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Tropical Storm Sergio Forecast Discussion Number 4
2018-09-30 10:54:29| National Hurricane Center (East Pacific)
Issued at 400 AM CDT Sun Sep 30 2018 130 WTPZ41 KNHC 300854 TCDEP1 Tropical Storm Sergio Discussion Number 4 NWS National Hurricane Center Miami FL EP212018 400 AM CDT Sun Sep 30 2018 Microwave and infrared satellite images, along with scatterometer wind data, indicate that Sergio has continued to become better organized, with strong convection now more concentrated near the center. However, scatterometer data showed that the inner-core wind field is still rather loose and not particularly well-defined quite yet. The initial intensity has been increased to 45 kt based on a Dvorak satellite intensity estimate of T3.0/45 kt from TAFB and SAB, which is supported by a 0536 UTC ASCAT pass that contained a few 45-kt vectors east of the center. The initial motion estimate is 270/10 kt. There is no significant change to the previous forecast track or reasoning. For the next 48 hours or so, Sergio should move generally westward along the southern periphery of a large, deep-layer subtropical ridge that extends from Mexico westward across the eastern and central Pacific. By 72 hours, a broad mid- to upper-level trough is forecast to drop southward and southeastward over the southwestern U.S. and the northeastern Pacific, producing a break in the ridge which will allow Sergio to move toward the northwest at a slower forward speed through 120 hours. The new NHC track forecast is similar to but slightly south of the previous advisory track, and lies close to an average of the consensus models TVCE, HCCA, and FSSE. Sergio is forecast to remain in a low-shear, warm-water, and high-moisture environment for the next 36 hours or so, during which time rapid intensification is expected. In the 48- to 72-hour period, the vertical shear is forecast by the GFS and ECMWF global models to increase from the northeast at around 25 kt, which should act to cap the strengthening process, and possibly even induce some weakening. By 96 and 120 hours, the shear is expected to decrease to 5 kt or less, which would favor re-strengthening. However, due to the uncertainty in how much Sergio's inner-core wind field will be disrupted by the aforementioned strong shear, the intensity forecast is simply leveled off at 95 kt at 72 hours and beyond. The official intensity forecast is slightly higher than the previous advisory and closely follows the IVCN consensus model, which is lower than the more robust HCCA and FSSE models, which bring Sergio to near category 4 strength in 36-48 hours. FORECAST POSITIONS AND MAX WINDS INIT 30/0900Z 12.0N 104.3W 45 KT 50 MPH 12H 30/1800Z 12.0N 105.9W 60 KT 70 MPH 24H 01/0600Z 11.9N 108.2W 75 KT 85 MPH 36H 01/1800Z 11.6N 110.4W 90 KT 105 MPH 48H 02/0600Z 11.7N 112.4W 100 KT 115 MPH 72H 03/0600Z 13.0N 115.9W 95 KT 110 MPH 96H 04/0600Z 15.2N 118.6W 95 KT 110 MPH 120H 05/0600Z 17.1N 120.8W 95 KT 110 MPH $$ Forecaster Stewart
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