textproduct: San Juan

KEY MESSAGES

Issued at 314 AST Tue Dec 16 2025

* A prefrontal trough north of the area and a low level perturbation moving over the islands will continue to promote showers and isolated t-storms, steered by ESE flow, during the afternoon hours over Puerto Rico.

* Showers will move across the US Virgin Islands at times tonight, producing moderate to locally heavy rainfall.

* A long-period northerly swell will arrive late tonight, creating a high risk of rip currents along the north-facing beaches in PR (with High Surf Conditions likely tomorrow) and then along the rest of the north-facing beaches of PR/Northern USVI tomorrow through at least Thursday.

* Marine conditions will also deteriorate tonight through at least Thursday, prompting Small Craft Advisories over the Atlantic waters.

Short Term(This evening through Thursday)

Issued at 314 PM AST Tue Dec 16 2025

Current satellite derived precipitable water (PWAT) values indicate around 1.75 inches, above seasonal values, as moisture from a low level perturbation continues to be steered over the area under ESE steering flow. This ESE steering flow helped promote seasonal 925 mb temperatures, reported highs until now have reached the mid to upper 80s across urban and coastal sectors, with some stations reporting in the low 90s. A front, well north of the area is also inducing a prefrontal trough to our north. The low level perturbation brought showers over the USVI to start the day and then promoted afternoon showers and isolated t-storms over Puerto Rico, particularly over interior to north/northwestern PR, being steered by the moderate ESE flow.

Afternoon showers are also being detected downwind of the local islands and El Yunque towards eastern PR and over the southern plains. This afternoon activity will gradually dissipate and or move offshore this evening. However, showers will continue to be steered towards the eastern region tonight. Tomorrow, patches of more humid air due to low level perturbations will continue to filter into the islands with PWAT values around 1.6 to 1.75 inches. An upper level trough will also continue to move north of the area, aiding in instability. ESE flow will continue tomorrow as the result of two surface highs northwest and northeast of the islands, and their interaction with the prefrontal trough. This steering flow, sea breeze convergence and local effects, will promote afternoon shower activity mainly across western PR and downwind of El Yunque and the local islands, prompting a limited flooding risk. Passing showers over the eastern region are expected throughout the day and early tonight. Current model guidance has a drier air mass then moving over the islands late tomorrow night into Thursday. Another patch of more humid air is forecast to arrive on Thursday under more easterly steering flow (as the western Atlantic high establishes itself north of the region), once again increasing PWAT values to high end normal to above normal values to end the period. By Thursday, the upper level trough, however, will be further northeast and will continue moving away from the local area. Available moisture will continue to steer showers towards the eastern region, with afternoon showers also over interior to western PR. 925 temperatures will continue at seasonal to high end seasonal values.

Long Term(Friday through next Monday)

Issued at 426 AM AST Tue Dec 16 2025

Mid- to upper-level ridging will dominate the pattern at the beginning of the long-term period, maintaining generally stable conditions across the northeastern Caribbean. Subsidence associated with the ridge will limit vertical development for the most part, while mid-level relative humidity remains near climatological values, occasionally dropping to very dry levels near or below 10 percent. This will tend to cap convection and keep shower activity limited early on.

At the lower levels, precipitable water values will remain somewhat near normal for most of the period. Patches of moisture will continue to stream across the area from time to time, resulting in passing showers, mainly across windward sections and during the overnight and early morning hours. Rainfall amounts should remain light and localized with minimal impacts.

Toward the latter portion of the long-term forecast, the mid-level ridge begins to erode as a polar trough exiting the eastern U.S. seaboard approaches from the northwest. This will introduce cooler air aloft, with 500 mb temperatures falling into the -7 to -8 C range. As a result, lapse rates will steepen, and the environment will become increasingly unstable. At the surface, an associated frontal boundary is expected to approach from the northwest late in the forecast window. Columnar moisture will begin to increase, with PWAT values trending upward with this setup. The combination of cooling aloft, increasing moisture, and improved forcing suggests a more unsettled pattern developing heading into the Christmas period.

This evolving setup could support a higher potential for thunderstorms and periods of locally heavy rainfall, increasing the risk for flooding in prone areas. Confidence remains lower at this time, given that these features occur late in the forecast window, where uncertainty in timing and strength remains higher. Subsequent forecasts will continue to refine the potential impacts, if any.

AVIATION

(18Z TAFS) Issued at 314 PM AST Tue Dec 16 2025

The cold front moving eastward across the Western Atlantic with a prefrontal trough interacting with an Azores surface high will continue to promote a moderate to locally fresh easterly winds across the islands through tonight. The cold front will approach the northeast Caribbean by midweek and remain off to the northwest of the islands, bringing trade wind perturbations across the islands, resulting in showers and afternoon to early evening thunderstorms. A long period northerly swell will arrive across the local waters late tonight into Wednesday, deteriorating marine and coastal conditions. Thus, Small Craft Advisories will be in effect for the middle of the week.

MARINE

Issued at 314 PM AST Tue Dec 16 2025

The cold front moving eastward across the Western Atlantic with a prefrontal trough interacting with an Azores surface high will continue to promote a moderate to locally fresh easterly winds across the islands through tonight. The cold front will approach the northeast Caribbean by midweek and remain off to the northwest of the islands, bringing trade wind perturbations across the islands, resulting in showers and afternoon to early evening thunderstorms. A long period northerly swell will arrive across the local waters late tonight into Wednesday, deteriorating marine and coastal conditions. Thus, Small Craft Advisories will be in effect for the middle of the week.

BEACH FORECAST

Issued at 314 PM AST Tue Dec 16 2025

A long period northwesterly swell will deteriorate marine and coastal conditions over the Atlantic waters and coastline by tonight through at least Thursday. The northern PR coastline will have a high risk of rip currents tonight from Aguadilla to Fajardo. By tomorrow, Rincon to Fajardo, Culebra and the northern USVI will have a high risk of rip currents. We encourage beachgoers to monitor the beach forecast, as High Surf Conditions are also possible and a High Surf Advisory can be issued later this afternoon for tomorrow, pending observations from offshore buoys. Dangerous breaking waves, which could promote life- threatening coastal conditions are forecast for tomorrow, Wednesday.

SJU WATCHES/WARNINGS/ADVISORIES

PR...High Rip Current Risk from 6 PM AST this evening through late Wednesday night for PRZ001-002-005-008.

High Rip Current Risk from Wednesday morning through Wednesday evening for PRZ010.

High Rip Current Risk from Wednesday morning through late Wednesday night for PRZ012.

VI...High Rip Current Risk from Wednesday morning through late Wednesday night for VIZ001.

AM...Small Craft Advisory from midnight tonight to 6 AM AST Thursday for AMZ711.

Small Craft Advisory from 6 AM Wednesday to midnight AST Wednesday night for AMZ712.