Area Forecast Discussion

KEY MESSAGES

Issued at 1250 PM AST Tue Jul 21 2026

* Additional showers and thunderstorms through this afternoon may result in ponding of water on roads and localized urban and small-stream flooding, particularly across southwestern Puerto Rico.

* Another round of showers and thunderstorms is expected on Wednesday. A shift to a more southeasterly wind flow will favor activity across the northern half of Puerto Rico, including the San Juan metropolitan area and the northwestern municipalities.

* Across the U.S. Virgin Islands, passing showers and brief periods of gusty winds will continue through at least Wednesday. * Hotter conditions are expected on Wednesday, with heat advisory/warning criteria possible across urban and coastal areas.

* Hazy skies are expected to return late this week as moderate Saharan dust concentrations move into the region.

Short Term(This evening through Thursday)

Issued at 1250 PM AST Tue Jul 21 2026

An induced surface trough brought showers and thunderstorms across the northern half of Puerto Rico, eastern Puerto Rico, and the U.S. Virgin Islands this morning, producing locally heavy rainfall and ponding of water in some areas. This activity was fast-moving. Meanwhile, southern Puerto Rico remained mostly sunny and warm, with maximum temperatures reaching the lower to mid-90s along the southern coast and across the USVI. In contrast, northern coastal areas remained in the mid to upper 80s. Northeast winds prevailed at 5 to 15 mph.

This afternoon, daytime heating, local sea breeze convergence, and lingering moisture will support another round of showers and thunderstorms across the interior and southwestern Puerto Rico, while a drier air mass trailing this trough will filter over the USVI and the rest of PR. Locally heavy rainfall may result in ponding of water on roads and localized urban and small-stream flooding.

Tonight through Wednesday, an upper-level low north of the region and the induced surface trough will maintain an unsettled pattern. As the low-level flow shifts to the east-southeast, shower and thunderstorm activity will become more focused across northern Puerto Rico, including the San Juan metropolitan area and the northwestern municipalities. Passing showers and a few isolated thunderstorms will also affect the U.S. Virgin Islands late tonight into Wednesday morning.

Hotter conditions on Wednesday may push heat indices into Heat Advisory territory across urban and coastal areas. By late Wednesday night into Thursday, a Saharan Air Layer will bring a drier and more stable air mass, resulting in hazy skies, degraded air quality, and limited shower activity.

LONG TERM

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

(18Z TAFS) Issued at 1250 PM AST Tue Jul 21 2026

Mostly VFR conditions across most TAF sites. A trough will continue to affect TJBQ, TIST, TISX and TJPS with VCSH. Winds will remain from the E-NE through 21/23Z and from 22/00Z onwards winds will gradually shift from the E-SE.

MARINE

Issued at 1250 PM AST Tue Jul 21 2026

The induced surface trough will continue to move west of the region tonight while Atlantic high pressure gradually rebuilds. As a result, winds will shift from the northeast today to the east- southeast by Wednesday, increasing to moderate to locally fresh speeds. Showers and thunderstorms will remain possible through Wednesday, with locally higher winds and seas near stronger convection. By Thursday, a drier air mass accompanied by moderate concentrations of Saharan dust will overspread the region, resulting in hazy skies and reduced visibility. A tropical wave approaching the Lesser Antilles is expected to increase moisture and shower activity by Friday.

BEACH FORECAST

Issued at 1250 PM AST Tue Jul 21 2026

A low risk of rip currents will continue through Wednesday. However, life-threatening rip currents can still occur near groins, jetties, reefs, and piers. Beachgoers should always swim near lifeguards, follow posted warning flags, and never swim alone.

Showers and thunderstorms will remain possible through Wednesday. Passing showers will affect windward beaches tonight into Wednesday morning. At the same time, afternoon thunderstorms are expected to develop mainly across northern Puerto Rico on Wednesday as winds shift to the east-southeast. Beachgoers should remain weather-aware, as lightning can strike far from rainfall. If thunder is heard, leave the beach and seek shelter in a substantial building or hard-topped vehicle.

An elevated risk of heat-related illnesses will continue on Wednesday, especially from late morning through the afternoon across urban and coastal areas. Stay hydrated, seek shade whenever possible, and apply sunscreen regularly.

SJU WATCHES / WARNINGS / ADVISORIES

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