textproduct: San Juan

KEY MESSAGES

Updated at 210 AM AST Mon Apr 13 2026

* A wet and unstable weather pattern will bring widespread showers and thunderstorms across the area, with a Flash Flood Watch in effect from 10 AM today through Wednesday.

* An elevated to significant flooding risk is expected across the islands; residents are urged to exercise caution, avoid flood- prone areas, and never cross flooded roads.

* The wettest period is expected from tonight into early Tuesday morning, when the heaviest rainfall and most persistent shower activity are likely.

* A moderate risk of rip currents will persist across all local beaches, creating hazardous conditions for swimmers.

* Across the U.S. Virgin Islands, an elevated to significant flooding risk is expected from early Tuesday morning through the day as periods of heavy rainfall impact the area.

Short Term(Today through Wednesday)

Issued at 210 AM AST Mon Apr 13 2026

Moderate to strong showers affected the Atlantic offshore and nearshore waters for most of the night, resulting in hazardous marine conditions due to locally higher seas and gusty winds. Given the prevailing conditions, several Special Marine Warnings and Marine Weather Statements were issued throughout the night. As the night progressed, by 2 AM, showers shifted southward into coastal areas, affecting portions of eastern and northeastern Puerto Rico, including municipalities near Naguabo, Fajardo, and Rio Grande. Overnight temperatures remained in the lower 70s across coastal areas, with even cooler conditions across the mountains.

For today, the forecast of a wetter and more unstable weather pattern remains on track. As a result, a Flash Flood Watch will be in effect from 10 AM today through Wednesday across the entire area. At upper levels, a deep trough with its axis just over Hispaniola places the islands under a divergent region, favoring vertical development and widespread thunderstorm activity. This upper-level feature will allow colder temperatures at 500 mb, ranging between -9C and -10C, which are around two standard deviations below climatological normals. At the surface, a frontal boundary and the associated pre-frontal trough will reach the local area. Winds will shift to a northerly component from this afternoon into early Tuesday as the frontal boundary moves southward across the region. Additionally, abundant deep moisture will be in place, with relative humidity values from 850 mb up to 500 mb near the 75th percentile. All of these factors provide a well-defined setup for a very convective day. Model guidance suggests an increase in showers during the morning hours, with activity expanding and intensifying through the afternoon. According to the highest-resolution models, a very active weather day is expected, with the strongest convection developing across the interior and then moving into northwestern and interior Puerto Rico. By late afternoon into the evening, showers will affect northeastern areas, extending into Culebra, Vieques, and the U.S. Virgin Islands. An important factor for today will be the surface wind pattern, as slow-moving showers will favor higher rainfall accumulations in addition to the widespread shower activity. As a result, an elevated to significant flooding threat is expected across the entire forecast area today. Residents are advised to avoid rivers and not to cross flooded roads or low-lying areas.

On Tuesday in the early morning hours into Wednesday, conditions will remain on the unstable side, as the upper-level trough continues to deepen, leaving a cut-off low just over the area. This feature will be reflected through the column, resulting in a low at 500 mb and an induced surface trough extending from around 1000 mb up to 700 mb. The presence of colder temperatures aloft and the slow movement of this feature will continue to support enhanced instability and prolonged convective activity across the region. Given the expected conditions deteriorate weather conditions are forecast across the islands. Given the expected rainfall accumulations and the already saturated soils, this pattern will maintain an elevated to significant risk of urban and river flooding, as well as potential mudslides in areas of steep terrain. Again, a Flood Watch is in effect from 10 AM this morning through 8 AM Wednesday for all of Puerto Rico and the U.S. Virgin Islands. Residents and visitors should continue to monitor the latest weather information.

Long Term(Thursday through Monday)

Issued at 210 AM AST Mon Apr 13 2026

No major changes were made to the long-term forecast, as the wet and unsettled weather pattern is likely to persist into the weekend, gradually improving by the first part of the next workweek. A surface high pressure is expected to build over the Western extending into the Central Atlantic, promoting SE-E winds through Sunday. As a surface trough over the Hispaniola weakens, the pressure gradient will likely increase, with winds gradually strengthening throughout the period, resulting in breezy to windy conditions across the coastal areas of the islands. In the mid to upper levels, the deep-layered trough may linger and deepen into the tropics, with our CWA under the favorable side for convection. From the latest model guidance, moisture content will remain higher for this time of the year, with PWAT values likely between 1.8 and 2.0 inches low chance of higher values. Additionally, the low to mid- level moisture content should remain higher than normal, with model soundings suggesting skinny profiles, particularly on Friday and Saturday. In terms of convection, the influence of the trough will maintain slightly cooler than normal mid-level temperatures (500 mb temperatures around -8 degrees Celsius), a jet streak with 250 mb winds between 60 and 80 knots, and divergence aloft, triggering upper-level dynamics. Under these conditions, forced vertical motion will result in cloud growth and ventilation, favorable for deep convection. Additionally, the Galvez-Davison Index (GDI) continues to suggest the potential for isolated to scattered thunderstorms across the CWA, particularly in the afternoons over interior into north/northwestern Puerto Rico and in the night across regional waters, eastern Puerto Rico, and the U.S. Virgin Islands. As mentioned in the previous discussion, the weather event in the short- term forecast may result in saturated soils and higher-than-normal river levels. A factor that may slightly inhibit deeper convection is the winds, which may lead to more progressive showers instead of stationary ones. Nevertheless, with the expected weather forecast through Sunday, showers and thunderstorms over the CWA will increase the flooding and lightning potential. Rainfall accumulations will likely result in urban and small stream flooding, including localized flash floods, including landslides, and rapid river rises. Additionally, thunderstorms may result in gusty winds, reduced visibility, and lightning. Residents and visitors of the islands are encouraged to keep monitoring conditions and the next issuance of the long-term forecast. By Monday, as another surface high pressure builds over the western Atlantic and wind shifts from the NE, the weather pattern should gradually improve with a drier air mass filtering into the region.

AVIATION

(06Z TAFS) Issued at 210 AM AST Mon Apr 13 2026

MVFR to IFR conditions are expected across all TAF sites today due to an increase in SHRA and TSRA activity. Periods of reduced VIS and lowering CIGs are anticipated, with tempo IFR conditions possible in heavier showers and thunderstorms. Mountain obscurations will persist, especially across interior and western sections. Winds will remain VRB through 13/15Z, then increase to around 10 KT from the E-SE as the surface trough lifts north of the area. Thereafter, winds will become light and variable again, before gradually shifting to the NE around 14/03Z.

MARINE

Issued at 210 AM AST Mon Apr 13 2026

Moderate easterly trades will persist through the period as high pressure remains over the central Atlantic, with periods of locally fresh winds. Combined with ongoing northerly swell, this will maintain choppy conditions across the Atlantic waters and passages. Increasing showers and thunderstorms early next week will lead to locally hazardous marine conditions, including gusty winds and reduced visibility, though impacts will be brief and variable. By midweek, a combination of stronger winds and building swell may result in hazardous conditions for small craft, particularly across the Atlantic waters.

BEACH FORECAST

Issued at 210 AM AST Mon Apr 13 2026

Rip current conditions have improved as wave energy has decreased, no longer supporting a High Rip Current Risk. However, a moderate risk of rip currents is in place for most north- and east-facing beaches of Puerto Rico, including Vieques and Culebra, as well as the U.S. Virgin Islands, meaning rip currents are still possible and can be life-threatening. A low risk will likely persist across western Puerto Rico, from Anasco and Mayaguez southward and east along the Caribbean coast through southeastern Puerto Rico, including Patillas, Maunabo, and Yabucoa. Increasing winds early to midweek could lead to a return of more hazardous marine and coastal conditions. Exercise caution at the beach, especially along exposed coasts. Sharing is saving.