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Tropical Storm Karen Forecast Discussion Number 13
2019-09-25 10:52:51| National Hurricane Center (Atlantic)
Issued at 500 AM AST Wed Sep 25 2019 000 WTNT42 KNHC 250852 TCDAT2 Tropical Storm Karen Discussion Number 13 NWS National Hurricane Center Miami FL AL122019 500 AM AST Wed Sep 25 2019 Karen's convection has increased significantly and become better organized since the previous advisory. The well-defined low-level circulation center that previously had been fully exposed now has intense convection with cloud tops of -85C to -90C almost completely encircling the center at times. During the last leg of an earlier Air Force Reserve reconnaissance mission, an 850-mb flight-level wind of 46 kt and SFMR surface winds of 37 kt were observed in the southeastern quadrant, suggesting that Karen may have weakened slightly during an earlier convective hiatus. However, the recent increase in deep convection over the center, along with a pronounced increase in the Doppler velocities and the development of a smaller radius of maximum winds of less than 10 nmi above 20,000 ft, suggest that Karen has likely strengthened. For now, however, the intensity is being maintained at 40 kt. The secondary mid-level circulation that had developed north of Puerto Rico several hours ago has since weakened based on San Juan Doppler radar data and satellite images. Now that deep convection has redeveloped, resulting in a stronger and deeper the vortex column, the motion has steadied off in a northerly direction of 360/12 kt over the past 6 hours. A northward motion is forecast to continue this morning, followed by a turn toward the north-northeast by this afternoon. A north-northeastward to northeastward motion accompanied by a gradual decrease in forward speed is expected in the 12-48 hour period. By 72 hours, steering currents are forecast to collapse and Karen is expected to stall or make a clockwise loop. By 96 and beyond, the global models forecast that a ridge will build eastward from the southeastern United States to Bermuda, forcing Karen in a slow westward to west-southwestward direction. The new NHC track forecast is very close to the previous advisory track, and lies close to the simple consensus track model TVCN, which is about midway between the corrected-consensus models FSSE located to the north and NOAA-HCCA to the south. The intensity forecast remains somewhat perplexing with the dynamical global and regional models showing no strengthening for the next 4 days, followed by weakening and even dissipation by day 5, whereas the GFS- and ECMWF-based statistical-dynamical models show slow but steady strengthening during the forecast period with Karen becoming a hurricane by 120 hours. The main reasons for global models weakening the cyclone is due to the low- and upper-level circulations decoupling in about 3 days, followed by very dry mid-level overspreading of the low-level circulation, shutting off convective development. The problem with that scenario is that Karen will be moving into very low vertical shear conditions and underneath an upper-level anticyclone by 36 hours, which favors strengthening since the cyclone will also be sitting over 29 deg C water of considerable depth. For now, the new official intensity forecast remains a compromise between the weak dynamical model solutions and the stronger SHIPS intensity forecasts, which is a little above the intensity consensus models IVCN, HCCA, and FSSE. Key Messages: 1. Karen will continue to produce rainfall causing flash floods and mudslides across Puerto Rico, Vieques and the U.S. and British Virgin Islands today even as the center moves away from the region. FORECAST POSITIONS AND MAX WINDS INIT 25/0900Z 20.5N 65.2W 40 KT 45 MPH 12H 25/1800Z 22.3N 64.5W 45 KT 50 MPH 24H 26/0600Z 24.5N 63.9W 45 KT 50 MPH 36H 26/1800Z 26.4N 63.2W 50 KT 60 MPH 48H 27/0600Z 27.3N 62.5W 55 KT 65 MPH 72H 28/0600Z 27.1N 61.9W 55 KT 65 MPH 96H 29/0600Z 26.3N 64.4W 55 KT 65 MPH 120H 30/0600Z 25.9N 67.8W 55 KT 65 MPH $$ Forecaster Stewart
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