Flood Event of November 11 to 15, 2004 in Puerto Rico
 
Flooding and landslides affected a significant portion of northern, central and northeast Puerto Rico during the period from November 11 to 15, 2004. The sustained rainfall, mostly moderate in intensity, seemed to be particularly effective in producing landslides in the steep terrain of the north facing slopes. The river flooding in the major basins of north central Puerto Rico, including the Rio Grande de Arecibo, Rio Grande de Manati, Rio Cibuco and Rio de La Plata was exacerbated by a persistent strong northerly flow and high surf that prevented normal drainage of these basins into the Atlantic Ocean. One flood death was reported after this event and one person died in a landslide.
 
A persistent upper trough to the west of Puerto Rico and an extremely moist low level air mass (nearly saturated from 600 mb to the surface) shifted slightly westward to encompass Puerto Rico early on the 12th, bringing widespread shower activity. By 1200 UTC on the 12th rainfall was already fairly impressive, with 2.73" at Represa de Comerio (CMRP4); 2.66" at Aguas Buenas (BZBP4), 2.59" at Rio Canas and 2.40" at Naranjito (NAMP4), along with numerous 1 to 2 inch rains over the northeast quarter of Puerto Rico. The heavy rainfall prompted issuance of an Urban and Small Stream Flood Advisory around the San Juan metro area, after reports of flooding in a residential area of Toa Baja were received. Reports of flooding were received along the Rio Cibuco in Vega Alta, Sector Fatima, prompting issuance of a Flood Warning for that river. Moderate to occasionally heavy rains persisted throughout much of the 12th under strong north to northeast surface flow that produced substantial orographic enhancement over the foothills and mountains of central and eastern Puerto Rico. Rainfall intensity remained fairly low, but the persistence produced impressive totals in these areas, with 3 to 6 inches across a huge swath of central Puerto Rico for the 24 hours ending at 1200 UTC on the 13th (see Table 1 below). Flood Warnings for river basins draining north from the mountains, including the Rio Grande de Arecibo, Rio Grande de Manati, Rio de La Plata and Rio de Bayamon were issued early in the day and remained in effect for up to two days in some locations. Crest forecasts were complicated considerably by the high surf and north winds along the coast which hindered normal discharge of rivers into the sea.
 
The activity continued to be widespread and intense on the 13th as a Flash Flood Warning was issued toward mid-morning for 14 northeast municipalities from Fajardo southwest to Gurabo and Juncos. Reports of flash flooding became more numerous throughout the day and another Flash Flood Warning was issued for 7 municipalities in the central mountains in the afternoon. This warning was extended into the evening as moderate to heavy rainfall persisted across the same area. Numerous landslides were also reported at this time, especially in Cayey, Aguas Buenas and Naranjito. Bridges were reported overtopped, collapsed or washed out in several locations, including Naranjito, Orocovis, Morovis, Aguas Buenas, Cidra, and Utuado. Another Flash Flood Warning was issued for Dorado and Toa Baja, primarily in the highly saturated Rio de La Plata basin during the evening. 24-hour rainfall ending at 1200 UTC on the 14th was very comparable to the previous day, with highest amounts, in excess of 5 inches, centered over the area around Comerio, Aibonito and Aguas Buenas. The 14th saw little break in the rainfall activity as the upper trough remained anchored to the west of the island and the very strong and moist low level northeast flow continued bringing showers over the island. Rainfall intensity remained moderate throughout much of this event, with hourly amounts rarely exceeding one inch and often remaining below 0.5 inches. The persistent moderate rainfall appears to have been ideal for the production of landslides around the island. Landslides covered numerous roads and several damaged or collapsed bridges were also reported. Around 1700 UTC on the 14th a landslide in Adjuntas, Barrio Yahuecas, pushed a vehicle with three persons into the Rio Blanco, causing a 28-year old man to drown and injuring two young women. A direct flood death occurred in Guaynabo around the same time when a 74-year old woman was reportedly swept from near her home into the Rio Guaynabo, a tributary of the Rio de Bayamon. Her body was recovered five days later. Meanwhile, Flood Warnings remained in effect for many of the river basins draining from the northern slopes of the central mountains. The most intense rains on the 14th occurred over the east central and southeast parts of the island, prompting a Flash Flood Warning for 10 municipalities in that region. During the late afternoon through the overnight hours of the 14th the heaviest rains fell across Culebra, Vieques and the U.S. Virgin Islands, prompting several Flood Advisories and a Flash Flood Warning for St. Croix.
 
Flood Warnings along the Rio Cibuco, Rio de La Plata, Rio Grande de Manati and Rio Grande de Arecibo continued into the 15th, as the strong onshore flow continued to restrict outflow of these rivers into the Atlantic Ocean. Rainfall amounts were much less than previous days however, with only isolated amounts in excess of one inch and many locations dry for the first time in nearly a week.
 
 
Table 1. Top Twenty-five Stations: 24-hour rainfall and 4-day storm total ending at 1200 UTC:
 
Station Name (NWS ID)
Nov 12
Nov 13
Nov 14
Nov 15
Total
Rio Icacos nr Naguabo (NGIP4)
2.23
4.63
6.12
4.24
17.22
Lago Icacos at Damsite (LICP4)
1.84
4.47
5.37
3.97
15.65
Boca Grande nr Jayuya (JACP4)
1.67
6.41
3.25
3.33
14.66
Represa De Comerio (CMRP4)
2.73
4.62
5.55
1.51
14.41
R. De La Plata - Comerio (COMP4)
1.82
5.47
4.88
1.15
13.32
Bairoa Arriba RG (BZBP4)
2.66
5.49
3.71
0.81
12.67
R. Guadiana nr Naranjito (NMAP4)
2.40
4.63
3.93
1.68
12.64
Aibonito Coop (ALPP4)
1.20
4.20
5.80
1.03
12.23
Rio Bauta  Orocovis (BAUP4)
1.05
6.15
3.15
1.84
12.19
Bo. Mamays Ab. nr Jayuya (JABP4)
1.28
4.93
2.59
2.42
11.22
R. Limon ab. L. Dos Bocas (ARHP4)
1.23
3.89
3.13
2.89
11.14
Bo. Saliente nr Jayuya (JAZP4)
0.96
4.50
3.08
2.46
11.00
Bo. Fronton nr Ciales (CIBP4)
1.08
4.79
2.67
2.31
10.85
R. Caonillas at Paso Palma (JAXP4)
0.85
5.43
2.40
2.16
10.84
Rio Orocovis at Orocovis (RORP4)
0.72
4.99
2.55
2.18
10.44
Rio Bayamon at Arenas (CIFP4)
0.51
4.11
4.57
0.76
9.95
Gurabo Abajo RG (GARP4)
1.10
4.58
2.97
1.23
9.88
Bo. Beatriz RG (BZDP4)
0.50
4.19
4.32
0.85
9.86
Rio Jauca at Paso Palma (JAPP4)
0.55
5.62
1.81
1.44
9.42
Canaboncito RG (CAHP4)
0.85
3.61
4.35
0.36
9.17
Adjuntas 2 NW (ADKP4)
1.22
3.73
2.07
2.03
9.05
R. Gde. De Arec.  Utuado (UTXP4)
1.91
2.67
2.33
2.11
9.02
Rio Turabo abv Borinquen (CAKP4)
0.40
4.91
2.55
1.07
8.93
Bo. Consejo nr Adjuntas (ADNP4)
1.21
4.45
1.81
1.34
8.81
Rio Yunes at Hwy 140 (ADMP4)
1.23
4.51
1.78
1.23
8.75
 
River flooding from this event was significant but quite ordinary by Puerto Rico standards and was considerably less than during Tropical Storm Jeanne in September 2004. The rainfall and subsequent flooding was also somewhat less than occurred in November 2003, which affected a larger area and had slightly higher rainfall amounts. Eighteen (18) river gage locations in Puerto Rico (out of over 60 locations with defined Flood Stages) exceeded Flood Stage (see NWS E-3 Report for crest details) and of these only three (3) reached Moderate Flood Stage and one (1) Major Flood Stage. This occurred at Rio de La Plata at Comerio (COMP4), where records have been kept for only 15 years, so this may be subject to revision as more data becomes available. Return period data, where available, shows that the largest basin discharges were less than a 10-year frequency and most less than a 5-year frequency.
 
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