textproduct: San Juan

KEY MESSAGES

Issued at 440 AM AST Mon May 18 2026

* An upper-level trough northwest of the islands will aid in the formation of showers and isolated thunderstorms, particularly during the afternoons across the interior and western PR. Limited to elevated flooding risk.

* A limited heat risk will continue this week, heat indices expected at the mid 90s to low 100s across urban and coastal areas.

* Breezy conditions will maintain choppy seas. A Small Craft Advisory is in effect for the offshore Atlantic waters through at least Tuesday afternoon.

* Localized flooding concerns may gradually increase through the holiday weekend, especially in urban and poor drainage areas

* A moderate risk of rip currents will continue this week, mainly along the north and east facing beaches of Puerto Rico (southern beaches at times), as well as beaches of Vieques, Culebra, and the U.S. Virgin Islands.

Short Term(Today through Wednesday)

Issued at 440 AM AST Mon May 18 2026

Current satellite derived precipitable water (PWAT) values indicate a band of moisture over the USVI and eastern PR, moving westward towards the rest of the region and steered by easterly winds. PWAT values reach 1.50 inches over the above mentioned areas gradually decreasing westward and reaching around 1.33 inches over western PR. Fast moving showers continue to be steered towards windward areas, some being moderate to locally heavy. At least minimal accumulations have been detected since midnight over the eastern third of PR, Vieques, Culebra, and the USVI. T-storms developed earlier tonight mainly west of Mona Island. Patchy fog was also detected over sectors of interior PR while higher clouds 9from t-storms south of Hispaniola have crossed the Mona Passage and reached western PR. Lows have been in the low 70s to locally around 80 over urban and coastal sectors of the islands, and in the upper 50s to the 60s at higher elevations of PR.

A broad surface high over the Atlantic will promote up to breezy easterly winds. This easterly flow will continue to steer the above mentioned band of moisture over the islands during the morning, with PWAT values reaching normal values for this time of the year. A band of drier air (with below normal PWAT) will then be steered towards the islands and reach the eastern region late morning into the afternoon and then western PR tonight. Current model guidance suggests that more broad moisture with seasonal PWAT values will then reach the islands tomorrow morning and Wednesday under more east to east-southeast breezy flow. PWAT values can locally reach above normal values during afternoon convection. An amplifying upper level trough from W-NW of the region during the period can promote increased instability and increase the frequency of showers. The presence of patches of drier air in the mid- levels can help inhibit this activity, however. Fast moving showers will continue to be steered towards windward sectors, resulting in periods of variable to showery weather. This can result in ponding of water over roads and poorly drained areas with a chance of urban and small stream flooding. Conditions should improve over windward sectors today as the band of drier air gradually moves over the area. The upper trough will also aid in the development of afternoon showers and t-storms, in addition to diurnal heating, local orographic effects and sea breeze convergence. These will develop over interior towards the western half of Puerto Rico, as well as downwind of El Yunque, and the USVI towards eastern PR. This will result in a limited to elevated flooding risk and flood advisories might need to be issued each afternoon. Although this pattern is forecast to continue tomorrow and Wednesday, the band of drier air approaching the area can tapper off convection this afternoon as it moves westward, however current model guidance indicates that it should still be over eastern PR as afternoon convection develops. Galvez- Davidson Indices are forecast at up to around 30 each afternoon.

More seasonal, but still warm, highs are expected during the period, as indicated by 925 mb temperature guidance. Highs will reach the upper 80s to lower 90s across urban and coastal areas while heat indices exceed 100 degrees F during the afternoon hours. This will result in a limited heat risk. Low concentrations of saharan dust will reach the region from time to time.

Long Term(Thursday through Monday)

Issued at 440 AM AST Mon May 18 2026

Model guidance has remained fairly consistent through the long-term period, supporting a persistent late spring weather pattern across the local islands and surrounding waters through the Memorial Day weekend. Broad surface high pressure over the central Atlantic will maintain moderate to fresh easterly winds, becoming breezy to locally windy at times late in the week and during the weekend. Moisture embedded within the trade wind flow will continue to fluctuate between drier and wetter periods, with some moisture patches lingering longer from Friday onward and supporting a gradual increase in shower activity. Minor traces of Saharan dust will occasionally filter across the area with little impact expected. Temperatures will remain seasonably warm to hot, while marine and beach conditions gradually deteriorate through the weekend with increasing rip current and marine hazards.

At upper levels, the area will remain between troughing over the western Atlantic and ridging to the southeast, with the trough gradually shifting closer to the region late in the period. This pattern will support periods of enhanced upper-level winds and more favorable conditions for afternoon convection by the weekend and early next week. Passing showers will continue across windward areas during the overnight and morning hours, followed by afternoon showers and isolated thunderstorms mainly across interior and western Puerto Rico. Localized flooding concerns may gradually increase through the holiday weekend, especially in urban and poor drainage areas, along with lightning and gusty winds in the strongest activity.

AVIATION

(06Z TAFS) Issued at 440 AM AST Mon May 18 2026

Mainly VFR conditions continue. E winds will steer -SHRA/VCSH towards windward terminals during the period. Btwn 18/16-23Z, SHRA/+SHRA, with isolated TS, developing across interior to W/NW PR. This can cause brief MVFR conditions over TJBQ. Lines of -SHRA /SHRA can also develop from the local islands and El Yunque, affecting TJSJ. Breezy to Windy E winds up to 15-20 kts, with higher gusts and sea breeze variations, decreasing after 18/23Z and gradually becoming more E to ESE.

MARINE

Issued at 440 AM AST Mon May 18 2026

A strong surface high pressure over the western Atlantic will promote moderate to fresh trades through midweek. Winds will turn more east to southeast thereafter as the surface high moves into the central Atlantic. These breezy conditions will maintain choppy seas, particularly across the offshore and exposed Atlantic and Caribbean waters. A Small Craft Advisory is in effect for the offshore Atlantic waters through at least late tomorrow. Afternoon and early evening isolated thunderstorms can be expected each day across the northwestern waters of Puerto Rico.

BEACH FORECAST

Issued at 440 AM AST Mon May 18 2026

A moderate risk of life-threatening rip currents will continue to prevail across the east, north, and southern beaches of Puerto Rico, Culebra, Vieques and the U.S. Virgin Islands during the next several days. Beachgoers should exercise caution when entering these beaches. This means life-threatening rip currents are possible in the surf zones, and beachgoers should exercise caution when entering these beaches.

If you become caught in a rip current, yell for help. Remain calm, do not exhaust yourself and stay afloat while waiting for help. If you have to swim out of a rip current, swim parallel to shore and back toward the beach when possible. Do not attempt to swim directly against a rip current as you will tire quickly. For additional information and location-specific rip current details, visit weather.gov/beach/sju.

SJU WATCHES/WARNINGS/ADVISORIES

PR...None. VI...None. AM...Small Craft Advisory until 6 PM AST Tuesday for AMZ711.