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Tropical Depression Eight Forecast Discussion Number 1
2021-08-16 05:01:31| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Aug 15 2021 643 WTNT43 KNHC 160301 TCDAT3 Tropical Depression Eight Discussion Number 1 NWS National Hurricane Center Miami FL AL082021 1100 PM AST Sun Aug 15 2021 A small well-defined low pressure system located just east-northeast of Bermuda has produced persistent deep convection since this morning. Radar imagery from Bermuda along with geostationary and polar orbiting microwave satellite data show that the convection is sufficiently well organized to meet the definition for a tropical cyclone. Therefore advisories have been started on Tropical Depression Eight. The initial intensity is set at 25 kt, based on believable winds near that value in a pair of recent ASCAT overpasses. Dvorak estimates from TAFB and SAB were a little higher, so this estimate could be slightly conservative. Confidence in the forecast is reasonably high for the next 48 to 72 h. The depression should make a slow clockwise turn around Bermuda, steered by a mid-level ridge currently centered off the coast of the Carolinas. Warm SSTs should provide ample fuel to support at least slight strengthening during this period, despite expected northerly shear, and this is shown by all of the intensity guidance. The NHC forecast is very near the multi-model consensus for both track and intensity through that period. Beyond 72 h, the track and intensity models diverge quickly. The HWRF and COAMPS-TC models rapidly intensify the system to major hurricane strength. A stronger, deeper vortex would likely be steered generally westward through the end of the forecast period, influenced by a deeper-layer steering flow. However, the global models have a much weaker, shallower system which turns northward, and then possibly accelerates northeastward by day 5. As a result, the spread in the track guidance is nearly 800 n mi by day 5, with the GFS and HWRF taking the extreme positions to the northeast and southwest, respectively. The official track forecast takes a middle approach, and is between the various consensus aids at days 4 and 5. The NHC intensity forecast is slightly more conservative, and is below the consensus at the end of the period. Significant changes to the forecast may be required in subsequent advisory packages. FORECAST POSITIONS AND MAX WINDS INIT 16/0300Z 33.2N 62.7W 25 KT 30 MPH 12H 16/1200Z 32.4N 62.8W 30 KT 35 MPH 24H 17/0000Z 31.5N 63.3W 35 KT 40 MPH 36H 17/1200Z 31.1N 64.0W 40 KT 45 MPH 48H 18/0000Z 31.0N 64.9W 45 KT 50 MPH 60H 18/1200Z 31.0N 66.1W 50 KT 60 MPH 72H 19/0000Z 31.2N 67.4W 55 KT 65 MPH 96H 20/0000Z 32.0N 68.5W 55 KT 65 MPH 120H 21/0000Z 33.5N 68.0W 55 KT 65 MPH $$ Forecaster Zelinsky
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