textproduct: San Juan

KEY MESSAGES

Updated at 459 AM AST Sat Apr 11 2026

* A mid to upper level trough approaching from the northwest will result in wetter and more unstable conditions with an increased flooding risk as it reaches the islands late Sunday into the next workweek. The heaviest activity is forecast for Monday night into Tuesday. A Flood Watch (FFASJU) will likely be issued later today.

* A High Risk of Rip Currents is in effect for the northern coast of Puerto Rico and Culebra through early next workweek due to a long- period NNW swell.

* A limited to elevated flooding risk will be present today, mainly due to afternoon showers and t-storms over interior to W and NW PR, with showers also affecting the eastern region today and tonight.

* Limited heat risk today for coastal and urban areas.

Short Term(Today through Monday)

Issued at 459 AM AST Sat Apr 11 2026

Since midnight, radar estimated rainfall accumulations showed minimal amounts over eastern to southeastern PR, including Vieques. Official and unofficial stations reported lows in the 70s across coastal areas, with some stations reaching up to 80 degrees, and in the 60s across interior PR. Current satellite derived precipitable water (PWAT) values indicate a band of moisture approaching from the east with values up to 1.65 in (at normal values for this time of the year). A broad surface high over the central Atlantic interacting with a frontal low over the western to central Atlantic will continue to result in E to ESE steering flow through the weekend; 925 wind speeds are forecast to continue to around 15 kts through the weekend. Another building high and a trough near the area will then result in winds backing to ENE and increasing to breezy to locally windy on Monday. PWAT values will increase from high end normal to above normal (PWAT values above 1.75 in) as the period progresses with model guidance suggesting a peak in PWAT (above 2 inches) Monday night into the long term period. Drier air in the mid levels will continue to intrude through Sunday afternoon, with Sunday night into Monday having deep columnar moisture. ESE flow will steer moisture and showers towards the region today and tomorrow Sunday.

For today, a limited (ponding of water in roads and poorly drained areas with a low chance of urban and small stream flooding) to elevated (flooding in urban areas, roads, small streams and washes with a low chance of isolated flash floods) flooding risk will is forecast. This is mainly due to afternoon convective showers and t- storms over interior to W and NW PR due to diurnal heating, local orographic effects and sea breeze convergence, with advective showers also affecting the eastern region. Afternoon showers can also develop downwind of the local islands and El Yunque. The ESE steering flow will also result in a limited heat risk, with 925 mb temperatures forecast to be warmer than normal, with highs reaching the upper 80s to around 90 at urban and coastal areas.

For tomorrow, Sunday, an enhanced pattern will be present with broader limited to elevated flooding risk forecast during the afternoon over W-NW PR due to convective showers and t-storms. Showers can also develop downwind of the local islands. Advective showers will also continue to reach windward sectors of the islands during the morning and overnight hours. At the same time a mid to upper level trough (with a surface reflection) is forecast to approach the region from the NE on Sunday, reaching the local islands late Sunday into Monday. Conditions will gradually become more wet and unstable as the short term period progresses. Galvez- Davison Index (GDI) values are forecast at up to 30 today, up to 40 on Sunday and up to 40 at most of the region on Monday. 500 mb temperatures will be at normal values today but cool to below normal values as the period progresses. 250 heights will also continue to decrease during the period and 850 to 500 relative humidities will increase to above normal values (possibly even 2 standard deviations above normal) as the period continues. Similarly other variables and general model guidance indicate these increasingly unstable conditions with shower and t-storm frequency increasing. This will ultimately promote an elevated flooding risk for most of the area during this period.

The first round of showers related to this trough are expected to reach the region Sunday night into early Monday. Showers and thunderstorms will then increase in frequency on Monday as the above mentioned deep moisture and the approaching trough continue to result in wet and unsettled conditions. Afternoon convection is forecast to be further enhanced on Monday resulting in showers and t- storms developing under the above mentioned NE steering flow over the interior, and moving into south-central, west and southwest Puerto Rico. The heaviest activity and the highest potential for flooding is still forecast for Monday night into Tuesday (long term period) as the trough axis reaches the islands. Model guidance continues to suggest that this activity will be particularly strong around St. Thomas, St John, Vieques, Culebra, eastern and northern Puerto Rico. The likelihood of urban and small stream flooding, isolated flash flooding, landslides and rapid river rises is increasing to start the workweek. For additional details, including rainfall expectations and hydrologic conditions Sunday onwards, refer to the latest Hydrologic Outlook (ESFSJU). A Flood Watch (FFASJU) will also likely be issued later today. Patchy fog over will be present during the overnight hours over areas of the interior. Satellite and model guidance also suggest low concentrations of Saharan Dust over the area and gradually decreasing during the period. Although Sunday could still see above normal 925 mb temperatures, these will decrease to normal and possibly below normal values as the period continues.

Long Term(Tuesday through Saturday)

Issued at 459 AM AST Sat Apr 11 2026

A wetter and unstable pattern will continue from Monday night into Tuesday as a deep-layered trough and upper-level low remain northwest of the region, supporting favorable conditions aloft. Deep tropical moisture will remain in place through midweek, with precipitable water values at times exceeding climatological maxima. Although some drying is possible late Wednesday into early Thursday, moisture is expected to increase again late week, maintaining a very moist pattern through at least Saturday.

The primary concern is an elevated to locally significant flooding risk, with the highest impacts expected Monday evening through Tuesday. Periods of heavy and repeated rainfall may result in urban and small stream flooding, isolated flash flooding, and localized landslides. A Hydrologic Outlook (ESFSJU) is currently in effect to highlight this potential. Rainfall amounts will be refined in later forecasts as confidence increases. While it is still too early for more specific flood headlines, a Flood Watch may be needed if conditions warrant.

Although activity may become somewhat less organized at times late week, sufficient moisture and instability will persist to support continued showers and thunderstorms with localized flooding impacts, especially over areas that become saturated earlier in the period. Confidence is moderate overall.

AVIATION

(06Z TAFS) Issued at 459 AM AST Sat Apr 11 2026

Mainly VFR conditions. ESE winds today, bcmg E by around 11/22Z. Light and variable winds for TJSJ/TJPS/TJBQ; winds up to around 10 kts for TIST/TISX through 11/13Z. Winds increasing to around 15 kts with higher gusts and sea breeze variations, before decreasing again after 11/23Z. VCSH/-SHRA to continue mainly over eastern/southern terminals from time to time throughout the period. TSRA in the VCTY or at JBQ at 11/17-22Z, this could promote brief MVFR conditions with lower cigs and reduced visibilities.

MARINE

Issued at 459 AM AST Sat Apr 11 2026

Moderate trades will prevail through the weekend as a broad surface high pressure builds from the Central Atlantic. A 4 to 5 feet northwest to north swell will continue to spread across the Atlantic waters and passages, building seas up to 6 feet for the next few days. Therefore, small craft should continue to exercise caution, mainly across the Atlantic waters. Another pulse of the northerly swell is expected on Sunday. By early next week, a frontal boundary and a pre-frontal trough will increase showers and thunderstorms across the regional waters. Increasing winds and seas building up to 7 feet are also expected, and Small Craft Advisories could be issued for the Atlantic waters during this period.

BEACH FORECAST

Issued at 459 AM AST Sat Apr 11 2026

Buoy data from Rincon and Arecibo indicated a NNW swell at 4 to 5 feet with wave periods of 11 to 12 seconds. This swell is expected to linger today across the Atlantic waters and passages, while another pulse arrives on Sunday. Therefore, a High Rip Current Risk is in effect for the northwest to northeast beaches of Puerto Rico and Culebra through at least Monday. Across the USVI and Vieques, a moderate risk of rip currents is expected to persist over the next several days, while a low risk will continue along the southern and more protected beaches of Puerto Rico. More information is available at weather.gov/beach.sju.

HYDROLOGY

Issued at 459 AM AST Sat Apr 11 2026

As a mid to upper level trough approaches from the northwest, the likelihood of flooding is increasing for late Sunday into the next workweek. An elevated flooding risk is forecast through at least midweek. The first round of showers are expected to reach the region Sunday night into early Monday. Showers and thunderstorms will then increase in frequency on Monday, but the heaviest activity is anticipated for Monday night into Tuesday. Although a level of uncertainty is still present, the days with the highest potential for flooding are Monday and Tuesday. However, above normal moisture will persist through late in the week. The likelihood of urban and small stream flooding, isolated flash flooding, landslides and rapid river rises is increasing. Residents and visitors are urged to remain alert to changing weather conditions and to avoid flooded roadways. A Hydrologic Outlook (ESFSJU) is currently in effect to highlight this potential. A Flood Watch (FFASJU) will also likely be issued later today.

SJU WATCHES/WARNINGS/ADVISORIES

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

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