textproduct: San Juan

KEY MESSAGES

Issued at 437 AM AST Thu Jan 1 2026

* Periods of unsettled and wet weather will continue through Friday, with moderate to heavy showers and isolated thunderstorms, increasing the risk of ponding of water on roads and localized urban and small-stream flooding, especially across western and interior Puerto Rico.

* A brief improvement in weather conditions is expected late Saturday into Sunday, with fewer showers and more stable conditions.

* Another wet and unsettled pattern is likely from late Monday into Tuesday as a cold front approaches, bringing increased rainfall potential, isolated thunderstorms, and a renewed flooding risk due to slow-moving showers.

* For U.S. Virgin Islands, the current trough and associated shower and thunderstorm activity will remain mainly north over the Atlantic waters, with generally calmer conditions locally and only brief passing showers expected through the short term.

Short Term(Today through Saturday)

Issued at 437 AM AST Thu Jan 1 2026

During New Years Eve night, mostly cloudy skies were observed across Puerto Rico and the U.S. Virgin Islands. Shower activity was detected by Doppler radar between the Mona Passage and the western coastal sections of Puerto Rico. After midnight, shower activity gradually dissipated, with mainly light rain observed and minimal rainfall accumulations. Nighttime temperatures were slightly warmer than seasonal normals due to abundant cloud cover across the islands. Across the San Juan metropolitan area, minimum temperatures ranged between 75 and 78 degrees. Winds remained generally light, below 5 mph.

Unstable weather will persist, mainly due to a mid to upper level trough moving over the region, which will enhance instability aloft and create very favorable conditions for thunderstorm development, supported by colder temperatures at the 500 mb level ranging between -8 and -10 degrees Celsius. At the same time, surface conditions will also become favorable as a deep column of moisture associated with a frontal boundary reaches the islands during the day. Given these factors, a mostly cloudy day is expected, with moderate to heavy showers developing during the afternoon and evening hours. High resolution model guidance indicates that the peak of shower activity will occur during the afternoon, continuing into the evening hours, particularly across interior sections. Similar mid to upper level conditions will persist on Friday, however, at the4 surfaced, a weak surface disturbance moves eastward across the area and winds shift to an east-northeast direction, although global model guidance, including the GFS and ECMWF, suggests reduced mid level moisture, which may limit overall shower coverage. On Saturday, mid to upper level conditions will become more stable as the trough shifts eastward, leaving the subsident side over the region, resulting in mostly fair weather conditions with only passing showers expected.

Long Term(Sunday through Thursday)

Issued at 437 AM AST Thu Jan 1 2026

The long-term forecast period will begin in the wake of the previous polar trough passage, with conditions trending toward a more seasonable pattern across the northeastern Caribbean. At the onset of the period, near-normal to slightly above-normal moisture is expected across the region, while weak mid- to upper-level ridging briefly establishes aloft. Low-level east-southeast flow will dominate, maintaining generally benign conditions with limited convective activity.

This relative lull will be short-lived. By late Monday into Tuesday, another amplifying polar trough is forecast to dig southeastward into the northeastern Caribbean. As this system approaches, upper- level dynamics will once again become increasingly favorable for convective development. Model guidance suggests the development of a 75-90 kt jet streak near the 250 mb level, enhancing upper-level divergence and ventilation over the region.

At the lower levels, an associated frontal boundary is expected to approach from the northwest, promoting moisture convergence across the local area. Precipitable water values are forecast to increase to above climatological normals, resulting in a deepening moist column. As a result, a renewed period of unsettled and wet weather is anticipated across the northeastern Caribbean, with increasing coverage of showers and isolated thunderstorms.

Considerable uncertainty remains regarding the exact placement of the most active region and organized convection. However, as the frontal boundary nears, surface winds are expected to weaken and become more variable. This may favor slower-moving showers and thunderstorms, increasing the potential for locally heavy rainfall accumulations and urban and small-stream flooding.

Following the passage of the polar trough and its associated frontal boundary late Tuesday into Wednesday, a drier and more stable air mass is expected to overspread the region. Subsidence aloft and decreasing moisture will lead to a gradual reduction in shower and thunderstorm activity through the latter portion of the forecast period. Trade winds are forecast to reestablish, maintaining more stable conditions and a reduced risk of convective development by Wednesday into Thursday.

AVIATION

(06Z TAFS) Issued at 437 AM AST Thu Jan 1 2026

VFR conditions are expected to prevail for most of the period, with brief MVFR conditions possible due to temporary reductions in ceilings and visibility associated with increased cloudiness and passing SHRA and isolated TSRA between 01/18Z and 01/22Z. Surface winds will remain light and variable through 01/13Z, becoming established from the east to east-southeast thereafter at 10 to 15 knots, with occasional higher gusts and sea breeze influences during the afternoon hours.

MARINE

Issued at 437 AM AST Thu Jan 1 2026

Light east southeast winds will continue through early today under the influence of a surface high over the central Atlantic, and an approaching frontal boundary from the western Atlantic. This front and associated upper level trough is currently promoting showers and thunderstorms over the Mona Passage and will continue to spread across the regional waters through at least Friday. Winds are expected to increase and turn more east to northeast through Saturday resulting in choppy seas at times.

BEACH FORECAST

Issued at 437 AM AST Thu Jan 1 2026

The long-period northerly swell that affected the region has subsided, resulting in improved local beach conditions. Despite this improvement, a moderate risk of rip currents will persist across the north-facing beaches of Puerto Rico and most beaches of the smaller islands. Therefore, Life-threatening rip currents remain possible along northern exposed beaches, particularly near reefs and jetties.

Southern beaches of Puerto Rico will continue to experience a low risk of rip currents due to more sheltered coastal conditions.

There is a limited potential for showers and isolated thunderstorms near the western and northern coastal waters of Puerto Rico, mainly during the afternoon and evening hours. Any nearby thunderstorms may produce lightning and brief gusty winds. Remember that lightning can strike several miles away from a thunderstorm. Beachgoers should seek shelter immediately if thunder is heard.

For localized and updated risk levels, visit weather.gov/beach/sju.

SJU WATCHES/WARNINGS/ADVISORIES

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