Preliminary Report on Heavy Rainfall Event across the U.S. Virgin Islands and Puerto Rico from October 4-8, 2010


Rainfall graphic from NCEP associated with Hurricane Otto.

Rainfall graphic courtesy of the National Centers for Environmental Prediction (NCEP) from October 3-8, 2010


          During the period from October 4-8, 2010, the U.S. Virgin Islands and Puerto Rico endured a period of persistent heavy rains that caused major flooding and landslides, leading to a declaration of a state of emergency by both the U.S. Virgin Islands and Puerto Rican governments.
          It’s not unusual for prolonged wet spells to affect the local area this time of the year, when tropical waves and upper cold lows interact to generate abundant rains and significant flooding. October 2010 was no exception, and this rainy event comes close to past October events, similar to events in October 1985 and October 2005. 
          Impressive rainfall amounts accumulated across the islands during the five day period, which began on the October 4th and ended on October 8th (see graphic above). This rainfall was associated with the passage of a tropical wave that eventually became Subtropical Depression Seventeen on the morning of Wednesday October 6th, when it was located about 270 miles north northwest of San Juan, Puerto Rico. Later that same day, this system strengthened into Subtropical Storm Otto and completed its transition to Tropical Storm Otto on October 7th. Already the 15th named storm of the season, this system became Hurricane Otto on the afternoon of October 8th, the eighth hurricane of the season. Although the U.S. Virgin Islands and Puerto Rico remained well to the southeast and south of the tropical storm wind field, the distant rain bands which formed Otto’s tail impacted the local area for many days, pulling moist and unstable tropical air northward across the islands. This resulted in numerous showers and thunderstorms, as well as strong gusty winds both over land and over water.


5-Day rainfall totals across the USVI associated with Hurricane Otto

Station Location
Oct. 4, 2010
Oct. 5, 2010
Oct. 6, 2010
Oct. 7, 2010
Oct. 8, 2010
Event Total Rainfall
5.71 inches*
0.56 inches
10.30 inches
4.95 inches**
21.52 inches
0.65 inches
6.61 inches
1.58 inches
3.23 inches
0.87 inches
12.94 inches
Cotton Valley, Saint Croix
0.59 inches
4.23 inches
4.37 inches
2.49 inches
0.09 inches
11.77 inches
0.27 inches
2.85 inches
3.75 inches
1.50 inches
0.44 inches
8.81 inches
0.08 inches
1.80 inches
3.56 inches
1.44 inches
0.22 inches
7.10 inches
0.15 inches
0.63 inches
2.00 inches
2.30 inches
1.74 inches
6.82 inches

*Three day total from October 2-4, 2010    **Two day total from October 7-8, 2010
5-Day rainfall totals across Puerto Rico associated with Hurricane Otto

Station Location
October 4, 2010
October 5, 2010
October 6, 2010
October 7, 2010
October 8, 2010
Event Total Rainfall
Rio Portugues, Ponce, PR
0.79 inches
1.30 inches
1.35 inches
5.84 inches
6.35 inches
15.63 inches
Lago Patillas, Patillas, PR
1.42 inches
0.00 inches
2.70 inches
5.70 inches
5.73 inches
15.55 inches
Rio Inabon, Ponce, PR
2.36 inches
2.67 inches
0.91 inches
3.48 inches
5.31 inches
14.85 inches
Lago Cerillos, Ponce, PR
2.36 inches
2.05 inches
0.87 inches
2.37 inches
4.83 inches
12.48 inches
Rio Cerillos, Ponce, PR
0.62 inches
0.66 inches
1.30 inches
4.38 inches
4.69 inches
11.65 inches
Aibonito, PR
0.06 inches
0.04 inches
0.75 inches
4.08 inches
6.01 inches
10.94 inches
Bo. Marin, Patillas, PR
0.60 inches
0.00 inches
2.30 inches
4.08 inches
3.81 inches
10.79 inches
 Lago Guayabal, Villalba, PR
1.31 inches
1.00 inches
0.86 inches
3.79 inches
3.74 inches
10.70 inches
Lago Toa Vaca, Villalba, PR
0.96 inches
0.96 inches
0.27 inches
3.63 inches
4.65 inches
10.47 inches
Patillas, PR
1.17 inches
0.00 inches
1.27 inches
3.05 inches
4.92 inches
10.41 inches
Vieques, PR
0.13 inches
0.00 inches
6.83 inches
1.07 inches
2.31 inches
10.34 inches
Lago Adjuntas, Adjuntas, PR
0.19 inches
0.14 inches
0.95 inches
3.57 inches
5.07 inches
9.92 inches
Rio Coamo, Coamo, PR
0.26 inches
0.00 inches
1.68 inches
3.62 inches
4.28 inches
9.84 inches
Lago Prieto, Adjuntas, PR
0.06 inches
0.18 inches
0.83 inches
2.43 inches
6.15 inches
9.65 inches
0.15 inches
0.95 inches
0.27 inches
1.13 inches
0.75 inches
3.10 inches


          For Puerto Rico, several of these rainfall totals, such as the 14.84 inches that fell at the Rio Portugues near Tibes over the 4 day period ending October 8th, reflect a recurrence interval of approximately 10 years (based on the point precipitation frequency estimates from the NOAA Atlas 14).

         For the U.S. Virgin Islands, the 4 day rainfall total of 12.29 inches for Charlotte Amalie (October 5th - 8th), reflects a recurrence interval between 10 and 25 years, while the highest 24-hour rainfall total of 6.61 inches, reflects a recurrence interval of between 5 and 10 years. 


The Event:

          As early as Friday morning on October 1st, 2010, NWS San Juan anticipated the possibility of a period of heavy rains associated with the arrival of a large area of disturbed weather associated with a pair of tropical waves that extended from the Lesser Antilles into the Central Atlantic. In addition, the NHC had indicated this area of disturbed weather area had a 40 percent chance of becoming a tropical cyclone over the next 48 hours. This large and complex area of disturbed weather, located around 900 miles east of the Lesser Antilles, was expected to track westward or west northwest across the U.S. Virgin Islands and Puerto Rico October 4th - 6th, with the potential to produce heavy and persistent rains.

          An excessive rainfall precipitation outlook was issued by the NWS San Juan more than 72 hours in advance of the rains, indicating that rainfall amounts of 6 to 10 inches with localized higher amounts were likely from October 4th - 7th. Also, given the prolonged nature of this potential event and the very wet antecedent conditions, it was recognized that this event presented a moderate threat of flooding (meaning that flash flooding as well as flooding of larger rivers with inundation of structures, and near streams and rivers was possible).

          Due to this potential threat of flooding, a flash flood watch was issued for the U.S. Virgin Islands and Puerto Rico at 2:42 pm on Friday October 1st, extending into the following week. By the morning October 2nd, an area of disorganized showers and thunderstorms formed across Puerto Rico and the U.S. Virgin Islands, associated with a surface trough that extended from the Leeward Islands into the Atlantic Ocean. By Sunday morning October 3rd, additional showers and thunderstorms continued over the Leeward Islands and the adjacent Atlantic and Caribbean waters, in association with this surface trough. In addition, scattered showers and isolated thunderstorms did develop across the local islands over the weekend period, with some areas of heavy rain and localized flooding.

         It was not until Tuesday October 5th that the U.S. Virgin Islands, Culebra and Vieques began to experience the very heavy rains and bands of showers and thunderstorms associated with this slowly organizing system. At 9:34 pm October 5th, the first flash flood warning was issued for Saint Thomas and Saint John, followed by a flash flood warning for Vieques and Culebra at 10:03 pm. By the end of the day, the NWS rain gage at the Cyril E. King Airport on Saint Thomas recorded a total of 6.61 inches, the 5th wettest day ever recorded at the site. 

         On Wednesday October 6th, Subtropical Depression Seventeen formed well to the west northwest, with its tail continuing to produce impressive rainfall amounts across the U.S. Virgin Islands along with numerous reports of flash flooding and mudslides. Soil conditions across the islands remained at or near saturation, and creeks and guts were at high levels, and a flash flood warning was eventually added for Saint Croix at 8:49 am October 6th. Over mainland Puerto Rico, relatively dry conditions prevailed for much of the day on October 6th. Some showers did develop across the west and southeast sections of the island, but moved rapidly eastward. The flash flood watch for Puerto Rico was redefined and extended to cover Vieques, Culebra and the eastern third of the island at 3 pm on Wednesday, and remained in effect for all of the U.S. Virgin Islands. Later that evening, scattered to numerous showers developed, some with very heavy rain, across the southern half of Puerto Rico, Culebra and Vieques. The Rio La Plata in Aibonito, Rio Majada in Salinas, and Rio Grande de Patillas, were all reported out of their banks.

         On Thursday October 7th, periods of heavy rainfall affected much of Puerto Rico and the U.S. Virgin Islands. At 3:07 am, the flash flood watch was then expanded to include all of Puerto Rico and the U.S. Virgin Islands through 6 pm that afternoon. Flash flood warnings were issued for numerous municipalities in Puerto Rico, and were kept in effect for Saint Thomas and Saint John. Flash flooding, river flooding, and mudslides occurred over much of the area, as the large circulation associated with Tropical Storm Otto, kept drawing deep tropical moisture across the local islands.

         On Friday October 8th, the rains continued to affect Puerto Rico and the U.S. Virgin Islands. Flood warnings were now in effect for many of the main rivers in Puerto Rico. Flash flood warnings were reissued for the U.S. Virgin Islands during the afternoon, as bands of showers and thunderstorms, some with gusty winds and heavy rainfall, continued to move across the local islands. However, compared to previous days, a significant improvement in weather conditions was noted, since most of the steady rains had dissipated. The flash flood watch was eventually extended until 4 am Saturday, but it was discontinued for the U.S. Virgin Islands and some sections of Puerto Rico at 12:35 am Saturday, and at 4:11 am Saturday for the remainder of Puerto Rico.


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