textproduct: San Juan

KEY MESSAGES

Issued at 210 PM AST Tue Oct 20 2026

* Afternoon thunderstorms will persist through this evening producing lightning, locally heavy rainfall, and urban or small- stream flooding across northern Puerto Rico. Activity should decrease by Wednesday before a surge of tropical moisture brings more widespread showers and renewed flooding concerns Thursday into the weekend.

- Heat Threat (Level 1): A Level 1 heat threat is expected across most of the region throughout the week. Warm temperatures and elevated heat indices may primarily affect individuals who are highly sensitive to heat, particularly those spending extended periods outdoors without adequate hydration or effective cooling.

- High Surf Advisory will be in effect through 6 PM AST this evening. However, hazardous coastal conditions will persist, with a high risk of rip currents along northern exposed beaches due to incoming northerly swells. Therefore, a high risk of rip currents will continue across the northern exposed beaches for the rest of the week.

Short Term(This evening through Thursday)

Issued at 210 PM AST Tue Oct 20 2026

Relatively quiet and typical weather conditions prevailed this morning, with a few towering cumulus clouds and passing showers mainly over the surrounding waters. However, afternoon convection has already developed across northern Puerto Rico, prompting the issuance of a Special Weather Statement for several north-central municipalities. The 20/12Z TJSJ sounding indicated sufficient moisture and instability to support this activity, with precipitable water near 1.84 inches and seasonal low- to mid-level lapse rates. Additionally, a surface-to-3 km mean wind of only 6 knots suggests slow-moving convection. A weak pressure gradient continues to maintain light southeast to southerly flow, allowing sea-breeze convergence and local terrain effects to dominate convective development.

For the remainder of the afternoon, showers and thunderstorms will continue to affect portions of the interior and northern Puerto Rico. Given the weak steering flow, slow-moving thunderstorms may produce locally heavy rainfall, frequent lightning, and localized urban and small-stream flooding, particularly where showers persist over the same areas. Across the U.S. Virgin Islands, passing showers and isolated thunderstorms should remain the predominant weather feature, although most activity will remain over the surrounding waters.

Wednesday continues to look like the most stable and least active day of the short-term period. GFS guidance suggests precipitable water values decreasing to around 1.4 to 1.5 inches, while 700-500 mb relative humidity drops to near 20 percent, well below climatological normals. Although mid-level lapse rates will remain near or slightly above seasonal values, the drier air aloft should limit the coverage and persistence of deep convection. Nevertheless, afternoon showers and isolated thunderstorms remain possible across Puerto Rico due to local effects. Greater sunshine and reduced cloud coverage may also make Wednesday the warmest day of the period, particularly across coastal and urban areas.

A significant increase in tropical moisture remains on track for late Wednesday into Thursday. Model guidance indicates precipitable water values rising to around 2.2 to 2.3 inches, while 700-500 mb relative humidity increases to around 75 to 85 percent, both exceeding climatological normals. Despite weakening mid-level lapse rates and slightly warmer temperatures aloft, the deeper moisture should favor more widespread and persistent shower activity across Puerto Rico and the U.S. Virgin Islands. Increasing cloud cover may limit daytime heating and thunderstorm development in some areas; however, local convergence and breaks in the clouds could still support heavier convection. The primary concern by Thursday will shift toward an increasing flooding threat, including urban and small-stream flooding, rapid river rises, and isolated flash flooding where persistent or repeated heavy rainfall develops. This wetter pattern should continue into the latter part of the workweek.

Long Term(Friday through Tuesday)

Issued at 145 AM AST Tue Oct 20 2026

No major changes were introduced to the long-term forecast. Wet and unsettled weather conditions are expected across Puerto Rico and the U.S. Virgin Islands through much of the weekend, with the greatest potential for widespread showers and thunderstorms Friday into Saturday, followed by gradually improving conditions beginning Sunday.

A southeast to southerly steering flow will persist across the region due to the interaction between a deep-layer low moving across the eastern United States and surface high pressure over the central Atlantic. This pattern will allow deep tropical moisture to spread across the region, with precipitable water values around or exceeding 2 inches. Latest model guidance also indicates anomalously high mid-level relative humidity, favoring enhanced shower and thunderstorm development and locally heavy rainfall. Despite a building mid-level ridge over the eastern Caribbean, daytime heating, sea breeze convergence, and local topographic effects will support passing showers across windward areas during the overnight and morning hours, followed by scattered to numerous afternoon thunderstorms across interior, northern, and western Puerto Rico. Unsettled conditions will likely continue into Saturday, although increasing steering winds should favor more progressive showers and thunderstorms. By Sunday, a gradual reduction in atmospheric moisture is expected, leading to decreasing shower coverage. However, weakening steering winds late in the weekend could allow thunderstorms to move more slowly, increasing the potential for localized rainfall accumulations. The main hazards Friday through the weekend will include lightning, urban and small-stream flooding, and isolated flash flooding, particularly where repeated or persistent heavy showers occur. Meanwhile, persistent southeast to southerly winds will maintain hot and humid conditions, with the greatest heat impacts expected across northern coastal and urban areas, particularly for individuals spending extended periods outdoors without adequate cooling or hydration.

A more noticeable improvement in weather conditions is forecast for Monday into Tuesday as drier air spreads through the atmospheric column and increasing subsidence associated with the mid-level ridge limits convective development. This should favor a drier and more stable weather pattern across the region early next week.

AVIATION

(18Z TAFS) Issued at 210 PM AST Tue Oct 20 2026

SHRA/TSRA expected across the interior and northern PR thru 20/22Z. VCTS at TJSJ/TJBQ thru early evening. VFR conds expected to prevail overnight, with VCSH across TJSJ and USVI terminals. Winds mainly ESE-SE at 8-11 kt, becoming VRB or calm overnight. E-SE winds at 10- 13 kt expected after 21/13Z.

MARINE

Issued at 210 PM AST Tue Oct 20 2026

A long-period northerly swell continues to spread across the local Atlantic waters and passages, resulting in hazardous marine conditions. Recent observations from the San Juan buoy indicated seas up to 6 feet with a dominant period of 13 seconds, while buoy 41043, located approximately 170 nautical miles north- northeast of San Juan, reported seas near 6 feet with a period of 13 seconds around 11 AM AST. This additional pulse of northerly swell will continue to propagate across the regional waters, maintaining seas up to 6 feet, particularly across the offshore Atlantic waters and northern passages. Although a gradual improvement is anticipated, another northerly swell is expected to arrive late Thursday and persist into the upcoming weekend, maintaining elevated seas and hazardous conditions for small craft.

BEACH FORECAST

Issued at 210 PM AST Tue Oct 20 2026

A long-period northerly swell continues to generate large breaking waves along the northern exposed beaches of Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands. Recent observations from the San Juan buoy indicated seas up to 5 feet with a dominant period of 13 seconds, supporting breaking waves up to 10 feet. These hazardous surf conditions are expected to persist through at least 6 PM AST. Therefore, a High Surf Advisory remains in effect until 6 PM AST.

Additionally, a new pulse of northerly swell was observed around 11 AM AST at buoy 41043, located approximately 170 nautical miles north- northeast of San Juan, with significant wave heights of 5.9 feet and a dominant period of 13 seconds. This additional swell energy may maintain borderline High Surf Advisory conditions through the overnight hours. Although surf conditions are expected to gradually improve, hazardous marine and beach conditions will likely persist as another northerly swell arrives late Thursday and continues into the upcoming weekend. As a result, a high risk of rip currents will remain across all northern exposed coastal areas, posing life- threatening conditions for swimmers and beachgoers.

SJU WATCHES/WARNINGS/ADVISORIES

PR...High Rip Current Risk through Wednesday afternoon for PRZ001-002- 005-008-010-012.

High Surf Advisory until 6 PM AST this evening for PRZ001-002- 005-008-010.

VI...High Rip Current Risk from 6 PM AST this evening through Wednesday afternoon for VIZ001.

AM...None.