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Tropical Storm Eta Forecast Discussion Number 37
2020-11-09 15:50:08| National Hurricane Center (Atlantic)
Issued at 1000 AM EST Mon Nov 09 2020 000 WTNT44 KNHC 091449 TCDAT4 Tropical Storm Eta Discussion Number 37 NWS National Hurricane Center Miami FL AL292020 1000 AM EST Mon Nov 09 2020 Eta's overall appearance in satellite radar imagery has become quite fragmented, with inner-core convection having become vertically shallow and broken due to mid-level dry air entrainment. Some of the recent Air Force Reserve reconnaissance flight-level data indicate that dry air has worked way down to the 850-mb level based dewpoint spreads of more than 8 deg C. In the northeastern quadrant, a large curved convective band persists that has produced widespread rainfall amounts exceeding 10 inches along with strong gusty winds across portions of the southern Florida peninsula. However, even that band of thunderstorms has become less pronounced in both satellite radar imagery over the past few hours. Doppler velocities over land and over water have also come decreased significantly, and the latest reconnaissance flight-level (58 kt) and SFMR surface wind data (49 kt) supports lowering the intensity to 50 kt. The decrease in intensity is also supported by the gradual rise in the central pressure, which is now up to 994 mb. Eta has finally made the expected southwestward turn, and the initial motion is now 235/12 kt. A strong deep-layer ridge across the Gulf of Mexico, Florida, and near the U.S. east coast is expected to keep Eta moving southwestward for the next 24-36 hours, accompanied by a steady decrease in forward speed. Steering currents are forecast to collapse by 36-48 hours, causing Eta to possibly stall and/or make a small loop just northwest of western Cuba. By 60 hours and beyond, a broad mid-latitude trough currently located over the Rocky Mountains is forecast to move eastward and gradually erode the portion of the ridge over the Gulf of Mexico, allowing Eta slowly move northward to north-northeastward through 20 hours. However, there remains considerable divergence between the global models on days 4 and 5 with regards to how far north Eta will move, with the ECMWF showing a more northward progression while the GFS and UKMET models show the trough lifting out and not eroding the ridge as much. For now, the official forecast track remains a compromise of these two extremes, and shows a slow poleward progression on days 3-5, similar to the consensus models TCVA and NOAA-HCCA. Eta could still re-strengthen some during the 24-48 hour period when the cyclone will be moving over the warm waters of the Gulf of Mexico Loop Current and the wind shear gradually decrease to less than 10 kt. However, occasional intrusions of very dry air will likely continue to plague the system, which would prevent any rapid intensification from occurring and could keep Eta from regaining hurricane status. By 48 hours and beyond, increasing vertical wind shear and dry air should cause a gradual weakening of the cyclone through end of the forecast period. The new NHC intensity forecast is a little lower than the previous advisory, and closely follows the intensity consensus models IVCN and HCCA. Key Messages: 1. Heavy rainfall from Eta will continue across portions of Cuba, the Bahamas, and southern and central Florida. Life-threatening flash flooding will be possible across urban areas of southeast Florida today. Flash and urban flooding will also be possible for Cuba, the Bahamas and the remainder of southern Florida, along with potential minor river flooding in central Florida. 2. Eta could approach the Florida Gulf Coast later this week as a tropical storm, and possibly bring impacts from rain, wind, and storm surge. Interests in this area should monitor the progress of Eta and updates to the forecast this week. FORECAST POSITIONS AND MAX WINDS INIT 09/1500Z 24.6N 83.4W 50 KT 60 MPH 12H 10/0000Z 23.9N 84.7W 50 KT 60 MPH 24H 10/1200Z 23.6N 85.3W 55 KT 65 MPH 36H 11/0000Z 24.5N 85.4W 60 KT 70 MPH 48H 11/1200Z 25.6N 85.5W 55 KT 65 MPH 60H 12/0000Z 26.5N 85.3W 55 KT 65 MPH 72H 12/1200Z 27.2N 85.1W 50 KT 60 MPH 96H 13/1200Z 28.4N 84.4W 45 KT 50 MPH 120H 14/1200Z 29.6N 83.7W 35 KT 40 MPH...INLAND $$ Forecaster Stewart
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