textproduct: San Juan

KEY MESSAGES

Updated at 305 AM AST Mon Jul 27 2026

* Dangerous heat will persist across Puerto Rico and the U.S. Virgin Islands through much of the week. Heat Advisories remain in effect today, and additional Heat Advisories or even Extreme Heat Warnings may be needed on subsequent days, especially where rainfall is limited.

* A tropical wave will increase the chance of numerous showers and thunderstorms through Tuesday, elevating the risk of urban and small-stream flooding, with isolated flash flooding possible where heavy rainfall persists.

* A gradual decrease in moisture is expected by Wednesday, reducing the flooding threat to a more localized level, mainly across northwestern Puerto Rico during the afternoon.

* Drier air, accompanied by periodic pulses of Saharan dust, will return during the second half of the week, reducing rainfall coverage while maintaining hot and hazy conditions across Puerto Rico and the U.S. Virgin Islands.

Short Term(Today through Wednesday)

Issued at 305 AM AST Mon Jul 27 2026

Subsidence ahead of today's tropical wave dominated overnight, limiting rain activity across Puerto Rico and the US Virgin Islands. Additionally, we observed another warmer-than-normal night and early morning, with lows in the low 80s and upper 70s in coastal and urban areas of the USVI and PR. However, the Doppler Radar detected a few showers across the local waters, some of which moved briefly across the USVI. GOES-19-derived Total Precipitable Water (TPW) showed the leading edge of the tropical wave approaching the region, as the Doppler detected those showers. Winds were mainly from the east to east-southeast across most local stations, at 5 to 10 mph, with land-breeze fluctuations.

Increasing tropical moisture associated with an approaching tropical wave, combined with another warmer-than-normal day, is expected to lead to dangerous heat across PR and the USVI. Therefore, a Heat Advisory remains in effect from 9 am to 5 pm AST for all of PR and the USVI, where heat indices are likely to reach hazardous levels, especially in areas that receive little or no rainfall.

Throughout the morning, bands of showers associated with the tropical wave will likely move across the USVI and portions of eastern and southeastern PR, producing periods of moderate to locally heavy rainfall. Low-level convergence and orographic lifting may locally enhance this activity, while areas that remain dry are expected to experience rapidly rising temperatures. As the tropical wave axis continues moving westward through the day, shower and thunderstorm coverage is forecast to increase across the interior and western portions of Puerto Rico. There is a medium to high chance that some storms will produce torrential rainfall, frequent lightning, and gusty winds. Consequently, an elevated risk of urban and small-stream flooding exists this afternoon, with isolated flash flooding possible where heavier or repeated rainfall occurs. Residents and visitors should continue to monitor the forecast and remain prepared for additional short- fused advisories, or warnings if hazardous conditions develop.

The tropical wave is forecast to linger across the region through at least Tuesday as it interacts with an upper-level trough, resulting in the wettest period of the short-term forecast. Although the anticipated rainfall could provide beneficial moisture to areas experiencing significant rainfall deficits over the past several months, it will also increase the risk of flooding, particularly in urban and poorly drained areas. As the wave moves west-northwestward across the northeastern Caribbean, deep tropical moisture and atmospheric instability are expected to remain above normal through Tuesday, supporting numerous showers and scattered thunderstorms. Precipitable water values will likely remain near 2.0 inches, favoring efficient rainfall. Consequently, there is a moderate to high likelihood of urban and small-stream flooding through Tuesday, while localized flash flooding will be possible where repeated rounds of heavy rainfall or persistent convection occur. Stronger thunderstorms may also produce frequent lightning and locally gusty winds.

By Wednesday afternoon, moisture is forecast to gradually decrease as a drier air mass, with precipitable water values falling to around 1.4 to 1.5 inches, overspreads the region. Periodic pulses of Saharan dust are also expected to return during the second half of the week, resulting in hazier skies and a lower probability of widespread shower and thunderstorm activity.

Long Term(Thursday through Monday)

Issued at 305 AM AST Mon Jul 27 2026

The latest model guidance continues to suggest the arrival of a drier air mass across the region, dropping precipitable water values below normal to around 1.30 to 1.40 inches on Thursday. Along with this drier air, NASA GEOS-5 model guidance indicates low-to-moderate concentrations of Saharan Dust arriving through the first half of the period.

Strengthening surface high pressure over the central Atlantic will create a tight pressure gradient, bringing breezy to locally windy east-southeast winds through Saturday. Aloft, a mid-to-upper-level ridge will promote stable conditions, limiting widespread shower activity. However, the forecast remains on track for patches of low- level moisture embedded in the trade winds to move across the area. This will bring passing showers, particularly over the windward portions of the islands during the morning hours. In the afternoons, this moisture combined with daytime heating and local effects could trigger shower activity along the interior and northwestern portions of Puerto Rico.

By late Sunday into Monday, an approaching tropical wave will increase moisture alongside the potential for widespread showers and thunderstorms. Elevated temperatures will persist through the period, driven by normal to above-normal temperatures at the 925 mb level, with daytime highs reaching the upper 80s to low 90s across coastal and urban areas. Residents and visitors are encouraged to monitor forecasts and stay updated on changing conditions as the week progresses.

AVIATION

(06Z TAFS) Issued at 305 AM AST Mon Jul 27 2026

VFR conds will prevail today, although passing SHRA may bring brief MVFR cigs/vsbys to TIST/TISX and TJSJ after 27/06Z. SHRA/TSRA coverage will increase after 27/16Z as a tropical wave moves across the area, with TEMPO MVFR/IFR conds likely at TJBQ and possible at TJSJ due to +SHRA/TSRA, reduced vsbys, mtn obscurations, and gusty winds. ESE sfc winds 10-18 kt with gusts to 28 kt, becoming light/vrb after 27/23Z. Stronger convection may produce gusts above 30 kt and frequent lightning.

MARINE

Issued at 305 AM AST Mon Jul 27 2026

A tropical wave will bring increasing showers and isolated thunderstorms through Tuesday, producing locally higher winds and seas, frequent lightning, and reduced visibility. However, in general, expect moderate east to east-southeast winds today, becoming moderate to locally fresh Tuesday into Wednesday. These winds will maintain choppy seas across portions of the Atlantic waters, offshore Caribbean waters, and local passages, with seas of 6 feet limited to localized areas.

BEACH FORECAST

Issued at 305 AM AST Mon Jul 27 2026

A moderate risk of rip currents continues across northern Puerto Rico. As winds decrease early this week, the rip current risk should gradually lessen, resulting in improving beach conditions through midweek. However, life-threatening rip currents will remain possible near reefs, jetties, groins, and piers. The rip current risk is expected to increase again later in the workweek as easterly winds strengthen. Beachgoers should also remain alert for thunderstorms and heat.

FIRE WEATHER

Issued at 305 AM AST Mon Jul 27 2026

Despite increasing moisture associated with a tropical wave keeping minimum relative humidity values above critical thresholds, critically dry fuels persist across southern Puerto Rico and St. Croix. As a result, any fire that develops could still spread through dry vegetation, particularly where locally breezy winds occur. Residents and visitors are encouraged to avoid outdoor burning and use caution with activities that could generate sparks or open flames in areas with critically dry fuels.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 9 AM this morning to 5 PM AST this afternoon for PRZ001>005-007-008-010>013.

VI...Heat Advisory from 9 AM this morning to 5 PM AST this afternoon for VIZ001-002.

AM...None.