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Tropical Storm Irwin Forecast Discussion Number 4
2017-07-23 10:53:43| National Hurricane Center (East Pacific)
Issued at 200 AM PDT Sun Jul 23 2017 000 WTPZ45 KNHC 230853 TCDEP5 Tropical Storm Irwin Discussion Number 4 NWS National Hurricane Center Miami FL EP102017 200 AM PDT Sun Jul 23 2017 An ASCAT-B overpass at 0516 UTC showed several vectors of 32-33 kt winds and a max value of 34 kt in the northeast quadrant of the tropical cyclone. Based on that data, the system has been named Tropical Storm Irwin. Irwin is the fifth system to be named in the eastern North Pacific so far in July. Although the system has been upgraded based on the ASCAT data, the structure has not improved. Shortwave-IR imagery shows that the low-level center is clearly exposed to the northwest of the deep convection, and Dvorak intensity estimates from TAFB and SAB have not increased. The global models suggest that the shear should decrease quite a bit after about 12 to 24 hours, which should allow for some intensification. The regional dynamical models HWRF and COAMPS-TC depict Irwin quickly recovering from its current sheared state and intensifying to a category 2 or 3 hurricane. On the other hand, the statistical models still only show modest strengthening, and keep Irwin below hurricane strength. As a course of least regret, the intensity forecast splits these two scenarios, but slightly favors the lower statistical guidance since the dynamical models may show the structure of Irwin recovering from the current shear too quickly. Near the end of the forecast period, Irwin is expected to encounter a higher shear environment, due in part to outflow from Hilary to the east, so the forecast depicts gradual weakening. The initial motion is 280/6 kt. Irwin continues to be steered by a low- to mid-level ridge to the northwest. Although no significant changes have been made to the track forecast, confidence is still somewhat low since there is a fair amount of speed-spread in the models. The GFS remains an outlier in being the only model that shows substantial binary interaction with Hilary, and is thus the farthest east, though the 00 UTC ECMWF is also a little slower than before. The official forecast is a little slower at the end of forecast period than the previous advisory, but still assumes that any significant track deviation due to interactions with Hilary will occur beyond 120 h. FORECAST POSITIONS AND MAX WINDS INIT 23/0900Z 14.7N 115.2W 35 KT 40 MPH 12H 23/1800Z 14.8N 116.0W 40 KT 45 MPH 24H 24/0600Z 14.9N 117.0W 45 KT 50 MPH 36H 24/1800Z 15.0N 118.0W 50 KT 60 MPH 48H 25/0600Z 15.0N 118.8W 60 KT 70 MPH 72H 26/0600Z 14.7N 121.0W 65 KT 75 MPH 96H 27/0600Z 14.5N 123.1W 60 KT 70 MPH 120H 28/0600Z 14.0N 125.5W 55 KT 65 MPH $$ Forecaster Zelinsky
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