textproduct: San Juan

KEY MESSAGES

Issued at 258 AM AST Fri Aug 21 2026

* A Heat Advisory is in effect from 10 AM to 5 PM due to hot temperatures and heat indices between 108 and 111 degrees. Similar conditions are forecast from Saturday into Sunday with a heat threat across the urban and coastal areas.

* A stable day is expected, with mostly clear skies and limited shower activity. However, rainfall accumulations are not expected to result in flooding threat.

* A more variable weather pattern is expected from Saturday into Sunday, with a slight increase in the frequency of passing showers. * Saharan dust particles will remain in place at least through this evening.

* A low to moderate risk of rip currents is expected for the upcoming days.

Short Term(Today through Sunday)

Issued at 258 AM AST Fri Aug 21 2026

During the overnight hours, mostly clear skies were observed across Puerto Rico. However, some high-level cloudiness was observed over portions of the island municipalities of Vieques and Culebra, as well as across the U.S. Virgin Islands, associated with a cutoff low in the upper levels over the region. In terms of rainfall, no shower activity was observed overnight across the forecast area (CWA). Minimum temperatures were in the low 80s across coastal and urban areas and in the mid 70s across the higher elevations. Winds were generally from the east at speeds up to 10 mph, with some variations associated with the land breeze along coastal areas.

For the rest of the morning, conditions similar to those observed overnight are expected to prevail, but with less high- to mid- level cloudiness as it gradually moves north of the region. At the upper levels, the cutoff low will continue to favor some high- level cloudiness during the early morning hours as it gradually shifts north of the CWA. At the surface, a trade wind pattern will prevail under the influence of a surface high pressure over the Atlantic, promoting generally easterly winds across the region. Meanwhile, a relatively dry air mass, accompanied by moderate to locally high concentrations of Saharan dust particles, will continue to dominate the area. As a result, another hazy and hot day is expected, with limited shower activity. Warm temperatures and elevated heat indices could exacerbate heat-related impacts, particularly among vulnerable communities and people exposed to the sun for prolonged periods. Due to these conditions, a Heat Advisory is in effect for the coastal and urban areas of Puerto Rico, including the Caguas urban area and nearby municipalities.

By Saturday, the weather pattern will begin to change across the region as an increase in moisture associated with patches of moisture from a low-pressure system located well north of the islands over the north-central Atlantic is advected into the area by the easterly trade winds, while Saharan dust concentrations gradually decrease. Therefore, an increase in the potential for passing showers, resulting in a more variable weather pattern, is forecast from Saturday onward. Despite the expected increase in shower activity, significant rainfall accumulations are not anticipated, and the flooding potential should remain limited and mainly focused across western Puerto Rico with the strongest afternoon showers. By Sunday, the surface pattern will remain relatively similar, with patches of moisture moving in and out of the region under a prevailing southeasterly wind flow. This will continue to support periods of passing showers across the islands, particularly across windward areas during the morning hours, followed by afternoon shower development mainly across northwestern Puerto Rico. Overall, a more variable weather pattern is expected during the weekend, with periods of passing showers alternating with periods of drier conditions. During these days, the heat threat will remain across coastal and urban areas, with hot temperatures and heat indices ranging from 108 to 111 degrees.|

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.

MARINE

Issued at 258 AM AST Fri Aug 21 2026

Locally choppy conditions will prevail across portions of the offshore waters and local passages through the end of the workweek, due to moderate and locally fresh easterly winds. Small Craft should exercise caution remain in effect. Seas 4 to 5 feet, occasionally to 6 feet across offshore waters while 3 to 4 across coastal waters. Winds will gradually subside as we head into the weekend, though occasionally breezy conditions may persist across the more exposed waters.

BEACH FORECAST

Issued at 258 AM AST Fri Aug 21 2026

Expect moderate risk of rip currents along the beaches of northern and eastern Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands, meaning life-threatening rip currents through tonight. Lower risk will prevail elsewhere. However, remember that even when the risk is low, life-threatening rip currents can occur near groins, jetties, reefs, and piers. From Tuesday into midweek, winds are expected to gradually become breezier, with a moderate risk expanding to most local beaches.

FIRE WEATHER

Issued at 258 AM AST Fri Aug 21 2026

An elevated fire danger threat is expected today due to the combination of dry atmospheric conditions and breezy winds. Relative humidity (RH) values are forecast to drop into the mid to lower 40s during the peak heating hours of the afternoon. Combined with sustained east-southeast winds between 10 and 15 mph and dry fuels and soils, environmental conditions will be favorable for rapid fire ignition and spread. Therefore, a Fire Danger Statement (RFD) will be issued for the affected area.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001>005-007-008-010>013.

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

AM...None.