textproduct: San Juan

KEY MESSAGES

Issued at 305 PM AST Tue Aug 18 2026

- A drier air mass with Saharan Dust concentrations will continue over the region today, limiting any shower activity.

- A Heat Advisory continues through 5 PM AST for urban and coastal areas of Puerto Rico and the U.S. Virgin Islands.

- By this evening into tomorrow, Wednesday, a tropical wave (former Invest 94L) will approach the islands, increasing the potential for showers and t-storms.

- A low to moderate rip current risk is expected across local beaches through the rest of the week, with the exception of tomorrow where a low risk should prevail.

Short Term(This evening through Thursday)

Issued at 305 PM AST Tue Aug 18 2026

Warm to hot conditions prevailed over the islands today, as Heat indices have reached the mid to upper 100s to the low 110s across urban and coastal areas of the islands. A Heat Advisory remains in effect through 5 PM AST for these areas. Current satellite-derived precipitable water (PWAT) values indicate 1.35 to 1.55 inches over Puerto Rico, below normal values for this time of the year, as a dry and stable air mass with up to moderate saharan dust concentrations is over the region. However, satellite imagery already detects increasing PWAT values (1.65 to 1.85 in) over Vieques, Culebra and the USVI as the moisture field of a tropical wave (former Invest 94L) continues to approach the region from the east. Advective showers will continue to drift in from the Anegada Passage throughout the afternoon under easterly steering flow. Concurrently, diurnal heating, sea breeze convergence, and local terrain effects can result in limited convective activity over interior and western Puerto Rico.

As the tropical wave continues to approach and move over the region, a transition towards a wetter and more unstable pattern is forecast from this evening through Wednesday. As the moisture field engulfs the islands, PWAT values will continue to increase towards 2 to 2.15 inches over the islands by tomorrow morning, and then locally higher during tomorrow's afternoon convection. With the tropical wave interacting with an upper trough, the potential for moderate to heavy rain and t-storms will also increase across the region under more ESE steering flow. Although uncertainty remains, the current pattern should favor periods of moderate to heavy rain and isolated t-storms over the eastern region, and then during the afternoon over interior to western Puerto Rico, as well as downwind of the local islands and El Yunque. This will sustain a flooding risk tonight and tomorrow over the eastern region, possibly increasing to elevated over interior to western Puerto Rico tomorrow afternoon.

As the moisture field moves out of the islands during the overnight hours tomorrow night to Thursday, conditions will rapidly dry out once again (with PWAT values sharply decreasing to below normal) as a drier and more stable air mass once again establishes itself over the region. Breezy ESE flow will persist on Thursday while low to moderate Saharan Dust concentrations move over the region. Although tomorrow could provide some relief in terms of heat, an elevated to possibly significant heat risk will return on Thursday.

Long Term(Friday through next Monday)

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

(18Z TAFS) Issued at 305 PM AST Tue Aug 18 2026

VFR conds expected across all TAF sites through the forecast cycle. Fair weather conditions to continue across the region with aftn SHRA possible in and around TJBQ through about 18/23z. Winds E-ESE at 10-15 kt with sea breeze variations, becoming at 10 knots overnight. SHRA/VCSH expected to increase tonight into the overnight hours at TJSJ, TIST, TISX as a tropical wave approaches the islands.

MARINE

Issued at 305 PM AST Tue Aug 18 2026

Light to moderate easterly trade winds are expected to persist over the coming days. Seas at 2 to 4 feet, occasionally up to 5 feet across the offshore waters. Tonight through tomorrow, Wednesday, a tropical wave and its moisture field will move over the area, heightening the likelihood of showers and t-storms over most local waters. Additionally, up to moderate saharan dust concentrations will continue today and return following the wave's passage, on Thursday. A more moderate to fresh breeze is also forecast Thursday through the end of the week resulting in SCEC conditions as seas increase to up to 5 feet, occasionally higher, and possibly up to 6 feet over the Mona Passage, occasionally higher.

BEACH FORECAST

Issued at 305 PM AST Tue Aug 18 2026

A moderate risk of rip currents (rip currents are possible in the surf zone) is forecast tonight for north-central to northwestern Puerto Rico. A low risk of rip currents is forecast for the rest of the local beaches tonight and should persist for most of the local beaches tomorrow. Even if the risk is low, life-threatening rip currents often occur in the vicinity of groins, jetties, reefs, and piers. Lightning potential will increase tonight through tomorrow, Wednesday, due to thunderstorm activity associated with the arrival of a tropical wave. Hazy skies will persist prior to the passage of the wave due to low to moderate Saharan dust levels, with higher dust concentrations anticipated starting Thursday. A moderate risk of rip currents is also forecast to return across local beaches on Thursday and continue through the end of the week.

FIRE WEATHER

Issued at 305 PM AST Tue Aug 18 2026

Critical fuel conditions continue across the islands, with extreme drought affecting the southern coastal plains and eastern Puerto Rico. Generally dry conditions have prevailed today, with low relative humidity, stressed vegetation, and dry fuels continuing to support an 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 this afternoon to evening, as a tropical wave, and its moisture field, approaches the islands. Although some relief is possible, uncertainty persists regarding the coverage of rainfall. A Fire Danger Statement continues through 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 until 5 PM AST this afternoon for PRZ001>005-007- 008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.