textproduct: San Juan

KEY MESSAGES

Issued at 437 AM AST Fri Sep 4 2026

* A weak tropical wave will move across the region today, increasing shower activity particularly across interior and western PR during the afternoon. Locally heavy rainfall could result in ponding of water on roads and in poorly drained areas.

* Moisture will increase through the weekend, resulting in more frequent showers and a few thunderstorms, particularly during the afternoon across western PR. However, flooding impacts are expected to remain localized rather than widespread. Locally heavy rainfall could result in ponding of water on roads and in poorly drained areas.

* Dangerous heat conditions will continue through the weekend. A Heat Advisory is in effect for today, while even warmer and more humid conditions could result in significant heat impacts this weekend, particularly on Sunday.

* A low risk of rip currents will prevail across all beaches through the Holiday weekend. However, life-threatening rip currents can still occur when the risk is low, particularly near groins, jetties, reefs, and piers.

Short Term(Today through Sunday)

Issued at 437 AM AST Fri Sep 4 2026

Passing showers moved across the local waters during the overnight hours, with some reaching portions of the windward coastal areas. Despite the presence of a tropical wave moving across the region, shower activity remained generally brief and limited, as this is a fairly weak feature with little associated instability. A few thunderstorms were detected by the Geostationary Lightning Mapper satellite data over the offshore Atlantic waters, but remained well north of the forecast area.

For today, the tropical wave will continue to move across the forecast area, with precipitable water values increasing to around 1.9 inches by this evening. Passing showers are expected across the U.S. Virgin Islands and eastern Puerto Rico this morning, followed by showers and isolated thunderstorms across interior and western Puerto Rico this afternoon. Despite increasing moisture, relatively dry mid-level conditions and limited instability should inhibit widespread or vigorous convection. Still, locally heavy rainfall could result in ponding of water on roads and in poorly drained areas.

Deeper moisture will spread across the region tonight into Saturday, with precipitable water values increasing to around 2.0 inches and 700-500 mb relative humidity reaching 55 to 65 percent. Despite the increasing moisture, strengthening ridging aloft, as indicated by rising 250-mb heights through Sunday, will favor increasing stability. Nevertheless, the deeper moisture should increase shower frequency, with hi-res models suggesting greater areal coverage of rainfall across western Puerto Rico on Saturday. Abundant moisture will persist into Sunday as a broader tropical wave moves across the forecast area, supporting locally heavy rainfall and isolated afternoon thunderstorms as it combines with diurnal heating and sea- breeze convergence.

Heat will remain the primary hazard throughout the short-term period. A Heat Advisory is in effect for today, with elevated heat conditions expected to affect most individuals sensitive to heat, particularly those without effective cooling or adequate hydration. Even warmer conditions are possible this weekend as southerly winds bring a warmer air mass into the region. Of particular concern is Sunday, when well above normal 925-mb temperatures will combine with abundant moisture, and Extreme Heat Warning criteria cannot be ruled out. However, increased cloud coverage associated with the approaching tropical wave could limit daytime heating and provide some relief.

Long Term(Monday through Friday)

Issued at 437 AM AST Fri Sep 4 2026

Some changes were made to the long-term forecast, as conditions are likely to be variable across the CWA. As mentioned in the previous discussion, a surface high pressure will dominate the wind pattern, promoting an easterly-southeasterly flow and leading to breezy to locally windy conditions by mid next week. Under these conditions, any showers will become progressive rather than stationary. A tropical wave is still forecast to move across the islands by Tuesday, though the NASA GMAO Dust Extinction AOT suggests concentrations of Saharan Dust surrounding the wave, which may weaken it by the time it approaches the local area. From the latest model guidance, solutions show the most active area north of the CWA, though moisture content should remain seasonal to well above- climatological normal (PWATs between 1.9 - 2.1 inches) with the main bulk of moisture in the low and mid levels (RHs above 60%). Although the most likely scenario is to observe shower activity moving over windward sections in the morning and afternoon convection, the presence of a mid to high level ridge may limit thunderstorm activity in the afternoon, though Saharan Dust concentrations may boost the lightning activity.

Rainfall accumulations across interior and northwestern Puerto Rico could lead to ponding of water over roadways and poorly drained areas, along with minor flooding. As moisture content reduces and drier air filters into the mid levels, the weather pattern on Wednesday may be less convective, driven mainly by daytime heating and local effects, posing no flooding threat. A tropical wave is forecast to move near the CWA by late Thursday into Friday, pooling tropical moisture (PWATs once again between 1.9 - 2.2 inches), mainly over the eastern side of Puerto Rico. The most likely scenario is an advective pattern across windward sections of the islands early Friday, with afternoon convection if cloud cover allows it. These accumulations could enhance the flooding threat, particularly across the eastern side of Puerto Rico and the U.S. Virgin Islands, with the highest threat across northwestern Puerto Rico. The drier conditions mentioned in the previous conditions may be delayed, as guidance suggests the arrival of drier air by the end of the period. Warmer to hot conditions are still on forecast, with Monday and Tuesday likely to the warmest days of the long-term period.

AVIATION

(06Z TAFS) Issued at 437 AM AST Fri Sep 4 2026

Mainly VFR conds expected across all terminals thru the fcst period. A weak tropical wave could result in SHRA/iso TSRA across interior/NW PR aft 04/16Z, with tempo MVFR conds possible at TJBQ. Winds light/VRB overnight, becoming ESE-SE at 10-14 kt after 04/13Z, with sea breeze variations and stronger gusts possible near SHRA/TSRA.

MARINE

Issued at 437 AM AST Fri Sep 4 2026

Light to moderate east to southeast winds will prevail through Monday. Winds will strengthen and become moderate to fresh by Tuesday as surface high pressure builds across the central Atlantic, resulting in increasingly choppy seas. A weak tropical wave will bring scattered showers and isolated thunderstorms today into Saturday. Another tropical wave will increase shower and thunderstorm activity from Sunday into Monday.

BEACH FORECAST

Issued at 437 AM AST Fri Sep 4 2026

A low risk of rip currents is expected across most beaches through at least the middle of next week. However, life-threatening rip currents can still occur near groins, jetties, reefs, and piers, even when the risk is low. Beachgoers may enjoy generally favorable conditions but should remain alert for isolated rip currents and changing weather conditions. Afternoon thunderstorms with cloud-to- ground lightning are expected each day along the western coastal areas of Puerto Rico.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001>013.

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

AM...None.