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Tropical Storm Sergio Forecast Discussion Number 5
2018-09-30 16:58:05| National Hurricane Center (East Pacific)
Issued at 1000 AM CDT Sun Sep 30 2018 200 WTPZ41 KNHC 301457 TCDEP1 Tropical Storm Sergio Discussion Number 5 NWS National Hurricane Center Miami FL EP212018 1000 AM CDT Sun Sep 30 2018 Recent microwave data indicate that the inner core of Sergio continues to become better defined, with an eye-like feature evident. However, this eye feature is not yet apparent in geostationary satellite imagery. Deep convection with cloud top temperatures of -70 C are concentrated around the storm's center as well as along a prominent banding feature on the south side. The average of available satellite intensity estimates support an intensity of 50 kt, which will be the initial intensity for this advisory. The initial motion estimate is 270/11 kt. There continues to be no significant changes in the forecast track philosophy from the previous few forecasts. For the next couple of days, Sergio should move generally westward or just south of due west around the southern periphery of a large deep-layer subtropical ridge that extends from Mexico westward across the eastern Pacific. By 48 hours, a mid- to upper-level trough will move southeastward along and offshore the western U.S., which will erode the ridge from the west. This will allow the cyclone to turn west-northwestward then northwestward from 48- to 96-hours with a gradual decrease in forward speed. Thereafter, the trough should shift eastward and low- to mid-level ridging should build to the northwest of the cyclone. This pattern change should induce a more westward turn toward the end of the forecast period. Sergio will remain in an environment of low-shear, high-moisture, and over warm waters for the next 24 hours or so, which should result in rapid intensification. This is in agreement with rapid intensification indices from the SHIPS model of 40 to 45 percent over the next 24 hours. By 36 hours, northeasterly shear is expected to develop over the storm environment and increase to 25 kt by 48 hours. This should at least limit the rate of intensification if not weaken the cyclone slightly from 48- to 72-hours. After that time, global models indicate that the shear will lessen to under 10 kt by 96 hours, but by that time the system will be moving over waters of 26-27 C very near a gradient of cooler SSTs just to the north. Based on this, the intensity remains steady from 72- to 96-hours and then shows slow weakening by the end of the period. The intensity forecast is a little higher that the previous advisory, and is close to the dynamical model consensus. Based on this intensity forecast, Sergio is forecast to become a major hurricane by Tuesday. FORECAST POSITIONS AND MAX WINDS INIT 30/1500Z 12.1N 105.4W 50 KT 60 MPH 12H 01/0000Z 12.1N 107.0W 65 KT 75 MPH 24H 01/1200Z 11.8N 109.2W 80 KT 90 MPH 36H 02/0000Z 11.7N 111.4W 95 KT 110 MPH 48H 02/1200Z 12.0N 113.5W 105 KT 120 MPH 72H 03/1200Z 13.6N 116.6W 100 KT 115 MPH 96H 04/1200Z 16.0N 119.0W 100 KT 115 MPH 120H 05/1200Z 17.5N 121.5W 90 KT 105 MPH $$ Forecaster Latto/Cangialosi
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