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Tropical Storm Sergio Forecast Discussion Number 8
2018-10-01 10:54:10| National Hurricane Center (East Pacific)
Issued at 300 AM MDT Mon Oct 01 2018 428 WTPZ41 KNHC 010854 TCDEP1 Tropical Storm Sergio Discussion Number 8 NWS National Hurricane Center Miami FL EP212018 300 AM MDT Mon Oct 01 2018 Sergio's convective pattern in infrared satellite imagery has waxed and waned since the previous advisory. A previously large, solid curved band has become broken during the past couple of hours, but the comma head or CDO of the cloud band has increased in size while cloud tops have cooled to near -80C. Passive microwave imagery indicates that within the comma head feature an 18-20-nmi-wide closed eye has developed in the low-levels, while the mid- and upper-level eye remains open and ragged. Subjective intensity estimates remain T3.5/55 kt from TAFB and SAB. However, based on the well-defined, low-level eye, the intensity has been nudged upward to 60 kt for this advisory. The initial motion is now 270/12 kt, based primarily on microwave satellite fix positions. Sergio is forecast to move generally westward for the next 36 hours or so, under the influence of a strong, deep-layer ridge located to the north of the cyclone. By 48 hours, a mid- to upper-level trough moving southeastward along and offshore of the western U.S. is expected to gradually erode the western part of the ridge, allowing Sergio turn west-northwestward on day 2 and move northwestward on days 3-5. The new NHC track forecast is similar to the previous advisory track, and lies near a blend of the consensus models TCVA, TVCE, and HCCA. Both the statistical and dynamical intensity models have been too bullish on Sergio strengthening during the past 36 hours. Although the environment is quite moist with near 80 percent mid-level humidity values, a pronounced dry intrusion eroding the inner-core convection remains. By the time the dry air mixes out, modest northerly vertical wind shear is expected to hinder development in the 24-48 hour period. By 72 hours and beyond, the shear is forecast to decrease below 10 kt, but Sergio will be moving over cooler waters at that time. Given the mixed environmental signals, the official intensity is on the conservative side and remains well below the stronger HCCA and FSSE models, and closer to the simple consensus model IVCN. FORECAST POSITIONS AND MAX WINDS INIT 01/0900Z 11.5N 108.6W 60 KT 70 MPH 12H 01/1800Z 11.4N 110.3W 70 KT 80 MPH 24H 02/0600Z 11.5N 112.5W 85 KT 100 MPH 36H 02/1800Z 12.0N 114.5W 95 KT 110 MPH 48H 03/0600Z 12.8N 116.1W 100 KT 115 MPH 72H 04/0600Z 14.9N 118.8W 100 KT 115 MPH 96H 05/0600Z 16.8N 121.2W 95 KT 110 MPH 120H 06/0600Z 17.9N 123.2W 90 KT 105 MPH $$ Forecaster Stewart
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