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.
Figure 1. Infrared satellite imagery taken on May 2, 2008 superimposed
with a tropical surface analysis depicting a frontal boundary approaching Puerto Rico.
The yellow line with arrows represents the cold front advancing east southeastward.
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.
Figure 2. Infrared satellite imagery taken on May 6, 2001 depicting a frontal
boundary approaching Puerto Rico. The blue line represents the cold front advancing east southeastward.
Figure 3. Infrared satellite imagery taken on November 7, 2001 depicting a
frontal boundary approaching Puerto Rico. The blue line represents the cold front advancing east southeastward.
Figure 4. Visible satellite imagery superimposed with low-level satellite
derived winds taken on April 18, 2003 depicting a frontal boundary approaching Puerto Rico.
The blue line represents the cold front advancing east southeastward.
Figure 5. Water vapor satellite imagery superimposed with upper-level
streamlines and wind analyses taken on October 11, 2005 depicting a frontal boundary
approaching Puerto Rico. The blue line represents the cold front advancing east southeastward.
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.
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