textproduct: San Juan

KEY MESSAGES

Issued at 237 PM AST Fri Aug 21 2026

- A Heat Advisory remains in effect until 5 PM AST due to above- normal temperatures and elevated heat indices across urban and coastal areas of PR and USVI. Similar conditions are expected to persist over the coming days.

- Limited showers possible across western PR this afternoon.

- Patches of moisture arriving Saturday and Sunday will likely increase the frequency of trade wind showers and afternoon convection, with a limited flood threat across western PR.

- A low risk of rip currents is expected through early next week, with moderate risk returning by midweek, along with breezier winds along most local beaches.

Short Term(This evening through Sunday)

Issued at 237 PM AST Fri Aug 21 2026

Mostly fair weather conditions prevailed across Puerto Rico and the U.S. Virgin Islands this morning, although hazy skies continued due to suspended Saharan dust particles. Meanwhile, the combination of warm temperatures and available low-level moisture has already resulted in heat index values above 100 degrees across many urban and coastal areas. Several stations across north-central, western, and southwestern Puerto Rico have already reported heat index values at or above 108 degrees. Therefore, limited to elevated heat impacts are expected to persist through the remainder of the afternoon, particularly across coastal and urban areas.

For the remainder of today, Saharan dust and a relatively stable air mass will continue to dominate the local weather pattern. Precipitable water values will generally remain between 1.4 and 1.6 inches, while very dry air in the mid-levels and relatively warm temperatures aloft should continue to suppress widespread or deep convective development. Nevertheless, daytime heating, local effects, and sea-breeze convergence may be sufficient to generate a few showers across portions of western and interior Puerto Rico this afternoon. Any activity that develops should remain limited in coverage and duration. Elsewhere, mostly fair and hazy conditions should prevail. Breezy easterly winds will continue, especially along coastal areas, with locally stronger gusts.

Tonight into Saturday, a gradual transition toward a somewhat wetter trade-wind pattern is expected as the concentration of Saharan dust diminishes and patches of low-level moisture approach from the east. Precipitable water values are forecast to increase, periodically reaching around 1.8 to 1.9 inches. However, moisture will remain concentrated primarily below 700 mb. In addition, warm 500-mb temperatures and generally weak low- to mid-level lapse rates will continue to limit atmospheric instability.

As a result, more frequent passing showers are expected across the U.S. Virgin Islands and eastern and southeastern Puerto Rico during the overnight and morning hours as moisture patches move through in the prevailing easterly flow. During the afternoon, the combination of increased low-level moisture, daytime heating, and local convergence should support additional shower development over the interior and western portions of Puerto Rico. A few thunderstorms cannot be ruled out with the strongest activity; however, the relatively stable mid-level environment should limit its overall intensity and longevity.

A similar variable trade-wind pattern is expected Sunday, with patches of moisture supporting passing showers across windward areas during the morning, followed by afternoon development mainly across interior and northwestern Puerto Rico. Although locally moderate to briefly heavy rainfall is possible, widespread heavy rainfall is not anticipated. Despite the increase in moisture and shower activity this weekend, hot conditions will persist. Heat index values of 108 to 111 degrees will remain possible each afternoon, particularly across coastal and urban areas. Overall, conditions will transition from today's hot, hazy, and relatively dry pattern to a more variable trade-wind shower regime this weekend.

Long Term(Monday through Friday)

Issued at 258 AM AST Fri Aug 21 2026

The main long-term forecast concerns are persistent excessive heat and the evolution of the mid-level ridge as a couple of weak tropical disturbances move through the Caribbean. Confidence remains moderate through Tuesday, then decreases somewhat for the latter half of the week as model differences become more pronounced.

On Monday, the deterministic GFS suggests the mid-level ridge will erode by the afternoon. If this occurs, reduced mid-level inhibition combined with daytime heating and local effects should support a better opportunity for afternoon convection across the interior and western portions of Puerto Rico. Farther east, the U.S. Virgin Islands and windward portions of Puerto Rico should experience mainly isolated, fast-moving showers. Limited to elevated excessive heat conditions will remain a concern, particularly across coastal and urban areas.

By Tuesday, a weak tropical wave is forecast to approach the region. However, the deterministic GFS and ECMWF differ on the strength of the mid-level ridge and associated moisture. Nevertheless, increasing moisture should support showers and isolated thunderstorms across the U.S. Virgin Islands and eastern Puerto Rico as early as Tuesday morning, followed by additional convection over the interior and western portions of Puerto Rico during the afternoon. Confidence in rainfall coverage and amounts remains limited, however. The GFS is more aggressive in weakening the mid- level ridge, while guidance also suggests increasing trade winds. A faster low-level flow could limit rainfall duration and reduce the potential for significant accumulations.

Any rainfall would be beneficial given the ongoing drought conditions across portions of Puerto Rico and the U.S. Virgin Islands, particularly if precipitation occurs within reservoir watersheds. However, given the expected progressive nature of the showers and current model uncertainty, confidence remains low that this wave will provide meaningful drought relief.

Guidance generally favors the mid-level ridge rebuilding over the region Wednesday, which should once again suppress widespread convection while maintaining hot conditions. Another disturbance is being monitored for Thursday, although current GFS guidance keeps the bulk of its moisture and associated activity south of the local islands over the Caribbean Sea. Another possible tropical wave may approach Friday, but current guidance suggests limited organization and moisture. Given the lead time and model variability, confidence in the Thursday-Friday evolution remains low.

Overall, periods of excessive heat will remain one of the primary hazards during the long-term period, particularly Monday and Tuesday and potentially into Wednesday. Rainfall opportunities will increase at times with passing disturbances, but confidence in significant or widespread accumulations remains low.

AVIATION

(18Z TAFS) Issued at 237 PM AST Fri Aug 21 2026

Mainly VFR conds expected across all terminals thru the period. Saharan dust will continue to promote HZ, with VIS generally P6SM through 21/22Z. SHRA/TSRA development over W PR may result in brief MVFR conds at/near TJBQ/TJPS aft 22/17Z. VCSH expected at USVI/E PR terminals overnight into Sat AM. E-ESE winds 13-17 kt with higher gusts, becoming lighter overnight.

MARINE

Issued at 237 PM AST Fri Aug 21 2026

Subtropical Atlantic high pressure is sustaining moderate easterly trade winds across the region. This pattern is expected to continue through the weekend, with seas of 3 to 4 feet, occasionally reaching 5 feet across offshore waters and local Caribbean passages, and 2 to 4 feet across coastal waters. Occasionally breezy conditions may persist across the more exposed waters, resulting in locally moderate chops. Around Tuesday, winds are expected to become moderate to fresh with the arrival of a wind surge, increasing seas to as high as 6 feet. The frequency of showers may also increase.

BEACH FORECAST

Issued at 237 PM AST Fri Aug 21 2026

A moderate risk of rip currents will prevail tonight along the beaches of northern and eastern Puerto Rico, meaning life- threatening rip currents are possible in the surf zones and beachgoers should exercise caution. By Saturday into early next week, with slightly lighter winds, the risk is forecast to drop to low across most beaches of Puerto Rico, Vieques, Culebra, and the U.S. Virgin Islands. However, remember that even when the risk is low, life-threatening rip currents can occur near groins, jetties, reefs, and piers. From Wednesday into the end of next week, winds are expected to gradually become breezier, with the risk returning to moderate levels and expanding to most local beaches. Additional hazards will include heat across all areas and isolated thunderstorms, particularly across the western beaches of Puerto Rico during the afternoons.

FIRE WEATHER

Issued at 237 PM AST Fri Aug 21 2026

Elevated fire weather conditions will continue through the remainder of the afternoon, particularly across the southern coastal plains and other fire-prone areas of Puerto Rico. Low RH values, breezy E-ESE winds, and dry fuels will continue to favor rapid fire spread. Conditions should gradually improve this evening as winds decrease and RH values recover. Increasing moisture and shower activity from Saturday onward should further reduce the fire weather threat.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007- 008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.