textproduct: San Juan

KEY MESSAGES

Issued at 424 AM AST Sun Sep 6 2026

* Dangerous heat is expected today and again on Monday. An Extreme Heat Warning is in effect for Puerto Rico beginning this morning, with a Heat Advisory for Vieques, Culebra, and the U.S. Virgin Islands. Although passing showers and clouds may provide some temporary relief, enough sunshine is expected for heat indices to reach dangerous levels.

* Showers and thunderstorms will increase at times today, with the heaviest activity expected this afternoon across the interior and northwestern Puerto Rico. Locally heavy rainfall may result in ponding of water and localized flooding. Shower and thunderstorm coverage may decrease somewhat on Monday.

* A low risk of rip currents will continue across most beaches. However, life-threatening rip currents remain possible, especially near jetties, reefs, groins, and other structures.

* A much more stable weather pattern is expected by Tuesday, with a noticeable decrease in shower and thunderstorm activity. Trace amounts of Saharan dust will also be present by midweek.

Short Term(Today through Tuesday)

Issued at 424 AM AST Sun Sep 6 2026

During the overnight hours, temperatures remained a few degrees warmer than the previous night, allowing the region to start the day from a warmer baseline. Well-above-normal 925-mb temperatures associated with southeasterly flow, combined with well-above-normal moisture, will support dangerous heat conditions later today. An Extreme Heat Warning for Puerto Rico and a Heat Advisory for Vieques, Culebra, and the U.S. Virgin Islands will go into effect by mid-morning. Some uncertainty remains, as showers and isolated thunderstorms continue to develop over the local waters early this morning, enhanced by the tropical wave and an upper-level trough slowly moving westward. However, this activity has not produced widespread cloud cover, and sufficient clearing after sunrise could still allow temperatures and heat indices to rise quickly.

For today, abundant moisture will persist as the tropical wave moves through the region, with precipitable water values around 2.1-2.2 inches. The upper-level trough will remain nearby through this afternoon, although it will shift farther west as ridging gradually builds over the northeastern Caribbean. Some ventilation aloft may therefore persist, allowing afternoon convection to become locally stronger across the interior and northwestern portions of Puerto Rico, where local and diurnal effects will provide additional forcing. Streamers may also develop downwind of the local islands and El Yunque, producing additional showers over portions of eastern Puerto Rico and the San Juan metro area. Locally heavy rainfall will result in ponding of water, and minor flooding in poor-drainage areas, particularly across northwestern Puerto Rico.

A similar pattern is expected Monday, with lingering moisture and above-normal temperatures supporting another day of dangerous heat. Afternoon convection is also expected, mainly over the interior and western to northwestern portions of Puerto Rico, while streamers may once again develop downwind of the islands and El Yunque. However, with the upper-level trough farther west and ridging aloft becoming more established, increasing mid-level stability may reduce the overall coverage and vertical extent of convection. Trace amounts of Saharan dust will also be present, particularly by Monday evening.

By Tuesday, a significant drying trend is expected as the ridge becomes firmly established over the region. Precipitable water values and lower- to mid-level relative humidity will fall sharply through the day, reaching well-below-normal levels by the evening. At the same time, 500-mb temperatures will warm to well-above-normal values, while both the 700-500 mb and, eventually, the 850-700 mb lapse rates decrease to below-normal levels, indicating increasing stability through a deeper layer of the atmosphere. Therefore, a much less active weather pattern is expected, with considerably lower shower and thunderstorm coverage by later in the day.

Long Term(Wednesday through Sunday)

Issued at 424 AM AST Sun Sep 6 2026

A surface high-pressure will move closer to the northeastern Caribbean, tightening the pressure gradient and resulting in breezy to locally windy east-southeasterly winds, especially along coastal and exposed areas. A weak tropical wave will move through the region Wednesday, with residual moisture persisting into Thursday. Precipitable water values are expected to increase to 2.1 to 2.3 inches, just above the 75th percentile for this time of year, and the 500 mb temperatures are expected to decrease to near -6 to -7 degrees Celsius, supporting an increase in shower and thunderstorm activity. However, the breezy conditions should limit the flood potential, with little relief expected for the ongoing drought and water-supply concerns.

A drier air mass, accompanied by low concentrations of Saharan dust, will spread across the region Friday and Saturday. Precipitable water values will decrease to around 1.5 inches, nearly two standard deviations below normal, favoring mostly stable and warmer conditions. Some uncertainty remains Friday, as the ECMWF maintains higher moisture across the region.

By Sunday, an upper-level low and associated lower-level disturbance may move closer to the northeastern Caribbean, gradually increasing moisture and instability. Precipitable water values are expected to rise to 1.6 to 1.8 inches, closer to climatological normals, while cooler 500-mb temperatures and more favorable mid-level conditions could further enhance instability and shower and thunderstorm development, especially in the afternoon along the western interior and northwestern quadrant.

Overall, the tropical wave may provide some temporary relief from the heat Wednesday and Thursday, but warmer conditions are expected to return late in the week and into the weekend, with limited to elevated heat risk persisting through the period.

AVIATION

(06Z TAFS) Issued at 424 AM AST Sun Sep 6 2026

Mainly VFR conditions are expected to prevail across all terminals through the fcst period. Passing -SHRA/VCSH and brief VCTS are expected across the USVI and E PR terminals through 06/15z. Convection may result in VCTS at TJPS aft 06/17z and tempo MVFR conditions at TJBQ aft 16/17z, with a PROB30 for TSRA between 18- 22z. Elsewhere, mainly VCSH expected. Winds light/VRB to calm through 06/13z, becoming E-ESE at 9-14 kt thereafter, with sea breeze variations and stronger gusts possible near SHRA/TSRA.

MARINE

Issued at 424 AM AST Sun Sep 6 2026

Light to moderate southeast winds are expected to continue through at least tomorrow, Monday. From Tuesday onward, winds are expected to become moderate to fresh as a surface high pressure strengthens over the central Atlantic, leading to choppy seas of up to 6 feet and east winds reaching 15 to 20 knots with higher gusts. Moisture associated with a tropical wave will continue to bring scattered showers and isolated thunderstorms over the regional waters today.

BEACH FORECAST

Issued at 424 AM AST Sun Sep 6 2026

A low risk of rip currents is expected to prevail through Monday night along all local beaches of Puerto Rico and U.S. Virgin Islands. However, even when the risk is low, life-threatening rip currents often occur near jetties, piers, reefs, and groins. Beachgoers should remain cautious always. By Tuesday, the risk will increase to moderate along the islands' northern and eastern beaches, as winds become moderate to fresh and seas gradually build to around 4 or 5 feet. By midweek onward, the moderate risk is anticipated to extend to most beaches. A moderate risk means life- threatening rip currents are possible and beachgoers are urged to exercise caution.

Another beach hazard will be the heat. Beachgoers should remain hydrated and limit prolonged sun exposure. Additionally, gusty winds and lightning due to afternoon showers and isolated thunderstorms, particularly across the western beaches of Puerto Rico. Beachgoers should seek shelter whenever they hear thunder.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Extreme Heat Warning from 10 AM this morning to 5 PM AST this afternoon for PRZ001>011.

Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ012-013.

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

AM...None.