Isaac (2018)

Isaac formed from a tropical wave that moved off West Africa on 2 September and became a tropical depression at 1200 UTC 7 September about 600 nautical miles west of the Cabo Verde Islands. It moved generally westward under a subtropical ridge, became a tropical storm on 8 September, and reached hurricane strength early on 10 September as it tracked across the central tropical Atlantic. Increasing dry air and strong upper-level wind shear from an amplified trough—partly influenced by nearby Hurricanes Florence and Helene—caused Isaac to weaken steadily as it accelerated westward. The system fell to a tropical depression by 0000 UTC 15 September and degenerated into a tropical wave by 0600 UTC that day about 200 n mi south-southwest of Puerto Rico.

Isaac passed through the Lesser Antilles as a tropical storm. Its center crossed the Windward Islands between Martinique and Dominica just after 1200 UTC 13 September. Tropical-storm watches and hurricane watches had been issued for parts of the northeastern Caribbean in the days prior, but all watches and warnings were discontinued by 2100 UTC 13 September as the cyclone weakened and moved away.

The storm’s peak intensity was estimated at 65 knots (75 mph) with a minimum central pressure of 995 mb, making Isaac a Category 1 hurricane at its strongest (valid 0000–1200 UTC 10 September). That peak was based on satellite estimates and adjusted analysis from Hurricane Hunter data. No sustained tropical-storm-force winds were reported on land; the strongest ship report was 37 kt (interpreted as about 30–35 kt at 10 m) from the cruise ship Carnival Fascination on 14 September.

Storm surge and rainfall impacts were limited. An automated station on Guadeloupe recorded 0.46 inches of rain from 1200 UTC 13 September to 1200 UTC 14 September; heavier rains were reported in local media on islands to the south such as St. Lucia and Dominica but official rain totals at those locations were not available in the report. High surf and localized flooding were reported in parts of Anse La Raye and Castries on St. Lucia; Pointe-á-Pitre International Airport (Guadeloupe) reported a gust of 36 kt.

There were no reported deaths attributed to Isaac. Impacts were generally limited to locally heavy rain, flooding, minor landslides, downed trees, and coastal surf damage in parts of the Lesser Antilles—particularly St. Lucia and Dominica. The NHC noted that track forecasts for Isaac were very good (errors well below recent averages), but intensity forecasts had a high bias and generally overestimated the storm’s strengthening and longevity because of Isaac’s small size and the unexpected increase in shear.