textproduct: San Juan

KEY MESSAGES

Issued at 215 AM AST Mon Sep 21 2026

- A seasonal weather pattern will persist across Puerto Rico and the U.S. Virgin Islands during the next few days, with afternoon showers and thunderstorms across central, northern, and western Puerto Rico.

- A Heat Advisory is in effect from 10 AM to 3 PM AST for all Puerto Rico, Vieques, Culebra and the U.S. Virgin Islands.

- A wetter and more unstable pattern will return by mid-week, increasing the potential for widespread heavy rainfall and stronger thunderstorms.

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

Short Term(Today through Wednesday)

Issued at 215 AM AST Mon Sep 21 2026

A seasonal weather pattern, with high heat indices and convection during the afternoon is expected today. A Heat Advisory will be in effect from 10 AM to 3 PM. Locals and visitors are encouraged to remain hydrated and avoid outdoor activities, especially during peak daytime heating hours. Seasonal to above-seasonal precipitable water (PWAT) values ranging from 1.70 to 2.10 inches will support passing morning showers during the morning over the USVI and eastern portions. Light winds from a southeasterly flow will promote showers and thunderstorm activity enhanced by local effects during the afternoon, especially over the northwestern portions of Puerto Rico. On Tuesday, a similar pattern is expected, however, a trough in mid- to-high levels should enhance instability, which could result in stronger afternoon convection over the CWA. In addition, a more easterly flow and prevailing light winds could allow showers and thunderstorms development over most of the local area. Moving into mid-week, a weak mid-level ridge will allow drier air intrusion and a surface high pressure will promote winds strengthening gradually to seasonal levels by the end of workweek. As a result, PWAT values are anticipated to diminish to sub-seasonal to seasonal ranges (1.6- 2.00 inches) by Wednesday. Nonetheless, afternoon convection could still be possible over western and interior portions.

Overall, the primary weather hazards will include heat-related risks, localized heavy rainfall, and isolated thunderstorms, which could be accompanied by frequent lightning and strong gusts. Tuesday is expected to be the most unstable day, as light winds could lead to slow-moving storms that heighten the risk of excessive rainfall, particularly in the afternoon. While Monday and Tuesday face a limited to elevated risk of excessive rainprimarily impacting interior, northwestern, and western areasthis risk will decrease to a limited level on Wednesday as mid-level moisture drops. Consequently, these conditions may lead to water ponding on roadways, urban and small stream flooding, and even isolated flash floods in areas with an elevated risk level, alongside a continued risk of landslides where terrain is already saturated from recent rainfall.

Long Term(Thursday through Monday)

Issued at 215 AM AST Mon Sep 21 2026

Surface high pressure across the central to eastern Atlantic will promote mainly southeasterly low-level wind flow during most of the long-term period, becoming more easterly early next week, at around 10 to 18 knots. The latest models show that precipitable water content will remain mainly near seasonal values, between 1.75 and 2.0 inches, except from Friday night into early Saturday, when values may decrease to around 1.5 inches (25th percentile), as low concentrations of Saharan dust particles are expected to start arriving.

By Thursday and Friday, models continue to indicate mid to upper- level lows, bringing some instability and 500-mb temperatures cooling to below-seasonal values, around -8 to -9 degrees Celsius. Models have been adjusting, bringing drier air into the 850 to 700- mb levels and also into the mid to upper levels during the weekend. With marginal instability aloft, daytime heating, and local effects, at this time Thursday and Friday look like two days with frequent passing showers across windward areas and the U.S. Virgin Islands, along with strong afternoon thunderstorms across parts of the interior into northwestern Puerto Rico. If this scenario materializes, it could produce heavy rainfall, frequent lightning, and locally gusty winds. A limited to elevated flooding risk is possible during this period, meaning that flooding in urban areas, on roads, and in small streams and washes is possible.

As previously mentioned, Saturday looks like the day Saharan dust will start to infiltrate the islands. Additional mid-level drying is expected. As a result, rain chances are expected to decrease, and afternoon showers and isolated thunderstorms, driven by local effects, should remain more confined to northwestern Puerto Rico and decrease in intensity. From Sunday into early next week, low-level winds are also expected to strengthen somewhat and available moisture gradually increase. That should allow for a more typical pattern, with morning showers across eastern Puerto Rico and the U.S. Virgin Islands, followed by afternoon convection across western sectors of Puerto Rico.

A persistent hazard during the long term will be the heat. It will largely depend on cloud cover and the coverage and timing of showers, but overall, the combination of seasonal moisture, mainly southeasterly winds, and 925-mb temperatures around two standard deviations above normal will likely result in warm to hot conditions, particularly from 11 AM to 4 PM, which are typically the peak heat hours.

AVIATION

(06Z TAFS) Issued at 215 AM AST Mon Sep 21 2026

Mostly VFR conds expected across all terminals thru the forecast period. Passing SHRA may result in VCSH and briefly lower VIS/CIGs at TJSJ/TIST/TISX thru 21/14Z. SHRA/TSRA will increase across central and western PR aft 21/16-17Z, affecting mostly TJBQ/TJSJ. Winds light/VRB overnight, becoming E-ESE 10-15 kt after 21/14Z with sea-breeze variations and higher gusts near TSRA. Light/VRB winds expected overnight aft 21/23Z.

MARINE

Issued at 215 AM AST Mon Sep 21 2026

A weak surface high pressure north of the region will maintain gentle to moderate easterly trades of 5 to 15 knots through Monday, with winds gradually becoming east by late Monday into 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 215 AM AST Mon Sep 21 2026

Expect the low rip current risk to prevail across 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 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.