Issued at 1100 AM AST Fri Sep 16 2022
468 WTNT42 KNHC 161449 TCDAT2 Tropical Storm Fiona Discussion Number 9 NWS National Hurricane Center Miami FL AL072022 1100 AM AST Fri Sep 16 2022 Convective activity is occurring much closer to the low-level center of Fiona this morning, suggesting that the deep-layer westerly shear has decreased a bit. Air Force Reserve and NOAA Hurricane Hunters have been flying through Fiona during the past several hours, and winds as high as 57 kt were reported about 2500 ft above the surface, along with SFMR winds slightly higher than 45 kt. The initial intensity remains 45 kt for now, but Fiona could easily attain 50 kt soon if the deep convection remains close to the center. After losing latitude most of the day yesterday and overnight, Fiona has started moving north of due west, with an initial motion of 275/12 kt. A north-of-due-west motion with some slowing of the forward speed is expected through early Sunday, with Fiona steered across the Leeward Islands and the far northeastern Caribbean Sea by subtropical ridging over the western Atlantic. Fiona then reaches the western extent of the ridge just beyond 48 hours, and it is expected to move more slowly and turn northwestward across Hispaniola in 3-4 days, and then be near the Turks and Caicos Islands and southeastern Bahamas in about 5 days. The spread among the deterministic track models and the ensembles has decreased since yesterday, and there's therefore a little bit more confidence in the NHC track prediction. In addition, there has not been much shift in the guidance since the previous forecast cycle, and the updated NHC forecast lies very close to the previous prediction, and in the middle of the guidance envelope close to the TVCN and HCCA consensus aids. Fiona is located near a tight gradient of deep-layer shear, and it's possible that its south-of-due-west motion since yesterday brought it into a lighter-shear environment for the convection to build back closer to the center. Model guidance indicates that the shear is likely to remain moderate for much of the forecast period, but at the same time, upper-level divergence and mid-level moisture are forecast to increase. Coupled with the expected decrease in forward speed, these factors could allow Fiona's structure to become more vertically stacked over the northeastern Caribbean Sea, and gradual strengthening is still anticipated during the next 3 days. Based on this forecast, Fiona could be very near hurricane strength as it approaches the southern coast of the Dominican Republic. The terrain of Hispaniola is likely to disrupt Fiona's circulation, but the global models suggest that Fiona shouldn't have much trouble reorganizing itself once over the far southwestern Atlantic, and the NHC forecast now calls for the cyclone to become a hurricane by the end of the 5-day forecast period. This prediction is slightly below the IVCN and HCCA consensus aids. Key Messages: 1. Tropical storm conditions are expected across the Leeward Islands within the warning area starting this afternoon, and will spread westward across the U.S. Virgin Islands on Saturday and Puerto Rico late Saturday and Saturday night. Tropical storm conditions are possible over Dominica tonight and in the British Virgin Islands on Saturday. 2. Heavy rains from Fiona will reach the Leeward Islands by this evening, spreading to the British and U.S. Virgin Islands and Puerto Rico Saturday, reaching the Dominican Republic Sunday, and the Turks and Caicos Monday night or Tuesday. This rainfall may produce considerable flood impacts including flash and urban flooding, along with mudslides in areas of higher terrain. 3. Fiona is expected to strengthen gradually over the northeastern Caribbean Sea, and it could be near hurricane strength as it approaches the southern coast of the Dominican Republic Sunday night and early Monday. Watches could be required for portions of the Dominican Republic later today. 4. Fiona is forecast to strengthen after moving across Hispaniola early next week, and interests in the Turks and Caicos Islands and southeastern Bahamas should continue to monitor forecasts for the storm. FORECAST POSITIONS AND MAX WINDS INIT 16/1500Z 16.0N 59.4W 45 KT 50 MPH 12H 17/0000Z 16.2N 61.2W 50 KT 60 MPH 24H 17/1200Z 16.6N 63.5W 50 KT 60 MPH 36H 18/0000Z 17.0N 65.6W 55 KT 65 MPH 48H 18/1200Z 17.3N 67.3W 55 KT 65 MPH 60H 19/0000Z 17.8N 68.6W 60 KT 70 MPH 72H 19/1200Z 18.5N 69.6W 60 KT 70 MPH...S COAST OF DOM. REP. 96H 20/1200Z 20.5N 71.3W 55 KT 65 MPH...OVER WATER 120H 21/1200Z 22.5N 72.4W 65 KT 75 MPH $$ Forecaster Berg