Tropical Storm Hanna Lashes Puerto Rico and the USVI

Overview

On August 28, 2008, Tropical Storm Hanna formed in the tropical Atlantic Ocean and became the eighth named system in the Atlantic Basin for the season. Climatologically, late August through early October is an especially active time for storms to develop in the Atlantic Ocean, with tropical cyclone development in the Atlantic Basin peaking during the second week of September.

As Hanna propagated west northwest, it encountered an upper level low pressure system that was located just to the north of Puerto Rico. This upper low actually protected Puerto Rico by exposing Hanna to southwest shear around its counterclockwise flow. This, in turn, caused Hanna's eye to become briefly exposed and convection around Hanna to favor its eastern quadrant.

As the storm passed to the north of Puerto Rico, an upper level high pressure system strengthened in the northern Atlantic Ocean, which helped to steer Hanna more to the west. Hanna briefly reached hurricane status on Monday, September 1, 2008, as it approached the Turks & Caicos Islands and the southeastern Bahamas. Eventually, a strong upper-level low pressure system and associated trough located near the east coast of the United States resulted in increased vertical wind shear that once again exposed Hanna's center, forcing much of its convection to form on its eastern side. This weakened the storm back to tropical storm status by the late morning of September 2, 2008. Eventually, Hanna was captured by the upper low and was entrained into the circulation of the trough allowed the storm to drift to the south toward Haiti causing massive mud slides and river flooding throughout the country.

By this time, deep, tropical atmospheric moisture engulfed the Caribbean. Precipitable water values over San Juan were greater than 2 inches, which is approximately 20 percent above normal.

This, coupled with deep moisture convergence along Hanna's southeastern quadrant, caused outer rain bands to develop, redevelop, and train over Puerto Rico and the U.S. Virgin Islands over the course of three days (September 1-3, 2008) with the peak and subsequent culmination of the event occurring on Wednesday, September 3, 2008. These rain bands brought with them thunderstorms that produced rainfall in excess of 3 inches per hour. Due to the fast moving nature of these storms, no one storm produced the flooding rains that were seen across most of Puerto Rico and the Virgin Islands, but rather a series of thunderstorms that kept repeatedly propagating over the same geographic area multiple times throughout the three day period. This process quickly increased the water levels of many fast-responding rivers in Puerto Rico and several reached moderate flood stage, causing flooding of lower lying areas.

Total rainfall for the first three days of September was high across many stations in Puerto Rico. While the airports across Puerto Rico and the U.S. Virgin Islands received in upwards of 2 inches throughout the three day period, many other stations received much more. Table 1 below depicts reported precipitation from various ASOS sites across the county warning area.

Table 2 below depicts reported precipitation for the three day period from some selected USGS and COOP sites across the island of Puerto Rico. Precipitation values varied greatly across the island. Some areas surprisingly received no rain at all, while others were deluged with rainfall reports nearing 14.50 inches.

In addition to the heavy rainfall throughout the three day period, strong directional wind shear, which is defined as air coming from different directions at different levels throughout the same column in the atmosphere, created an environment conducive to severe weather, including straight-line winds and tornadoes, on Wednesday, September 3, 2008. On this particular day, 0-3-km storm relative helicity values, which define how much winds are turning in the lower atmosphere, were approximated at 112 m*2/s*2. This is a very high, but rare, value to see this deep within the tropics. This subjectively can be confirmed by the sounding image above, where winds mostly veer with height all the way through the column. Rain bands and isolated storms, tapping deep atmospheric moisture already in place from Tropical Storm Hanna, responded to this environment. National Weather Service (NWS) Doppler Radar showed frequent severe thunderstorm characteristics throughout the day across the entire area, with thunderstorms developing across the offshore Caribbean waters and moving quickly northward across the local area. One damaging blow to the islands came when a strong rain band developed over the Caribbean waters just south of Puerto Rico, which crossed Puerto Rico and the adjacent northern Atlantic waters during the early afternoon hours on Wednesday. This line of storms developed a bowing segment on radar, effectively racing across the area, bringing with it heavy rainfall and high winds. These high winds, both in the form of straight line winds and weak tornado spin-ups at the apex of the bow, toppled trees, power lines, and sank small crafts out over the open waters. Towns located in areas of higher elevation, such as Cayey, within the central mountainous interior, were hardest hit by this feature, while cities closer to sea-level, such as San Juan, were impacted, but with much less ferocity. NWS Doppler Radar indicated winds in excess of 71 knots (82 mph) relative to the ground 3500 feet above sea level as the rain band approached the island. This feature crossed the island with a speed of roughly 40 knots (46 mph), adding to the straight line wind speed that occurred at the surface. While interaction with the land (friction) slowed the final wind speeds measured at the surface, they were still fast enough to cause widespread damage across the island. In the image below, you can see the straight-line winds actually occurring in the velocity product associated with the line of storms. The bright green colors indicate strong winds blowing toward the radar while the bright red colors indicate strong winds blowing away from the radar. Once the bright green signatures cross the radar�s path, they change color, indicating that the winds coming from the south southwest that were once blowing toward the radar, are now blowing away from the radar. The radar's location is depicted by the small black circle centered over Cayey, Puerto Rico, which is located within the island�s southeastern interior.

Numerous other features of interest included an isolated mesocyclone that formed and moved across the San Juan, Carolina, and the Trujillo Alto area. This storm had a well-defined mid-level rotation associated with it, which could easily be seen on NWS radar products. Storms usually exhibit signs of rotation and develop distinct mesocyclones when strong wind shear is present. Storms exhibiting these types of characteristics can produce hail, high winds, heavy rainfall, and even tornadoes. Tornadoes and water spouts (tornadoes over water) form when lift, deep moisture, and strong rotational wind shear combine. In this environment, Lifted Condensation Level (LCL) heights were just over 500 feet. That means that the low-level moisture (dew point) was so high, storm bases developed just over 500 feet off the ground. The smaller the distance between the storm's base and the ground allows any spin-ups that develop to reach the ground with relative ease. While no tornado was reported with this storm, strong rotational signatures indicated by radar over the Atlantic waters north of San Juan reveal the possibility of a potential water spout.

Many other storms, both in isolated form or in a line, affected the islands during this three day event while Tropical Storm Hanna meandered near Haiti and the Bahamas.

Impacts

While Puerto Rico and the U.S. Virgin Islands were not in the direct path of Tropical Storm Hanna, deep atmospheric moisture and directional shear coupled with surface convergence associated with Hanna's outer rain bands resulted in extensive flooding and damage to the islands. It was the perfect case to illustrate the dangers of tropical cyclones that are centered hundreds of miles away. Storms rarely need to make a landfall for its effects to be felt. Hanna's center, or eye, at its closest point to Puerto Rico on September 3, 2008, the day when the islands were most affected by the storm, was nearly 350 miles away to the west northwest, however the outer rain bands associated with the storm caused extensive damage to Puerto Rico and the U.S. Virgin Islands.

Damage Reports

Damage was extensive island-wide. Wind damage mainly was confined to fallen trees and downed power lines. Fallen trees accounted for 18 damage reports, blocking roads across many city and urban areas, including Mayaguez, San German, Cabo Rojo, Maricao, and Cayey. There also was one report of a home that lost its roof at barrio El Verde in Comerio due to wind. In addition, over 18 major roads across multiple towns were closed or diverted due to urban flooding. Heavy rain also was responsible for land/mudslides across the higher terrain, which damaged roads and bridges in Sabana Grande, Maricao, Yauco, Lajas, and San Carlos. Additional damage reports also came into the office from the city of San Juan and Carolina as well.

Additionally, homes were reported flooded at Barrio Pasarel in Comerio and Urb. San Jose Valverde, in San Juan.

River flooding also was an issue. In Vega Baja, road 160 was closed due to the river Rio Indio overflowing its banks. In Manati roads 667, 604 were flooded and impassable. In Camuy, road 4491 flooded due to the river Camuy overflowing its banks and in Arecibo, La Puntilla Sector, River Rio Grande close to overflowing its banks, and, as a result, families were advised to evacuate.