textproduct: San Juan

KEY MESSAGES

Issued at 350 AM AST Tue Oct 13 2026

- As a mid-to-upper level low lingers in our vicinity, enhanced instability will continue through at least midweek. This will maintain a limited to elevated flooding threat.

- Light winds today will promote slow-moving showers and thunderstorms, which will favor higher localized rainfall accumulations with the heaviest downpours.

- A limited to elevated Heat Risk is expected through the workweek. Sensitive individuals to these events should avoid outdoor activities during peak daytime heating hours and stay hydrated.

- A moderate Rip Current Risk will prevail along the northern and eastern-facing beaches of Puerto Rico through the workweek and will include the USVI by Saturday, elsewhere a low risk is expected.

Short Term(Today through Thursday)

Issued at 350 AM AST Tue Oct 13 2026

An upper level low west and southwest of the region continued to promote shower and t-storm development over the waters and windward sectors during the overnight hours under a weaker E to ENE steering flow. Since midnight, at least minimal accumulations were detected by radar over northern, eastern and southeastern Puerto Rico, as well as over western Vieques, Culebra and the USVI. Across interior Puerto Rico, overnight lows varied from the low 60s to low 70s, while lower-elevation areas throughout the islands generally dropped into the mid 70s to low 80s (though a few low-elevation spots in Puerto Rico dipped into the low 70s).

Current satellite derived precipitable water (PWAT) values indicate 1.85 to 2.13 inches of PWAT (normal to slightly above normal values for this time of the year). As the period progresses the upper low will continue moving away from Puerto Rico and the USVI, over Hispaniola and to the north and northwest. PWAT values today should stay at around 2 inches with a gradual decrease tonight onwards. 500 mb temperatures can reach -8 to -9C today (below normal values) before increasing to around -7 to -6C tomorrow (more seasonal values). Showers and t-storms will continue over the waters and reach some windward sectors of the islands to start the day. Apart from the favorable upper-level conditions, diurnal heating, sea breeze convergence and local effects will promote afternoon convection over mainly interior to western and southwestern Puerto Rico under a weaker E and ENE steering flow. The Galvez Davidson Index (GDI) further supports a high likelihood of isolated to scattered thunderstorms today with values between 40 to 45. Slower-moving t-storms and showers will also further promote urban and small stream flooding, along with rapid river rises. Afternoon showers and thunderstorms can also develop downwind of the USVI, Vieques and Culebra. As the low moves northward tonight, shower and t-storms will again develop during the overnight hours, particularly over the Mona Passage. Overall shower and t-storm coverage is expected to decrease by tomorrow, Wednesday as the low continues to move away, drier air leads to PWAT values to 1.6 to 1.8 inches (around 2 inches during afternoon convection) and as mid-level moisture decreases. Nevertheless, isolated heavy showers and t-storms remain likely across mainly interior to west and west-southwest Puerto Rico (as well as downwind of the USVI, Vieques and Culebra) under the weak ENE steering flow. GDI values between 35 and 40 will continue to support afternoon shower and thunderstorm development. A limited to elevated risk of excessive rainfall is forecast today and tomorrow.

As the upper-level low departs from the Eastern Caribbean, Thursday should be a transition day towards more stable conditions ahead. A mid-to-upper level ridge north of the area will promote more stable conditions; however, available moisture over the region (PWAT values ranging from 1.75 to 2.00 inches), local effects, and daytime heating will support shower and isolated thunderstorm activity. This activity will be steered by an ENE flow caused by a frontal boundary inducing a pre-frontal trough north of the islands. Therefore, activity will focus over the interior to western and southwestern portions of Puerto Rico, where a limited risk of excessive rainfall is expected.

Despite flooding and lightning being the main hazards through at least midweek, above normal 925 mb temperatures are still forecast to persist, especially across urban and coastal areas. Although a limited heat risk is forecast, from late morning into the afternoon, through the next few days, please continue to monitor the forecast for any updates.

Long Term(Friday through Tuesday)

Issued at 350 AM AST Tue Oct 13 2026

Some changes were introduced to the long term forecast, as the stable weather conditions may linger in the weekend. As mentioned in the previous discussion, winds on Friday should continue under a E- ENE pattern due a frontal boundary north of the CWA. As a surface high builds over the Western Atlantic, winds may veer and become from the E-SE by Saturday, becoming easterly on Sunday and Monday. In the previous discussion, Friday was forecast as the less "active" day due to a mid-level ridge dominating the weather pattern through Saturday, becoming unstable by Sunday due to an upper-level trough approaching the Caribbean. Nevertheless, the latest model solutions are tending to more stable conditions as the upper-level trough may linger over the Central Atlantic while the ridge gets closer to the local area. With the presence of the ridge, warmer than usual mid level temperatures (above -6 C) are very likely, along with subsidence, indicating stability aloft. Although moisture content remains near seasonal values (PWATs up to 1.75 inches), a dry slot into the mid levels (below 50%) filtering on Friday will also unfavor deep afternoon convection. Additionally, the Galvez-Davison Index (GDI) suggest that the best chance of observing convection activity should be in the afternoons over interior and western Puerto Rico, but mainly shallow. In general, daytime heating, local effects, and seabreeze convergence are likely the main drivers of the afternoon convection, though the weather conditions won't necessarily support long-lasting thunderstorms, lowering the flood threat. Over the weekend, these rainfall accumulations may lead to ponding of water over roadways and poorly drained areas over the aforementioned areas. The latest guidance suggest a warmer conditions this weekend, with 925 mb temperatures warmer for this time of the year. During peak hours of the day, particularly in urban and low-lying areas, heat indices could exceed 100 F, affecting mainly sensitive groups. Therefore, the heat threat will remain limited to elevated throughout the forecast period.

AVIATION

(06Z TAFS) Issued at 350 AM AST Tue Oct 13 2026

Mainly VFR conds to prevail. Pds of VCSH/-SHRA and/or VCTS/-TSRA will cont to reach mainly the E terminals thru at least 13/12Z. Aftn showers and t-storms to afct (psbly resulting in tempo MVFR to brf IFR conds) mnly vcty TJBQ/TJSJ and TJPS, while streamers also afct TIST/TISX from 13/17-22Z. E to ENE flow incrg to arnd 14-17 kts aft 13/13z, with higher gusts and sea breeze variations; bcmg light and vrbl after 13/23Z. VCSH/-SHRA and/or VCTS/-TSRA dvlpg over the wtrs once again aft 13/23Z and can afct N & E trmls.

MARINE

Issued at 350 AM AST Tue Oct 13 2026

A lingering upper-level low will continue to support showers and isolated thunderstorms across the regional waters through midweek, with lightning being the primary concern, particularly during the afternoon hours. Moderate east to east-southeast winds and generally tranquil seas will prevail across most regional waters. A northeasterly swell will persist across the Atlantic waters, while seas are expected to gradually increase across portions of the northern and offshore waters by the weekend. Mariners should remain alert for thunderstorms capable of producing frequent lightning, locally gusty winds, and reduced visibility.

BEACH FORECAST

Issued at 350 AM AST Tue Oct 13 2026

A moderate risk of rip currents returns today for most north- and east-facing beaches of Puerto Rico, including Culebra. The moderate risk is expected to persist through the workweek and expand to additional beaches, including portions of the U.S. Virgin Islands, by Saturday. Afternoon thunderstorms will also pose a lightning threat to beachgoers, particularly along the coast of Puerto Rico. Additionally, heat concerns are expected to return toward the end of the workweek and continue into the weekend, especially during the late morning and afternoon hours.

SJU WATCHES/WARNINGS/ADVISORIES

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