textproduct: San Juan

KEY MESSAGES

Issued at 445 AM AST Wed Sep 30 2026

- Passing showers will continue to affect the USVI and eastern/northern Puerto Rico during the night and morning hours, followed by afternoon showers and thunderstorms over the interior and western Puerto Rico each day.

- The potential for locally heavy rainfall and flooding will increase Thursday through Saturday, especially across the interior and western portions of Puerto Rico. Urban and small stream flooding will be possible.

- A few stronger thunderstorms are possible Friday and Saturday as conditions become more favorable for thunderstorm development. The strongest thunderstorms could produce frequent lightning and gusty winds. Gusty winds may knock down tree limbs and blow around unsecured objects.

- The heat threat will remain limited today and Thursday. By Friday, a warmer and more humid air mass may produce localized hazardous heat, particularly across urbanized portions of north-central and western coastal Puerto Rico. Stay hydrated, take frequent breaks, and limit strenuous outdoor activity during the hottest part of the day.

Short Term(Today through Friday)

Issued at 445 AM AST Wed Sep 30 2026

During the overnight hours, an advective weather pattern prevailed across the local area, with scattered showers moving across the northern and northeastern portions of Puerto Rico and the smaller islands. However, rainfall accumulations remained generally below two tenths of an inch.

For today, a similar weather pattern to the past few days is expected. East to east-northeast winds will maintain a relatively fast steering flow, with breezy conditions persisting, particularly across coastal areas. Precipitable water values will remain near seasonal levels, generally between 1.7 and 1.9 inches, providing sufficient moisture to support passing showers across windward areas during the morning hours and locally induced convection during the afternoon. However, warming at mid-levels will gradually weaken the 700-500 mb lapse rates, limiting the potential for deeper convection. Afternoon shower and isolated thunderstorm activity will favor the interior and western to southwestern portions of Puerto Rico. Given the relatively fast steering flow, showers and thunderstorms should move quickly, and with warming aloft, limit rainfall accumulations and the overall flooding threat. Nevertheless, ponding of water on roads and in poorly drained areas is possible, along with a low chance of urban and small stream flooding.

Late tonight into Thursday, GOES-19 precipitable water imagery shows a surface-induced perturbation approaching the local area, which is expected to move across the region during the day. Winds will gradually shift from the east-northeast to the east-southeast. Although this perturbation will provide some dynamical support and sufficient low- to mid-level moisture will be available, warm- air advection near 500 mb will result in temperatures further warming to around - 4 to -5 degrees Celsius, above normal for this time of year. Therefore, 700-500 mb lapse rates will decrease, providing less favorable conditions for deep convection. Nevertheless, daytime heating, local effects, and the passage of the perturbation should still promote showers and isolated thunderstorms, particularly across the interior and western portions of Puerto Rico during the afternoon. The flooding threat will be somewhat higher than today, with urban and small stream flooding possible where persistent or locally heavier rainfall occurs.

By Friday, weather conditions aloft will become more favorable for deeper convection as a series of mid- to upper-level troughs deepen and sink southward northwest of the region. Cold-air advection aloft will lower 500 mb temperatures to around -7 to -8 degrees Celsius, resulting in a renewed steepening of the 700-500 mb and the 850-700 mb lapse rates. At the same time, increasing moisture and east- southeast low-level flow will support additional shower and thunderstorm development. Afternoon convection will again favor the interior and western/northwestern portions of Puerto Rico, where an elevated flooding threat is expected. With more favorable dynamics aloft, some thunderstorms could become stronger, producing frequent lightning and gusty winds in addition to locally heavy rainfall. Breezy conditions will gradually diminish through the end of the short-term period as low-level winds weaken somewhat.

The heat threat will remain limited today and Thursday, primarily affecting individuals who are extremely sensitive to heat, especially those outdoors without effective cooling or adequate hydration. By Friday, east-southeast winds will advect a somewhat warmer and more humid air mass into the region, with 925 mb temperatures increasing to slightly above-normal levels. Although widespread conditions warranting Heat Advisories may not be necessary, hazardous heat may develop locally, particularly across urbanized portions of north-central and western coastal Puerto Rico. Individuals planning prolonged outdoor activities should take preventive actions, including staying well hydrated, taking frequent breaks in shaded or air-conditioned areas, and limiting strenuous activity during the hottest part of the day.

Long Term(Saturday through Wednesday)

Issued at 445 AM AST Wed Sep 30 2026

The long-term forecast remains on track, with slightly unstable conditions on Saturday, transitioning to a typical weather pattern by the beginning of next workweek. A surface high pressure over the western Atlantic is expected to promote a southeasterly wind flow on Saturday, becoming E-ESE later in the night. Abundant moisture content should linger on Saturday, as the latest model guidance suggests PWAT values between seasonal and near above climatological normal (1.8 and 2.0 inches), with low variability between ensemble members (around 0.3 inches). Additionally, the cross section over the San Juan area shows abundant low- and mid-level moisture content (RHs between 60 - 70%), with skinny sounding profiles and high CAPEs (around 2000 and 3000 J/kg). As mentioned in the previous discussion, the proximity of a series of upper-level troughs should introduce instability across the area, with the latest model guidance suggesting cooler-than-normal mid-level temperatures (between -7 and -8 C) and steeper lapse rates (between 6.0 and 6.5 C/km). These conditions, combined with the position of the troughs, should allow cloud growth and ventilation, increasing the likelihood of deeper convection. The most likely scenarios remain, with showers and isolated thunderstorms moving over windward sections of the islands during the morning, followed by deep afternoon convection over interior and western/northwestern Puerto Rico. Additionally, expected local island streamers are developing, moving over eastern Puerto Rico, including the San Juan Metro Area. Lingering moisture on Sunday combined with daytime heating, local effects, and sea breeze convergence should trigger afternoon convection as well, though a drier airmass filtering into the region and an approaching ridge may limit the flood threat.

Monday onward, a lingering upper-level trough in our vicinity will enhance instability through the end of the long-term period. Combined with daytime heating and local effects, available low-level moisture will drive localized shower and thunderstorm activity, mainly over the interior and western half of Puerto Rico. By Tuesday, precipitable water (PWAT) values are expected to return to seasonal levels. This moisture increase, along with a shift to a more east-southeasterly wind flow, will slightly enhance deeper convection over the interior and northwestern areas.

Overall, the primary hazards for the long-term period remain a limited-to-elevated risk of excessive rainfall, with the highest threat on Saturday, with a limited-to-elevated heat risk persisting throughout the forecast period. As a result, urban and small-stream flooding is possible over roadways and poorly drained areas, and flash flooding cannot be ruled out. For those sensitive to the heat, staying hydrated and avoiding prolonged outdoor activities during peak heating hours is highly recommended.

AVIATION

(06Z TAFS) Issued at 445 AM AST Wed Sep 30 2026

Mainly VFR conds are expected across all terminals thru the period. Passing SHRA may result in brief MVFR cigs/vsbys at TIST/TISX and occasionally TJSJ thru the morning. Winds will increase aft 13-14Z, with E-ENE winds 15-18 kt and hir gusts. Afternoon TSRA may result in VCTS at TJPS/TJBQ and mtn obsc after 17-18Z, with brief MVFR conds possible in TSRA and mtn obscurations developing along the Cordillera Central. The 30/00Z TJSJ sounding indicated ENE winds up to 22 kt blo FL050.

MARINE

Issued at 445 AM AST Wed Sep 30 2026

Building high pressure north of the region will maintain moderate to locally fresh easterly trade winds through Thursday, resulting in choppy conditions across the Atlantic and Caribbean waters and the local passages. Winds should become mainly moderate from Friday into the weekend and early next week. Seas will generally range from 3 to 5 feet, occasionally up to 6 feet across the exposed Atlantic and Caribbean waters and passages, with somewhat lower seas in protected coastal waters. Periods of passing showers will continue, while afternoon showers and isolated thunderstorms are expected mainly across the western and northwestern coastal waters of Puerto Rico. Thunderstorms may produce locally higher winds and seas, lightning, and reduced visibility.

BEACH FORECAST

Issued at 445 AM AST Wed Sep 30 2026

A moderate risk of rip currents is expected across most north- and east-facing beaches of Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands through much of the forecast period. Life-threatening rip currents will remain possible, especially near reefs, jetties, and piers.

Additionally, showers and isolated thunderstorms may develop each afternoon, particularly along the western coast of Puerto Rico and nearby waters. Beachgoers should remain alert for rapidly changing weather conditions and seek shelter if thunder is heard.

SJU WATCHES/WARNINGS/ADVISORIES

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