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Tropical Depression Eighteen-E Forecast Discussion Number 1

2017-09-23 22:43:23| National Hurricane Center (East Pacific)

Issued at 400 PM CDT Sat Sep 23 2017 000 WTPZ43 KNHC 232042 TCDEP3 Tropical Depression Eighteen-E Discussion Number 1 NWS National Hurricane Center Miami FL EP182017 400 PM CDT Sat Sep 23 2017 ASCAT-A/-B scatterometer data from around 16-17Z indicated that the low pressure system located just offshore of the southwestern coast of Mexico that the NHC has been tracking the past several days has become much better defined, and it also possessed surface winds of 30-33 kt. As a result, the system has been upgraded to a tropical depression, the eighteenth of the eastern Pacific hurricane season. The initial motion estimate is 335/04, based primarily on microwave satellite fixes. The latest NHC model guidance is in fairly good agreement on the cyclone moving slowly in a general north-northwestward direction around the western periphery of a deep-layer ridge for the next 5 days. Some of the models like the GFS, Canadian, and HCCA take the system just inland near Cabo Corrientes in about 24 hours, whereas the remainder of the guidance, especially the UKMET and ECMWF, keep the cyclone just offshore of the southwestern coast of Mexico. The forecast motion for the next few days is expected to be 5 kt or less, an indication that steering currents will be weak. Since there is no strong forcing that would want to drive the depression inland over the mountainous terrain of southwestern Mexico, the official forecast calls for the center of the cyclone to remain just offshore of the coast throughout the forecast period, similar to the ECMWF and UKMET solutions. Given the well-defined circulation noted in the aforementioned scatterometer data, along with vertical wind shear decreasing to less than 10 kt by 24 hours, steady strengthening is forecast for the next 24-36 hours. Afterwards, southeasterly to southerly shear is expected to gradually increase to 25 kt by 72 hours and more than 30 kt in the 96-120 hour period, which should induce steady to possible rapid weakening. The NHC intensity forecast is above most of the guidance through 48 hours, and then is a little lower than the guidance after that. Due to the depression being forecast to become a tropical storm by tonight, along with its proximity to the coast of Mexico, a tropical storm warning has been issued from Manzanillo northward to El Roblito, including the Islas Marias. Heavy rainfall causing flash floods and mudslides will also be possible within the warning area and extending well inland. FORECAST POSITIONS AND MAX WINDS INIT 23/2100Z 18.4N 105.3W 30 KT 35 MPH 12H 24/0600Z 18.8N 105.5W 40 KT 45 MPH 24H 24/1800Z 19.4N 105.7W 50 KT 60 MPH 36H 25/0600Z 20.2N 106.0W 50 KT 60 MPH 48H 25/1800Z 21.2N 106.2W 45 KT 50 MPH 72H 26/1800Z 22.7N 107.0W 40 KT 45 MPH 96H 27/1800Z 24.0N 108.4W 35 KT 40 MPH...POST-TROPICAL 120H 28/1800Z 26.0N 110.0W 30 KT 35 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart

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