textproduct: San Juan

KEY MESSAGES

Issued at 331 AM AST Tue Jul 21 2026

* Ponding of water on roads and minor flooding expected today across portions of the islands due to shower and thunderstorm activity.

* Temperatures reaching heat advisory/warning criteria expected on Wednesday.

* Hazy skies return by the end of the workweek due to moderate concentrations of Saharan dust.

* Across the USVI, scattered showers and brief periods of gusty winds can be expected today.

Short Term(Today through Thursday)

Issued at 331 AM AST Tue Jul 21 2026

An upper-level low north of the region, along with an induced surface trough, will bring an increase in shower and thunderstorm activity through Wednesday. This setup will pull a surge of deep tropical moisture across the northeastern Caribbean, pushing precipitable water values up to 1.80 to 2.00 inches. Favorable upper-level divergence combined with strong daytime heating and local sea breeze convergence will support shower and thunderstorm activity each afternoon, concentrated mainly over the interior and western Puerto Rico. The wind will prevail from the NE today, and this activity should favor mostly the west/southwest quadrant of PR, and over NW PR on Wednesday as winds shift from the SE behind the trough. The U.S. Virgin Islands and eastern Puerto Rico will see periods of showers, mainly this morning, with isolated thunderstorms developing across the coastal waters of the islands as the low-level trough propagates westward. Quick moderate to heavy downpours could lead to ponding of water on roads and poor drainage areas, as well as localized urban flooding.

By late Wednesday and through Thursday, a marked drying and stabilizing trend will take hold as a dense plume of the Saharan Air Layer (SAL) moves into the northeastern Caribbean. Precipitable water values are forecast to drop sharply back down toward 1.20 to 1.40 inches, accompanied by a robust low-to-mid level capping inversion and dry mid-level air. This dust/drier airmass will bring hazy skies, degraded air quality, and sharply suppressed convective development for Thursday. Shower activity on Thursday will be capped and restricted mainly to brief, isolated afternoon showers over western PR. Warm 925 mb temperatures combined with lower wind speeds and dewpoints in the upper 70s to low 80s will also favor elevated (106F-112F) heat indices across coastal and low-elevation areas, particularly on Wednesday, and heat advisories/extreme heat warnings could be issued for portions of the islands.

Long Term(Friday through Tuesday)

Issued at 331 AM AST Tue Jul 21 2026

By the end of the week and into the weekend, moderate concentrations of Saharan dust are expected to persist across the region. Surface high pressure over the Atlantic will reestablish itself as the upper- level low weakens and moves away, resulting in breezy easterly winds, particularly on Friday and Saturday. During this period, available moisture will remain primarily confined to the surface through the 850 mb layer, with PWAT values fluctuating between 1.6 and 1.9 inches. These values are considered seasonal for this time of year based on climatology, although locally above-normal values may develop during afternoon convection. The easterly steering flow will continue to favor passing showers across windward areas during the nighttime and morning hours, followed by afternoon showers over western Puerto Rico driven by local effects and available moisture. The flood risk is expected to remain low, although a few isolated thunderstorms will remain possible each afternoon across western Puerto Rico. Temperatures at 500 mb will remain around -6 to -7 C, which is near climatological normal, but still sufficient to support isolated thunderstorms associated with afternoon convection.

From Sunday through Tuesday, current guidance suggests that moisture associated with a tropical wave and an accompanying wind surge will reach the area, increasing PWAT values to above-normal levels while producing breezy to locally windy east-southeasterly flow. PWAT values are expected to remain above 1.8 inches from Sunday onward, peaking around 2.1 inches between Monday and Tuesday (near the 75th percentile). Additionally, current model guidance indicates that an upper-level low will develop and move across the northeastern Caribbean. This is reflected by lower 250 mb heights and cooler 500 mb temperatures, both decreasing toward the 25th percentile of the climatological distribution. As a result, more favorable upper-level dynamics are expected to enhance the potential for showers and isolated thunderstorms across the forecast area. Continue to monitor the forecast for updates.

Warm to hot conditions will persist through most of the forecast period, with an elevated heat risk expected across coastal and lower elevations of the islands. Residents and visitors are encouraged to continue practicing heat safety measures to reduce the risk of heat- related illnesses.

AVIATION

(06Z TAFS) Issued at 331 AM AST Tue Jul 21 2026

Tempo MVFR to brief IFR conds possible thru 21/12z across the USVI terminals and TJSJ due to -TSRA/SHRA from a broad sfc trough moving across the northeastern Caribbean. Similar conditions are possible at TJPS btw 21/17-21z, while VCTS could prevail across the rest of the PR terminals. SCT/BKN cigs btw FL025-FL150 and mtn tops obscd expected. Sfc winds NE at 12-16 kt with stronger gusts around 25-30 kt. A southerly sea breeze variation is expected at TJPS aft 21/14z.

MARINE

Issued at 331 AM AST Tue Jul 21 2026

An approaching induced surface trough will weaken the influence of a surface high pressure over the central Atlantic and promote more moderate east-northeast winds through late tonight. The surface high will then reestablish itself and promote moderate to fresh east to east-southeast winds early tomorrow, Wednesday, onwards. This induced surface trough will also enhance the likelihood of showers and thunderstorms, as well as increase sea conditions and wind strength near any thunderstorms today and tomorrow. Additionally, another tropical wave is forecast to move near the Lesser Antilles by Thursday and approach the region by Friday. Moderate concentrations of Saharan Dust will also reach the region on Thursday and into the weekend.

BEACH FORECAST

Issued at 331 AM AST Tue Jul 21 2026

A low risk of rip currents will persist today and tomorrow (Wednesday) however, life-threatening rip currents often occur in the vicinity of groins, jetties, reefs, and piers. A moderate risk of rip currents (life-threatening rip currents are possible in the surf zone) will return on Thursday and persist through the weekend. Beachgoers should swim near lifeguards, obey beach warning flags, and never swim alone. Showers and thunderstorm chances increase today and tomorrow, with showers moving towards windward sectors under east-northeast winds today and east-southeast winds tomorrow, while afternoon showers and t-storms are forecast mainly over western Puerto Rico. Beachgoers should remain weather-aware and be cautious of thunderstorms, as lightning can strike far from rain. If thunder is heard, leave the beach and seek shelter in a substantial building or hard-topped vehicle. There is also a risk of extreme heat, especially from late morning to afternoon, due to high temperatures, humidity, and limited shade. To minimize heat-related illnesses, beachgoers should stay hydrated, seek shade, and apply sunscreen regularly.

SJU WATCHES/WARNINGS/ADVISORIES

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