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Tropical Depression Thirteen Forecast Discussion Number 5
2020-08-21 04:49:47| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Thu Aug 20 2020 000 WTNT43 KNHC 210249 TCDAT3 Tropical Depression Thirteen Discussion Number 5 NWS National Hurricane Center Miami FL AL132020 1100 PM AST Thu Aug 20 2020 A NOAA Hurricane Hunter aircraft has been flying all around the circulation of the depression this evening, providing valuable information on the low-level structure. The strongest winds are primarily northeast of the center, with an elongated surface circulation and a mid-level swirl near the southern end. While the plane couldn't locate a definite center, there's enough uncertainty and curvature in the plane's wind field data to hold onto the system as 30-kt tropical depression for now. Another mission should be in the area around 1200 UTC. A late-arriving scatterometer pass confirms both the disorganization of the cyclone and the maximum winds. The initial motion continues about the same as before, 290/19. A strong subtropical ridge over the western Atlantic should remain north of the depression during the next few days, steering the cyclone at a fast pace to the west-northwest. After the weekend, the ridge weakens some over the eastern Gulf of Mexico, leading to depression probably turning more to the northwest. The guidance has trended to the south and west after 24 hours tonight, led by the GFS and HWRF models. I'm not inclined to make significant changes since the models are about to ingest the information from the reconnaissance mission, plus the initial disorganization of the center, but the new track is still adjusted slightly to the south and west, north of much of the guidance. The depression should be moving through mixed conditions over the next few days. While the water is very warm, GFS forecasts show that there is significant mid-level shear that could continue to mix in nearby dry air toward the center. This shear will probably relax late this weekend or early next week, but there is very poor agreement on how much shear remains and the timing of this event. In addition, a track any farther south would result in potentially mountainous land interaction, which also increases the intensity forecast uncertainty. No significant changes were made to the previous wind speed forecast, and the overall confidence in both the track and intensity forecasts remain lower than normal. Key Messages: 1. Tropical storm conditions are possible across portions of the northern Leeward Islands, the Virgin Islands, and Puerto Rico Friday night and Saturday, and Tropical Storm Watches have been issued for some of these islands. Heavy rainfall is likely across this area beginning late Friday and could cause mudslides and flash and urban flooding through Sunday. 2. The details of the long-range track and intensity forecasts are more uncertain than usual since the system could move over portions of the Greater Antilles this weekend. However, this system could bring some storm surge, rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida this weekend and early next week. Interests there should monitor this system's progress and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 21/0300Z 17.3N 56.4W 30 KT 35 MPH 12H 21/1200Z 17.9N 59.1W 30 KT 35 MPH 24H 22/0000Z 18.6N 62.4W 35 KT 40 MPH 36H 22/1200Z 19.4N 65.7W 40 KT 45 MPH 48H 23/0000Z 20.3N 69.2W 45 KT 50 MPH 60H 23/1200Z 21.3N 72.8W 50 KT 60 MPH 72H 24/0000Z 22.6N 76.4W 55 KT 65 MPH 96H 25/0000Z 26.0N 82.5W 65 KT 75 MPH 120H 26/0000Z 29.0N 85.5W 65 KT 75 MPH $$ Forecaster Blake
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