SYNOPTIC PATTERNS CONDUSIVE TO FLASH FLOODS OVER PUERTO RICO
An analysis is presented of the synoptic-scale conditions associated with heavy rainfall events over Puerto Rico. Although tropical waves and cyclones are known to have the potential to produce torrential rainfall amounts, this study investigates the flooding potential of quasi-stationary fronts, old frontal boundaries, upper level troughs, upper level jet-streak/streams, and their interactions with the islands local effects. The flooding events are identified by analyzing 24 hour precipitation data over a 10 year period. Synoptic analyses are based on NCEP's reanalisis data.
In cases where slow-moving fronts reach the island from the northwest, a surface trough developes ahead of the fronts initiating convection over the mountains of Puerto Rico. Typically rainfall continues as the mid-latitude cyclones move east or weaken, leaving a west to east stationary front over the area for 2-3 days. Other flood events are characterized by the proximity of upper-level low pressure systems on either side of the island. These upper level lows are typically associated with the Tropical Upper Tropospheric Trough or TUTT. The TUTT lows have cold core centers and can produce moderate to strong convection in the quadrant of divergence at upper levels. The divergent quadrant may be located on either side of the low pressure center depending on how the system tilts with height. Knowledge of synoptic weather types conducive to widespread heavy rain events can lead to better forecast decisions.