Cases of Heavy Precipitation and Flash Floods in the Caribbean during El Niño Winters

Abstract

The environments associated with three episodes of heavy precipitation and flash floods in the Caribbean are diagnosed. Analysis of the hydrometeorological conditions leading up to flash floods on 3?4 January 1998, 5?6 January 1992, and 4 March 1998 are focused on the synoptic features as well as the surface conditions. Subsequent flood mitigation efforts are briefly discussed. In the first case, deep convection and heavy precipitation were associated with a surface trough that developed in the wake of a quasi-stationary front. Warm, moist unstable air conveyed by a low-level jet and impinging on steep terrain created a quasi-stationary cloud cluster that produced more than 400 mm of rainfall in 2 days. Upper-level divergence and weak midtropospheric vorticity advection enhanced ascending motion. Antecedent precipitation from the front, the steep terrain, river basin topography, and human encroachment in the flood plain aggravated the flood hazard. The second case had similar conditions, with additional lift induced by an upper-level trough. The third case was weaker than the first two primarily because its low-level airflow was northerly and weaker. In all cases, the orography, the low-level wind velocity, a deep layer of moisture, and potential instability played important roles. These findings agree with other studies of heavy orographic precipitation from convection. Interestingly, these cases of heavy precipitation occurred during what is normally the dry season in the Caribbean. However, during El Niño years, midlatitude systems track well south of their normal tracks. Fronts, prefrontal troughs, and upper-level low pressure systems can then contribute to the development of deep convection and heavy precipitation in the Caribbean.