textproduct: San Juan

KEY MESSAGES

Issued at 235 AM AST Sun Oct 18 2026

* Showers and thunderstorms may cause localized flooding today and Monday. Flooding and lightning threats will decrease midweek before increasing again late in the workweek and through the weekend.

* A lingering northerly swell will subside today, followed by a stronger swell Monday night, bringing dangerous rip currents, higher surf, and hazardous seas, peaking Tuesday.

* Limited heat impacts will mainly affect heat-sensitive individuals during the late morning and early afternoon each day.

* Across the U.S. Virgin Islands, rainfall impacts will remain limited through Tuesday, followed by increasing showers and thunderstorms late in the workweek. Marine and beach conditions will deteriorate Monday night.

Short Term(Today through Tuesday)

Issued at 235 AM AST Sun Oct 18 2026

Isolated to scattered showers developed over the surrounding waters overnight, with some moving inland and producing minimal rainfall accumulations. This activity should continue through the morning, particularly across windward areas, as patches of moisture move along the prevailing east to southeast flow. Aloft, a northward-shifting mid-level ridge and weakening upper-level high, influenced by a lingering deep-layer trough northeast of the region, will gradually reduce subsidence. Although mid-level moisture remains limited, periodic moisture pulses should gradually improve moisture availability through Monday. With marginal instability and precipitable water values generally near to below the October climatological median of 1.9 inches, daytime heating, sea breeze convergence, and terrain effects will support afternoon showers and isolated thunderstorms across interior and northwestern Puerto Rico. Weak steering flow may favor slow-moving convection and locally heavy rainfall.

By Monday, further weakening of the mid-level ridge and improving moisture depth will support somewhat deeper and potentially more persistent afternoon convection. Showers and isolated to scattered thunderstorms will favor interior and western Puerto Rico, where NWS and ECMWF ensemble guidance highlight locally enhanced rainfall. By Tuesday, southeasterly low-level winds and increasingly southerly flow near 700 mb may shift afternoon activity toward northern Puerto Rico, although convective coverage and intensity may become more limited. A brief reduction in moisture is expected Tuesday night. Meanwhile, increasing low- level temperatures will favor a gradual warming trend, particularly across northern coastal and urban areas. Heat impacts should remain limited, mainly affecting individuals extremely sensitive to heat, especially during the late morning and early afternoon before increasing cloud cover and showers provide localized relief.

The main hazards will be locally heavy rainfall, lightning, and isolated flooding, particularly today and Monday. Ponding of water on roads and in poorly drained areas is likely beneath heavier showers, with a low chance of urban and small-stream flooding. Isolated flash flooding cannot be ruled out where slow-moving or repeatedly developing thunderstorms produce prolonged rainfall, especially across interior and western Puerto Rico. Monday may favor deeper and more persistent thunderstorms, while Tuesday's rainfall and lightning threats should become more limited, with any heavier activity potentially shifting toward northern Puerto Rico.

Long Term(Wednesday through Sunday)

Issued at 235 AM AST Sun Oct 18 2026

Relatively drier conditions on Wednesday will transition into a wetter and more unsettled pattern Thursday through Sunday. The interaction between a surface high over the central Atlantic and a deep-layer low over the eastern CONUS will maintain a generally southeast to southerly wind flow across the region, with steering winds strengthening late Thursday into Saturday. Wednesday appears to be the driest day of the period, as model guidance suggests below-normal low- to mid-level moisture, with precipitable water values below 1.6 inches. Nevertheless, daytime heating and local effects may still support afternoon showers across portions of Puerto Rico.

A considerable increase in moisture is expected by Thursday as deep tropical moisture spreads across the northeastern Caribbean. Model guidance suggests precipitable water values exceeding 2.0 inches through much of the remainder of the period, occasionally approaching 2.4 inches, with periods of above-normal moisture. Increasing mid-level humidity, cooling temperatures aloft, and the proximity of a deep-layer trough will support a more favorable environment for deeper convection. Although a mid-level ridge may gradually build over the eastern Caribbean, abundant moisture and local effects should continue to favor showers and thunderstorms. An advective shower pattern is expected across windward sections of the islands, particularly during the nighttime and morning hours, followed by afternoon convection across the interior and western portions of Puerto Rico. Increasing steering winds late Thursday through Saturday should allow convection to become more progressive, although locally heavy rainfall may still result in urban and small-stream flooding, isolated flash flooding, and rises in river levels. By Sunday, steering winds may weaken again, potentially favoring slower-moving thunderstorms and greater localized rainfall accumulations.

Under the prevailing southeast to southerly flow, warmer than normal temperatures are likely, particularly across northern coastal and urban areas of Puerto Rico. Combined with abundant moisture, heat indices may exceed 100 degrees Fahrenheit, mainly during the late morning and early afternoon. Heat impacts may range from limited to elevated, particularly for individuals sensitive to heat, and heat products may be issued if conditions warrant.

AVIATION

(06Z TAFS) Issued at 235 AM AST Sun Oct 18 2026

VFR conds expected to prevail across most terminals through the TAF prd. SCT -SHRA will continue across the local waters through the morning, with brief activity affecting TJSJ and USVI terminals through 18/14Z. Tempo MVFR conds possible with reduced VIS/CIGs in heavier SHRA. Aftn SHRA/TSRA expected across interior and NW PR, particularly aft 18/17Z, with VCTS possible at TJBQ and potentially TJSJ. Brief MVFR or locally IFR conds possible beneath heavier convection, along with mountain obscurations. E-ESE winds at 5-10 kt early, increasing to 8-12 kt aft 18/13Z with sea breeze variations.

MARINE

Issued at 235 AM AST Sun Oct 18 2026

A weak pressure gradient will maintain gentle to moderate east to southeast winds across the local waters through midweek. Relatively favorable marine conditions are expected today into Monday, before an energetic, long-period northwest to north swell begins arriving Monday evening. The swell will peak Tuesday, building seas to 5 to 7 feet, occasionally up to 9 feet, across the Atlantic waters, northern coastal waters, and exposed local passages. Hazardous conditions for small craft are possible, with elevated seas gradually subsiding later in the week. Meanwhile, passing showers and isolated thunderstorms may produce locally hazardous conditions, particularly near the northern and western coastal waters during the afternoons.

BEACH FORECAST

Issued at 235 AM AST Sun Oct 18 2026

A moderate risk of rip currents will continue today into early Monday along exposed northern beaches of Puerto Rico and much of the U.S. Virgin Islands, including eastern St. Croix. The risk may increase to high along portions of northern Puerto Rico on Monday, ahead of a more energetic, long-period northwest to north swell arriving Monday evening. This swell will build through Tuesday, producing increasingly dangerous rip currents and higher surf along exposed beaches, with hazardous conditions potentially persisting through midweek.

Breaking waves are currently forecast to reach 7 to 8 feet along portions of northern Puerto Rico. However, localized surf of 10 feet or higher cannot be ruled out at the most exposed beaches, potentially reaching High Surf Advisory criteria. Additionally, afternoon thunderstorms may produce lightning and locally gusty winds near beaches, particularly today and Monday.

SJU WATCHES/WARNINGS/ADVISORIES

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