textproduct: San Juan

KEY MESSAGES

Issued at 233 PM AST Mon Aug 17 2026

* For the rest of the week, a heat threat will persist due to high temperatures and heat indices ranging between 108 and 111 degrees.

* For the rest of the day into Tuesday, a drier weather pattern is expected as a drier air mass dominates the region.

* No significant rainfall accumulations are anticipated; however, by Wednesday, an increase in moisture is forecast, which will increase the potential for showers and a limited risk of flooding, particularly across the western and eastern sections of Puerto Rico.

* Unlike yesterday, model guidance suggests a different pattern from Thursday into Friday, with much drier air dominating the region.

* A low to moderate risk of rip currents is expected across local beaches throughout the workweek.

Short Term(This evening through Wednesday)

Issued at 233 PM AST Mon Aug 17 2026

Tranquil and generally fair weather conditions will continue across most of the region through this evening and much of Tuesday as a relatively dry and stable air mass remains in place. Precipitable water (PWAT) values will remain near 1.25 to 1.50 inches through Tuesday, near the lower end of climatological values for this time of year. This dry air is particularly evident in the mid-levels, where 700-500 mb relative humidity remains well below normal. Therefore, shower activity should remain limited despite sufficient low-level moisture and daytime heating. Nevertheless, local and diurnal effects could promote isolated to locally scattered showers across western Puerto Rico each afternoon. Otherwise, mostly fair weather is expected across the U.S. Virgin Islands and eastern Puerto Rico. Hot conditions will remain the main concern across coastal and urban areas, where elevated heat indices are expected.

A transition to wetter and more unsettled weather remains possible from late Tuesday night into Wednesday as a vigorous tropical wave, previously designated as Invest 94L, interacts with a deep-layer trough. The trough is expected to pull the wave and its associated tropical moisture northward toward the region. However, uncertainty remains regarding how much of this moisture reaches the local islands. The GFS has remained consistent in bringing the bulk of the moisture across the forecast area, while the ECMWF keeps the deepest moisture north of the region.

If the wetter solution materializes, PWAT values could increase to around 2.0-2.3 inches, with a substantial increase in moisture through the mid-levels. This would favor periods of moderate to locally heavy showers across the U.S. Virgin Islands and eastern Puerto Rico, followed by additional afternoon activity across interior and western Puerto Rico. Localized urban and small-stream flooding could occur where persistent heavy rainfall develops. Stronger thunderstorms could also produce frequent lightning and locally strong, gusty winds.

Rainfall associated with this wave would be beneficial given the worsening drought conditions across portions of the region, particularly if the wetter GFS solution materializes. Nevertheless, while this event could provide some temporary relief, it is not expected to be sufficient to end the ongoing drought conditions. Continue to monitor subsequent forecasts, as relatively small changes in the evolution of the wave and trough could significantly affect Wednesday's rainfall coverage and amounts.

Long Term(Thursday through next Sunday)

Issued at 317 AM AST Mon Aug 17 2026

A generally quiet weather pattern is expected through most of the period, with no significant rainfall hazards anticipated. Moisture should gradually decrease Thursday into Friday, favoring brief passing showers and limited afternoon convection over western Puerto Rico. Heat will likely remain the main concern, with elevated heat risk possible where sufficient sunshine occurs.

Forecast confidence decreases toward the end of the period as some guidance suggests an open tropical wave approaching the northeastern Caribbean by Sunday or the following weekend. At this range, however, the signal remains highly uncertain and may partly reflect climatological influences in the model solutions rather than a well- defined system. Trends will continue to be monitored.

AVIATION

(18Z TAFS) Issued at 233 PM AST Mon Aug 17 2026

Mainly VFR conds expected across all TAF sites thru the period. Winds E-ESE at 10-15 kt with sea breeze variations and ocnl gusts up to 25 kt, becoming light/VRB overnight. SHRA/VCSH expected to increase aft 18/14Z, mainly across interior and wrn PR. VCTS cannot be ruled out at TJPS.

MARINE

Issued at 233 PM AST Mon Aug 17 2026

A subtropical surface high-pressure ridge centered across the western Atlantic will remain the dominant feature controlling the regional pattern. This system will maintain gentle-to-moderate easterly trade winds across the local waters for the next few days. Tranquil to moderate marine conditions will prevail through much of the workweek, with seas generally ranging between 2 and 5 feet across the Atlantic and Caribbean waters. Moisture from an approaching tropical wave is forecast to build by Wednesday, bringing a higher potential for scattered showers and thunderstorms over coastal waters, which will elevate the localized lightning hazard.

BEACH FORECAST

Issued at 233 PM AST Mon Aug 17 2026

There is a moderate risk of rip currents across the northern, eastern, and southern coastal areas of Puerto Rico, Culebra, and Vieques, as well as the eastern tip of St. Croix. Meanwhile, a low risk of rip currents is expected along the western sections of the region. Similar marine conditions, with a low-to-moderate risk of rip currents, will remain in place for the rest of the workweek. In addition, an increase in showers and thunderstorms is expected by Wednesday afternoon. This weather shift will raise the potential lightning threat across the coastal areas. Beachgoers and residents should remain cautious and stay informed on changing weather updates throughout the week.

FIRE WEATHER

Issued at 233 PM AST Mon Aug 17 2026

Critical fuel conditions continue across the islands, with extreme drought affecting portions of eastern Puerto Rico and the southern coastal plains. Dry conditions have prevailed through much of today, with low relative humidity, stressed vegetation, and dry fuels continuing to support elevated fire danger.

The greatest concern remains across southern and southwestern Puerto Rico, where fuels are exceptionally dry and recent fire activity has been frequent. Breezy easterly winds with occasional higher gusts this afternoon could support rapid fire spread and make any ongoing fires more difficult to control. Elevated fire danger also continues elsewhere across Puerto Rico, Vieques, Culebra, and the U.S. Virgin Islands.

Some relief remains possible by Wednesday as a tropical wave could bring an increase in showers and beneficial rainfall. However, uncertainty remains regarding rainfall coverage and amounts, and drought-stressed fuels will likely persist in areas that receive limited rainfall.

SJU WATCHES/WARNINGS/ADVISORIES

PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007- 008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.