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Tropical Storm Karen Forecast Discussion Number 11
2019-09-24 22:49:14| National Hurricane Center (Atlantic)
Issued at 500 PM AST Tue Sep 24 2019 000 WTNT42 KNHC 242049 TCDAT2 Tropical Storm Karen Discussion Number 11 NWS National Hurricane Center Miami FL AL122019 500 PM AST Tue Sep 24 2019 The satellite appearance of Karen has improved over the past several hours, with a large convective band wrapped about halfway around the center in the western semicircle and an outer band in the southeastern semicircle. However, surface observations, Doppler radar data, and reports from an Air Force Reserve Hurricane Hunter aircraft indicate that the low-level circulation is elongated from south to north and that multiple vorticity centers are present. The lowest pressures and the strongest winds are at the southern end of the elongation near the eastern end of the convective band, and this is the center used for this advisory. The Hurricane Hunter aircraft reported 53 kt flight-level winds at 850 mb and data suggesting a central pressure near 1005 mb, so the initial intensity is increased to 40 kt. The initial motion is again 360/7, with some uncertainty due to the multiple vorticity centers and an apparent re-formation of the center closer to the convection between 15Z-18Z. Karen remains in a complex steering environment that includes a low- to mid-level ridge to the east and northeast, Tropical Storm Jerry to the north- northwest, and a large mid- to upper-level trough extending from near Jerry to eastern Cuba. These weather systems should steer Karen generally north-northeastward for the next couple of days. This motion should bring the center of Karen near or over Puerto Rico and the Virgin Islands during the next few hours and then into the open Atlantic tonight. During the 72-120 h period, the large-scale models forecast a low- to mid-level ridge to build near Karen, which will significantly slow its forward motion. The track guidance is in somewhat better agreement that this ridge will cause Karen to turn westward near the end of the forecast period, although there remains a lot of uncertainty about when and how fast. The new forecast track keeps Karen a little farther south before this turn occurs, and thus the 120-h position is a little to the south of the previous forecast. Karen is moving into an area of weaker shear, and should remain in that environment for the next 2-3 days. Thus, strengthening is expected, with the main limiting factor being the current poor organization of the circulation. From 72-120 h, the storm is expected to start entraining dry air, which could limit intensification despite the otherwise favorable shear and sea surface temperature environment. In addition, several of the global models suggest that another round of shear could affect Karen near the 120 h point. If this occurs, the cyclone could end up weaker than forecast in this advisory. The new intensity forecast has minor changes from the previous forecast, and it again lies between the weaker dynamical models and the stronger statistical-dynamical models. Key Messages: 1. Karen is expected to bring heavy rainfall, flash floods and mudslides to Puerto Rico and the U.S. and British Virgin Islands today, where a tropical storm warning is in effect. The rainfall and potential flooding will likely continue on Wednesday even as the center of Karen moves away from Puerto Rico and the Virgin Islands. FORECAST POSITIONS AND MAX WINDS INIT 24/2100Z 18.0N 65.8W 40 KT 45 MPH 12H 25/0600Z 19.6N 65.5W 40 KT 45 MPH 24H 25/1800Z 21.9N 64.9W 45 KT 50 MPH 36H 26/0600Z 24.2N 64.2W 50 KT 60 MPH 48H 26/1800Z 25.9N 63.5W 50 KT 60 MPH 72H 27/1800Z 27.2N 61.9W 55 KT 65 MPH 96H 28/1800Z 27.0N 62.5W 60 KT 70 MPH 120H 29/1800Z 26.5N 65.5W 60 KT 70 MPH $$ Forecaster Beven
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