Vigorous Tropical Wave Keeps 21 Municipalities Under Flood Alert With 40 mph Gusts in the West

A vigorous tropical wave kept Puerto Rico under a pattern of heavy rain this week and triggered a chain of flood warnings that, at its widest point on Wednesday night, September 16, covered 21 municipalities in the east and northeast of the island ¹. The National Weather Service (NWS) in San Juan extended the alerts several times within minutes as Doppler radar detected accumulations of up to four inches of rain in some areas ¹.

The system was first felt Wednesday afternoon, when the wave approached Puerto Rico and raised humidity across the entire island, according to the NWS as cited by El Nuevo Dia ³. The official forecast anticipated that the rainy pattern would continue "at least through Friday," September 18, with morning showers favoring the east and southeast, and afternoon activity concentrating over the interior and west due to daytime heat and local effects ³. Already on Monday, September 14, the NWS had warned that heavy rain could cause urban and river flooding "particularly in the interior west of Puerto Rico," listing fifteen municipalities at elevated risk: Jayuya, Utuado, Adjuntas, Lares, Yauco, Maricao, Las Marias, San Sebastian, Sabana Grande, San German, Mayaguez, Anasco, Moca, Aguada and Rincon ⁴.

On Wednesday night, the NWS issued two consecutive flood warnings for the metropolitan area and the east. The first, valid until 7:00 p.m., covered Caguas, Gurabo, Juncos, Las Piedras and San Lorenzo, where one to two inches of rain had already fallen with one to two additional inches expected, carrying a risk of urban flooding and stream overflow ¹. Minutes later, a second warning, valid until 7:30 p.m., extended the alert to Canovanas, Carolina, Loiza, Luquillo, Rio Grande, Trujillo Alto, Catano, Dorado, Guaynabo, San Juan and Toa Baja, with similar accumulations and a risk of minor flooding in low-lying areas with poor drainage ¹. In total, 21 municipalities in the east and northeast were under alert that night ¹.

At the same time, the west faced its own round of severe weather. A severe thunderstorm watch and a flood warning were in effect until 5:00 p.m. Wednesday for Aguada, Anasco, Moca and Rincon, with wind gusts up to 40 miles per hour and one to two additional inches of rain falling on ground already saturated from previous days of downpours ². The NWS repeated its most insistent message: never cross flooded roads, turn around and find an alternate route ². That same Monday, east winds between 9 and 16 miles per hour with gusts up to 22 miles per hour had already accompanied the first round of showers over the interior west ⁴.

What this means for you

If you live in any of the more than 25 municipalities named in these alerts, from the metro area to the central mountain range and the west coast, this week's real risk isn't just the rain itself, but saturated ground that absorbs it increasingly poorly. A one-inch downpour that would normally drain without issue can now back up streams, flood secondary roads and trigger landslides in mountainous sectors of Utuado, Adjuntas or Las Marias. For those living near rivers or in low-lying zones of San Juan, Carolina, Toa Baja or Guaynabo, the danger concentrates in the afternoon hours, when heat combined with the tropical wave's humidity sets off fast-developing thunderstorms.

Checklist / What to do

1. Check the NWS San Juan forecast (weather.gov/sju) before heading out, especially between midday and 8:00 p.m., when thunderstorm activity peaks.
2. If driving and you encounter a road with running or standing water, do not cross it; find an alternate route even if it takes longer.
3. Move vehicles and valuables away from streams, gutters and low-lying areas known to flood in your neighborhood.
4. If you live in the mountainous interior (Utuado, Jayuya, Adjuntas, Lares, Las Marias, San Sebastian), have an evacuation plan ready in case of any landslide alert.
5. Charge your phones and keep flashlights handy; strong thunderstorms can cause brief LUMA power interruptions.
6. Avoid outdoor activities in the afternoon while the rainy pattern persists, and remember lightning can strike more than ten miles from where it's raining.
7. If your area has a history of urban flooding, consider sandbags or temporary barriers before the next round of downpours arrives.

What's next

The NWS projected that the rainy pattern tied to this tropical wave would remain active at least through Friday, September 18, with the possibility that follow-on waves keep humidity elevated over the weekend ³. Meteorologists urged particular attention to the west, interior west and interior northeast, where soil remains saturated from recent days' accumulated rainfall and where one more inch or two of rain could be enough to trigger flash flooding or landslides ³⁴.

Sources

¹ Primera Hora, "Emiten varias advertencias de inundaciones para la noche del miercoles" — https://www.primerahora.com/noticias/puerto-rico/notas/emiten-varias-advertencias-de-inundaciones-para-la-noche-del-miercoles/ ² Primera Hora, "Advertencia de inundaciones y tronada fuerte para varios pueblos del oeste" — https://www.primerahora.com/noticias/el-tiempo/notas/advertencia-de-inundaciones-y-tronada-fuerte-para-varios-pueblos-del-oeste/ ³ El Nuevo Dia, "How long will it rain? Vigorous tropical wave approaches Puerto Rico" — https://www.elnuevodia.com/noticias/el-tiempo/notas/hasta-cuando-llovera-onda-tropical-vigorosa-se-acerca-a-puerto-rico/ ⁴ Metro Puerto Rico, "Downpours could cause flooding in the interior and west" — https://www.metro.pr/noticias/2026/09/14/aguaceros-podrian-provocar-inundaciones-en-el-interior-y-oeste