A Strong Tropical Wave Brings Beneficial Rains

Overview

On August 14, 2008, a strong tropical wave moved across the northeast Caribbean, passing westward across the U. S. Virgin Islands during the late afternoon and early evening hours, and then across Puerto Rico during the overnight and morning hours of August 15, 2008. Visible satellite imagery on the afternoon of the 14th appeared to show a tropical depression or tropical storm developing, with well defined cyclonic turning at mid levels, and anticyclonic outflow directly above it. When a hurricane hunter aircraft flew into this system, however, it found the seemingly classic storm structure occurring only at the middle levels of the atmosphere, over the northern Leeward Islands, with a weak lower level cyclone over the Anegada Passage. The lowest surface pressures were found even farther to the west of the main visible mid-level circulation across the Virgin Islands.

The main question then seemed to be when and where would these differing circulation centers merge. During the late evening hours strong thunderstorm activity shifted from the Virgin Islands westward across Culebra, Vieques, and the adjacent Caribbean waters, exploding after midnight as this activity shifted westward into eastern Puerto Rico and the Caribbean waters just to the south. This produced an electrifying lightning show, torrential rains, and gusty winds, which turned out to be the beginning of the merger of the mid level circulation and lower level low to its west.

As this system exited Puerto Rico and moved across the Mona Passage on the afternoon of the 15th, the National Hurricane Center (NHC) found sufficient evidence to call this area of disturbed weather Tropical Storm Fay.

Impacts

Most of the impacts across the islands were generally mild. Precipitation reports ranging from 1-3 inches were recorded area-wide with locally higher amounts ranging from 2-4 inches across the eastern sections of Puerto Rico, the islands of Culebra and Vieques, and the U.S. Virgin Islands. A few small isolated areas along the extreme southern coast of Puerto Rico were estimated by the National Weather Service (NWS) Doppler to have received 6-7 inches during the overnight hours. Most notably in the image below is the fact that the majority of the precipitation fell over the Caribbean Sea south of the island. Radar estimated greater than 10 inches fell over the water south of the island. If those stronger bands of precipitation fell over the island, severe flooding would have been observed island-wide.

The NWS Doppler Radar precipitation estimates also revealed one-hour rainfall rates in excess of 3-4 inches/hour in some very localized spots, however, due to the precipitation banding affect, the heaviest rainfall did not fall in one place for long periods of time. As a result, only localized flooding was reported.

Specifically, the most notable report received included a river rise on the Rio Fajardo. Normally, this river flows at approximately 4.5 feet. During the height of the storm, the river increased its flow to approximately 10 feet, which is 2 feet above flood stage. This caused Route 3, a major artery through the heart of Fajardo to be shut down for a period of time before the water eventually, but quickly, receded.

Wind reports were very scarce. Generally, sustained winds ranged from 15-25 mph across much of the islands during the passage of the wave. Some notable gusts included Cabo Rojo at 36 mph, St. Thomas at 35 mph, Vieques at 32 mph, and San Juan at 21 mph.

No damage, deaths, or injuries were reported.

For most of the residents on the islands, this tropical wave was extremely beneficial. Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands continue to be very dry, with one of the driest summers recorded on record through the beginning of August. Reservoirs have been running low and water restrictions were about to take place before the wave eventually hit. Consequently, this wave was welcomed by most residents, which helped cut the edge off the ongoing short-term drought during the early summer months.