textproduct: San Juan

KEY MESSAGES

Issued at 145 AM AST Sun Sep 20 2026

- Hot and humid conditions will continue through much of the week, with extreme heat concerns each day.

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

- Increasing winds mid to late next week may bring a moderate rip current risk to north- and east-facing beaches.

Short Term(Today through Tuesday)

Issued at 145 AM AST Sun Sep 20 2026

A generally seasonal but locally active weather pattern is expected today, with relatively light winds allowing local effects to play a larger role in shower and thunderstorm development. Precipitable water values will remain near to locally above normal, supporting passing morning showers across the U.S. Virgin Islands and eastern Puerto Rico, followed by afternoon showers and thunderstorms across the interior and western to northwestern Puerto Rico. Streamers are also expected to develop downstream of the islands and higher terrain, and may produce locally significant rainfall where they persist. With relatively weak steering winds, showers and thunderstorms may linger somewhat later than usual this evening before transitioning to more typical passing overnight showers across the local waters, U.S. Virgin Islands, and eastern Puerto Rico. Low concentrations of Saharan dust will remain across the region.

On Monday and Tuesday, a weak mid-level ridge will help introduce some drier air, while surface high pressure over the Atlantic becomes better organized and winds gradually strengthen. Despite the drying, enough low-level moisture will remain and, combined with strong daytime heating, will support active afternoons Monday and Tuesday. Winds will shift from east-southeast Monday to more easterly Tuesday, favoring afternoon convection from the interior into western Puerto Rico with a more westward movement of showers and thunderstorms. The driest period is expected Tuesday evening, while colder temperatures aloft associated with an approaching mid- to upper-level trough begin the transition toward a more unstable pattern Tuesday night into Wednesday.

The main hazards through the period will be heat, locally heavy rainfall leading to flooding, lightning, and gusty winds. A Heat Advisory is in effect today, with extreme heat concerns likely to continue Monday and Tuesday. The greatest rainfall threat will occur during the afternoon, mainly from the interior into western Puerto Rico, where slow-moving or repeated convection may produce urban and small-stream flooding, isolated flash flooding, and landslides in steep terrain, especially where soils remain locally saturated from recent heavy rainfall.

Long Term(Wednesday through Sunday)

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

(06Z TAFS) Issued at 145 AM AST Sun Sep 20 2026

Mostly VFR conds expected thru the period. Passing SHRA may briefly lower VIS/CIGs at TJSJ/TIST/TISX overnight. SHRA/TSRA increase across PR after 20/16-17Z, with greatest impacts at TJBQ/TJSJ around 20/17-20Z; PROB30 groups for 4SM TSRA BKN/OVC020-030. VCTS psbl at TJPS. Winds light/VRB overnight, becoming E-ESE 10-15 kt after 20/13Z with sea-breeze variations and higher gusts near TSRA, then returning to light/VRB after 20/22Z.

MARINE

Issued at 145 AM AST Sun Sep 20 2026

Weak surface high pressure north of the region will maintain gentle to moderate easterly trades of 5 to 15 knots today, with winds gradually becoming 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 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 145 AM AST Sun Sep 20 2026

A low risk of rip currents is expected across the beaches of Puerto Rico and the U.S. Virgin Islands through around midweek. However, life-threatening rip currents can still occur near groins, jetties, reefs, and piers. Mid to late week, a moderate risk is expected to return across some north- and east-facing beaches as winds increase and a small long-period northeasterly swell reaches the region.

Other beach hazards will include heat and afternoon thunderstorms, particularly across western Puerto Rico. Thunderstorms may produce gusty winds and frequent lightning. Beachgoers should remain hydrated, limit prolonged sun exposure, and seek shelter indoors whenever thunder is heard.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory from 10 AM this morning to 3 PM AST this afternoon for PRZ001>013.

VI...Heat Advisory from 10 AM this morning to 3 PM AST this afternoon for VIZ001-002.

AM...None.