textproduct: San Juan

KEY MESSAGES

Issued at 354 AM AST Wed Sep 2 2026

- Afternoon showers and thunderstorms are expected, with the heaviest rainfall across interior and western Puerto Rico. Ponding of water and localized flooding are possible in urban areas, on roads, and near small streams. The U.S. Virgin Islands will see passing showers.

- Hot temperatures combined with high humidity will result in elevated heat indices today. A Heat Advisory is in effect for Puerto Rico and the U.S. Virgin Islands from 10 AM to 4 PM AST. Heat will remain one of the primary hazards in the coming days, particularly this next weekend.

- A low risk of rip currents will prevail over the coming days. However, even when the overall risk is low, life-threatening rip currents can occur near groins, jetties, reefs, and piers.

- A weak tropical wave will increase the chances of rain by next Friday. Then, early next week, additional moisture could bring more rainfall chances to the region.

Short Term(Today through Friday)

Issued at 354 AM AST Wed Sep 2 2026

Fair weather conditions prevailed over most land areas overnight under clear to partly cloudy skies. One exception was along the coasts of Maunabo and Patillas, where scattered showers moved through, leaving around 1 inch of rain based on radar estimates. The rest of the showers observed were moving across the local waters. Surface observations recorded minimum temperatures in the upper 70s to low 80s across most coastal areas, while temperatures across the mountainous range remained in the upper 60s to low 70s. Light variable winds dominated across the islands.

Satellite-derived data indicate that a moist air mass persists across the region, with precipitable water (PWAT) values exceeding 2 inches. Moisture is mainly from the surface through 500 mb, as drier air intrusion is observed aloft in the latest cross- sections. Surface high pressure across the eastern Caribbean will support light easterly winds today, while an upper-level trough will maintain local instability across the northeast Caribbean before gradually weakening throughout the day. Today, passing showers are expected this morning across windward areas of Puerto Rico and the U.S. Virgin Islands. During the afternoon, showers and thunderstorms are expected, with the heaviest rainfall across interior and western Puerto Rico. Ponding of water and localized flooding are possible in urban areas, on roads, and near small streams. With light winds prevailing, showers are expected to move slowly across the mountainous region, favoring higher rainfall accumulations, particularly across the interior and west-central areas. The other main hazard today will be the heat. Less cloudiness compared to the past few days will allow land areas to warm up. Also, 925 mb temperatures are observed in the 75th percentile based on climatology. Hot temperatures combined with high humidity will likely result in elevated heat indices, especially in areas with limited rain. As a result, a Heat Advisory was issued and is in effect for Puerto Rico and the U.S. Virgin Islands from 10 AM to 4 PM AST. Heat will remain one of the primary hazards throughout the short-term.

Precipitable water values are forecast to decrease to seasonal levels on Thursday. Patches of moisture embedded in the trade winds will affect the windward areas, followed by more localized afternoon convection across western Puerto Rico and downwind of the U.S. Virgin Islands. As a result, the primary hazard will be warm to hot temperatures, with the potential for additional heat- related products to be issued. By Thursday night into Friday morning, a patch of below-normal moisture will move across the region, leading to generally fair weather conditions. From mid- morning Friday onward, winds will develop a more southeasterly component and slightly increase. Moisture will also trend upward, with PWAT values likely reaching around 2.0 inches as a weak tropical wave approaches the Caribbean. Aloft, an upper-level low will remain nearby, resulting in periods of upper-level cloudiness, while a mid-level ridge near 500 mb should provide some stability. Low to slightly moderate concentrations of Saharan dust will also be present. These conditions are expected to limit widespread shower development, however, the available moisture, combined with local effects, may enhance afternoon convection across western Puerto Rico. At this time, a limited flood threat is highlighted for these areas. Residents and visitors are encouraged to monitor the forecast for any adjustments and updates as they become available.

Long Term(Saturday through Wednesday)

Issued at 354 AM AST Wed Sep 2 2026

A surface high pressure over the central Atlantic, a mid-level ridge, and the passage of a tropical wave by Tuesday will be the main weather features during the long-term period. On Saturday, the surface high pressure over the central Atlantic will maintain a southeasterly wind flow across the region. Moisture will remain relatively high, with precipitable water values around 1.9 inches. However, at the mid-levels, a ridge near 500 mb will promote warmer temperatures and a strong trade wind cap, providing enough stability to limit the vertical development of showers. At the upper levels, southerly winds around 250 mb will continue to bring periods of upper-level cloudiness across the region. Therefore, despite the available moisture, generally stable conditions should limit widespread rainfall activity. The main concern on Saturday will continue to be the heat threat, particularly across coastal and urban areas.

From Sunday into Monday, the surface pressure gradient will weaken as the high pressure remains over the central Atlantic, resulting in lighter winds across the islands. Winds are expected to become more southerly to southeasterly at the lower levels, while southwesterly winds prevail near 250 mb. Meanwhile, moisture will gradually increase, with precipitable water values approaching 2.0 inches. The combination of weak steering winds, abundant moisture, and local effects should favor afternoon convection, particularly across the interior and northern portions of Puerto Rico. Slow-moving showers could result in locally higher rainfall accumulations, although widespread activity is not anticipated at this time. By Tuesday, weather conditions are expected to become more unstable as the moisture field associated with a tropical wave moves across the region. Model guidance suggests precipitable water values could increase to around the 75th percentile for this time of year, indicating an anomalously moist environment. In addition, an upper- level low is forecast to become positioned near the region, increasing instability aloft and providing better upper-level support for convective development. Upper-level disturbances generally favor enhanced clouds and precipitation by increasing instability and lift. Therefore, Tuesday currently appears to have the greatest potential for more widespread shower and thunderstorm activity, along with an increasing potential for locally heavy rainfall.

AVIATION

(06Z TAFS) Issued at 354 AM AST Wed Sep 2 2026

Mainly VFR conditions are expected through the forecast period. VCSH and/or brief -RA will move across the USVI and TJSJ this morning. Afternoon VCTS/-TSRA are expected mainly over interior and W PR, potentially causing brief/intermittent MVFR conditions and mountain obscurations, particularly near TJBQ but also TJPS. Winds will remain from the E at 10-12 kt and sea-breeze variations after 02/14Z. Lighter winds (6kt or less) aft 02/23Z.

MARINE

Issued at 354 AM AST Wed Sep 2 2026

A surface high pressure will continue to build over the far eastern Atlantic and promote gentle to moderate trades through the workweek, with seas between 2 and 4 feet, occasionally 5 feet. The next tropical waves are forecast to reach the local waters and passages by Friday into the weekend, increasing shower and thunderstorm activity and bringing gusty winds and lightning.

BEACH FORECAST

Issued at 354 AM AST Wed Sep 2 2026

The beach forecast remains on track. As winds gradually weaken, breaking wave should remain between 2 and 3 feet, lowering the rip current risk. Therefore, the risk will downgrade to low today across all beaches of Puerto Rico and the U.S. Virgin Islands, though beachgoers must pay attention to the beach conditions as life- threatening rip current are often seen in the vicinity of groins, piers, reefs, and jetties. The low risk is expected to continue throughout the week, likely to become moderate early next week. Aside from rip currents, beachgoers should stay weather alert, particularly each afternoon, due to shower and thunderstorm activity that may move near the western and northern coastal areas of Puerto Rico, producing gusty winds and lightning. Beachgoers must seek shelter whenever they hear thunder.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 10 AM this morning to 4 PM AST this afternoon for PRZ001>013.

VI...Heat Advisory from 10 AM this morning to 4 PM AST this afternoon for VIZ001-002.

AM...None.