textproduct: San Juan

KEY MESSAGES

Issued at 346 AM AST Sun Sep 13 2026

- Residual Saharan dust will maintain light to moderate haze across the region before gradually diminishing later today.

- The heat threat will persist mainly across coastal and low-elevation areas of Puerto Rico and the U.S. Virgin Islands, where daily heat indices could range between 102 and 108 degrees Fahrenheit.

- Afternoon thunderstorms developing over the interior and western Puerto Rico may produce strong wind gusts, frequent lightning, and localized urban and small-stream flooding through early week.

- A moderate risk of rip currents continues across exposed northern, eastern, and southern beaches of the local islands.

Short Term(Today through Tuesday)

Issued at 346 AM AST Sun Sep 13 2026

Satellite observations and aerosol extinction models indicate that the main plume of Saharan dust will slowly clear the region westward, allowing air quality and visibility to improve gradually into early next week. Even with dry air lingering aloft, low- level moisture remains near seasonal levels, with precipitable water values fluctuating between 1.60 and 1.80 inches. Strong solar heating combined with increasing surface humidity will keep temperatures elevated across low-lying coastal regions, where afternoon heat indices are expected to exceed 100 degrees Fahrenheit, maintaining a slightly elevated heat risk across coastal urban areas.

For the rest of the short-term period, atmospheric conditions aloft will turn favorable for convective development as an upper- level low remains anchored to the west-northwest of the forecast area. Upper-level divergence and 500 mb temperatures cooling between -7 and -8 degrees Celsius will promote mid-level instability. Diurnal heating coupled with local sea breeze convergence and terrain forcing will generate scattered afternoon showers and strong thunderstorms across the interior, northwestern, and western sections of Puerto Rico each afternoon. With somewhat lighter mid-level steering winds (around 15 kt), slower-moving cells can produce localized rainfall accumulations of 1 to 2 inches, increasing the potential for urban and small- stream flooding, as well as ponding of water on roadways and in poor drainage areas.

By Tuesday, a surge in low-level tropical moisture will push precipitable water values near to slightly above 2.00 inches across the local area. Passing easterly trade wind showers will increase in frequency during the overnight and early morning hours across eastern Puerto Rico and the U.S. Virgin Islands, followed by stronger afternoon convection than previous days, particularly over western PR, where the flood threat will remain elevated.

Long Term(Wednesday through Sunday)

Issued at 346 AM AST Sun Sep 13 2026

Confidence is increasing in the long-term forecast, with Thursday being the wettest day of the period. A surface high pressure building in the Central Atlantic should maintain east-southeast winds, leading to breezy conditions across coastal areas. Slightly drier conditions are expected on Wednesday, as the latest NASA GMAO AOT model suggests low to moderate concentrations of Saharan Dust moving across the CWA. From the latest model guidance, PWAT values may drop below climatological normal (below 1.70 inches), highlighted in the 850 - 700 mb. Additionally, a mid-to-upper-level ridge lingering across the eastern Caribbean will likely lead to warmer-than-normal mid-level temperatures (500 mb temperatures above -6 degrees Celsius), subsidence, and stability aloft. Although local effects and daytime heating will drive the shower and thunderstorm activity, the most likely scenario is for it to be localized and short-lived. As mentioned in the previous discussion, a tropical wave is forecast to move across the islands, pooling well above normal tropical moisture by late Wednesday night and Thursday. From the latest deterministic guidance of the GFS and ECMWF, PWAT values are likely to increase up to 2.20 inches, well above climatological normal, mainly in the low and mid levels. Additionally, theres more agreement between global models, as ensemble members are showing less variability between them, tending to wetter conditions. In terms of instability, the latest guidance continues to suggest areas of enhanced vorticity associated with the wave, increasing the likelihood of an advective pattern across windward sections of the islands, followed by afternoon convection. As the tropical wave departs from the region, atypical conditions for this time of the year may be expected due to another SAL with low to moderate concentrations moving across the area, introducing a slot of drier air in the low levels and limiting shower and thunderstorm activity. Concentrations could lower by Saturday, with conditions becoming more seasonal.

Overall, afternoon showers and thunderstorms can be expected each day, particularly across interior and western/northwestern Puerto Rico, though the flooding threat is low on Wednesday and Friday. Saturday conditions should lead to ponding of water over roadways and poorly drained areas. Meanwhile, the tropical wave on Thursday should increase both the lightning and flooding threat, with the potential of urban and small stream flooding across interior and northwestern Puerto Rico and minor flooding across the U.S. Virgin Islands and eastern/southeastern Puerto Rico.

Under an E-SE wind pattern, seasonal to warmer conditions are very likely across the CWA, particularly by the end of the long-term forecast. During the peak hours of the day, urban and low-lying areas of the islands could easily reach 108 degrees Fahrenheit, increasing the heat threat. Therefore, an elevated heat threat is expected throughout the forecast period, though it may be limited on Thursday given the weather forecast.

AVIATION

(06Z TAFS) Issued at 346 AM AST Sun Sep 13 2026

Mainly VFR conditions are expected to prevail across all terminals during the next 24 hours. However, early morning trade wind showers could move briefly the USVI and eastern PR terminals. Tempo MVFR conds possible at TJBQ btw 13/18-22z due to locally induced TSRA. HZ due to Saharan dust will continue across the region, but VSBY will improve throughout the day. East winds expected to increase between 12-16 kt with sea breeze variations and stronger gusts at times aft 13/13z.

MARINE

Issued at 346 AM AST Sun Sep 13 2026

A weak surface high north of the region and the Colombian low will maintain moderate to fresh easterly trade winds across the Caribbean through today, creating choppy seas of 4 to 6 feet. Saharan dust levels will decrease, and a tropical wave will move mainly south of the region, bringing increased showers and isolated thunderstorms. Moderate easterly winds will continue through midweek, with another tropical wave expected to bring more rain and thunderstorms from Thursday into Friday.

BEACH FORECAST

Issued at 346 AM AST Sun Sep 13 2026

A moderate risk of rip currents will continue today along portions of Puerto Rico's northern coast, with additional localized areas across the offshore islands and U.S. Virgin Islands. The risk will become more confined to north-facing beaches of Puerto Rico Monday and Tuesday, with a low risk elsewhere.

From Wednesday through Friday, the moderate rip current risk is forecast to expand again, affecting portions of the northern and southern coasts of Puerto Rico, as well as Culebra and parts of the U.S. Virgin Islands. Even where a low risk is forecast, stronger rip currents can still occur near groins, jetties, reefs, and piers. Swim near a lifeguard whenever possible.

Saharan dust concentrations will linger through today, creating some haziness. Moisture will gradually increase this afternoon into Monday, prompting a better chance of showers and more frequent thunderstorms. Beachgoers should remain alert for rapidly changing conditions and seek shelter immediately if thunder is heard.

SJU WATCHES/WARNINGS/ADVISORIES

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