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Tropical Storm Willa Forecast Discussion Number 2
2018-10-20 16:46:36| National Hurricane Center (East Pacific)
Issued at 1000 AM CDT Sat Oct 20 2018 000 WTPZ44 KNHC 201446 TCDEP4 Tropical Storm Willa Discussion Number 2 NWS National Hurricane Center Miami FL EP242018 1000 AM CDT Sat Oct 20 2018 A GMI microwave pass over the depression at 0916 UTC revealed that a tight inner core has developed, with a cyan and pink ring noted in the low-level 37-GHz channel. Outer convective banding has also been increasing, and a consensus of the latest subjective and objective satellite intensity estimates supports raising the maximum winds to 35 kt and designating the cyclone as Tropical Storm Willa. On an historical note, this is the first time that at least 21 named storms have formed within the eastern Pacific basin since the 1992 season. The microwave data suggest that Willa's center is a little farther south than previously estimated, and the initial motion is calculated to be westward, or 270/8 kt. The storm is already located near the western periphery of a mid-level ridge which extends across northern and central Mexico, and Willa is expected to slowly recurve around the ridge axis during the next 4 days. By the end of the forecast period, a mid-latitude trough located over the western U.S. is likely to cause Willa to accelerate toward the northeast in the direction of mainland Mexico. All of the track models agree on this general scenario, but there are some differences on how wide a turn Willa makes while it recurves. There are also some speed differences that appear to be related to how strong Willa will be when the mid-latitude trough becomes the main steering driver. The NHC forecast is very close to the previous forecast during the first 48 hours. After that time, the forecast has been nudged eastward since the GFS and ECMWF models are both near or east of the multi-model consensus aids. With a low-level ring already observed in microwave imagery, and Willa located in an environment of low shear and over very warm ocean water, the cyclone appears poised to go through a period of rapid intensification (RI). Several of the various RI indices are over 50 percent, and thus the NHC forecast favors the high end of the intensity guidance and explicitly shows rapid strengthening over the next 48 hours. Willa is expected to be moving more slowly by days 3 and 4 (about 3 kt), and upwelling of colder water could induce some weakening by that time. An increase in shear is likely to cause more significant weakening by the end of the forecast period. The new NHC intensity forecast is a bit higher than the previous forecast to account for the possibility of rapid intensification, and it shifts Willa's expected peak intensity about a day earlier. FORECAST POSITIONS AND MAX WINDS INIT 20/1500Z 14.8N 105.7W 35 KT 40 MPH 12H 21/0000Z 15.0N 106.5W 50 KT 60 MPH 24H 21/1200Z 15.3N 107.3W 65 KT 75 MPH 36H 22/0000Z 15.8N 107.9W 80 KT 90 MPH 48H 22/1200Z 16.6N 108.5W 95 KT 110 MPH 72H 23/1200Z 17.7N 109.0W 100 KT 115 MPH 96H 24/1200Z 19.0N 109.0W 90 KT 105 MPH 120H 25/1200Z 22.0N 107.5W 75 KT 85 MPH $$ Forecaster Berg
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