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Tropical Depression FOUR-E Forecast Discussion Number 1
2015-07-08 04:44:48| National Hurricane Center (East Pacific)
Issued at 800 PM PDT TUE JUL 07 2015 000 WTPZ44 KNHC 080244 TCDEP4 TROPICAL DEPRESSION FOUR-E DISCUSSION NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL EP042015 800 PM PDT TUE JUL 07 2015 Visible satellite images late this afternoon indicate that curved bands of convection have developed around the center of the well-defined low pressure system located more than 1000 n mi east-southeast of the Hawaiian Islands. Therefore the system now qualifies as a tropical cyclone, and advisories are being initiated at this time. SAB/TAFB Dvorak satellite intensity estimates range from T1.5/25 kt to T2.5/35 kt, and the system has a pattern T-number of T2.0, so the initial intensity is set at 30 kt. The initial motion estimate is 300/15 kt. During the next 72 hours, the cyclone is forecast to move in a general west-northwestward direction along the southern periphery of a strong deep-layer ridge located to its north. By 96 hours, the system is expected to weaken and become more vertically shallow, and be steered westward by the low-level easterly trade wind flow. The models are in general agreement through 72 hours, but then diverge significantly after that, with most of the NHC guidance moving the cyclone or its remnants west-northwestward to northwestward at 96 and 120 hours. The exception is the ECMWF model, which turns the system west-southwestward by 96 hours and beyond. The official forecast track is similar to the consensus model TVCE through 72 hours, and then follows the ECMWF trend after that since this model maintains a larger and more realistic vortex on days 4 and 5. The SHIPS guidance indicates that the deep-layer vertical wind shear is expected to remain below 10 kt while the system is over 26C or greater sea-surface temperatures for the next 24 hours or so, which should allow for gradual strengthening into a tropical storm. By 48 hours, the shear is forecast to increase to more than 20 kt from the southwest, which should cap any intensification and induce a steady weakening trend after that. However, the GFS-based SHIPS model is forecasting stronger vertical shear than the ECMWF model and, as a result, shows complete dissipation of the cyclone by 96 hours. Given the reliability of the ECMWF, the official intensity forecast has incorporated a blend of these two models' shear computations, and maintains the system as a tropical cyclone through 96 hours, and a remnant low at 120 hours. This scenario seems more likely given the rather large size of the circulation, which will make the vortex more shear resistant and also take longer to spin down and dissipate. The depression has crossed 140W longitude as of the 0300 UTC advisory time, so this will be the last advisory issued by the National Hurricane Center on this system. Future information on the depression will be provided by the Central Pacific Hurricane Center in Honolulu Hawaii. FORECAST POSITIONS AND MAX WINDS INIT 08/0300Z 15.4N 140.2W 30 KT 35 MPH 12H 08/1200Z 16.6N 142.2W 35 KT 40 MPH 24H 09/0000Z 17.6N 144.5W 40 KT 45 MPH 36H 09/1200Z 18.6N 146.5W 45 KT 50 MPH 48H 10/0000Z 19.6N 148.3W 45 KT 50 MPH 72H 11/0000Z 21.5N 152.2W 35 KT 40 MPH 96H 12/0000Z 22.7N 156.3W 30 KT 35 MPH 120H 13/0000Z 23.0N 160.0W 25 KT 30 MPH...POST-TROP/REMNT LOW $$ Forecaster Stewart
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