textproduct: San Juan

KEY MESSAGES

Issued at 204 PM AST Wed Sep 9 2026

- The tropical waves moving across the region today and tomorrow will increase the potential for rainfall and thunderstorm activity. Flood risk remains limited.

- Moderate to high Saharan Dust concentrations will gradually filter across the US Virgin Islands and Puerto Rico from tonight onwards.

- Heat risk remains limited for today and Thursday due to higher cloud cover, with a warmer pattern returning by Friday along with the dry air mass and Saharan Air Layer (SAL).

- Breezy to locally windy conditions will continue to deteriorate marine and coastal conditions. Therefore, Small Craft Operators should continue to exercise caution, and beach goers should continue to monitor beach conditions as the evolve.

Short Term(This evening through Friday)

Issued at 204 PM AST Wed Sep 9 2026

The passage of a weak tropical wave over the US Virgin Islands and Puerto Rico has brought TPW values to 2 inches and has favored the development of isolated showers over the US Virgin Islands, the windward waters and offshore Puerto Rico. Nonetheless, the rain activity over the islands has remained limited with low concentrations of Saharan dust particles. Passing showers and thunderstorms are forecast to develop across the interior and northwestern regions of Puerto Rico aided by the elevated humidity levels, daytime heating and local effects. Breezy to locally windy conditions prevailed today due to a low-level high pressure over the Atlantic waters which is strengthening the pressure gradient and leading to a ESE wind flow. Heat risk remains limited due to the increasing cloudiness over the forecast region with heat indices ranging from 90 - 105 degrees Fahrenheit along urban and coastal sections of the US Virgin Islands and Puerto Rico and around mid 80s to mid 90s degrees Fahrenheit across the interior of Puerto Rico. While heat risk remains limited, individuals should be cautious when outside and should remain well hydrated.

Another tropical wave is expected to arrive to the windward waters between tonight and Thursday morning along with moderate to high concentrations of Saharan Dust. This will lead to variable conditions on Thursday over the US Virgin Islands and Puerto Rico. We can expect similar conditions to those seen today, with some passing showers and thunderstorms over the US Virgin Islands and eastern Puerto Rico in the morning hours, with the main activity gradually shifting to the interior and western Puerto Rico in the afternoon. A low-level high pressure over the Atlantic waters coupled with the wave activity over the region will continue to favor breezy to locally windy ESE conditions across the forecast region, particularly across coastal and exposed areas, limiting the potential for significant rainfall accumulations. The 500 millibar temperatures will remain around -6 to -8 degrees Celsius providing favorable conditions for convective activity. By late Thursday into Friday, we expect the wave to move outside of the forecast region and be displaced by a drier air-mass accompanied by a Saharan Air Layer (SAL) that will drop TPW values from 2 inches to 1.5 inches. We suggest citizens and visitors to continue to monitor conditions as the evolve and follow the recommendations of health officials.

Warmer conditions may be expected by Friday aided by the limited shower activity over the islands. Even if conditions are unfavorable for the development of convection on Friday, the ESE wind flow accompanied by daytime heating and local effects might lead to isolated showers and thunderstorms during the morning hours over the US Virgin Islands and eastern Puerto Rico and in the afternoon for the interior and northwestern regions of Puerto Rico.

Long Term(Saturday through next Tuesday)

Issued at 204 PM AST Wed Sep 9 2026

A surface high-pressure is expected to promote easterly breezy winds across the islands for most of the forecast period. The Saharan dust layer will persist during the weekend, with concentration values reaching high levels on Saturday. This will promote hazy skies, reduced visibility, and poor air quality. Residents and visitors are encouraged to follow medical recommendations and closely monitor weather forecasts for updates. Precipitable water values are expected to drop to around 1.25 to 1.5 inches over the weekend, which is considered below-normal to seasonal values. Nonetheless, passing showers during the morning cannot be ruled out in portions of eastern Puerto Rico and the U.S. Virgin Islands. Later on, afternoon showers and isolated thunderstorms triggered by daytime heating and local effects are possible each day, with a limited flood threat expected on Sunday afternoon across western Puerto Rico.

A transition to a wetter pattern from Monday through the rest of the long term, associated with a series of upper-level lows. Current model guidance shows a drier air slot moving across the area Tuesday morning. The persistent breezy easterly flow will likely bring a few passing showers across windward areas of the islands during the morning, followed by afternoon convection across the western interior and western sectors of Puerto Rico. Due to the aforementioned lingering upper-level lows, 500-mb temperatures are expected to remain at or below -6 degrees Celsius, which will likely trigger isolated thunderstorms during the afternoon hours in combination with diurnal heating and local effects.

A change in the pattern is anticipated by Tuesday night into midweek as a tropical wave brings above-normal moisture, with PWAT values exceeding 2 inches. This will bring an increasing chance of showers and thunderstorms, along with a potentially elevated flood risk across portions of the islands. Continue to monitor the forecast as adjustments and updates become available.

Overall, hazy, drier, and warmer conditions are expected on Saturday, followed by variable conditions Sunday into Tuesday, with seasonal moisture and instability fueled by lingering upper-level lows. By Tuesday night into Wednesday, moisture associated with a tropical wave will increase the chance of showers and thunderstorms, along with an increasing flood threat.

AVIATION

(18Z TAFS) Issued at 204 PM AST Wed Sep 9 2026

VFR conditions will persist across all TAF sites during the period with surface winds from the E-Se at 15 to 20 knots and gusty winds. Some VCSH to SHRA will move near the area mostly across eastern TAF sites. Saharan dust particles will remain in place reducing VIS. CIGS and VIS will also be reduced due to an increase in showers and thunderstorms at around 10/03Z 10/12Z near TISX, TIST & TJPS.

MARINE

Issued at 204 PM AST Wed Sep 9 2026

A surface high pressure strengthening across the central Atlantic will promote moderate to fresh winds, with stronger gusts for the rest of the week. This will result in choppy to locally rough marine conditions. Seas will build up to 6 feet across the offshore waters and local passages, with occasional seas up to 8 feet. A passing tropical wave will increase showers and thunderstorms across the regional waters through this afternoon. However, a second tropical wave will arrive tonight through Thursday afternoon promoting similar weather conditions. Gusty winds, brief periods of torrential rainfall, and locally higher seas are expected with these tropical waves.

BEACH FORECAST

Issued at 204 PM AST Wed Sep 9 2026

A moderate risk of rip currents will persist across most east, south, and north-facing beaches of the islands through the end of the week due to increasing winds and building seas across the local coastal waters. Life-threatening rip currents are possible in the surf zone, and beachgoers are urged to exercise caution.

A passing tropical wave will continue to enhance the development of showers and thunderstorms across the region, promoting gusty winds and lightning strikes during the afternoon hours. Beachgoers should remain hydrated and limit prolonged sun exposure, and seek shelter whenever they hear thunder.Expect another tropical wave to reach the islands from tonight into Thursday.

SJU WATCHES/WARNINGS/ADVISORIES

PR...None. VI...None. AM...None.