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Tropical Depression Sixteen Forecast Discussion Number 3
2017-10-05 04:48:43| National Hurricane Center (Atlantic)
Issued at 1100 PM EDT Wed Oct 04 2017 000 WTNT41 KNHC 050248 TCDAT1 Tropical Depression Sixteen Discussion Number 3 NWS National Hurricane Center Miami FL AL162017 1100 PM EDT Wed Oct 04 2017 There have been some structural changes to the depression during the past few hours. Inner-core convection began developing just after the issuance of the previous advisory, and Colombian radar images from San Andres are now showing a well-defined convective band to the east and southeast of the center. Despite these changes, Dvorak satellite estimates remain T2.0 from TAFB and SAB, so the initial intensity is held at 30 kt. The depression probably only has another 12 hours or so before its center moves inland over northeastern Nicaragua, but it is still expected to reach tropical storm strength before that happens. Once the center re-emerges over the waters of the northwestern Caribbean Sea a little after 24 hours from now, high oceanic heat content and low shear should contribute to strengthening. Despite these favorable conditions, the amount of strengthening will be unclear until we know how well the inner core survives crossing over Nicaragua and Honduras. Strengthening is likely to continue through at least day 3 up until the time the cyclone reaches the central Gulf of Mexico. After day 3, there are some indications that higher shear and/or cooler shelf waters over the northern Gulf of Mexico could lead to some weakening, but that scenario is by no means a definite one at this time. Needless to say, there continues to be greater-than-normal uncertainty in the intensity forecast. The updated NHC intensity forecast has been adjusted downward just a bit through day 3 to follow an overall shift in the guidance, although it should be noted that the official forecast still lies above the normally skillful HCCA model. If the intensity forecast is complex, the track forecast is not much easier. For the first 48 hours, the models appear split on how the depression will interact with a disturbance currently located near the Straits of Florida. For example, the ECMWF model shows some interaction with the disturbance's low-level vorticity, which swings the depression more to the east on the right side of the guidance envelope. The GFS, on the other hand, shows no such interaction and has the cyclone on the western side of the guidance envelope. This setup has significant downstream effects after 48 hours because it keeps the ECMWF on an eastern route and the GFS on a western route as the cyclone heads toward the U.S. Gulf coast. The new NHC track forecast has been shifted slightly westward, although it is still not as far west as the consensus aids or the HCCA model. Interestingly, although the ECMWF ensemble mean is close to the operational run on the eastern side of the guidance envelope, there is a high density of members to the left close to the consensus aids, which lends additional support for the westward adjustment. A G-IV mission and Florida special soundings will begin tomorrow to better determine the synoptic steering flow around the cyclone. KEY MESSAGES: 1. The depression is forecast to strengthen and bring tropical storm conditions to portions of Nicaragua and Honduras through early Friday. Heavy rainfall could produce life-threatening flash flooding and mud slides in portions of Nicaragua, Honduras, Costa Rica, and Panama through Friday night. 2. The system could be near hurricane intensity when it approaches the Yucatan Peninsula late Friday or Saturday, bringing direct impacts from wind, storm surge, and heavy rainfall, and a hurricane watch has been issued for a portion of this area. 3. The system is forecast to continue strengthening over the Gulf of Mexico and could affect portions of the northern Gulf Coast as a hurricane this weekend, with direct impacts from wind, storm surge, and heavy rainfall. However, it is too early to specify the timing, location, or magnitude of these impacts. Residents along the Gulf Coast from Louisiana to Florida should monitor the progress of this system for the next several days and heed any advice given by local officials. FORECAST POSITIONS AND MAX WINDS INIT 05/0300Z 12.8N 82.7W 30 KT 35 MPH 12H 05/1200Z 13.5N 83.2W 35 KT 40 MPH 24H 06/0000Z 15.1N 83.7W 35 KT 40 MPH...INLAND 36H 06/1200Z 17.6N 84.7W 45 KT 50 MPH 48H 07/0000Z 20.2N 86.3W 55 KT 65 MPH 72H 08/0000Z 25.5N 88.6W 70 KT 80 MPH 96H 09/0000Z 30.5N 87.0W 65 KT 75 MPH...INLAND 120H 10/0000Z 36.5N 82.5W 30 KT 35 MPH...POST-TROP/INLAND $$ Forecaster Berg
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