Preliminary Report on the Floods over Puerto Rico during 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 4^th into Saturday May 5^th . 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 6^th 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 6^th. 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 Precipitaion 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.