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Tropical Storm Grace Forecast Discussion Number 18
2021-08-17 22:56:06| National Hurricane Center (Atlantic)
Issued at 500 PM EDT Tue Aug 17 2021 272 WTNT42 KNHC 172056 TCDAT2 Tropical Storm Grace Discussion Number 18 NWS National Hurricane Center Miami FL AL072021 500 PM EDT Tue Aug 17 2021 Radar imagery and earlier Hurricane Hunter aircraft and ASCAT observations indicate that the center of Grace has been moving over Jamaica. The aircraft and scatterometer data indicate that the maximum winds are near 45 kt. Cirrus-level outflow from the system remains quite pronounced, suggestive of weak vertical shear at this time. Grace will be moving over the waters of the northwestern Caribbean Sea beginning later tonight. These waters are of very high oceanic heat content. Although mid-level environmental humidities are shown by the SHIPS diagnoses to be only marginally moist for the next couple of days, other factors should be conducive for strengthening. The official intensity forecast, which shows Grace becoming a hurricane tomorrow, is in reasonable agreement with the latest multi-model consensus. Some slight weakening should occur due to passage over the Yucatan Peninsula, followed by re-intensification over the Bay of Campeche. There is, however, significant uncertainty in the intensity prediction for this latter part of the forecast period. The storm continues on a heading just north of west, or 280/13 kt. A strong 500-mb ridge should be maintained to the north of Grace for essentially the entire forecast period. There is good agreement among the track models on a continued generally westward to west-northwestward motion for the next 5 days. Little change has been made to the official track forecast in comparison to the previous one. The new NHC track is, again, very close to the dynamical model consensus. 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 late Wednesday night. 2. Heavy rainfall across Haiti, Cuba, Jamaica, the Cayman Islands, and the Yucatan Peninsula is likely to cause to flash, urban, and small stream flooding, with the potential for mudslides highest in Haiti. 3. Tropical storm conditions are expected over portions of Jamaica through this evening, and over the Cayman Islands tonight and early Wednesday. 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 tonight through Wednesday. FORECAST POSITIONS AND MAX WINDS INIT 17/2100Z 18.4N 77.9W 45 KT 50 MPH 12H 18/0600Z 18.9N 80.3W 60 KT 70 MPH 24H 18/1800Z 19.4N 83.4W 70 KT 80 MPH 36H 19/0600Z 20.0N 86.7W 75 KT 85 MPH 48H 19/1800Z 20.5N 89.7W 60 KT 70 MPH...INLAND 60H 20/0600Z 20.9N 92.2W 65 KT 75 MPH...OVER WATER 72H 20/1800Z 21.2N 94.8W 75 KT 85 MPH 96H 21/1800Z 21.0N 99.5W 40 KT 45 MPH...INLAND 120H 22/1800Z 21.0N 103.0W 20 KT 25 MPH...POST-TROP/INLAND $$ Forecaster Pasch
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