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Tropical Depression Nineteen Forecast Discussion Number 3
2020-09-12 11:03:10| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Sat Sep 12 2020 958 WTNT44 KNHC 120903 TCDAT4 Tropical Depression Nineteen Discussion Number 3 NWS National Hurricane Center Miami FL AL192020 500 AM EDT Sat Sep 12 2020 Surface observations and Doppler radar data indicate that Tropical Depression Nineteen was close to tropical storm strength when it moved ashore in Miami-Dade county just after 06Z. The central pressure had dropped to near 1004 mb, and the radar showed winds of 45-50 kt above the surface just to the northeast of the center, associated with a strong convective burst. However, there were no surface observations of 35-kt sustained winds, and the highest reported gusts were in the 35-40 kt range. Based on the available data, the initial intensity remains 30 kt. It should be noted that since landfall, the strong convective burst near the low-level center has weakened considerably. The cyclone has turned left during the past several hours, and the initial motion is a somewhat uncertain 270/8 kt. The subtropical ridge extending from the southeastern United States eastward over the Atlantic should steer the cyclone generally west-northwestward through Sunday. After that time, the ridge is forecast to weaken due to an approaching mid-latitude trough. This should result in a northwestward motion through from Sunday night into the middle portion of next week, with the cyclone expected to reach the northern Gulf coast around the 96 h point. The latest track guidance now has the cyclone responding more strongly to the trough and turning northward by 120 h. The new forecast track is shifted a little south of the previous track through 72 h based on the initial position and motion. At 120 h, the new track is nudged to the east of the old track, but it still lies to the west of the consensus models. The depression is currently experiencing some northerly vertical wind shear, and the latest global model runs are showing more shear affecting the system as it crosses the Gulf of Mexico than earlier. However, since the cyclone is going to be over very warm sea surface temperatures and in a moist environment, the intensity guidance still shows it strengthening to near hurricane strength, or stronger, before it reaches the northern Gulf coast. Due to the uncertainty about the amount of shear, the new intensity forecast has only minor adjustments from the previous forecast. However, it lies below the various intensity consensus models. Users are reminded to not to focus on the exact details of the track or intensity forecast as the average NHC track error at 96 h is around 150 miles and the average intensity error is around 15 mph. In addition, winds, storm surge, and rainfall hazards will extend far from the center. KEY MESSAGES: 1. Tropical Depression Nineteen is expected to produce isolated to scattered flash flooding across portions of west-central and southern Florida and prolong existing minor river flooding across Central Florida through Sunday. Scattered flash flooding is also possible across portions of the central Gulf Coast Sunday through Tuesday morning. 2. Tropical storm conditions are possible by Sunday night in portions of the Florida Panhandle, where a Tropical Storm Watch has been issued. Wind gusts to tropical-storm force could occur over portions of the southern Florida Peninsula today. 3. The system is forecast to strengthen to near hurricane intensity by early next week as it moves across the northeastern Gulf of Mexico. Dangerous impacts from storm surge, wind, and heavy rainfall will be possible along the Gulf Coast from the Florida Panhandle to southeastern Louisiana late this weekend and early next week. Residents in these areas should monitor the progress of this system and updates to the forecast, as Storm Surge, Tropical Storm or Hurricane watches could be issued later today. FORECAST POSITIONS AND MAX WINDS INIT 12/0900Z 25.6N 80.7W 30 KT 35 MPH...INLAND 12H 12/1800Z 26.1N 82.0W 30 KT 35 MPH...OVER WATER 24H 13/0600Z 26.9N 83.8W 35 KT 40 MPH 36H 13/1800Z 27.9N 85.4W 40 KT 45 MPH 48H 14/0600Z 28.7N 86.6W 50 KT 60 MPH 60H 14/1800Z 29.3N 87.7W 60 KT 70 MPH 72H 15/0600Z 29.6N 88.5W 60 KT 70 MPH 96H 16/0600Z 30.5N 90.0W 55 KT 65 MPH...INLAND 120H 17/0600Z 31.5N 90.5W 25 KT 30 MPH...INLAND $$ Forecaster Beven
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