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Potential Tropical Cyclone Nine Forecast Discussion Number 3
2020-07-29 04:53:21| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Tue Jul 28 2020 000 WTNT44 KNHC 290253 TCDAT4 Potential Tropical Cyclone Nine Discussion Number 3 NWS National Hurricane Center Miami FL AL092020 1100 PM AST Tue Jul 28 2020 Earlier wind data from an Air Force reconnaissance aircraft and a recent 0038Z ASCAT-A overpass indicate that the disturbance has not become any better organized since the previous advisory. There were indications of a circulation center located near the position estimate used in this advisory. However, there was a sharp cusp noted in the ASCAT wind field, and that was used for positioning the disturbance since it lies closest to the strong convective band and best upper-level divergence. The initial intensity of 35 kt is based on several ASCAT wind vectors of 35-36 kt located well north of the center. The 35-kt intensity is also consistent with a 0000Z TAFB Dvorak satellite intensity estimate of T2.5/35 kt. The initial motion estimate is an uncertain 295/22 kt. The ridge to the north of the disturbance is forecast to remain strong for the next 36-48 h, which keeps the system moving in a general west-northwestward direction across the Lesser Antilles tonight and Wednesday, and near or over the Greater Antilles Wednesday night and Thursday. For such a loosely organized system at this time, the models are in fairly good agreement on the large disturbance slowing down significantly after 48 h, reaching forward speeds of only 10-12 kt when it reaches the very warm waters of the Straits of Florida in 72-96 h. On days 4 and 5, the system is expected to slow even further and turn northward into a break in the subtropical ridge that is expected to develop across Florida and the Bahamas. The new NHC track forecast is similar to but a little south of the previous advisory track, mainly due to the more westward initial position, and lies along the southern portion of the guidance envelope near the middle of the consensus models. Regardless of the exact track, the system is expected to bring locally heavy rainfall to much of the Lesser Antilles, and tropical-storm-force winds to portions of the Leeward Islands, the Virgin Islands, and Puerto Rico during the next 24 hours. The intensity forecast remains problematic for primarily two reasons: 1) the lack of a well-defined center and inner-core wind field and 2) likely land interaction to some degree. In the short-term, a bonafide center could develop tonight in response to the expected development of intense convection caused by orographic forcing by the mountainous islands of the central and northern Leeward Islands. Once a center closes off, which has likely been inhibited from doing so due to the disturbance's fast forward speed in excess of 20 kt, the low-level convergence will improve and convection will become more organized and symmetrical, allowing for strengthening to occur. The main question is how much land interaction with Puerto Rico and Hispaniola will disrupt the circulation in the 36-48-hour period. Assuming the system remains intact after emerging off the coast of Hispaniola, the slow track over the very warm waters of the Straits of Florida would result in more strengthening, assuming the system doesn't interact with the Cuban landmass. Although the GFS-and ECMWF-based SHIPS intensity guidance shows considerable southwesterly vertical wind shear of 20-30 kt in the 72-96 h period when the disturbance is over the Straits, the global model fields show that this is self-induced shear caused by the SHIPS model incorporating the system's impressive upper-level outflow winds in its shear calculations. As a result, this is not being considered a negative intensity factor compared to land interaction. Due to aforementioned uncertainties, the new NHC intensity forecast remains on the conservative side, and lies between the slightly weaker IVCN and stronger NOAA-HCCA consensus models. Interests in Hispaniola, the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both the track and intensity are likely. Key Messages: 1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands, Puerto Rico, and the Dominican Republic. 2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands, Puerto Rico, and portions of the Dominican Republic beginning Wednesday and spreading westward through Thursday. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system. 3. The details of the long-range track and intensity forecasts are more uncertain than usual since the system does not have a well-defined center and it could move over portions of the Greater Antilles later this week. However, this system could bring some rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida by the end of the week. Interests there should monitor its progress and updates to the forecast over the next few days. FORECAST POSITIONS AND MAX WINDS INIT 29/0300Z 14.6N 59.4W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 29/1200Z 15.7N 62.3W 40 KT 45 MPH 24H 30/0000Z 17.4N 65.9W 45 KT 50 MPH 36H 30/1200Z 18.7N 69.3W 45 KT 50 MPH...INLAND 48H 31/0000Z 20.1N 72.6W 45 KT 50 MPH...OVER WATER 60H 31/1200Z 21.5N 75.5W 50 KT 60 MPH 72H 01/0000Z 22.8N 77.7W 55 KT 65 MPH 96H 02/0000Z 25.7N 80.7W 55 KT 65 MPH...INLAND 120H 03/0000Z 27.9N 82.0W 45 KT 50 MPH...INLAND $$ Forecaster Stewart
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