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Tropical Storm Nana Forecast Discussion Number 1
2020-09-01 17:00:13| National Hurricane Center (Atlantic)
Issued at 1100 AM EDT Tue Sep 01 2020 000 WTNT41 KNHC 011500 TCDAT1 Potential Tropical Cyclone Sixteen Discussion Number 1 NWS National Hurricane Center Miami FL AL162020 1100 AM EDT Tue Sep 01 2020 An Air Force Reserve reconnaissance aircraft is currently investigating the vigorous tropical disturbance located south of Jamaica that the NHC has been tracking the past few days across the Caribbean Sea. An earlier report of 35 kt winds this morning from ship MAOR4 is the basis for the 35-kt initial intensity on Potential Tropical Cyclone Sixteen. (PTC-16) The initial motion estimate is an uncertain 285/16 kt, based mainly on past scatterometer surface wind data and passive microwave satellite fix data. A strong deep-layer to the north of the system is expected to build slowly westward over the next few days, keeping the disturbance moving in a general west-northwestward to westward direction through 48 hours, followed by a turn toward the west-southwest shortly thereafter when the system will be inland over Central America. The NHC official forecast track lies close to a blend of the simple consensus models TVCN and GFEX, and the corrected-consensus model HCCA. The disturbance has already developed an upper-level anticyclone, with outflow only restricted in the northeastern quadrant due to northeasterly shear of about 15 kt. This modest northeasterly to easterly vertical wind shear is expected to gradually subside to less than 10 kt by 48 hours, which should allow for at least some slow but steady strengthening, given that sea-surface temperatures (SST) will be 29.5-30.5 deg C along the track. Occasional intrusions of dry mid-level air might hinder development for the next 24-36 hours, but model guidance shows that the atmosphere is expected to moisten thereafter and right up until landfall. The official intensity forecast is similar to the consensus of the intensity guidance through 24 hours, and then is a little above the consensus at 36 h and 48 h due to the very warm waters, moistening atmosphere, and low shear conditions. The statistical SHIPS guidance suggests that the cyclone could reach hurricane strength just prior to landfall. This alternate scenario will be evaluated on the next advisory cycle. Due to the system's expected proximity to the north coast of Honduras and Roatan Island, a Tropical Storm Watch has been issued for those areas. Additional watches and possible warnings will likely be required later today for Guatemala, Belize, and the southern portion of the Yucatan Peninsula. FORECAST POSITIONS AND MAX WINDS INIT 01/1500Z 16.1N 77.5W 35 KT 40 MPH...POTENTIAL TROP CYCLONE 12H 02/0000Z 16.4N 79.6W 35 KT 40 MPH...TROPICAL STORM 24H 02/1200Z 16.6N 82.3W 40 KT 45 MPH 36H 03/0000Z 16.7N 85.1W 45 KT 50 MPH 48H 03/1200Z 16.8N 87.4W 50 KT 60 MPH 60H 04/0000Z 16.5N 89.6W 25 KT 30 MPH...INLAND 72H 04/1200Z 15.7N 91.7W 15 KT 15 MPH...POST-TROP/REMNT LOW 96H 05/1200Z...DISSIPATED $$ Forecaster Stewart
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