textproduct: San Juan

KEY MESSAGES

Updated at 255 AM AST Thu Aug 27 2026

* Dangerous Heat Today: Heat indices will reach dangerous levels, particularly across urban and coastal areas. A Heat Advisory is in effect from 10 AM through 5 PM today.

* Showers and Thunderstorms Today: Increasing moisture will support passing showers this morning across the windward sections, followed by afternoon showers and isolated thunderstorms across interior and western Puerto Rico. A limited to elevated flooding risk is expected.

* Drier Friday: A strengthening mid-level ridge and drier air will limit rainfall tonight through much of Friday, although afternoon showers may develop across western Puerto Rico.

* Unsettled Pattern Returns: A weak tropical wave will bring showers and isolated thunderstorms late Friday night into early Saturday. Additional moisture associated with AL96 may support a wetter pattern Sunday into early next week, but uncertainty remains regarding its evolution.

* Increasing Rip Current Risk: A moderate risk of life- threatening rip currents is expected across most exposed beaches of Puerto Rico and the U.S. Virgin Islands as wind-driven seas increase, combined with a small northeasterly swell.

Short Term(Today through Saturday)

Issued at 255 AM AST Thu Aug 27 2026

A trade-wind perturbation brought increased shower activity across the local waters overnight, while low-level convergence supported showers across the U.S. Virgin Islands and eastern Puerto Rico. However, most of the activity remained over the waters. Although temperatures are somewhat cooler than last night, warmer-than-normal conditions persist, with readings generally in the upper 70s to low 80s across coastal areas and locally reaching the mid-80s. Cooler temperatures prevail across higher elevations and sheltered valleys. Winds remain mainly from the east to east-northeast at 10 mph or less, with somewhat stronger winds along exposed coastal areas.

The trade-wind perturbation will continue to increase tropical moisture across the region today, resulting in more frequent passing showers across windward areas during the morning. Meanwhile, sea surface temperatures surrounding the islands remain around 1 to 2 degrees Celsius above normal. Given the strong influence of the surrounding waters on island temperatures, above- normal air temperatures will persist. The combination of warmer temperatures and increasing moisture will once again produce dangerous heat indices, particularly across urban and coastal areas. A Heat Advisory is in effect from 10 AM through 5 PM today.

By this afternoon, daytime heating and local effects will promote shower development across the interior and western portions of Puerto Rico, with isolated thunderstorms possible. The strongest and most persistent activity may cause ponding on roads and localized urban and small-stream flooding, supporting a limited to elevated flooding risk.

Most of the associated moisture will move west of the region by this evening. In its wake, a drier air mass and a strengthening mid-level ridge will promote subsidence aloft and reinforce the trade-wind inversion, limiting shower development tonight through much of Friday. A modest increase in low-level moisture on Friday may support a few passing showers, followed by locally induced afternoon convection across western Puerto Rico. Nevertheless, significant rainfall is not anticipated.

Late Friday night into early Saturday, a weak tropical wave will cross the region, bringing another increase in moisture and a better opportunity for passing showers and isolated thunderstorms. The wave's progressive nature should favor fast-moving activity, limiting the overall flooding threat. However, low-level convergence and orographic enhancement could support locally heavier downpours across eastern Puerto Rico, where a limited flooding risk is expected. A few brief downpours are also possible across the U.S. Virgin Islands. The wave and its associated moisture will quickly move westward, with another drier air mass arriving by mid-morning Saturday and limiting shower activity through the afternoon. Despite the drying trend, above-normal temperatures are expected to persist, and additional heat products may be required where dangerous heat indices develop.

Long Term(Sunday through Thursday)

Issued at 255 AM AST Thu Aug 27 2026

Forecast uncertainty remains higher than usual heading into early next week, primarily associated with the evolution of AL96, currently over the central tropical Atlantic. According to the latest Tropical Weather Outlook from the National Hurricane Center, the disturbance has become somewhat better organized and has a high chance of tropical cyclone formation during the next 2 to 7 days. However, environmental conditions are expected to become less conducive for development this weekend. Regardless of development, the evolution of AL96 and its associated moisture field will continue to be monitored for potential impacts across Puerto Rico and the U.S. Virgin Islands.

The current forecast generally follows the timing depicted by the GFS and ECMWF, both of which indicate increasing moisture and precipitable water values from late Sunday through at least Tuesday. Although their timing is relatively similar, the GFS remains more aggressive with moisture and rainfall than the ECMWF. Part of this difference appears related to the evolution of the mid-level ridge. The ECMWF maintains stronger ridging closer to the northeastern Caribbean Sunday into Monday, which would favor greater subsidence and potentially disrupt the disturbance and its moisture field. In contrast, the GFS shifts the ridge farther west ahead of AL96, allowing deeper moisture to approach the region.

Given these trends, the forecast favors a wetter pattern Sunday into Monday, with increasing shower and thunderstorm activity and a more widespread limited flooding risk. Steering flow should favor passing showers across windward areas, followed by afternoon convection mainly across the interior and western to northwestern portions of Puerto Rico. Locally heavier rainfall will ultimately depend on the track and structure of AL96 and the distribution of its associated moisture.

Moisture should gradually decrease by late Tuesday as guidance indicates low to moderate concentrations of Saharan dust reaching the region. Nevertheless, precipitable water values are forecast to remain near the upper end of climatological values or above normal through much of the remainder of the period. Lingering moisture associated with AL96, followed by another tropical wave passing mainly south of the islands Wednesday night into Thursday, should maintain periods of passing showers across windward areas and afternoon convection over Puerto Rico under persistent east- southeast flow.

Meanwhile, seasonal to above-seasonal 925-mb temperatures are expected to persist, maintaining the potential for elevated to significant heat impacts, particularly during periods of reduced cloud cover and rainfall. Given the uncertainty surrounding AL96, continue to monitor the latest Tropical Weather Outlook from the National Hurricane Center and subsequent forecasts from NWS San Juan for changes in its evolution and potential impacts.

AVIATION

(06Z TAFS) Issued at 255 AM AST Thu Aug 27 2026

Mainly VFR conditions are expected through the period. SHRA will increase across the USVI and eastern PR terminals this morning as a trade-wind perturbation moves through. Afternoon SHRA/isolated TSRA are expected mainly over interior and western PR, potentially causing brief MVFR conditions and mountain obscurations, particularly near TJBQ. Winds will remain from the E at 10-15 kt, with higher gusts and sea-breeze variations after 14Z.

MARINE

Issued at 255 AM AST Thu Aug 27 2026

Surface high pressure across the central Atlantic will promote a moderate east-to-east-southeast wind flow through the weekend. A weak long-period northeasterly swell is expected to arrive and spread across the regional waters this morning. By Friday night into Saturday morning, an increase in showers and isolated thunderstorms is anticipated across the regional waters as weak tropical wave approaches the area. Winds may briefly become moderate to locally fresh as the tropical wave moves across the area, with seas between 3 and 5 feet, occasionally 6 feet; small craft must exercise caution.

BEACH FORECAST

Issued at 255 AM AST Thu Aug 27 2026

A moderate risk of rip currents will continue along the northern and eastern exposed beaches of Puerto Rico and the U.S. Virgin Islands during the next several days. Beachgoers must exercise caution, as life-threatening rip currents are possible along the surf zone. A weak long-period northeasterly swell is expected to arrive and spread across the regional waters this morning, however, seas will remain at 3 to 5 feet in the nearshore waters. Beachgoers are urged to remain weather alert as showers and isolated thunderstorms are expected to develop each afternoon across western and northwestern coastal areas of Puerto Rico.

FIRE WEATHER

Issued at 255 AM AST Thu Aug 27 2026

Plenty of dry fuels remain across the region. However, a trade- wind perturbation will bring a slight increase in moisture today, maintaining relative humidity values above critical thresholds. Easterly winds will generally remain moderate, although locally higher gusts are possible. Therefore, despite the dry fuels, critical fire-weather conditions are not expected today. Afternoon showers may also provide localized wetting rainfall, mainly across interior and western Puerto Rico.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001>003-005-007-008-010>013.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ001-002.

AM...None.