Composite Meteorological Forcing of Puerto Rican Springtime Flood Events

Abstract

The central Antilles Islands experience short periods of heavy rainfall during the spring season (April and May) when trade winds weaken across the Caribbean Sea. Composite analysis of the top 10 flood events in the period 1979?2005 is carried out to understand the meteorological forcing. Cases are selected when mean rainfall over Puerto Rico exceeds 50 mm day?1 and emergency management reports indicate the day is a ?declared weather disaster.? In the NCEP?NCAR composite analyses, pulses of moisture shift westward across the tropical Atlantic about 10 days before a flood event. Five days before the composite flood a westerly trough penetrates eastward from the Gulf of Mexico. Northward flow develops over the Caribbean Sea and a southwest-oriented cloud band extends from Colombia toward Puerto Rico. A key feature of the midtropospheric circulation field is the development of anomalous twin rotors east of Florida in the mid- to upper troposphere. The flood events coincide with a change in zonal wind shear from westerly to easterly that is brought about by slow tropical and fast subtropical wave systems.