textproduct: San Juan

KEY MESSAGES

Issued at 1254 PM AST Sun Sep 20 2026

- Afternoon showers and thunderstorms may produce locally heavy rainfall that could lead to flooding, mainly across interior, northern, and western Puerto Rico.

- A wetter and more unstable pattern is expected mid to late next week, increasing the potential for more widespread heavy rainfall and stronger thunderstorms.

- Hot and humid conditions will continue through much of the week, with heat concerns each day before the onset of afternoon showers.

- A moderate risk of rip currents is expected from Wednesday onward across the north and eastern beaches of the islands.

Short Term(This evening through Tuesday)

Issued at 1254 PM AST Sun Sep 20 2026

Partly to mostly clear skies prevailed across the region during the morning hours, accompanied by isolated passing showers over eastern and southeastern Puerto Rico, Vieques, Culebra, and the U.S. Virgin Islands. Rainfall accumulations remained minimal in general, but the Doppler radar estimated up to three quarters of an inch if rain in Carolina. Light to moderate southeasterly winds are dominating the area at 5 to 15 mph with local sea breeze variations. Maximum temperatures so far are in the upper 80s and low 90s across lower elevations and valleys of Puerto Rico and the USVI. A Heat Advisory remains in effect through 3 PM AST this afternoon.

The 20/12Z TJSJ sounding showed a precipitable water (PWAT) value of 1.91 inches, which is within the normal range for this time of year, alongside a weak east-southeasterly steering flow of around 9 knots in the 1 to 3 km layer. These conditions will favor local effects playing a dominant role in shower and thunderstorm development today, especially across interior, western, north-central, and northwestern Puerto Rico. An elevated flood threat is expected today in these regions, as slow-moving or repeated convection could produce urban and small-stream flooding, isolated flash flooding, and landslides in steep terrain, particularly where soils remain saturated from recent heavy rainfall. Streamers have also developed downwind of El Yunque, Vieques, Culebra, and all of the USVI. Due to the ESE to SE wind flow, the El Yunque streamer will support shower and thunderstorm activity across the northern half of the San Juan metropolitan area, leading to potential ponding of water on roadways and in poorly drained areas. Some of this activity may persist into the evening hours. However, a seasonal pattern will return later this evening and overnight, bringing passing showers to portions of the USVI as well as eastern and southeastern Puerto Rico.

On Monday and Tuesday, weak mid-level ridging north of the region will help introduce slightly drier air, though PWAT values will generally remain between 1.8 and 2.0 inches. Meanwhile, surface high pressure over the Atlantic will become better organized, causing winds to gradually strengthen. Despite the subtle drying, sufficient low-level moisture combined with strong daytime heating will support active afternoons on both days. The 500 mb temperatures will stay around -6 degrees Celsius on Monday before decreasing to -7 degrees Celsius on Tuesday as the ridge retreats and a mid- to upper-level trough begins transitioning the pattern toward greater instability Tuesday night. This will support daily thunderstorm activity, with the potential for isolated strong thunderstorms capable of frequent lightning and gusty winds. Winds will shift from southeasterly on Monday to more easterly on Tuesday, focusing afternoon convection from the interior into western Puerto Rico with a more westward storm motion. More tranquil conditions are expected each night and morning, with passing showers across portions of eastern and southeastern Puerto Rico and the USVI. The driest period is expected Tuesday evening.

The main hazards through the period will continue to be heat, locally heavy rainfall leading to flooding, lightning, and gusty winds. Heat concerns will likely persist Monday and Tuesday across Puerto Rico and the USVI. The greatest rainfall threat will occur during the afternoon hours, mainly spanning the interior into the western and northwestern quadrants of Puerto Rico, and downwind from El Yunque.

Long Term(Wednesday through next Saturday)

Issued at 145 AM AST Sun Sep 20 2026

At this time, Wednesday is forecast to continue the transition toward a more unstable pattern as the mid- to upper-level trough exerts a greater influence across the region. By Thursday, a tropical wave is expected to reach the islands, providing additional moisture and low-level support while conditions aloft become increasingly favorable for convection. The latest model guidance shows mid-level temperatures falling to around -9 degrees Celsius. With favorable mid-level moisture also in place, Thursday into Friday appears to be the most favorable period for stronger convection.

More frequent passing showers are expected across windward areas and the U.S. Virgin Islands, while afternoon thunderstorms across Puerto Rico could produce heavy rainfall, frequent lightning, and locally gusty winds. An elevated flooding risk is possible during this period, particularly across urban areas, small streams, and locations receiving repeated heavy rainfall.

Although instability aloft will persist into Saturday, guidance indicates that mid-level moisture will begin to decrease, particularly within the 700-500 mb layer. Afternoon thunderstorms will remain possible, especially across the interior and western portions of Puerto Rico, but coverage should become more localized as moisture gradually decreases.

By Sunday, additional mid-level drying is anticipated while temperatures aloft remain relatively cold and lapse rates remain favorable. Therefore, sufficient instability should remain for afternoon convection, although reduced moisture should limit both thunderstorm coverage and lightning activity compared with Thursday and Friday. Low-level winds are also expected to strengthen somewhat toward the end of the period.

Heat risk will remain a concern throughout the period, but it will largely depend on cloud cover and the coverage and timing of showers associated with the upper-level trough and tropical wave. Greater cloudiness and earlier rainfall could limit daytime heating in some areas, while locations experiencing longer periods of sunshine could reach higher heat indices. Therefore, residents and visitors should continue monitoring the forecast as confidence increases regarding the evolution of the trough, tropical wave, rainfall, and heat risk.

AVIATION

(18Z TAFS) Issued at 1254 PM AST Sun Sep 20 2026

Mainly VFR conditions are expected to prevail across all terminals through most of the forecast period. However, tempo MVFR to brief IFR conds possible at TJSJ/TJBQ thru 20/22z due to locally induced TSRA/SHRA. Across the USVI terminals and TJPS, mostly VCSH expected. The 20/12z TJSJ sounding indicated ESE winds up to 12 kt blo FL050. A northerly component due to the sea breeze will continue at TJSJ/TJBQ with possible stronger gusts thru 20/21z. Light and variable wind expected overnight.

MARINE

Issued at 1254 PM AST Sun Sep 20 2026

A weak pressure gradient across the region will continue to promote light to moderate trades. Winds will gradually turn more east to southeast through midweek as high pressure over the northeastern Atlantic becomes better organized. Seas will remain around 2 to 4 feet across the Atlantic and Caribbean waters, including the local passages. Passing showers may briefly increase winds and seas, while afternoon thunderstorms moving into the Atlantic and western coastal waters of Puerto Rico may produce locally strong gusts, torrential rainfall, and frequent lightning through the early evening hours.

BEACH FORECAST

Issued at 1254 PM AST Sun Sep 20 2026

A low risk of rip currents is expected to continue across all beaches of Puerto Rico and the U.S. Virgin Islands through midweek. However, even with a low risk, life-threatening rip currents can occur near groins, jetties, reefs, and piers. From late Wednesday onward, a moderate risk is expected to return across the north and east-facing beaches of the islands, due to a small long-period northeasterly swell and as winds increase slightly across the coastal waters.

Other beach hazards will be heat, and thunderstorms with gusty winds and lightning strikes during the afternoon hours. Beachgoers should remain hydrated and limit prolonged sun exposure, and seek shelter whenever they hear thunder.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory until 3 PM AST this afternoon for PRZ001>013.

VI...Heat Advisory until 3 PM AST this afternoon for VIZ001-002.

AM...None.