textproduct: San Juan

KEY MESSAGES

Issued at 326 AM AST Tue Sep 8 2026

- A Heat Advisory will be in effect today from 10 AM to 5 PM AST for Puerto Rico and the U.S. Virgin Islands.

- A drier air mass filtering over the region will persist today, however locally induced afternoon showers and t-storms are still forecast under a breezy ESE flow.

- Tomorrow and Thursday, tropical waves is forecast to move over the islands, increasing the potential for showers and a limited flooding threat.

- Moderate Saharan Dust concentrations will arrive from time to time before a dense Saharan Air Layer (SAL) with high concentrations arrives Thursday night.

- A moderate risk of rip currents is expected today along the northern and eastern beaches of Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands.

Short Term(Today through Thursday)

Issued at 326 AM AST Tue Sep 8 2026

Since midnight, radar estimated accumulations were detected mainly over St. Croix and eastern to southeastern Puerto Rico. The highest accumulations were observed over the offshore Atlantic Waters where isolated t-storms managed to develop. Stations have reported lows in the upper 70s to low 80s, across urban and lower- elevation areas of northeastern, eastern, southern and southwestern Puerto Rico, in the mid to upper 70s across urban and lower-elevation areas of north-central to western Puerto Rico, in the low 80s across Vieques, Culebra and the USVI, and in the 60s to low 70s across higher elevations of Puerto Rico. A surface high pressure strengthening over the central Atlantic will continue tightening the local pressure gradient and result in breezy, locally windy, ESE flow, especially along coastal areas throughout the short term period. Drier air has been filtering over the islands during the overnight hours and will continue to filter in today and tonight. This drier air mass will continue to displace the moisture field and related disturbance to our north. Precipitable water (PWAT) values will decrease to between 1.45 to 1.8 inches, below normal to normal values, today and this evening. Available moisture will also be limited to mainly below 850 mb, as ridging establishes itself in the mid to upper levels. Low to locally moderate Saharan Dust concentrations will also move over the islands, particularly during the early morning hours. This will all promote a drier and more stable weather pattern compared to the rest of the short term period with 700 to 500 mb lapse rates decreasing to below-seasonal values and 500 mb temperatures warming up to above seasonal values today. Local patches of moisture moving over the region and afternoon convection over western PR are still forecast to result in locally higher and generally seasonal PWAT. Passing showers will continue to reach windward sectors under an ESE flow. Diurnal heating, sea breeze convergence and local effects will still result in afternoon showers and t-storms, over mainly interior to the NW quadrant of PR, with afternoon convection also possible downwind of the local islands and El Yunque. Even drier air is then forecast to arrive late tonight into the early overnight hours tomorrow, with PWAT values ranging from 1 to 1.4 inches. An elevated heat risk will be present today with heat indices reaching advisory criteria mainly from 10 AM to 5 PM AST. Although the above mentioned showers and t-storms can provide some relief this afternoon, a heat advisory will be in effect for the islands during that time.

A couple of tropical waves will increase moisture and shower activity across the islands tomorrow, Wednesday, and Thursday. The first tropical wave and its associated moisture field will move over the region early Wednesday. Although low to moderate saharan dust concentrations will also arrive at this time, these will be relatively short lived. The moisture surge will drive PWAT values to between 1.8 and around 2.0 inches across the area. However, current guidance indicates pockets of drier air will also filter in during the afternoon. Available moisture will somewhat expand back into the mid-levels, while 500 mb temperatures are expected to drop to around -6 to -7C. A favorable wind pattern will persist across the upper levels, supported by high pressure near 250 mb staying to the north of the area. These conditions will favor advective showers and thunderstorms across windward sectors driven by breezy to locally windy east-southeast flow. Convection is also anticipated during the afternoon over interior and west- northwest Puerto Rico, as well as downwind of El Yunque and the neighboring islands. Although a limited flooding potential remains, stronger wind speeds may act to restrict localized rainfall totals.

The moisture field from another tropical wave will then reach the area on Thursday, maintaining PWAT values up to around 2 inches once again. With 500 mb remaining similar and breezy to locally windy ESE flow continuing, a similar pattern is forecast with passing showers across the eastern region during the early morning hours. By the afternoon, convection is forecast to shift over mainly the western interior and northwestern Puerto Rico, and possibly downwind of the local islands and El Yunque. These regions are currently under a limited flood threat. Moderate concentrations of Saharan Dust will then filter in late Thursday afternoon and current guidance is suggesting that high concentrations due to a dense Saharan Air Layer will arrive Thursday night into the long term period. This will promote deteriorated air quality due to haze, reduced visibilities and a general drying pattern to end the short term period. A limited heat risk is forecast for Wednesday and Thursday as more seasonal 925 mb temperatures are forecast; please continue to monitor the forecast for any updates.

Long Term(Friday through Tuesday)

Issued at 326 AM AST Tue Sep 8 2026

A surface high-pressure system over the central Atlantic is expected to strengthen its pressure gradient, promoting breezy to locally windy southeasterly flow through Friday before shifting easterly from Saturday onward. On Friday, lingering moisture associated with a departing tropical wave will bring passing morning showers across the islands. However, a drier air mass will quickly move over the region, dropping precipitable water values to 1.30-1.40 inches, nearly two standard deviations below normal, and bringing stable conditions to the area. Stability will also build aloft as a mid- level ridge moves in. Moderate to locally high concentrations of Saharan dust will accompany the dry air mass, bringing hazy skies, reduced visibility, and poor air quality. Residents and visitors are urged to follow medical recommendations and closely monitor weather forecasts for updates.

Over the weekend, fast-moving morning showers are expected across portions of eastern Puerto Rico and the U.S. Virgin Islands. Afternoon showers and isolated thunderstorms fueled by daytime heating and local effects will follow, though impacts will remain limited with no flooding concerns. From Sunday through Tuesday, an upper-level low moving toward the northeastern Caribbean will gradually increase instability and bring moisture back up to seasonal levels across the region.

AVIATION

(06Z TAFS) Issued at 326 AM AST Tue Sep 8 2026

Mainly VFR conditions are expected across the terminals. Passing - SHRA/VCSH possible over the eastern terminals. HZ (Saharan Dust) gradually decreasing during the early morning hours. Locally induced SHRA/-TSRA can result in brief MVFR mainly over TJBQ btwn 08/17-21z, streamers can also form towards TJSJ at this time. Winds light and VRB to calm through 08/13z, then becoming E-ESE at up to around 15 kt, with sea breeze variations and stronger gusts.

MARINE

Issued at 326 AM AST Tue Sep 8 2026

Surface high pressure strengthening across the central Atlantic will promote moderate to fresh winds, with higher gusts, today and over the next few days. This will result in choppy to locally rough marine conditions. Seas will generally build to 4 to 6 feet across the offshore waters and local passages, with occasional seas up to 8 feet possible by tonight and through around next Thursday. Passing showers will continue across portions of the regional waters today, with locally higher winds and seas possible in and near thunderstorms. A tropical wave arriving by midweek will bring additional periods of showers and thunderstorms Wednesday and Thursday.

BEACH FORECAST

Issued at 326 AM AST Tue Sep 8 2026

Today, there is a moderate risk of rip currents along the northern and eastern beaches of Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands, due to increasing winds and building seas. A moderate risk means life-threatening rip currents are possible and beachgoers are urged to exercise caution. By midweek through next weekend, the moderate risk is expected to persist and extend to most local beaches.

Another beach hazard will be the heat. Beachgoers should remain hydrated and limit prolonged sun exposure. Additionally, gusty winds and lightning due to afternoon showers and isolated thunderstorms, particularly across the western beaches of Puerto Rico. Beachgoers should seek shelter whenever they hear thunder.

FIRE WEATHER

Issued at 326 AM AST Tue Sep 8 2026

Elevated KBDI values signal persistent dry conditions in deeper soil and fuel layers across southwestern Puerto Rico, most notably around Cabo Rojo. Breezy ESE winds combined with incoming drier air maintain a moderate fire danger risk today, with locally elevated threat levels possible in Cabo Rojo and neighboring areas. Weather pattern updates and fuel moisture trends will be monitored closely for potential increases in fire risk.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001>013.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ001-002.

AM...None.