textproduct: San Juan

KEY MESSAGES

Issued at 329 AM AST Tue Aug 4 2026

- Saharan Dust concentrations will continue gradually decreasing today. Moderate concentrations are forecast to clear the USVI between sunrise and early morning, and subsequently clear western Puerto Rico between noon and early afternoon.

- A trade wind perturbation will somewhat increase moisture today while east-northeast flow results in afternoon convection over interior to west-southwest PR.

- A Heat Advisory has been issued for urban and coastal areas of Puerto Rico and the U.S. Virgin Islands from 10 AM to 4 PM AST. Additional Heat Products will likely be required in the next few days.

- Increasing easterly winds will deteriorate marine and beach conditions from Thursday onward, resulting in choppy seas and a moderate risk of life-threatening rip currents along north- and east- facing beaches.

- A weak tropical wave is forecast to move over the islands from Saturday afternoon into Sunday morning, slightly increasing moisture.

Short Term(Today through Thursday)

Issued at 329 AM AST Tue Aug 4 2026

Current satellite-derived precipitable water (PWAT) values over the region range from 1.4 to 1.7 inches, below normal to low-end normal values for this time of the year. Stations have reported minimum temperatures in the upper 70s to low 80s, across urban and lower- elevation areas of the islands, and in the 60s across higher elevations of Puerto Rico. Patchy fog was also detected over the interior to the eastern interior of Puerto Rico. A Saharan Air Layer (SAL) continued over the region during the overnight hours with moderate concentrations over the islands and high concentrations now primarily over the Mona Passage.

A steady trend toward improvement in terms of Saharan Dust continues to be forecast for today. According to the most recent guidance, however, a faster-than-anticipated improvement is now indicated, as moderate concentrations of Saharan Dust are projected to clear the USVI between sunrise and early morning, and subsequently clear western Puerto Rico between noon and early afternoon. Sustained low Saharan Dust concentrations continue to be forecast tomorrow and Thursday. Under east-northeast winds a surge in moisture due to a trade wind perturbation, will result in passing showers over windward areas of Puerto Rico and the USVI. This surge of moisture will help sustain more seasonal PWAT values, with localized above normal values possible during afternoon convection. Seasonal 500 mb temperatures, gradually decreasing Saharan Dust, diurnal heating, local effects, and sea breeze convergence, will result in afternoon convection (showers and isolated t-storms. Under the ENE flow, this activity is forecast to concentrate over interior to west-southwest Puerto Rico. While widespread flooding is not expected, localized ponding of water on roads and poorly drained areas are possible due to brief periods of moderate to heavy rainfall.

By Wednesday and Thursday, winds are expected to veer and become more east-southeasterly. Concurrently, 500 mb temperatures will increase to above-seasonal thresholds, while precipitable water (PWAT) values are projected to decline to low-end or below- seasonal values as the short term period progresses. Since available moisture will also be restricted primarily to the lower levels, a drier iteration of the typical weather pattern is anticipated. Advective passing showers across windward areas during the overnight and morning hours are still forecast, followed by limited afternoon convective activity over interior and west-northwest Puerto Rico. Due to the persistence of above- normal 925 mb temperatures, an elevated, to possibly significant, heat risk is anticipated to remain throughout the short term period. As a result, daily Heat Advisories will likely become necessary for urban and lower- elevation sectors of the islands. A Heat Advisory is already in effect for today from 10 AM to 4 PM AST, covering coastal and urban zones of Puerto Rico and the U.S. Virgin Islands, where heat indices can exceed the mid-100s.

Long Term(Friday through Tuesday)

Issued at 329 AM AST Tue Aug 4 2026

The long-term forecast remains on track, with variable weather conditions dominating the overall outlook. Surface high pressure extending across the western and central Atlantic will promote an easterly wind flow on Friday, and is expected to shift out of the southeast from Saturday onward as the high pressure migrates eastward. Upper-level low pressure moving across the region from Saturday into Monday will provide instability aloft, reflected in cooler 500 mb temperatures dropping to between -6C and -7C, particularly Saturday through Sunday.

Generally drier conditions are expected overall, though patches of low-level moisture carried by the trade winds will move across the region from time to time. The next tropical wave will pass from Saturday afternoon into Sunday morning, slightly increasing moisture content across the local islands. The latest model guidance continues to suggest precipitable water (PWAT) values of 1.75 to 2.00 inches on Saturday. However, because this wave is embedded in drier air and Saharan dust, it will move through relatively quickly, and PWAT values will drop rapidly by Sunday morning to below-normal levels around 1.25 inches. Consequently, overall rainfall with this wave will not be significant, though a few passing showers are likely over windward sections of the islands. During the afternoon hours, available moisture combined with diurnal heating and local sea-breeze effects could trigger localized convective activity across central and western Puerto Rico. Warmer-than-normal temperatures will persist throughout the forecast period. Model guidance indicates 925 mb temperatures near two standard deviations for this time of the year, which combined with lingering moisture could result in Heat Advisories or Extreme Heat Warnings, particularly over the weekend.

AVIATION

(06Z TAFS) Issued at 329 AM AST Tue Aug 4 2026

Mainly VFR conditions will prevail. HZ will gradually decrease after 04/10Z. After 04/12Z, ENE surface winds increasing to 10-17 kts with higher gusts and sea breeze variations, before becoming light and variable after 04/23Z. Passing SHRA can move over windward areas under the ENE flow and, after 04/16Z, SHRA/VCTS can develop over interior to west-southwest PR, possibly reducing visibilities and ceilings at TJPS.

MARINE

Issued at 329 AM AST Tue Aug 4 2026

Mainly easterly winds will prevail throughout the workweek, increasing to locally fresh late in the week as high pressure strengthens over the central Atlantic. Despite the increase in winds, marine conditions will remain generally favorable for small craft throughout the period. Saharan dust will linger today, with moderate concentrations expected to return on Friday. Isolated afternoon thunderstorms may generate locally hazardous marine conditions, mainly across the coastal waters of western Puerto Rico. Another tropical wave is forecast to approach and move across the local waters during the upcoming weekend.

BEACH FORECAST

Issued at 329 AM AST Tue Aug 4 2026

A low risk of life-threatening rip currents is expected today through most of Thursday. A moderate risk will begin to develop late Thursday and expand to many exposed beaches along the north and east coasts of Puerto Rico, including Vieques and Culebra, as well as the U.S. Virgin Islands, from Friday through the weekend as easterly winds strengthen.

Other beach hazards will include elevated to locally significant heat risks through midweek and afternoon thunderstorms capable of producing frequent lightning and gusty winds, particularly across western Puerto Rico. Saharan dust will linger today and return in moderate concentrations on Friday.

FIRE WEATHER

Issued at 329 AM AST Tue Aug 4 2026

Critically dry fuels persist across southern Puerto Rico and St. Croix, with Keetch-Byram Drought Index (KBDI) values remaining well above the critical threshold. Local wind speeds near or above threshold values which could promote the rapid spread of any fire that develops. East-northeast steering flow can result in drier air moving over southern sectors of PR, however afternoon convection is also possible over interior to west-southwest PR which could provide relief and decrease the fire weather risk. Fire weather conditions will continue to be monitored closely. Residents and visitors should avoid outdoor burning and exercise caution with activities that could generate sparks or open flames, especially across areas with critically dry vegetation.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 10 AM this morning to 4 PM AST this afternoon for PRZ001>005-007-008-010>013.

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

AM...None.