Frontal Boundaries and Associated Effects

Significant rains fell across the western half of Puerto Rico on May 2, 2008, causing widespread flooding across both urban and rural areas of Puerto Rico's western interior. These heavy rains were a direct result of a frontal boundary, which extended far beyond the subtropical latitudes where most cold fronts normally generate and reside.

A cold front is nothing more than a density discontinuity between colder, drier, denser air and warmer, moister, lighter air. When cold fronts push into the tropics, two things normally occur: 1) the forward momentum of the front tends to slow as the front interacts with tropical circulation patterns, and 2) the density discontinuity becomes more ill-defined with time as the two air masses that once made up opposite sides of the front begin to moderate over the warm waters of the tropical oceans.

In the tropics, ample low-level moisture and instability, generated by warm daytime surface temperatures in contrast with the temperatures above the surface, are available on a nearly daily basis for the generation of cumulus clouds and precipitation. Usually, the limiting factor for widespread rains is lift, or broad-scale upward vertical motion that leads to thunderstorms. Mesoscale features such as converging sea breezes and the interaction between the trade winds and terrain account for smaller-scale lift across tropical islands, however larger-scale lift is often lacking. Aside from tropical waves and disturbances, synoptically-driven troughs and cold fronts that advance southeastward from the mid-latitudes across Puerto Rico account for the majority of this necessary component needed for widespread rainfall to occur.

Advancing cold fronts into the tropics are not rare phenomena for Puerto Rico in the transitional spring and fall months. Similar weather conditions to this May 2, 2008 event have occurred over Puerto Rico within the past decade, causing significant flooding over sections of the island. Below are several images depicting past cold fronts that have advanced toward Puerto Rico, causing significant rainfall to occur. Captions below the images describe the date of the event.

Rainfall climatology across Puerto Rico suggests that May and November are typically the two wettest months of the year, outside of hurricane season. During these transition months, mid-latitude frontal boundaries exit the eastern United States as strong high pressure systems build across the Central Plains. As high pressure pushes eastward, the frontal boundary is forced to do the same, stretching southward toward the tropics. These boundaries eventually slow in speed and can linger in the tropics for days. Depending on the moisture available, these fronts can typically cause torrential and devastating rainfall for sustained 12-24 hour periods before finally waning.

In the May 2, 2008 cold front case, numerous cases of flash flooding were reported as a slow-moving frontal boundary crossed Puerto Rico from northwest to southeast. In addition, many rivers, including the Rio Culebrinas and Rio Grande de Arecibo reached or surpassed bank-full stages. Multiple landslides were reported by Emergency Management as well as eroded streets and broken bridges caused by swift-moving flood waters. These headwaters, unfortunately, also forced several families to be relocated from their homes. Tragically, two people died during this event as they tried to cross flooded areas with their vehicles.

Many of these tragic incidents occur as synoptically-driven weather patterns, such as cold fronts, coupled with ample tropical moisture, interact with the island's flashy landscape and high terrain. While synoptically-induced flooding does not occur with every transitional weather period, which is defined here as the change in seasons between spring and summer or between fall and winter, neither does a hurricane or tropical system affect the island during every hurricane season. The climatology of the period simply suggests an increased opportunity for such systems to occur.