shear. This process could be completed in 96 h, especially if Tammy moves further north like the GFS and ECMWF models are forecasting. However, there also remains some uncertainty about this evolution, as a further south track before the storm pivots west could allow it to remain over warmer sea-surface temperatures and potentially prevent extratropical transition from completing, as suggested by the HAFS-A/B runs. Even as Tammy moves away from the Leeward islands, a prominent feeder band has formed over these islands, likely prolonging the heavy rain threat for these areas through this morning. KEY MESSAGES: 1. Heavy rain from Tammy will remain possible over the British Virgin Islands, Leeward Islands, and northern Windward Islands through this morning. This rainfall may produce isolated flash and urban flooding, along with isolated mudslides in areas of higher terrain. FORECAST POSITIONS AND MAX WINDS INIT 23/0900Z 21.4N 64.0W 70 KT 80 MPH 12H 23/1800Z 22.2N 63.7W 70 KT 80 MPH 24H 24/0600Z 23.2N 62.9W 75 KT 85 MPH 36H 24/1800Z 24.1N 61.7W 80 KT 90 MPH 48H 25/0600Z 25.1N 60.5W 80 KT 90 MPH 60H 25/1800Z 26.5N 59.3W 75 KT 85 MPH 72H 26/0600Z 28.0N 58.7W 65 KT 75 MPH 96H 27/0600Z 30.4N 59.4W 55 KT 65 MPH...POST-TROP/EXTRATROP 120H 28/0600Z 32.0N 62.9W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Papin |
