Hurricane Otto produced days of torrential rain over much of the northeastern Caribbean in October 2010. Otto originated as a subtropical cyclone lingering north of Puerto Rico on October 6, and transitioned into a tropical storm the next day, the fifteenth of the 2010 hurricane season. Accelerating toward the northeast, Otto strengthened into a Category 1 hurricane on the Saffir–Simpson scale on October 8, attaining peak winds of 85 mph (140 km/h). The storm began weakening due to incompatible surroundings and became extratropical west of the Azores on October 10. Otto was the first Atlantic tropical cyclone on record to transition from a subtropical storm since Tropical Storm Laura in 2008. Drifting near the northeastern Caribbean for several days, Otto and its precursor disturbance brought prolonged rainfall to the Leeward Islands, the Virgin Islands, and Puerto Rico, triggering widespread flooding and numerous mudslides. Damage from the storm—in particular to roads, property and infrastructure—exceeded $22.5 million (2010 USD), but there were no fatalities. After recurving toward the northeast, Otto proceeded across the open Atlantic without affecting any other landmasses. Meteorological history In late September, a large area of disturbed weather associated with two tropical waves formed just east of the Lesser Antilles. The disturbance drifted very slowly west-northwestward, inhibited by strong wind shear that eventually caused the westernmost wave to dissipate. Albeit weak, an elongated remnant trough, or area of low pressure, persisted for several days, stalling over the extreme eastern Caribbean Sea as a nearby upper-level cyclonic vortex retrograded to its north. By October 5, the trough had drifted northward into a more favorable atmospheric environment over the Virgin Islands, which allowed the lowest pressures to extend to the surface. The low became well defined, interacting with the adjacent upper vortex to produce a wide convective band—a concentrated region in which strong showers and thunderstorms can form—over the northeastern Caribbean. Over the next day, the system continued to acquire a prominent subtropical structure, developing a markedly larger wind radius than typically seen in pure tropical cyclones. Satellite observations also revealed that although the lower wind circulation was distinct, it had become intertwined with the cyclonic vortex aloft to its southwest, confirming the hybrid nature of the system. The National Hurricane Center (NHC) thus classified the system as a subtropical depression at 06:00 UTC on October 6, when it was 265 mi (425 km) north-northwest of San Juan, Puerto Rico. The