textproduct: San Juan

KEY MESSAGES

Issued at 315 AM AST Thu Sep 17 2026

- The tropical wave to the west of Puerto Rico will continue to drive rainfall and thunderstorm activity over the US Virgin Islands and Puerto Rico throughout Thursday.

- Lingering moisture from the wave and a low levels of Saharan Dust are expected by Friday. With rainfall and thunderstorms are expected over the western side of Puerto Rico during the afternoon hours.

- Extreme heat risk is expected to gradually increase across the CWA, with risk levels expected to be limited today, moderate on Friday and significant by Saturday.

- There is a moderate risk of rip currents through Friday for some beaches across the US Virgin Islands and Puerto Rico.

Short Term(Today through Saturday)

Issued at 315 AM AST Thu Sep 17 2026

The past 24 hours have been active in terms of rainfall activity for the US Virgin Islands and Puerto Rico due to the passage of a tropical wave over the forecast area. Radar estimates show some municipalities across the eastern (e.g., Canovanas, Rio Grande, Gurabo, Juncos, San Lorenzo, Caguas, Cidra, Cayey, Las Piedras and Humacao), northwestern (e.g., Aguada, Anasco, Moca, San Sebastian and Las Marias), and interior (e.g., Adjuntas) regions of Puerto Rico receiving from 2-4 inches of rain. While the rain associated to the passage of the system does not put an end to the ongoing drought, it does provide some respite to to the Carraizo and La Plata Basins which are currently under observation and adjustment levels respectively. The streamflow of the rivers over the eastern side of Puerto Rico for today is much above normal, we might see some of the effects from the rainfall to the water levels in the days that follow as the river runoffs make their way to the dams.

The rainfall and thunderstorm activity is forecasted to continue today as the wave axis is currently to the east of Puerto Rico and continues to bring an influx of moisture and instability to the region. Precipitable water (PWAT) values continue to be from seasonal to above seasonal levels which is favorable for the development of rainfall activity. Breezy conditions along with the ESE windflow may also lead to the development of advective showers over eastern Puerto Rico during the morning hours. Mid-tropospheric temperatures will be warmer than normal, which may act as a limiting factor for the development of deep convection. Nonetheless, local effects (i.e., daytime heating, orographic effects, and sea breeze convergence) may couple with the instability brought by the wave to produce active periods of widespread rainfall and localized thunderstorm activity over the US Virgin Islands and Puerto Rico throughout the day. The areas that will see the highest rainfall activity are eastern, interior and northwestern portions of Puerto Rico. Nonetheless rainfall activity is expected across the entire CWA. The elevated risk for excessive rainfall means that flooding in urban areas, roads, small streams and washes along with a low change of isolated flash floods is possible. Given the enhanced rainfall received over the past 72 hours, we cannot rule out the possibility of landslides with increased risk over the regions of high terrain, as well as regions of the east, northwest and interior of Puerto Rico where soil remains saturated from recent rainfall activity.

While the highest PWAT values will be seen today lingering moisture from the tropical wave will persist into Friday. This will be accompanied by low Saharan Dust concentrations. An upper-level ridge will also make its way to the south of the US Virgin Islands and Puerto Rico. Warmer than average mid-tropospheric temperatures will also persist into Friday. These atmospheric conditions will act as limiting factors for the development of rainfall activity as they will lead to more stable mid-levels. Nonetheless we expect that the lingering moisture from the wave coupled to local effects will lead to passing showers particularly across northwestern Puerto Rico in the afternoon as well as advective showers over the east during the morning. The risk for excessive rainfall for Friday is also elevated. Conditions are expected to become more seasonal by Saturday. With a less saturated low-to-mid atmospheric column, rainfall is primarily forecasted for the west and interior of Puerto Rico during the afternoon hours driven by local effects and seasonal PWAT values.

The heat risk for today is limited and will gradually increase to moderate on Friday and Significant on Saturday. The significant level of heat risk affects anyone without effective cooling and/or adequate hydration. Impacts likely in some health systems, heat sensitive industries and infrastructure. We recommend that individuals be cautions when pursuing outdoor activities and to remain well hydrated as they go into the weekend.

Long Term(Sunday through Thursday)

Issued at 315 AM AST Thu Sep 17 2026

To start the next week, lingering tropical moisture will continue over the islands under an east-southeast steering flow. Precipitable water (PWAT) values will start at around 2 to 2.25 inches, high-end normal to above normal values for this time of the year, before becoming more spotty as patches of drier air filter in from Thursday onwards. Throughout the period, areas of low pressure and surface high pressure will move into the western Atlantic, sustaining a generally light to moderate east-southeast steering flow. An upper- level low to the northeast will be present to start the week and will continue to move southward and approach the islands. Current model guidance suggests that this system will be east and northeast of the islands by early next workweek and last through the long term period. As of the latest guidance, 500 mb temperatures are forecast at around -6 degrees Celsius while available moisture reaches the upper levels through at least midweek, before retreating to the mid levels. Uncertainty remains regarding this and the available moisture reaching the islands midweek onwards. Please continue to monitor the forecast for future updates. Light to moderate east- southeasterly winds will steer ongoing showers toward windward eastern areas, primarily during nighttime and morning hours. The positioning of an upper-level low to the east and southeast brings the potential for thunderstorm activity alongside these showers. In the afternoons, shower and thunderstorm development is forecast across interior and western Puerto Rico, as well as downwind of El Yunque, Vieques, Culebra, and the US Virgin Islands. Light to moderate steering flow will allow rainfall to linger, also maintaining a risk of flooding, particularly during the afternoon hours across interior to western Puerto Rico. Additionally, persistent moisture combined with the east-southeast wind flow may result in limited to elevated heat risks.

AVIATION

(06Z TAFS) Issued at 315 AM AST Thu Sep 17 2026

06Z TAFs

Mainly VFR conds early. SHRA/TSRA will increase again as the day progresses, leading to TEMPO MVFR conds across all terminals at times. Light and variable winds increasing to 15-20 kt with higher and becoming more easterly through 17/23Z. Light and VRB aft 17/23Z.

MARINE

Issued at 315 AM AST Thu Sep 17 2026

A departing tropical wave will continue to promote showers and thunderstorms today into tonight, producing locally hazardous marine conditions, including wind gusts, frequent lightning, reduced visibility, and higher seas. A surface high pressure system over the western Atlantic will then dominate the regional weather pattern, yielding moderate to locally fresh easterly winds. Mariners should remain alert, as conditions could rapidly deteriorate in and around stronger thunderstorms.

BEACH FORECAST

Issued at 315 AM AST Thu Sep 17 2026

A moderate risk of rip currents is expected along the northern and eastern beaches of Puerto Rico, Vieques, Culebra, and St Croix. As winds weaken by Friday into early next week, the rip current risk will drop to low across all local beaches. Meanwhile, a departing tropical wave will continue to promote showers and thunderstorms along and near coastal areas today. Beachgoers should remain weather aware and seek shelter if thunderstorms develop nearby.

SJU WATCHES/WARNINGS/ADVISORIES

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