textproduct: San Juan

KEY MESSAGES

Issued at 143 PM AST Thu Jun 18 2026

* Periods of moderate to locally heavy rainfall, with isolated thunderstorms are still expected for the rest of today and later on Fathers Day weekend, with the highest chance of urban and small stream flooding on Sunday.

* Moderate to high concentrations of Saharan dust are expected early Friday into Fathers Day weekend, deteriorating air quality and bringing hazy skies and reduced visibility. Exercise caution and follow medical recommendations.

* Seasonal temperatures will continue this Fathers Day weekend through next week, with heat indices ranging from the mid 90s and mid 100s across urban and low-lying areas of Puerto Rico and the U.S. Virgin Islands.

Short Term(This evening through Saturday)

Issued at 143 PM AST Thu Jun 18 2026

While a weak tropical wave is tracking mainly south of the region, it is encountering a hostile environment characterized by a strong trade wind inversion near the 850 mb layer and significant mid-to-upper level drying. The 18/12z upper-air sounding indicated a precipitable water (PWAT) of 1.37 inches, well below seasonal climo. While a brief, localized bump in low- level moisture may support isolated afternoon convection across western Puerto Rico today, a substantial advection of dry air will rapidly overspread the islands tonight through Saturday. Total precipitable water is projected to remain suppressed between 1.30 and 1.60 inches, keeping shower activity limited to fast moving, shallow trade wind showers across windward areas overnight, followed by daytime driven convective activity over western Puerto Rico. High-res guidance indicates no significant wetting rains for most of the region through the first half of the weekend.

The primary feature affecting regional conditions will be a dense plume of Saharan dust spreading across the Atlantic into the northeastern Caribbean. Total precipitable water imagery and aerosol optical thickness models indicate a deep layer of suspended particulates accompanying a very dry air mass. This dense Saharan Air Layer (SAL) will fully filter into the region by early Friday and persist through Saturday, leading to widespread hazy skies, notably reduced horizontal visibilities, and a severe suppression of vertical cloud growth. Therefore, relatively dry and a stable weather pattern will dominate the local area through Saturday.

A strong surface high pressure over the central Atlantic will maintain moderate to fresh easterly to east-southeasterly trade winds at 15 to 20 knots, with localized sea breeze variations and higher gusts up to 2530 knots. Suppressed cloud cover, combined with 0-3 km steering winds with an ESE wind component, will drive intense daytime heating, pushing afternoon high temperatures into the lower 90s across coastal and urban areas, resulting in heat indices of 100108F. Furthermore, due to a prolonged lack of significant rainfall, critically dry fuels, and persistent breezy conditions, an elevated wildfire threat will persist, particularly across the southern hills and coastal plains of PR.

Long Term(Sunday through next Wednesday)

Issued at 303 AM AST Thu Jun 18 2026

A tropical wave is expected to approach and move across the northeastern Caribbean from late Sunday into Monday, bringing a significant increase in moisture across the region. Model guidance indicates precipitable water values rising to around 2 inches, with surface and mid-level relative humidity increasing to above-normal levels for this time of year. Combined with 500 mb temperatures near -7 degrees Celsius, these conditions will support a more unstable atmosphere and enhance the potential for showers and thunderstorms. As a result, periods of moderate to heavy rainfall are expected from late Sunday through late Monday, with the highest coverage likely during the afternoon hours across western and interior Puerto Rico. Locally heavy downpours or frequent showers may lead to ponding of water in urban and poorly drained areas, as well as isolated flooding concerns.

By Tuesday into midweek, a drier air mass will gradually filter into the region, improving weather conditions. Although passing trade- wind showers will remain possible across windward areas, a return to a more typical weather pattern is anticipated by midweek.

The primary hazards during the long-term period will be ponding of water and flooding, along with isolated thunderstorms.

AVIATION

(18Z TAFS) Issued at 143 PM AST Thu Jun 18 2026

Mainly VFR conditions are expected to prevail across all terminals during the next 24 hours across all terminals. However, iso -TSRA expected to develop in and around TJBQ thru 18/22z, which may lead to tempo MVFR conds. Low-level winds will continue E-ESE at 15-20 kt with sea breeze variations at TJBQ/TJSJ and stronger gusts thru 18/21z. HZ due to Saharan dust will gradually spread across the USVI and eastern PR terminals from this evening through Friday, and VSBY could drop to near 6 SM.

MARINE

Issued at 143 PM AST Thu Jun 18 2026

A broad surface high pressure building over the Central Atlantic will continue to promote moderate to locally fresh east to east- southeast winds over the next several days, resulting in choppy seas across local waters and Caribbean passages; small craft must exercise caution. Showers and isolated thunderstorms should move later today and early Friday, as a tropical wave moves across Caribbean Sea, resulting in a period of localized heavy rainfall, lightning activity, and gusty winds. A dense layer of Saharan Dust will gradually filter into the region late tonight into early Friday, resulting in hazy skies, reduced visibility, and poor air quality through Fathers Day weekend. Another tropical wave is expected to move across the Caribbean Sea by Sunday into Monday.

BEACH FORECAST

Issued at 143 PM AST Thu Jun 18 2026

The beach forecast remains on track. Theres a moderate risk of rip currents across beaches over north central, northwestern, and southeastern Puerto Rico, including Vieques and Culebra, and St. Croix in the U.S. Virgin Islands. Low risk elsewhere. Beachgoers must exercise caution, as life-threatening rip currents are possible along the beaches under the moderate risk. As mentioned in the previous discussions, the threat should gradually spread across the islands as winds continue to strengthen, leading to breaking waves around 4 feet. In addition to rip currents, residents and visitors are encouraged to follow the latest weather forecast, as afternoon showers and isolated thunderstorms could be expected this Fathers Day weekend and early next week, particularly near coastal areas of western/northwestern Puerto Rico. Gusty winds, along with possible lightning, can be expected near thunderstorms. Additionally, moderate to high concentrations of Saharan Dust will gradually filter into the region late tonight into early Friday, resulting in hazy skies, reduced visibility, and poor air quality. Beachgoers sensitive to these particles should follow medical recommendations.

SJU WATCHES/WARNINGS/ADVISORIES

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