textproduct: San Juan

KEY MESSAGES

Issued at 330 AM AST Wed Sep 9 2026

- Two tropical waves will move over the local area today and tomorrow increasing the potential for shower and thunderstorm activity, including a limited flooding threat.

- Moderate concentrations of Saharan dust to affect the area today and tomorrow, with drier conditions and high concentrations expected late Thursday into Friday. Prepare for deteriorated air quality and reduced visibilities.

- Breezy to locally windy conditions expected over the next few days, especially along coastal and exposed areas.

- Moderate risk of rip currents expected today along the northern, eastern and southeastern beaches of Puerto Rico, Culebra, Vieques and the U.S. Virgin Islands. Increasing winds will expand the moderate risk for most beaches tonight.

- A limited to elevated heat threat will continue through the rest of the workweek; please continue to monitor the forecast for updates.

Short Term(Today through Friday)

Issued at 330 AM AST Wed Sep 9 2026

The rest of the workweek will be influenced by a couple of tropical waves and a Saharan Air Layer (SAL) moving over the islands. Both tropical waves, along with a strengthening surface high pressure across the central Atlantic, are tightening the pressure gradient and will maintain breezy to locally windy ESE conditions through the rest of the workweek, particularly across coastal and exposed areas. The night started with mainly tranquil conditions, but an increase in shower and isolated t-storm activity has been observed over the U.S. Virgin Islands (USVI) and eastern Puerto Rico as the first tropical wave begins to move across the region. Overnight low temperatures have remained in the mid 70s to low 80s in the urban areas and lower elevations of Puerto Rico, in the low 80s across Vieques, Culebra and the USVI, and in the mid 60s to lower 70s across the interior sections of Puerto Rico. Satellite-derived precipitable water (PWAT) values are increasing from around 1.41.5 inches (below normal for this time of the year) to near 2.0 inches (high-end seasonal levels) as the moisture field from the first wave moves in from the east. Therefore, variable weather conditions with frequent fast-moving passing showers are expected this morning across the USVI and eastern Puerto Rico, with activity shifting toward the interior, western and northwestern sections of Puerto Rico during the afternoon, where scattered showers and isolated thunderstorms are expected. Streamers downwind of El Yunque and the local islands will also be possible. The bulk of the moisture field from this wave (with PWAT values around 2.0 inches) is forecast to be over western Puerto Rico and the Mona Passage by this afternoon, with lingering moisture (PWAT values of around 1.7 inches) persisting through the evening. This wave will favor advective showers and t-storms across the windward sections, driven by the breezy to locally windy ESE flow, which could limit rainfall totals despite a continued limited flooding risk. 500 mb temperatures are expected to drop to around -6 to -7C, supporting shower and t-storm development. Available moisture will somewhat expand back into the mid-levels, but will however be limited by low to moderate concentrations of Saharan Dust also arriving from this morning onwards. This drier air intrusion can also limit the potential for widespread shower activity, and therefore, a significant rainfall event is not anticipated. In terms of heat, for now we are leaning toward a limited heat risk today. However, depending on the evolution of the cloud cover, heat impacts could be higher later today and a Heat Advisory could be issued. Please continue to monitor the forecast for any updates.

The second tropical wave and its associated moisture field will reach the region tonight into tomorrow, Thursday. Current guidance indicates PWAT values near 2.0 inches moving in from the southeast, reaching the USVI first before crossing Puerto Rico overnight and Thursday morning. The bulk of the moisture is expected to shift toward western Puerto Rico, the Caribbean waters, and the Mona Passage by Thursday afternoon. More pronounced rainfall activity is forecast with this wave due to its broader moisture field and a more substantial mid-level moisture intrusion. Passing showers and isolated t-storms are expected across eastern areas during the overnight to morning hours, followed by afternoon convective showers and t-storms mainly across interior to western Puerto Rico, with streamers again possible downwind of El Yunque and the local islands. With 500 mb temperatures continuing at -6 to -7C and breezy to locally windy ESE flow persisting, a similar pattern to Wednesday is forecast. However, low to moderate Saharan Dust concentrations will continue to affect the region. Current guidance suggests high Saharan Dust concentrations could reach the USVI by around 5 to 6 PM tomorrow. An earlier arrival of these higher concentrations could disrupt tomorrow's afternoon convection, leading to some uncertainty.

These high concentrations of Saharan Dust will be the dominant feature Thursday night and Friday. PWAT values will rapidly decrease (as drier and more stable air related to this SAL filters in) and be at generally around 1.3 to 1.5 inches by Friday evening (below normal for this time of the year). This will promote hazy skies, deteriorated air quality and reduced visibilities. The still breezy to locally windy ESE flow could steer some passing showers along windward sectors as lingering moisture exits the area and local effects can still result in more limited afternoon convection. Overnight trapping by the SAL will lead to warmer lows Thursday night and Friday. An elevated heat risk is forecast for Thursday and Friday as above normal 925 mb temperatures continue to be expected.

Long Term(Saturday through Wednesday)

Issued at 330 AM 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

(06Z TAFS) Issued at 330 AM AST Wed Sep 9 2026

Mainly VFR conditions. Tropical waves will increase SHRA and isold -TSRA over the region and can mainly affect the eastern and southern terminals through the period. Locally induced VCTS/-TSRA can develop and affect TJBQ btwn 09/17-21z, streamers can also form towards TJSJ at this time. HZ (Saharan Dust) will also be present and can reduce VIS. Winds up to around 10 kts through 09/13z, then ESE at up to around 15 to 18 kt, with sea breeze variations and stronger gusts (locally higher near SHRA/TSRA).

MARINE

Issued at 330 AM 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 later 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 330 AM 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 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.

SJU WATCHES/WARNINGS/ADVISORIES

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