textproduct: San Juan

KEY MESSAGES

Issued at 248 AM AST Tue Aug 18 2026

- Dry conditions will persist today as a drier air mass dominates the region, limiting the shower activity across the islands.

- A Heat Advisory is in effect from 10 AM to 5 PM AST for urban and coastal areas of Puerto Rico and the U.S. Virgin Islands.

- By tonight into Wednesday, a vigorous tropical (formerly Invest 94L) will approach the islands, increasing the potential for showers and thunderstorms.

- A low to moderate risk of rip currents is expected across local beaches throughout the workweek.

Short Term(Today through Thursday)

Issued at 248 AM AST Tue Aug 18 2026

Fair weather conditions prevailed overnight across Puerto Rico and the U.S. Virgin Islands, as a dry, stable air mass continues to dominate the region. Once again, it was another warm night across the islands, with overnight lows in the upper 70s to low 80s across lower elevations and in the 70s across higher elevations. Winds were light and variable at 10 knots or less.

Another dry, warm day is expected today; however, a gradual transition to a wetter, more unstable pattern is anticipated beginning this evening into Wednesday as a vigorous tropical wave (formerly Invest 94L) approaches the region. Precipitable water (PWAT) values will remain below normal today at 1.35 to 1.50 inches, limiting overall shower activity. Nevertheless, localized afternoon convective activity remains possible across portions of western and central Puerto Rico due to the combination of daytime heating and local effects. As the day progresses, PWAT values will gradually increase with the arrival of the tropical wave, increasing to above- normal values near 2.25 inches by Wednesday morning.

This tropical wave is expected to interact with a deep-layer trough, increasing the potential for moderate to heavy rainfall and thunderstorms across the region. Recent model guidance continues to show some uncertainty regarding how much of the moisture will directly impact the islands, and the amount of rain. However, if the current forecast materializes, this setup will favor periods of moderate to locally heavy showers across the U.S. Virgin Islands and eastern Puerto Rico, followed by additional afternoon activity across interior and western Puerto Rico. Localized urban and small- stream flooding is possible if the heavy rainfall develops. Stronger thunderstorms could also produce frequent lightning and locally strong, gusty winds. Any rainfall associated with this wave would be beneficial given ongoing dry conditions across portions of the region, providing welcome relief. Please continue to monitor forecast updates closely today as conditions evolve.

Following the wave's passage by Thursday, conditions will rapidly dry out as a drier, stable air mass establishes over the islands. This air mass, accompanied by low concentrations of Saharan dust, will persist through the weekend.

An elevated heat risk will affect coastal and urban sections of Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands today, where a Heat Advisory remains in effect from 10 AM to 5 PM AST. These conditions significantly increase the risk of heat exhaustion and heat stroke, especially for young children, the elderly, and anyone spending extended time outdoors. Stay hydrated, limit direct sun exposure, and take frequent breaks in the shade or air conditioning. Brief relief from the hot temperatures is expected on Wednesday if the forecast materializes and cloudiness with widespread rain persists over the region.

Long Term(Friday through Tuesday)

Issued at 248 AM AST Tue Aug 18 2026

Drier to seasonal conditions may persist this weekend, with uncertainty increasing by the end of the period. The wind pattern may vary, as a surface high building over the Western Atlantic will likely promote E-ENE winds on Friday, becoming from the E-ESE Sunday onwards as a weak tropical wave approaches the local area. Friday will likely be the "driest" day of the period, as model guidance suggests a drier airmass, along with concentrations of Saharan Dust, filtering into the region. Based on the latest deterministic guidance of the GFS and ECMWF, PWAT values should drop to below climatological normals (between 1.3 and 1.5 inches), as moisture content drops particularly in the mid levels (below 40%). Additionally, the proximity of a mid-level ridge (north of the Hispaniola) should lead to warmer-than-normal mid-level temperatures (500 mb temperatures up to -4 degrees Celsius), subsidence and stability aloft. Although daytime heating combined with local effects should trigger afternoon convection over the western side of Puerto Rico and local island streamers, rainfall accumulations will likely be limited under this pattern, resulting in no flooding threat. By Saturday, moisture content is likely to increase as a perturbation in the trades moves over the CWA, with PWAT values up to 1.80 inches, mainly observed in the 850 - 700 mb layer. Based on the latest model guidance, passing showers should move over windward sections of the islands, during the morning, with afternoon convection over interior and western Puerto Rico. With Saharan Dust concentrations diminishing, conditions look more favorable for showers, without ruling out isolated thunderstorms, over the aforementioned areas. Nevertheless, rainfall accumulations should lead to ponding of water over roadways and poorly drained areas. A similar pattern is expected on Sunday as lingering moisture and winds veering from the E-ESE may enhance deep convection over the northwestern side of Puerto Rico. Monday's forecast remains uncertainty as global model solutions suggests different outlook. From the latest GFS solution, a tropical wave moving over the Central Atlantic may become a cyclonic system and move northeast of the CWA, while the ECMWF's solution suggests that this system shifting northward, far away of the region. Therefore, uncertainty will remain high for the beginning of the next workweek.

Seasonal to warmer-than-normal conditions are very likely in the long-term, with Sunday being the "hottest" day of the week as winds shift from the E-ESE. With the available moisture content, heat indices will very likely exceed 100 degrees Fahrenheit, particularly over urban and low-lying areas. Therefore, the heat threat will remain elevated to significant throughout the forecast period. Residents and visitors should stay hydrated and avoid prolonged outdoor activities.

AVIATION

(06Z TAFS) Issued at 248 AM AST Tue Aug 18 2026

Mainly VFR conds expected across all TAF sites. Generally fair weather conditions will prevail across the region with aftn SHRA across western PR. Light winds at 10 kt or less through 18/13Z. Winds E-ESE at 10-15 kt with sea breeze variations and ocnl gusts up to 25 kt through 18/23Z. SHRA/VCSH expected to increase aft 18/23Z at TJSJ, TIST, TISX as a vigorous tropical wave approaches the islands.

MARINE

Issued at 248 AM AST Tue Aug 18 2026

Light to moderate easterly trade winds will continue to prevail across the region over the next few days. Seas peaking up to 5 feet today will gradually ease to 2 to 4 feet later in the workweek. By late tonight into Wednesday, an approaching tropical wave will draw in deep moisture, increasing the potential for showers and thunderstorms across most local waters. This will elevate localized lightning hazards and bring gusty winds over the marine zones through midweek. Also, expect hazy skies across the northeastern Caribbean due to suspended Saharan dust, with higher concentrations after the wave passage.

BEACH FORECAST

Issued at 248 AM AST Tue Aug 18 2026

A low to moderate risk of rip currents is expected through most of the workweek as seas continue to subside to around 2-3 feet across the coastal waters of the islands. However, remember that even when the risk is low, life-threatening rip currents can occur near groins, jetties, reefs, and piers. The lightning potential will increase on Wednesday due to thunderstorm activity associated with the arrival of a tropical wave. Low to moderate concentrations of Saharan dust will keep sky hazy before the wave passage, with higher concentrations expected from Thursday onward.

FIRE WEATHER

Issued at 248 AM AST Tue Aug 18 2026

Critical fuel conditions persist across the islands, with extreme drought affecting portions of eastern Puerto Rico and the southern coastal plains. Dry conditions will persist across the region with low relative humidity, stressed vegetation, and dry fuels continuing to support elevated fire danger.

The greatest concern remains across southern and southwestern Puerto Rico, where fuels are exceptionally dry and recent fire activity has been frequent. A gradual increase in moisture is expected later in the afternoon, as a vigorous tropical wave approaches the islands. Therefore, a Fire Danger Statement is in effect from 10 AM to 5 PM AST for the southern and southwestern plains of Puerto Rico. Residents and visitors are strongly urged to avoid outdoor burning, as conditions will easily support wildfire ignition and rapid spread.

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.