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Tropical Storm Karen Forecast Discussion Number 9
2019-09-24 10:51:32| National Hurricane Center (Atlantic)
Issued at 500 AM AST Tue Sep 24 2019 000 WTNT42 KNHC 240851 TCDAT2 Tropical Storm Karen Discussion Number 9 NWS National Hurricane Center Miami FL AL122019 500 AM AST Tue Sep 24 2019 During the last pass through Karen's center around 0430 UTC, the Air Force reconnaissance aircraft reported some believable SFMR winds of 34-35 kt in the southeastern quadrant and an extrapolated central pressure of 1006 mb. The 925-mb flight-level height had also decreased by almost 20 meters in about 2 hours, an indication that the lower pressure estimate was legitimate. Reflectivity and velocity data from the San Juan NOAA WSR-88D Doppler radar also enunciate that Karen's inner-core has become better defined over the past few hours. Satellite intensity estimates at 0600 UTC were T2.5/35 kt from TAFB, and T2.4/34 kt and 40 kt from UW-CIMSS ADT and SATCON, respectively. Based on the recon and satellite intensity data, Karen has re-strengthened to tropical storm status. The initial motion is now northward or 360/06 kt. Karen's forecast track hinges heavily on the future intensity and associated vertical structure of the cyclone. The GFS, GFS-Legacy, UKMET, and HRWF dynamical models take a much weaker and more vertically shallow cyclone northeastward after 48 hours and either continue with that motion through day 5 or dissipate the system. In contrast, the ECMWF and many of its stronger ensemble members stall Karen around days 3-4 and then turn the somewhat stronger and deeper cyclone westward to west-southwestward to the south of a building ridge. Given that Karen is forecast to be stronger and vertically deeper than the weaker models, the current track forecast leans more toward the stronger ECMWF and ECMWF-Ensemble model solutions. The new NHC forecast is to the right of the previous advisory track and slower, especially on days 3-5, but the new track does not extend as far east as the weaker GFS, UKMET, and HWRF models or the consensus models that incorporate those three models. None of the dynamical models, including the HWRF and HMON hurricane models, show much in the way of strengthening once Karen moves north of Puerto Rico and Virgin Islands except for the ECMWF model, despite the very low vertical wind shear conditions that the cyclone will be moving into by 24 hours and beyond. Given Karen's decent wind field and vertical structure, the expected low shear conditions, moist mid-level environment, and SSTs of more than 29 deg C for the next 48 hours or so, forecasting at least slow but steady strengthening seems to be quite reasonable. By days 3-5, the mid-level environment dries out significantly, which the weaker models seem to be keying on. However, if Karen strengthens as currently expected, then the cyclone's robust circulation should be able to mix out any dry air intrusions, allowing for at least additional modest intensification to occur in the 72-120 hour period. The official intensity forecast is a little above the previous advisory, and is basically an average of the weaker dynamical models and the stronger GFS- and ECMWF-based statistical-dynamical models Decay-SHIPS and LGEM. 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 24/0900Z 16.8N 65.8W 35 KT 40 MPH 12H 24/1800Z 18.1N 65.9W 35 KT 40 MPH 24H 25/0600Z 20.1N 65.5W 40 KT 45 MPH 36H 25/1800Z 22.6N 64.7W 45 KT 50 MPH 48H 26/0600Z 24.6N 64.0W 50 KT 60 MPH 72H 27/0600Z 27.3N 62.7W 55 KT 65 MPH 96H 28/0600Z 28.0N 63.0W 60 KT 70 MPH 120H 29/0600Z 27.7N 65.5W 60 KT 70 MPH $$ Forecaster Stewart
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