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Tropical Storm Kate Forecast Discussion Number 13
2021-08-31 10:44:43| National Hurricane Center (Atlantic)
Issued at 500 AM AST Tue Aug 31 2021 000 WTNT45 KNHC 310844 TCDAT5 Tropical Storm Kate Discussion Number 13 NWS National Hurricane Center Miami FL AL102021 500 AM AST Tue Aug 31 2021 Kate remains a strongly sheared tropical storm with the associated deep convection occurring in intermittent bursts in the eastern semicircle of the cyclone, owing to 850-200-mb west-northwesterly vertical wind shear of at least 30 kt. The most recent Dvorak satellite current intensity (CI) estimates remain at 35 kt, and that is the initial intensity set for this advisory. This intensity is consistent with an earlier SSMI-S passive microwave satellite overpass that showed wind speeds of 30-35 kt on the west side of Kate where no convection/rain was present. The initial motion estimate is 360/04 kt. Kate's forward motion has slowed to as low 2 kt during the psst 6 hours, likely due to the hindering effects of the west-northwesterly shear direction and weak steering currents since Kate has recently moved into a weakness in the low- mid-level subtropical ridge. However, the latest NHC track guidance is in good agreement that the cyclone will begin moving northward at a slightly faster forward speed as Kate moves through a break in the ridge. In about 24 hours, a mid-level ridge to the northeast and east of Kate is forecast to build westward, forcing the cyclone on a more northwestward trajectory that should continue through Thursday. By Friday, a mid- to upper-level trough moving eastward over the western Atlantic is forecast to gradually lift out Kate toward the north, and accelerate the cyclone toward the northeast on Saturday. The new NHC forecast track is similar to but slightly east or right of the previous advisory track, and lies near the left edge of the tightly packed simple- and corrected-consensus track-model envelope. The intensity forecast is not as straightforward as the track forecast. The aforementioned hostile shear conditions are predicted by the regional and global models, especially the ECMWF model, to gradually abate over the next 18-24 hours, with the magnitude of the shear to decrease to less than 10 kt by 24-36 hours when Kate will be moving over 28.5 deg C water temperatures. If Kate can survive the next 18 hours or so, which is possible since the 850-300-mb shear is much less at only 10-15 kt, then some restrengthening could occur, especially in the 24-48-hour period when the cyclone will be located underneath a small upper-level anticyclone. Countering that, however, is the somewhat dry air (near-50-percent humidity) in the mid-levels of the atmosphere that could limit the formation of inner-core convection. Thus for now, the new official intensity forecast remains similar to the previous advisory, which shows Kate remaining as a low-end tropical storm through the 96-hour period. This scenario is consistent with the IVCN and HCCA intensity consensus models. On day 5, Kate is forecast to dissipate when the cyclone or its remnants are expected to merge with an extratropical low and associated frontal system. FORECAST POSITIONS AND MAX WINDS INIT 31/0900Z 23.0N 50.9W 35 KT 40 MPH 12H 31/1800Z 23.9N 50.9W 35 KT 40 MPH 24H 01/0600Z 25.2N 51.5W 35 KT 40 MPH 36H 01/1800Z 26.6N 52.4W 35 KT 40 MPH 48H 02/0600Z 28.3N 53.4W 35 KT 40 MPH 60H 02/1800Z 29.9N 54.2W 35 KT 40 MPH 72H 03/0600Z 31.5N 54.8W 35 KT 40 MPH 96H 04/0600Z 35.1N 53.6W 35 KT 40 MPH 120H 05/0600Z...DISSIPATED $$ Forecaster Stewart
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