Isaias has strengthened to become the first Atlantic hurricane of the 2026 season, reaching wind speeds of 80mph and a minimum pressure of 975mb.
The storm is expected to make landfall on the US's northern Gulf coast, carrying a risk of flash floods and isolated river flooding.
The first hurricane of the season has not formed this late since 8 October 1905, a period that pre-dates the satellite era of the 1960s used for tracking hurricanes and discovering more non-landfall hurricanes.
A strong El Niño has resulted in a notably quiet Atlantic hurricane season.