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Tropical Storm Grace Forecast Discussion Number 20
2021-08-18 10:52:16| National Hurricane Center (Atlantic)
Issued at 500 AM EDT Wed Aug 18 2021 000 WTNT42 KNHC 180852 TCDAT2 Tropical Storm Grace Discussion Number 20 NWS National Hurricane Center Miami FL AL072021 500 AM EDT Wed Aug 18 2021 An Air Force Reserve reconnaissance aircraft investigating Grace earlier this morning observed a peak 850-mb flight-level wind of 61 kt in the northeastern quadrant along with reliable SFMR surface wind speeds of 50-52 kt. The aircraft measured a central pressure of about 1000 mb, a reading that had been steady for about 3 hours. Those data were the basis for maintaining the 0600 UTC intensity at 50 kt. Since the last recon mission, however, deep convection has increased markedly over the low-level center, with cloud tops exceeding -75 deg C. Although the center is just inside the western edge of convective cloud canopy due to modest west-northwesterly vertical wind shear, Dvorak satellite intensity estimates from TAFB and SAB are a consensus T3.5/55 kt. These intensity estimates, along with the increase in convection over the center is the justification for increasing the 0900 UTC advisory intensity to 55 kt. Grace has been maintaining a steady course north of due west, or 280/14 kt for more than 18 hours. A strong deep-layer ridge to the north of the cyclone is expected to keep Grace moving in direction between west and west-northwest through the 96-hour forecast period. The latest NHC model guidance remain in strong agreement on Grace making landfall along the northeastern coast of the Yucatan Peninsula of Mexico shortly after the 24-hour forecast period, followed by a motion across the peninsula and emerging over the Bay of Campeche around 36 hours. Thereafter, the model guidance begins to diverge, with the HWRF, HMON, and COAMPS-TC regional models along the northern side of the guidance envelope, the GFS and UKMET global models down the middle, and the ECMWF model along the southern edge of the guidance suite. The new NHC forecast lies close to the previous advisory track and the consensus models TVCA and HCCA, which are a little south of the FSSE consensus model. Although the upper-level wind environment isn't ideal for rapid strengthening, it is conducive enough for steady strengthening to occur due to the flow forecast to be strongly diffluent. Sea-surface temperatures are expected to be near 30 deg C and the water is also deep and warm, which should prevent any cold upwelling from occurring beneath the cyclone. As a result, a peak intensity of 75 kt just prior to landfall has been carried over from the previous advisory, which is a little above all of the 0600 UTC intensity guidance. Although Grace will weaken substantially after passing over the Yucatan peninsula, restrengthening is expected over the warm waters of thew Bay of Campeche in the 48-72- hour period where the upper-level flow regime is forecast to be more conducive for intensification. Dissipation is now expected over the mountains of central Mexico by 120 hours. The NHC intensity forecast is in best agreement with the HCCA and IVCN consensus aids at 36 hours and beyond. Key Messages: 1. Hurricane conditions and a dangerous storm surge are expected over portions of the Hurricane Warning area in the eastern Yucatan Peninsula of Mexico beginning late tonight. 2. Over the next few days, heavy rainfall across the Cayman Islands as well as portions of the Yucatan Peninsula and Vera Cruz State should lead to flash and urban flooding. Mudslides will be possible in Jamaica. 3. Tropical storm conditions are expected over the Cayman Islands through this morning. Hurricane conditions are possible in the Cayman Islands later this morning. Tropical storm conditions are expected to spread westward from portions of the southern coast of Cuba in the warning area to, possibly, other portions of the southern coast of Cuba in the watch area today. FORECAST POSITIONS AND MAX WINDS INIT 18/0900Z 18.8N 80.9W 55 KT 65 MPH 12H 18/1800Z 19.3N 83.2W 60 KT 70 MPH 24H 19/0600Z 19.9N 86.4W 75 KT 85 MPH 36H 19/1800Z 20.3N 89.4W 50 KT 60 MPH...INLAND 48H 20/0600Z 20.6N 92.1W 50 KT 60 MPH...OVER WATER 60H 20/1800Z 20.7N 94.4W 65 KT 75 MPH 72H 21/0600Z 20.7N 96.8W 75 KT 85 MPH 96H 22/0600Z 20.6N 101.4W 35 KT 40 MPH...INLAND 120H 23/0600Z...DISSIPATED $$ Forecaster Stewart
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