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Tropical Storm Eta Forecast Discussion Number 41
2020-11-10 15:55:27| National Hurricane Center (Atlantic)
Issued at 900 AM CST Tue Nov 10 2020 000 WTNT44 KNHC 101455 TCDAT4 Tropical Storm Eta Discussion Number 41 NWS National Hurricane Center Miami FL AL292020 900 AM CST Tue Nov 10 2020 A significant increase in deep convection has occurred since the previous advisory this morning, including a CDO-like feature with cloud tops of -85C to -87C very near the low-level center. However, recent passive microwave satellite images indicate that the center is displaced to the northwest of the coldest cloud tops due to modest northwesterly mid- and upper-level vertical wind shear. The initial intensity has been increased to 50 kt, which is based on an average of Dvorak satellite intensity estimates of T3.0/45 kt from TAFB and T3.5/55 kt from SAB. Although the initial motion estimate is 360/02 kt, Eta has essentially been stationary for the past 9 hours or so. Radar data from Casablanca, Cuba, and satellite fixes suggest that Eta has been making a small cyclonic/counter-clockwise loop within the larger gyre in which the small center is embedded. Until the gyre breaks down or moves northward, there will be little poleward motion by Eta today. By tonight, however, the global and regional models are in good agreement on a broad mid-tropospheric trough moving eastward across the central and eastern United States, which is expected to erode a subtropical ridge to the north of the Eta, allowing both the larger gyre and Eta to move slowly northward. This steering pattern is expected to continue through about 72 hours. Thereafter, the model guidance diverges significantly between motions ranging from westward (GFS/GFS-ensemble) to northward (ECMWF) to northeastward (COAMPS-TC). The 96-120 hour motions are directly related to the strength of the cyclone, with a much weaker Eta forecast to move westward and a stronger hurricane solution moving northeastward. The latter scenario seems unlikely given that the vertical shear is forecast to increase from the northwest and west at more than 25 kt, which acts to weaken Eta and also impart a slight eastward tug on the system. As a result, the official forecast track calls for Eta to basically move slowly northward through the 120-h forecast period and gradually weaken into a shallow cyclone that drifts northward. The new NHC track forecast is similar to the previous advisory track, and lies close to a blend of the consensus models TVCA, NOAA-HCCA, and the Florida Superensemble (FSSE). Eta is expected to remain in a low-to-moderate vertical wind shear environment and over warm SSTs of 27-28 deg C for the next couple of days. Although the surrounding environment is expected to be somewhat dry, the other two favorable environmental factors should allow for some strengthening into Thursday, Thereafter, increasing shear from the northwest and west, along with drier mid-level air and cooler SSTs are likely to cause Eta to weaken. The rate of this weakening remains uncertain, and depends heavily on how much Eta re-intensifies over the next couple of days. The new intensity forecast is essentially the same as the previous advisory, and is a little below the consensus models IVCN, HCCA, and FSSE, all of which make Eta a hurricane again by 36 hours. Key Messages: 1. Heavy rainfall from Eta will continue across western Cuba and South Florida today and tonight. Additional flash and urban flooding, especially across previously inundated areas, will be possible in South Florida. Flash and urban flooding will also be possible for western Cuba. 2. Eta could approach the northeastern or north-central U.S. Gulf Coast later this week as a tropical storm, and possibly bring impacts from rain, wind, and storm surge. Interests in this area should continue to monitor the progress of Eta and updates to the forecast this week. FORECAST POSITIONS AND MAX WINDS INIT 10/1500Z 22.7N 85.3W 50 KT 60 MPH 12H 11/0000Z 23.1N 85.4W 55 KT 65 MPH 24H 11/1200Z 24.0N 85.3W 60 KT 70 MPH 36H 12/0000Z 25.0N 85.2W 60 KT 70 MPH 48H 12/1200Z 25.9N 85.1W 55 KT 65 MPH 60H 13/0000Z 26.6N 85.2W 45 KT 50 MPH 72H 13/1200Z 27.2N 85.5W 40 KT 45 MPH 96H 14/1200Z 28.6N 86.4W 35 KT 40 MPH 120H 15/1200Z 30.1N 86.4W 30 KT 35 MPH $$ Forecaster Stewart
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