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Tropical Storm Peter Forecast Discussion Number 6
2021-09-20 04:51:09| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Sep 19 2021 000 WTNT41 KNHC 200251 TCDAT1 Tropical Storm Peter Discussion Number 6 NWS National Hurricane Center Miami FL AL162021 1100 PM AST Sun Sep 19 2021 Peter is a sheared tropical cyclone. Satellite imagery and microwave data indicate that an earlier convective burst near the center of Peter has collapsed tonight. Although the low-level center is now displaced at least 60 n mi west of the edge of the convective cloud mass, an Air Force Reserve Hurricane Hunter aircraft investigating Peter has found the cyclone is a bit stronger tonight. The aircraft measured flight-level (925 mb) winds of 58 kt and SFMR winds of 45 kt or so. These data support raising the initial intensity to 45 kt for this advisory. Peter's wind field is very asymmetric, with tropical-storm-force winds extending outward up to 120 n mi from the center only in its northeast quadrant. The moderate to strong southwesterly vertical wind shear that is plaguing the system is forecast to persist during the next several days. Thus, despite sufficient oceanic heat content along its forecast track, the official NHC intensity forecast does not show any further intensification. In fact, some gradual weakening is forecast since the system appears likely to struggle sustaining organized convection near its center, as suggested by GFS and ECMWF model simulated satellite imagery. The official NHC intensity forecast has been adjusted upward in the near-term to account for the stronger initial intensity, but otherwise closely follows the HCCA and IVCN aids and shows gradual weakening this week. While the official NHC forecast shows Peter remaining a tropical cyclone through the forecast period, the GFS suggests Peter could struggle to even maintain its closed low-level circulation in the coming days. Therefore, it is plausible that the cyclone could degenerate into an open wave and weaken somewhat quicker than forecast. Peter is moving west-northwestward, or 295/12 kt, along the southwestern periphery of a subtropical ridge over the central Atlantic. This general motion should continue for the next couple of days. Thereafter, the track forecast becomes more challenging. A low- to mid-level ridge is expected to build over the western Atlantic by midweek, which would keep the weakening cyclone on a more northwestward trajectory. But, a mid-level shortwave is forecast to drop southward and erode the southern extent of the ridge, which should eventually draw Peter more northward during the middle and latter parts of the week. There is more spread noted in the track guidance at days 3-5, as the timing of this northward turn is uncertain. The official NHC track forecast has been adjusted slightly to the left of the previous one, and it lies near the center of the guidance envelope and closer to the TVCA and HCCA aids. Based on the latest track, intensity, and wind radii forecasts, no tropical storm watches or warnings are required for the northern Leeward Islands, Virgin Islands, or Puerto Rico at this time. However, locally heavy rain is possible on Monday and Tuesday when Peter is expected to pass to the north of these locations. Key Messages: 1. Rainfall around the southern periphery of Tropical Storm Peter may lead to areas of urban and small stream flooding through Tuesday across Puerto Rico, the Virgin Islands, and the Northern Leeward Islands. FORECAST POSITIONS AND MAX WINDS INIT 20/0300Z 18.6N 58.5W 45 KT 50 MPH 12H 20/1200Z 19.3N 60.6W 40 KT 45 MPH 24H 21/0000Z 20.1N 62.9W 40 KT 45 MPH 36H 21/1200Z 21.0N 65.0W 35 KT 40 MPH 48H 22/0000Z 22.0N 66.7W 35 KT 40 MPH 60H 22/1200Z 23.0N 67.9W 30 KT 35 MPH 72H 23/0000Z 23.9N 68.6W 30 KT 35 MPH 96H 24/0000Z 25.7N 68.5W 25 KT 30 MPH 120H 25/0000Z 28.0N 67.5W 25 KT 30 MPH $$ Forecaster Reinhart
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