textproduct: San Juan

KEY MESSAGES

Issued at 356 AM AST Mon Sep 7 2026

- An Extreme Heat Warning is in effect for Puerto Rico from 10 AM to 5 PM AST, with a Heat Advisory also in effect for Vieques, Culebra, and the U.S. Virgin Islands. Although passing showers, t-storms and clouds can provide some relief, enough sunshine and moisture is expected for heat indices to reach dangerous levels.

- Afternoon convection is also forecast to develop mainly over the interior to W and NW Puerto Rico, while streamers can once again develop downwind of the local islands and El Yunque.

- Low to locally moderate concentrations of Saharan Dust will filter over the islands later today into overnight tomorrow.

- A low risk of rip currents will persist along all coastal areas today and tonight, before increasing to moderate tomorrow along the northern and eastern coastal areas of Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands.

- Drier and more stable conditions are forecast tomorrow, before a tropical wave increases moisture once again on Wednesday. Low to briefly moderate concentrations of Saharan Dust will also filter over the islands on Wednesday.

Short Term(Today through Wednesday)

Issued at 356 AM AST Mon Sep 7 2026

During the overnight hours, a lingering moisture field from yesterday's tropical wave continued over the islands. Showers and isolated t-storms continued moving across the region under an ESE steering flow. Since midnight, radar estimated accumulations were detected over interior to eastern PR, and over Vieques, Culebra and the USVI. Stations have reported minimum temperatures in the upper 70s to low 80s, across urban and lower-elevation areas of Puerto Rico, in the low 80s across Vieques, Culebra and the USVI, and in the 60s across higher elevations of Puerto Rico. This lingering moisture from yesterday's tropical wave will continue over the local islands, with Precipitable water (PWAT) values over 2 inches, through at least tonight under an ESE steering flow. With above normal moisture, ESE flow persisting, and satellite imagery not suggesting widespread cloud cover, another day with high heat indices is forecast. 925 mb temperatures are forecast to reach 2 standard deviations above normal while wind speeds remain seasonal as a surface high gradually approaches the area. An extreme heat warning will be in effect from 10 AM to 5 PM AST today for Puerto Rico while a heat advisory will be in effect for the USVI, Vieques and Culebra from 10 AM to 5 PM AST today. Although passing showers, t-storms and clouds can provide some relief, enough sunshine and moisture is expected for heat indices to rapidly rise during the day and reach dangerous levels. These passing showers and isolated t- storms can continue to reach windward sectors of the islands to start the day under the ESE flow. Afternoon convection is also forecast due to available moisture, diurnal heating, sea breeze convergence and local orographic effects. This convective activity, showers and t-storms, is forecast to develop mainly over the interior to W and NW Puerto Rico, while streamers can once again develop downwind of the local islands and El Yunque. This will all result in a limited flooding risk over mainly those areas as locally heavy rainfall is forecast. An upper level trough west of the region will continue moving away from the eastern Caribbean, resulting in upper level ridging over the local area. 500 mb temperatures will reach high end seasonal to warmer than seasonal values while 850 mb to 700 mb lapse rates increase to seasonal and above seasonal values. Low to locally moderate concentrations of Saharan Dust will also filter over the islands later today into overnight tomorrow.

Tomorrow, Tuesday, and Wednesday, a surface high-pressure will continue moving closer to the region, tightening the local pressure gradient and resulting in breezy, to locally windy, ESE flow, especially along coastal areas. Drier air will filter in later tonight into tomorrow with PWAT values decreasing to between 1.5 inches and 1.8 inches, below normal to normal values for this time of the year. Available moisture will also be limited to mainly below 850 mb, as ridging continues to establish itself and drier air becomes present above 850 mb. 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. Conditions will be more stable on Tuesday however, as 700 to 500 mb lapse rates decrease to below-seasonal values and 500 mb temperatures warm up to above seasonal values. Passing showers will continue to reach windward sectors under an ESE flow. Although diurnal heating and local effects will still result in afternoon showers and t-storms, their coverage (over mainly interior to W-NW PR) is forecast to be more limited on Tuesday compared to the rest of the short term period. Current model guidance has PWAT values decreasing even further, ranging between around 1 to 1.4 inches, tomorrow night into the early overnight hours on Wednesday.

This decrease in moisture will be short-lived however, as a tropical wave (currently approaching 50W) and its associated moisture field move over the region from the east by the late Tuesday overnight hours into Wednesday morning. This moisture field along with trailing moisture will once again increase PWAT values to between 1.8 to around 2 inches over the region, with current model guidance suggesting patches of lower moisture also reaching the region. 500 mb temperatures are also forecast to decrease to around -6 and -7C on Wednesday while available moisture once again reaches the mid- levels. This can support advective showers and t-storms over windward sectors under a breezy, to locally windy ESE, flow and afternoon showers (more widespread compared to Tuesday) and t- storms, mainly for interior to W-NW PR as well as downwind of the local islands and El Yunque. Although a limited flooding risk is forecast, wind speeds, however, can serve to limit rainfall accumulations. Low to briefly moderate concentrations of Saharan Dust will also filter over the islands on Wednesday. A limited to elevated heat risk is forecast to persist Tuesday and Wednesday as ESE flow continues, however, 925 mb temperatures are forecast at slightly above normal values.

Long Term(Thursday through Monday)

Issued at 356 AM AST Mon Sep 7 2026

A surface high-pressure across the Atlantic will continue to promote breezy to locally windy southeast winds on Thursday, becoming more easterly Friday, and then east to northeasterly Saturday through Monday. On Thursday, residual moisture from a tropical wave will remain over the region, with precipitable water values around 1.9 to 2.2 inches, seasonal to near-above-average for this time of the year based on climatology. Model guidance suggests that 500mb temperatures will remain near -6 to -7 degrees Celsius, which will likely support thunderstorm development. As a result, expect passing showers across eastern Puerto Rico and the U.S. Virgin Islands early in the morning, followed by afternoon convection across the western interior to northwestern Puerto Rico. That activity will likely cause ponding on roads and in poorly drained areas. At the moment, those areas are being highlighted with a limited flood threat.

By the end of the week into Saturday, a drier weather pattern with hazy skies will dominate. A drier air mass, accompanied by moderate to locally high concentrations of Saharan dust particles, will approach the northeastern Caribbean. Precipitable water values are expected to drop below 1.5 inches, ranging from below normal to nearly two standard deviations below normal. In terms of rainfall, a few afternoon showers will remain possible across western Puerto Rico, fueled by local effects, but shower activity should remain limited, with no flooding concerns. Heat will be the primary hazard, as warmer conditions are expected, along with the potential need for heat-related products.

By Sunday into early next week, an upper-level low and an associated lower-level disturbance may move closer to the northeastern Caribbean, gradually increasing moisture and instability. Precipitable water values are expected to rise to seasonal levels, while cooler 500-mb temperatures and more favorable mid-level conditions could further enhance instability and shower and thunderstorm development, especially during the morning across eastern areas and in the afternoon across the western interior and northwestern quadrant of Puerto Rico.

Overall, residual moisture on Thursday will result in afternoon showers and thunderstorms. However, hazy, drier, and warmer conditions are expected to return by the end of the week and into Saturday, followed by increasing instability from Sunday into early next week.

AVIATION

(06Z TAFS) Issued at 356 AM AST Mon Sep 7 2026

Mainly VFR conditions are expected across the terminals. Passing - SHRA/VCSH and isold-VCTS can continue moving over mainly the eastern terminals. Locally induced SHRA/TSRA can result in brief MVFR conditions at TJSJ/TJBQ btwn 07/17-21z where PROB30s are in effect. Winds light and VRB to calm through 07/13z, then becoming E-ESE at 10 to 15 kt, with sea breeze variations and stronger gusts near SHRA/TSRA.

MARINE

Issued at 356 AM AST Mon Sep 7 2026

Moderate easterly winds will gradually strengthen tonight into Tuesday as a surface high pressure strengthens across the central Atlantic. Winds of 15 to 20 knots, with higher gusts, will become increasingly widespread from Tuesday onward, resulting 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 Tuesday night and through midweek. Passing showers will continue across portions of the regional waters, with locally higher winds and seas possible in and near thunderstorms.

BEACH FORECAST

Issued at 356 AM AST Mon Sep 7 2026

A low risk of rip currents will continue across all beaches through today. Increasing easterly winds and building seas will result in a moderate risk beginning tomorrow, Tuesday, mainly along the northern and eastern beaches of Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands. A low risk will persist along the southern and western beaches of Puerto Rico through at least tomorrow night. A moderate risk is forecast for most local beaches by Wednesday night.

Heat will remain an additional concern for beachgoers. Showers and isolated thunderstorms may also produce brief gusty winds and lightning, especially during the afternoon.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Extreme Heat Warning from 10 AM this morning to 5 PM AST this afternoon for PRZ001>011.

Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ012-013.

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

AM...None.