Area Forecast Discussion

KEY MESSAGES

Issued at 253 AM AST Tue Sep 15 2026

- Showers and localized thunderstorms are expected over the US Virgin Islands and Puerto Rico during the morning hours and for the interior and western portions of Puerto Rico during the afternoon.

- A tropical wave is expected to reach the region by Wednesday afternoon increasing the potential for the development of deep convection into Thursday.

- With increased chances for rainfall expected today, Wednesday and Thursday, the chances for flooding will be the highest on Thursday.

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

Short Term(Today through Thursday)

Issued at 253 AM AST Tue Sep 15 2026

Atmospheric conditions for today are expected to be favorable for the development of showers and thunderstorms. An upper-level low located to the west-northwest of the islands continues to drive upper level divergence across the CWA bringing instability to the region. Additionally, a surge in moisture at low levels to the south of the region continues to maintain seasonal precipitable water (PWAT) values (1.75-2.2 inches). With moist low levels and sub- saturated mid-levels, along with with mid-tropospheric temperatures nearing the -8 degrees Celsius, the thermodynamic environment also favors the potential for the development of deep convection. Thus, we forecast that these synoptic, thermodynamic and local effect features (i.e., diurnal heating, local orographic effects, and sea- breeze convergence) will lead to the development of widespread shower and localized thunderstorm activity primarily focused on the interior and western regions of Puerto Rico during the afternoon hours. Furthermore, a surge in PWAT values located to the southeast of the windward islands, in combination with the ESE wind flow, may also favor the development of advective showers over the region during the morning hours with a focus over the US Virgin Islands, windward waters, and eastern-southeastern sectors of Puerto Rico. Thus, the excessive rainfall risk for today is elevated. This means that flooding in urban areas, roads, small streams, and washes is possible, with a low chance of isolated flash flooding. This elevated risk for excessive rainfall will persist for the remainder of the short term forecast.

By early Wednesday morning a drier airmass will reach the forecast region bringing PWAT values down (1.33-1.7 inches). While this may limit shower activity during the early morning, the fair conditions will be short-lived as by late Wednesday morning into the afternoon a moist air mass associated with a tropical wave will make its way into the CWA bringing above seasonal PWAT values (2.0-2.5 inches) to the region. The tropical wave in combination with the upper-level low to the west-northwest, low-level moisture surge to the south, ESE wind flow, and local effects will increase the potential for the development of convection by Wednesday afternoon with the primary focus being across the US Virgin Islands, as well as the eastern, interior and northwestern sectors of Puerto Rico. However, the bulk of the activity associated to the tropical wave is expected by Thursday. With the synoptic and thermodynamic environment favoring the development of widespread convection and localized thunderstorm activity across eastern, interior and northwestern Puerto Rico.

With increased cloud coverage expected through Thursday, the heat risk for today and the remainder of the short term forecast remains limited. This level of heat affects primarily those individuals with extreme sensitivity to heat, especially when outdoors without effective cooling and/or adequate hydration. We recommend that individuals be cautions when pursuing outdoor activities and to remain well hydrated.

Long Term(Friday through Tuesday)

Issued at 253 AM AST Tue Sep 15 2026

For the first part of the long-term forecast period, model guidance suggests that moisture will remain generally near or above climatological normals. From Friday, residual moisture associated with the previous tropical wave will linger across the region, with precipitable water values remaining mainly between 1.7 and 2.0 inches, which is slightly above climatological values. Although the NASA Goddard Earth Observing System Model V5 suggest some light concentrations of Saharan Dust particles. Mid-level relative humidity will also remain relatively high, staying mostly near the 75th percentile and at times approaching +2 standard deviation threshold, supporting sufficient moisture for shower development. However, a broad mid-level ridge over the western Atlantic may enhance relatively warm 500 mb temperatures in the -5 degrees Celsius range, favoring some mid-level stability and limiting widespread deep convection. Easterly winds will prevail through Saturday, with some upper-level cloudiness also expected. Therefore, passing showers will remain possible across windward areas, followed by afternoon convection mainly across interior and western Puerto Rico due to local and diurnal effects.

From Sunday through Tuesday, a surface high pressure building across the central Atlantic will promote a gradual veering of the winds, becoming more southeasterly. Model guidance suggests that moisture will remain available across the region, with precipitable water generally near or above the 50th percentile and occasionally approaching the 75th percentile. This moisture will be partly associated with an induced surface trough over the central Atlantic, north of the region, which will promote periods of cloudiness and showers across the area. At the mid-levels, marginally favorable conditions will persist, with 500 mb temperatures near -6 degrees Celsius potentially supporting a few thunderstorms where sufficient moisture and instability are present. In general, the combination of sufficient moisture, moderate southeasterly winds, and local effects will maintain the potential for passing showers and afternoon convection through early next week. Additionally, the available low- level moisture and southeasterly winds will favor warmer temperatures, particularly along northern coastal and urban areas. Therefore, a limited to locally significant heat threat can be expected during this portion of the period. Residents are urged to stay hydrated and follow recommendations from local health officials.

AVIATION

(06Z TAFS) Issued at 253 AM AST Tue Sep 15 2026

Mainly VFR conditions are expected to prevail across all terminals during the next 24 hours. However, brief -SHRA possible at TJSJ/TIST/TISX thru 15/11z. Locally induced SHRA/TSRA developing over the interior and Western PR may lead to tempo MVFR conds at TJBQ/TJPS btw 15/18-22z. Mostly VCTS expected at TJSJ btw 15/16z-20z. East winds 10-15 kt with sea breeze variations and stronger gusts at times expected aft 15/13z.

MARINE

Issued at 253 AM AST Tue Sep 15 2026

A weak surface high north of the region and lower pressure across the southwestern Caribbean will maintain moderate easterly trade winds through midweek. Seas will generally range from 3 to 5 feet, with the highest seas across the offshore waters and local passages. A tropical wave over the central tropical Atlantic will continue westward and approach the eastern Caribbean Wednesday into Thursday, increasing the frequency of showers and thunderstorms across the regional waters. Moderate easterly trade winds are expected to persist into the weekend.

BEACH FORECAST

Issued at 253 AM AST Tue Sep 15 2026

A moderate risk of rip currents will continue today along much of the north-facing coast of Puerto Rico, becoming more localized Wednesday and Thursday. Another increase is expected along the north coast on Friday, with generally lower risk conditions over the weekend.

Afternoon thunderstorms may affect western Puerto Rico, while a tropical wave will increase showers and thunderstorms Wednesday into Thursday. Beachgoers should seek shelter immediately if thunder is heard.

Hot and humid conditions will also persist. Stay hydrated and limit prolonged sun exposure during the hottest part of the day.

SJU WATCHES / WARNINGS / ADVISORIES

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