A Case of Extreme Rainfall over the Eastern Caribbean in December 2013

The synoptic and mesoscale influences on the initiation and evolution of heavy precipitation that caused fatalities and extensive damage in the eastern Caribbean are examined.  Heavy rainfall and severe weather events in the Caribbean, while drawing less international attention than hurricanes, are much more frequent; endangering lives, destroying infrastructure, and threatening the vulnerable economies of these small islands.

The study focuses on 24-25 December 2013, when torrential rain (> 400 mm in 24 h at one station) and severe thunderstorms led to 18 deaths and widespread destruction, particularly in St. Lucia and St. Vincent and the Grenadines. An assessment is made of the ability of numerical models to represent the evolution and intensity of precipitation and, thereby, to alert forecasters to the potential for this extreme event.

Forecasting convective precipitation in mountainous islands is a major challenge because rainfall can be intense and localized. Radar, satellite, point observations, and model analyses reveal interactions on multiple scales that produced areas of heavy rainfall and severe weather over the Eastern Caribbean. Contributing factors at the synoptic-scale include: (1) an anti-cyclonic Rossby wave break at low-latitude, with an intensifying upper-tropospheric cyclone drifting west; (2) a low-level trough moving west from the tropical Atlantic into the Caribbean; and (3) a deep layer of moist air flowing from the tropical Atlantic and South America. Mesoscale deep convection was supported by upper level divergence that was induced by a jet streak; low-level warm advection; strong low-level winds; and weak mid-level shear. Low-level moisture convergence and large CAPE were also critical in initiating and sustaining mesoscale convective precipitation over and near the islands. On the local scale, the distribution of precipitation over the islands was influenced by orography.