textproduct: San Juan

KEY MESSAGES

Issued at 149 PM AST Sun Jun 28 2026

* Abundant tropical moisture will enhance shower and thunderstorm activity late tonight and Monday, with ponding of water over roadways and poorly drained areas, including urban and small stream flooding, across the islands.

* Warmer to hot conditions are expected for the next few days, with heat indices exceeding 100 degrees Fahrenheit across urban and low- lying areas of Puerto Rico and the U.S. Virgin Islands. Stay hydrated.

* Moderate to high concentrations of Saharan Dust will likely return by early Tuesday, resulting in hazy skies, reduced visibility, and poor air quality. Sensitive groups must follow medical recommendations.

Short Term(This evening through Tuesday)

Issued at 149 PM AST Sun Jun 28 2026

A relatively dry and stable air mass is currently in place across the region, influenced by a mid-level ridge and a trade wind inversion. This will keep shower activity fairly isolated through the remainder of the day, with locally induced showers developing briefly this afternoon due to local effects. However, conditions will shift gradually into a wetter pattern as the leading edge of a tropical wave is currently producing scattered showers and brief isolated thunderstorms across the Anegada Passage. This surge in moisture will continue through Monday as the tropical wave crosses the region. Latest satellite-derived total precipitable water imagery highlights this deep surge of moisture, with PWAT values exceeding 2 inches across portions of the Leeward Islands and the southeastern Caribbean.

As the axis of this deep tropical moisture moves across the islands on Monday, we expect a considerable increase in the coverage and intensity of showers and thunderstorms across the U.S. Virgin Islands and Puerto Rico. The activity will gradually increase from the local waters into the USVI and eastern Puerto Rico during the overnight and early morning hours. By Monday afternoon, daytime heating and local orographic effects will trigger thunderstorm development across the eastern interior, central, and western sections of Puerto Rico. High-res model guidance suggests between 1 and 2 inches of rain across these areas, which will elevate the threat of urban and small-stream flooding.

By Tuesday, as the tropical wave exits to the west, a strong Saharan Air Layer with a dense plume of Saharan dust will spread across the eastern Caribbean. This will cause rapid drying and more stable conditions, limiting shower activity in general across all the islands. The primary impacts will be noticeably hazy skies and reduced air quality. Also, the combination of suppressed cloud cover from the dry, dusty Saharan Air Layer and lingering surface humidity could push heat indices between 102F and 110F across the lower elevations and urban areas of the islands.

Long Term(Wednesday through next Saturday)

Issued at 149 PM AST Sun Jun 28 2026

A broad surface high pressure system centered over the central Atlantic and extending into the Caribbean will remain the dominant weather feature through the long-term period, maintaining a predominantly southeasterly low-level wind flow across Puerto Rico and the U.S. Virgin Islands. Embedded patches of moisture with precipitable water values fluctuating near climatological normals will continue to move across the local area from Wednesday into Thursday, promoting a variable weather pattern. While widespread rainfall is not anticipated, daytime heating, local effects, and available moisture will support the development of afternoon showers and isolated thunderstorms across portions of western Puerto Rico each day. Some of the strongest showers could produce localized ponding of water on roadways and in poorly drained areas. Aloft, a cutoff low at 250 mb will gradually approach the northeastern Caribbean, lowering temperatures near the 250 mb level to around -9 C. This feature will provide modest upper-level support and slightly enhance atmospheric instability, favoring locally stronger convection during the afternoon hours. In addition, a small area of Saharan dust is expected to move across the region on Wednesday, resulting in hazy skies and reduced visibility at times before concentrations gradually diminish on Thursday.

From Friday through Sunday, a much drier air mass will filter into the region from the surface to around 700 mb, reducing moisture availability and overall shower coverage. Precipitable water values are expected to range between 1.5 and 1.7 inches, while relative humidity between 850 and 500 mb will remain near to slightly below climatological normals. Another pulse of Saharan dust is also forecast to move across the northeastern Caribbean during the weekend, leading to hazy skies and further suppressing shower activity. Despite the drier conditions, isolated afternoon showers cannot be completely ruled out over the western interior of Puerto Rico due to local sea breeze convergence and daytime heating. Warm to hot conditions will persist throughout the forecast period, with a daily elevated heat threat expected across the islands. Heat index values will generally range from 100 to 108 degrees, with isolated locations reaching around 110 degrees, particularly across the north- central coastal plain and the western interior of Puerto Rico during the peak heating hours between 10 AM and 5 PM each day.

AVIATION

(18Z TAFS) Issued at 149 PM AST Sun Jun 28 2026

Mainly VFR conditions are expected to prevail across most terminals through the rest of the day. However, SHRA and iso TSTMs are expected to increase from this evening through Monday as a tropical wave moves across the region. Mtn tops obscd and BKN cigs near FL030 are expected with the wave passage. The 28/12z TJSJ sounding indicated ESE winds up to 19 kt blo FL060.

MARINE

Issued at 149 PM AST Sun Jun 28 2026

A tropical wave located over the Lesser Antilles will move across the region tonight and Monday, increasing shower and thunderstorm activity across local waters and Caribbean Passages. Small craft must exercise caution near thunderstorms, as they may bring gusty winds, lightning, and reduce visibility, creating localized hazardous conditions. As the tropical wave continues its way into the Caribbean basin, the surface high pressure lingering over the Central Atlantic will increase the local pressure gradient, leading to moderate to locally fresh E-ESE winds and choppy seas. A brief, dense plume of Saharan dust will arrive and filter into the region late Monday night into Tuesday, bringing hazy skies, reduced visibility, and poor air quality.

BEACH FORECAST

Issued at 149 PM AST Sun Jun 28 2026

The beach forecast remains on track, with the low risk of rip currents persisting across most beaches of Puerto Rico and the U.S. Virgin Islands through the forecast period, as seas remain between 2 and 3 feet based on the latest nearshore buoy reports. Although the risk will remain low, life-threatening rip currents can be present in the vicinity of groins, jetties, reefs, and piers. As winds gradually strengthen, the risk may increase to moderate over exposed beaches in the northern/northwestern side of Puerto Rico, with life- threatening rip currents possible along the surf zone. In addition to rip currents, beachgoers should remain weather alert if planning on going to the beach, particularly tonight and Monday, as showers and thunderstorms associated with a tropical wave moving across the region may bring gusty winds and lightning. On early Tuesday, moderate to high concentrations of SAL will result in hazy skies, reduced visibility, and poor air quality.

SJU WATCHES/WARNINGS/ADVISORIES

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