Fiona continued to steadily strengthen overnight, with its winds approaching hurricane strength (75 mph or stronger) this morning as its formative eyewall begins to scrape the southern reaches of Puerto Rico.
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The biggest concern ahead for Puerto Rico and the eastern Dominican Republic thereafter is flooding and mudslides from torrential rainfall, up to 25 inches forecast in spots, rivaling historic island rainmakers of the past like Hurricane Hortense in 1996, which devastated parts of the U.S. island territory.
After central pressures hovering in the 1000-1005 mb range for days – the storm held at bay by persistent wind shear – the upper-level pattern around Fiona relaxed its grip yesterday, allowing storms to wrap around its circulation and pressures to fall by this morning into the upper 980s as of the latest vortex data message from hurricane hunters. Both microwave satellite and traditional radar from Puerto Rico show a formative eyewall wrapping nearly entirely around Fiona’s deepening center, with its strongest winds roughly 35 miles to the northeast and less than 50 miles south of Puerto Rico.
Winds have at times gusted from 50-60 mph on the south and east sides of the island this morning. Much of the southern half of Puerto Rico sits 1,000 feet or higher above sea level, which means winds may be 25-30 percent higher at these elevated spots than maximum surface winds reported in forecast advisories.
While the fragile power grid in Puerto Rico remains a concern from the strong and gusty winds, the primary threat in the coming hours will be potentially catastrophic rainfall. In Friday’s newsletter we discussed the destructive water impacts of even mid-grade tropical storms like Erika in 2015 which wrought catastrophic flooding to Dominica in the eastern Caribbean. Fiona wrung out nearly 20 inches of rain into the early morning hours Saturday across Guadeloupe, Dominica’s northern French neighbor, producing destructive flash flooding and mudslides that chewed up roads and homes, killing at least one person. Fiona is forecast to bring widespread heavy rainfall to Puerto Rico through Monday, with up to two feet of rain possible in eastern and southern parts of the island.
Beyond Puerto Rico and the Dominican Republic, Fiona will bend northwestward towards the Turks and Caicos and southeastern Bahamas by late Monday and early Tuesday as a strengthening hurricane.
As we mentioned in newsletters earlier this week, high-altitude missions from the Hurricane Hunters, which commenced last evening and went into the overnight computer model runs, have helped settle down the models, giving us a clearer picture of the forecast for next week. Fiona is not expected to pose a threat to South Florida or the mainland U.S., but the Bahamas will need to monitor its progress into the middle and latter part of the week.
Besides Fiona, the tropics look mercifully quiet this week, with the only area of potential development from a north-moving system over the central open Atlantic.