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Tropical Storm Sam Forecast Discussion Number 6
2021-09-24 04:40:34| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Thu Sep 23 2021 000 WTNT43 KNHC 240240 TCDAT3 Tropical Storm Sam Discussion Number 6 NWS National Hurricane Center Miami FL AL182021 1100 PM AST Thu Sep 23 2021 Microwave data from around the time of the previous advisory revealed that Sam has developed a well-defined inner core, including a nearly closed eyewall feature in the 91-GHz channel. Dvorak estimates from TAFB and SAB have increased to T3.5 and T4.0, respectively, and the initial intensity is therefore estimated to be 60 kt. Sam's 24-hour intensity change qualifies as rapid intensification, since the cyclone was only a 30-kt tropical depression at this time yesterday. Sam is small, however. Recent scatterometer data showed that tropical-storm-force winds only extend a maximum of 40 n mi from the center, and the radius of maximum winds is estimated to be about 15 n mi. This small size is likely to have implications on Sam's future intensity. Sam is moving a little slower toward the west (280/13 kt). A mid-tropospheric ridge is expected to build across the Caribbean Sea and extend northeastward over the central Atlantic during the next few days. This blocking high is expected to cause Sam to slow down to 6 to 7 kt by late Saturday and Sunday while it maintains a westward to west-northwestward heading. Global models suggest the ridge may weaken and shift eastward a bit by days 4 and 5, allowing Sam to turn to the northwest and speed up a little by the end of the forecast period. The ECMWF, HWRF, and HCCA models lie along the southern edge of the main pack of models, while the ECENS ensemble mean is even farther south. The GFS and the HMON are on the northern side. Partially due to an adjustment of the initial position, the new NHC track forecast has been shifted southward a bit from the previous forecast, and it lies just south of the TVCA consensus aid--but not as far to the left as the previously mentioned southern models. Environmental conditions and Sam's structure and size are ideal for continued rapid intensification (RI). The SHIPS RI index for a 30-kt increase in 24 hours is nearly 50 percent. The DTOPS methodology, which uses inputs from the statistical-dynamical and deterministic models, shows RI indices for various forecast periods exceeding 90 percent. Therefore, rapid strengthening is being forecast at least for the next 36 hours, with Sam becoming a hurricane very soon and then a major hurricane by Friday night or Saturday morning. One caveat to this forecast is that although Sam's small size more readily allows for RI to occur, it can also make the cyclone more prone to weakening if, for example, vertical shear increases. By 48 hours, the intensity models show the strengthening trend leveling off, and the NHC intensity forecast follows suit. Fluctuations in intensity from days 3 through 5 are likely, both due to normal internal dynamics within the storm and the cyclone's response to the surrounding environment in relation to its small size. FORECAST POSITIONS AND MAX WINDS INIT 24/0300Z 11.2N 40.9W 60 KT 70 MPH 12H 24/1200Z 11.4N 42.9W 75 KT 85 MPH 24H 25/0000Z 11.8N 45.0W 90 KT 105 MPH 36H 25/1200Z 12.1N 46.6W 105 KT 120 MPH 48H 26/0000Z 12.5N 47.8W 110 KT 125 MPH 60H 26/1200Z 13.0N 49.1W 110 KT 125 MPH 72H 27/0000Z 13.5N 50.4W 110 KT 125 MPH 96H 28/0000Z 15.3N 53.2W 110 KT 125 MPH 120H 29/0000Z 17.3N 56.1W 110 KT 125 MPH $$ Forecaster Berg
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