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Tropical Storm Sergio Forecast Discussion Number 6
2018-09-30 22:35:33| National Hurricane Center (East Pacific)
Issued at 300 PM MDT Sun Sep 30 2018 110 WTPZ41 KNHC 302035 TCDEP1 Tropical Storm Sergio Discussion Number 6 NWS National Hurricane Center Miami FL EP212018 300 PM MDT Sun Sep 30 2018 Satellite imagery over the past several hours as well as microwave and scatterometer data indicate that the center of Sergio is farther south than previously analyzed. The deep convection near the center has become ragged in appearance and elongated from east-west. However, microwave data continues to show a well-defined mid-level circulation, and satellite intensity estimates are unanimously supporting 55 kt, which will be the initial intensity for this advisory. The initial motion estimate is now 260/11 kt, partially due to adjustment of the center farther south. Otherwise, there are no changes in the forecast track philosophy from the previous advisory. 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 extending from Mexico westward across the eastern Pacific. By 48 hours, a mid- to upper-level trough moving southeastward along and offshore of the western U.S. will erode the western part of the ridge. This will allow the cyclone to turn west-northwestward to northwestward from 48- to 96-hours with a gradual decrease in forward speed. Very near the end of the forecast period, 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 by 120 hours. Despite the current ragged appearance of the cyclone, the environment of low-shear, high-moisture, and warm waters are expected to support a period of rapid intensification sometime within the next 36 hours or so. By 36 hours, northeasterly shear is expected to develop over the storm 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 for the remainder of the forecast period. There may be a window of time beginning after 72 hours where some additional strengthening could occur. By 96 hours, the system will then be moving over waters of 26-27 C very near a gradient of cooler SSTs just to the north. The current intensity forecast calls for a steadying of intensity after the shear abates, then a slow weakening trend commences by the end of the period due to cooler SSTs. Other than a slight delay in Sergio becoming a hurricane, the intensity forecast is essentially the same as the previous forecast and remains close to the dynamical model consensus. Based on this intensity forecast, Sergio is still forecast to become a major hurricane on Tuesday. FORECAST POSITIONS AND MAX WINDS INIT 30/2100Z 11.5N 106.3W 55 KT 65 MPH 12H 01/0600Z 11.4N 108.1W 65 KT 75 MPH 24H 01/1800Z 11.2N 110.4W 80 KT 90 MPH 36H 02/0600Z 11.4N 112.6W 95 KT 110 MPH 48H 02/1800Z 11.9N 114.5W 105 KT 120 MPH 72H 03/1800Z 13.8N 117.5W 100 KT 115 MPH 96H 04/1800Z 16.0N 120.0W 100 KT 115 MPH 120H 05/1800Z 17.3N 122.0W 90 KT 105 MPH $$ Forecaster Latto/Cangialosi
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