Preliminary Report on the Floods over Puerto Rico

May 6-7 2001


During the period May 6-7 2001, Puerto Rico received torrential rainfall, resulting in widespread flooding of streams, rivers and roads, and also landslides in many municipalities. The heaviest rainfall and most severe effects occurred in the municipalities of Western, Southwestern, Southern, and the Western Interior of Puerto Rico. Two flood related fatalities in Yauco, and an estimated 146 million dollars in property damage area wide were the most tragic consequences of this event. The government of Puerto Rico requested that 22 municipalities be declared Federal Disaster Areas.

The rains were associated with a weather system referred to as a strong mid to upper level trough. This type of weather system occurs often during May and results in significant rainfall over the island. Upper level troughs usually develop in the mid latitudes and move east southeastward, penetrating as far south as the Caribbean. On occasions, upper level troughs combine with low level or surface troughs and fronts or other weather systems such as tropical waves. The combined effects can create devastating weather events or flooding over our local area.

On the days leading up to the event, an intense mid to upper level trough northeast of the Bahamas deepened and produced an associated low level trough extending southward across the Bahamas and the Southwest Atlantic. The system progressed very slowly eastward and remained nearly stationary across the Dominican Republic and to the west northwest of Puerto Rico Friday May 4th into Saturday May 5th . Therefore, most of the associated weather remained over the Dominican Republic during the days prior to the event.

The upper trough was forecast to move close enough to Puerto Rico on Sunday through Tuesday, to produce showers and thunderstorms across the area, as compared to the previous days with strong dry southerly winds and little to no rainfall. By early Sunday morning May 6th the upper trough moved east southeast and was in place just to our west, and then combined with this intense low level or surface trough which traversed across Puerto Rico in a west northwest to east southeast direction from Sunday afternoon into Sunday evening.

The combined effects of this deep system were felt mainly along the west half of Puerto Rico beginning on the afternoon of Sunday May 6th. The first line of intense showers and thunderstorms developed over the Mona Channel during Sunday morning. It moved steadily eastward into Western Puerto Rico during the afternoon, and produced copious amounts of rainfall along the west and southwest sections. Additional lines of showers and thunderstorms continued to develop and affected the island Sunday night through Monday morning. Some areas received rainfall rates of 1 to 2 inches or more per hour.

The highest 24 hour rainfall accumulation of 14.19 inches was recorded at the Lajas AES, followed by 12.75 inches at Yauco, and 9.36 inches at the Adjuntas AES. In general, from 7 to over 12 inches of rain fell in a 24 hour period over sections of the South, West, and Western Interior of Puerto Rico. Elsewhere rainfall varied from 1 to near 7 inches.

Moderate to extreme river rises were observed during the event. The majority of the highest river rises occurred in the southwest. See table 5. Gaged sites that exceeded 15 feet were; Rio Culebrinas near Moca,18 feet; Rio Guanajibo at San German, 16 feet; Rio Guanajibo near Hormigueros, 16 feet; and Rio Guayanilla near Guayanilla, 15 feet. As a result of this event the Luchetti Lake near Yauco rose 15 feet, and Rio Grande de Manati near Manati along the north coast rose 18 feet in stage.

Rainfall values around the vicinity of Lajas reached the 50-year 24-hour accumulations and the 100-year 12-hour accumulations. Comparison with the TP-42 charts indicate the 24-hour duration rainfall did not exceed the 100-yr return period. Nevertheless, radar imagery shows that the actual duration of the event lasted less than 12 hours. If we compare the actual rainfall with the 100-year 12-hour rainfall, we can infer that the 100-year 12-hour rainfall was exceeded.

Rainfall values in text format and in graphical format and values plotted on a map from the Southeast River Forecast Center in Atlanta and from the WFO San Juan's Daily Temperature and Precipitation product showed a maximum value of 14.19 inches in Lajas. The Storm Total Precipitation Estimates from our Doppler Radar tended to underestimate the rainfall through the event.

Doppler Radar Loop from 08 UTC (4 am) through 02 UTC (10 pm) on Sunday May 6th as the trough swept across the western half of Puerto Rico.

Visible Satellite Loop from 1245 UTC (845 am) through 2145 UTC (545 pm) through the daytime hours on Sunday May 6th.

Watch and Warning Summary May 6-8 2001 Flood Event

Date/Time AST Action Location
06/0215 pm Flash Flood Watch issued until 4 am Monday Southwest, Central Interior, Western Interior, Northwest, Mayaguez and Vicinity, including the municipalities of Moca, San Sebastian, Isabela, Maricao, Yauco, Anasco, Rincon, Guanica, Hormigueros, Las Marias, Mayaguez, Quebradillas, Camuy, Lares, Lajas, San German, Aguadilla, Cabo Rojo, Sabana Grande, Aguada
06/0425 pm Flash Flood Watch extended to cover all of Puerto Rico until 4 am Monday All of Puerto Rico except Vieques and Culebra
06/0433 pm Severe Thunderstorm Warning issued until 6 pm Guanica, Guayanilla, Lajas, Sabana Grande, San German, Yauco, Adjuntas,
06/0433 pm Flash Flood Warning until 730 pm Yauco, Sabana Grande, Cabo Rojo, Lajas, San German, Hormigueros
06/0700 pm Flash Flood Warning extended until 10 pm Yauco, Sabana Grande, cabo Rojo, Lajas, San German, Hormigueros, Guayanilla
06/0718 pm Flash Flood Warning extended until 1015 pm Jayuya, San Sebastian, Maricao, Guayanilla, Yauco,


Anasco, Aguada, Rincon, Guanica, Hormigueros, Penuelas, Las Marias, Mayaguez, Moca, Lares, Lajas, San German, cabo Rojo, Adjuntas, Utuado, Sabana Grande

06/0920 pm Flash Flood Warning issued until1215 am Monday Ponce, Villalba, Arecibo, Orocovis, Juana Diaz, Florida, Morovis, Ciales
06/1025 pm Flood Warning issued until 430 am Monday Jayuya, Arecibo, Adjuntas, Utuado, and the following rivers, Rio Grande de Arecibo, Caonillas, Tanama, and tributaries
06/1030 pm Flood Warning issue until 430 am Monday Hormigueros, cabo Rojo, San German, Guanajibo rivers and tributaries
06/1030 pm Flood Warning issued until 430 am Monday Aguada, Moca, San Sebastian, and river Culebrinas and tributaries
06/1055 pm Flash Flood Warning issued until 2 am Monday Cayey, Aibonito, Barranquitas, Caguas, Coamo, Cidra
07/1230 am Flash Flood Warning issued until 4 am Ponce, Villalba, Juana Diaz, Coamo, Santa Isabel, Aibonito, Salinas, Cayey, Barranquitas, Cidra, Guayama
07/0100 am Flash Flood Warning issued until 5 am Arroyo, Patillas, San Lorenzo, Yabucoa, Maunabo, Humacao, Las Piedras, Juncos, Gurabo, Naguabo
07/0325 am Flash Flood Watch extended until 4 pm All of Puerto Rico, Vieques and Culebra
07/0539 am Flash Flood Statement All of Puerto Rico, Vieques and Culebra
07/0638 am Urban and Small Stream Flood Advisory until 10 am All of U.S. Virgin Islands
07/0930 am Flash flood Watch issued until 4 pm All of U.S. Virgin Islands
07/1000 am Flood Warning extended until 4 pm  
07/1032 am Flash Flood Warning issued until St Croix
07/0400 pm Flash Flood Watch extended until 10 pm All of U.S. Virgin Islands
07/0400 pm River Flood Statement to discontinue Flood Warning Manati, Barceloneta, rivers Rio Grande de Manati, Hormigueros, Cabo Rojo, and river Guanajibo,
07/0400 pm Flash Flood Statement to discontinue Flash Flood Watch All of Puerto Rico
07/0410 pm Flash Flood Warning extended until 715 pm St. Croix
07/0715 pm Flash Flood Warning extended until 915 pm St. Croix
07/0905 pm Flash Flood Warning extended until 1215 am Tuesday St Croix
07/1000 pm Flash Flood Statement U.S. Virgin Islands
08/1212 am Flash Flood Warning extended until 3 am St. Croix
08/0207 am Flood Statement until 10 am River Guanajibo
08/0259 am Flash Flood Warning was extended until 6 am St. Croix
08/0400 am Flash Flood Watch extended until 11 am All of U.S. Virgin Islands
08/0422 am Special Marine Warning until 620 am St. Croix coastal waters
08/0618 am Flash Flood Statement to discontinue Flash Flood Warning St. Croix
08/1100 am Flash Flood Statement to discontinue Flash Flood Watch All of U.S. Virgin Islands

Prepared by:
Rafael Mojica
Warning and Coordination Meteorologist

Rachel Gross
Science and Operarations Officer

Eloy Colon
Service Hydrologist

National Weather Service Forecast Office
San Juan, Puerto Rico

Disclaimer: This is a preliminary report and is subject to change is the U.S. government's official web portal to all federal, state and local government web resources and services.