textproduct: San Juan

KEY MESSAGES

Updated at 450 AM AST Tue May 5 2026

* A modest increase in moisture combined with an approaching trough will bring a higher chance of isolated to scattered showers this afternoon. Isolated thunderstorms cannot be ruled out.

* Mostly fair weather will prevail for the rest of the period, with passing trade wind moisture bringing isolated showers, mainly during overnight and morning hours.

* Above-normal temperatures will persist, promoting elevated heat indices across urban and coastal areas.

* Moderate rip current risk will continue along north- and east- facing beaches of the islands at times.

Short Term(Today through Thursday)

Issued at 450 AM AST Tue May 5 2026

During the overnight hours, weather conditions remained fairly tranquil across Puerto Rico and the U.S. Virgin Islands, with mostly clear to partly cloudy skies and only a few very isolated showers observed over the surrounding waters.

For the morning hours, a slight uptick in passing showers is expected across windward coastal areas as a patch of moisture moves over the region, as indicated by GOES-19 imagery. Precipitable water (PWAT) values are forecast to increase to around 1.8 inches, which is near normal for this time of year. This moisture increase will also allow 700-500 mb relative humidity values to recover closer to climatological levels, potentially weakening the trade wind inversion that was more persistent yesterday.

From a dynamical perspective, instability aloft is expected to increase as an upper-level trough approaches from the west. This feature will promote a noticeable drop in 250 mb height fields and support mid-level cooling, with 500 mb temperatures decreasing to below-normal values. As a result, 700-500 mb lapse rates are expected to steepen to above-normal values, while low-level lapse rates remain comparatively weaker. This vertical pattern suggests that any deep convection developing this afternoon will still rely heavily on sea breeze convergence and local effects. The combination of slightly higher moisture content, a temporarily weakened inversion cap, and above-normal low-level temperatures driven by persistent southeasterly flow could provide sufficient support for isolated convective activity. However, the overall moisture increase remains modest, and some mid-level drying is still present, which will act as a limiting factor for widespread or sustained convection.

Therefore, afternoon activity is expected to remain localized, particularly across the northwestern quadrant of Puerto Rico. Streamer development downwind of the U.S. Virgin Islands and El Yunque, moving northwestward, is also likely. At most, a limited and very localized flooding risk is anticipated, with the potential for a few isolated thunderstorms. For the remainder of the short-term period, although some degree of dynamical instability aloft may persist, especially into Wednesday, the primary limiting factor will continue to be moisture availability. Wednesday afternoon could feature a similar pattern to today, though outcomes will depend on the timing and extent of drier air intrusions, which may re-establish inversion caps and suppress convection. By Thursday, conditions are expected to become more stable, resulting in the least active day of the period.

Overall, mostly fair weather conditions will prevail, but patches of moisture embedded in the southeasterly trades will modulate shower activity. These will bring isolated overnight and morning showers across windward areas at times, followed by localized afternoon convection, mainly over interior and northwestern Puerto Rico. Tuesday presents the highest chance for isolated thunderstorms, though activity will remain limited with minimal flooding risk.

Long Term(Friday through Tuesday)

Issued at 450 AM AST Tue May 5 2026

East to east-northeast flow to start the period will steer patches of both drier and more humid air towards the region. Precipitable Water (PWAT) values will generally be at below normal to low end normal values for this time of the year. Available moisture will also be shallow, mainly below 800 mb. With this below normal to normal moisture and increasing subsidence (an upper level trough will be moving away from the region to start the period and mid- level ridging will build) a general reduction in shower activity and rainfall coverage is forecast. A limited heat risk is also forecast for most of the period, affecting sensitive individuals and individuals with prolonged exposure and/or inadequate hydration. Current model guidance suggests a more easterly steering flow to start the next week, veering to become more easterly to east- southeasterly to start the next workweek and continuing to bring patches of both drier and more humid air. Up to breezy conditions will be present, particularly to start the weekend onwards.

This will result in a limited non-thundersstorm wind risk, with gusts up to around 25 mph at times at coastal and windward areas. Unsecured items could blow around. Shower activity can continue to reach windward sectors of the islands during the morning and overnight hours while diurnal heating and local effects can promote afternoon convection over mainly interior to western PR with steering flow each day determining a more NW or SW movement. Low concentrations of Saharan Dust will reach the area while the bulk of a Saharan Air Layer will stay south of the local islands. Under breezy conditions, increasing temperatures and with patches of drier air approaching, the potential for elevated fire danger remains, stay tuned for any future updates.

AVIATION

(06Z TAFS) Issued at 450 AM AST Tue May 5 2026

Mainly VFR conds expected to prevail across all terminals. However, VCSH possible aft 05/16Z across most sites, with brief SHRA mainly impacting TJBQ/TJSJ. A PROB30 group is in place for TJBQ btwn 17-21Z for TSRA with brief MVFR conds possible. Winds will E-SE 10-15 kt aft 13Z with occasional higher gusts and sea breeze variations.

MARINE

Issued at 450 AM AST Tue May 5 2026

Broad surface highs and frontal boundaries over the Atlantic will maintain an easterly to southeasterly flow through at least early Thursday, backing to become more east to northeast through at least Saturday. Up to moderate winds will continue, with periods of locally fresh winds across the offshore waters and passages at times. Seas will range between 2 and 4 feet, occasionally higher due to a combination of local wind waves and a few pulses of a small, long-period northeasterly swell spreading across the Atlantic waters. Generally favorable conditions for small craft, though operators should exercise caution at times due to wind-driven seas and passing showers that may produce locally higher winds and seas.

BEACH FORECAST

Issued at 450 AM AST Tue May 5 2026

Up to a moderate risk of rip currents (life-threatening rip currents are possible in the surf zone) will continue today for the north and east beaches of Puerto Rico, as well as exposed beaches of Culebra and St Croix. Pulses of small, long-period northeasterly swells will continue to occasionally promote higher breakers and stronger rip currents. A low risk of rip currents will continue elsewhere. However, even if the risk for rip currents is low, life-threatening rip currents often occur in the vicinity of groins, jetties, reefs, and piers. A similar pattern will continue at least through at least early Thursday. There is a chance of the low risk of rip currents dominating on Friday, however the moderate risk will return Saturday and spread to most beaches Sunday. For additional information, visit: weather.gov/beach/sju.

SJU WATCHES/WARNINGS/ADVISORIES

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