textproduct: San Juan

KEY MESSAGES

Issued at 132 PM AST Mon Dec 22 2025

* Marine and beach conditions will deteriorate by mid week and may extend for the weekend, becoming hazardous for small craft and beachgoers.

* Shower and thunderstorm activity will increase tomorrow, leading to ponding of water over roadways and poorly drained areas, gusty winds, and localized flooding over northern and eastern Puerto Rico.

* Below normal temperatures will likely enhance patchy fog across interior sections of Puerto Rico, reducing visibility and becoming hazardous for drivers by the end of the workweek.

Short Term(This evening through Wednesday)

Issued at 132 PM AST Mon Dec 22 2025

Variable weather conditions persisted during the early morning hours as a disturbance, and northeasterly wind flow resulted in a cool advective pattern, with showery weather affecting windward coastal areas of the islands. However, showers moved fairly quickly, leaving less than half an inch of rainfall. GOES-19 PWAT imagery shows a drier slot that will move over the region this afternoon and evening. Therefore, the frequency of showers will diminish for the rest of the day, although isolated to scattered showers are expected to develop over southwestern Puerto Rico this afternoon with no substantial impacts.

The remainder of the short-term forecast will continue to be dominated by the presence of a polar trough at upper levels. All ingredients necessary for instability aloft will be in place, as 250 mb height fields and 1000-500 mb thicknesses are expected to drop well below normal. Cold air advection will promote 500 mb temperatures cooling to between -9 and -10 degrees Celsius, which is at or below the 10th percentile of climatological normals. This setup will promote steep 700-500 mb lapse rates. Additionally, favorable jet dynamics will move over the forecast area, with a 65-80 kt jet at 250 mb promoting enhanced ventilation aloft with increased 0-500 mb bulk shear, a pattern often associated with organized convection.

However, the limiting factor will be available moisture across the northeastern Caribbean. The associated frontal boundary is currently northwest of the area and will gradually sink southward. Unlike previous model cycles that indicated abundant columnar moisture, recent runs now show fragmented pockets of moisture moving across the region from time to time, with PWAT values remaining near normal to below normal through Wednesday. Additionally, the northeasterly steering wind flow will keep temperatures below normal, which could reduce afternoon surface heating needed to fuel deep convection. As a result, any convection that develops is expected to be localized rather than widespread. Relatively fast steering winds will also limit rainfall accumulations, even in areas experiencing heavier showers.

Therefore, expect showery weather during the night and morning hours, with isolated thunderstorms developing mainly over the regional waters. Some of these thunderstorms could move close to coastal areas, particularly over the Atlantic waters. During the afternoon, showers and a few thunderstorms may develop over interior and southwestern Puerto Rico. Overall, a limited flooding threat is expected, mainly consisting of ponding of water on roads and in poorly drained areas.

Long Term(Thursday through next Sunday)

Issued at 530 AM AST Mon Dec 22 2025

Current model guidance suggest precipitable water (PWAT) values of around 1.4 inches to start the period. A series of mid to upper level troughs will move north of the region through the Friday, while a cold front approaches and likely moves over the region on Thursday (Christmas Day). Although they start an upward trend, instability is evident as 500 mb temperatures will still be below normal on Thursday at -10 to -8 degrees Celsius. A limited flooding risk will be present for most of the region, as the frontal boundary moves over the region, resulting in showers and possible isolated t-storms under north-northeast steering flow. Model guidance has been fairly consistent with each run regarding the approach of these features. With uncertainty remaining in the exact impacts across Puerto Rico and the U.S. Virgin Islands. As drier air behind the front will promote improved weather conditions with a cooling trend continuing Friday through Monday, 925 mb temperatures are forecast to be below normal for this period. Lows are forecast in the 50s to low 60s across higher elevations of PR, in the upper 60s to mid 70s across lower elevations of PR, and in the low to mid 70s across the USVI. Highs are forecast in the around the mid 80s across lower elevations of the islands, with isolated/urban areas a bit higher. Although PWAT values are forecast at around to below an inch, isolated patches of moisture will reach the area and promote passing showers from time to time.

AVIATION

(18Z TAFS) Issued at 132 PM AST Mon Dec 22 2025

VFR conds prevail at most TAF sites thru the fcst pd. SHRA/VCSH will continue to affect mainly TJSJ, TIST and TISX at times, with brief MVFR cigs/vsbys psbl, especially ovrngt and early mrng. Aft 23/14Z, SHRA may develop ovr interior and SW PR, with limited impacts expected. Winds ENE 6-15 kt, becoming locally gusty near SHRA.

MARINE

Issued at 132 PM AST Mon Dec 22 2025

A building surface high over the western Atlantic and a frontal low over the north-central Atlantic will promote moderate east to northeast winds to start the period. A fading northeasterly swell will promote seas up to 6 feet for the offshore Atlantic Waters for the rest of today; small craft should continue to exercise caution.

A series of surface highs and frontal lows will continue moving into the Atlantic during the period, resulting in moderate to locally fresh east to northeast winds through at least Wednesday, becoming moderate once again by Friday. Marine conditions will briefly improve tonight into early Tuesday; nevertheless, a long-period northerly swell, from one of the above mentioned frontal lows, will arrive and spread across the Atlantic waters and passages. This will result in choppy to rough seas over the offshore Atlantic Waters by Tuesday afternoon, becoming hazardous for small craft. These conditions will continue through the workweek as seas build to 6 to 9 feet and spread to the nearshore Atlantic Waters and local Passages.

BEACH FORECAST

Issued at 132 PM AST Mon Dec 22 2025

Beach conditions remain suitable for beachgoers, with moderate rip currents on the northern and eastern beaches of Puerto Rico, including Vieques and Culebra, as well as the U.S. Virgin Islands. Nevertheless, beachgoers are encouraged to exercise caution as life- threatening rip currents are possible along the surf zone. Although the risk will remain moderate, a long-period northerly- northeasterly is expected to arrive by Wednesday and spread across the local waters and passages, deteriorating beach and marine conditions. Hence, the risk of rip currents will likely be high on Wednesday, particularly along the north and east-facing beaches of Puerto Rico, Culebra, and the U.S. Virgin Islands. The risk is expected to remain high as pulses of another long-period northerly swell will deteriorate once again the beach conditions by the weekend. Due to previous swell activity across the islands, and large breaking waves expected, High Surf Advisories cannot be ruled out. Residents and visitors are encouraged to review the beach forecast at https://www.weather.gov/beach/forecast?site=sju&action= and stay tuned for the next updates.

SJU WATCHES/WARNINGS/ADVISORIES

PR...None. VI...None. AM...Small Craft Advisory from noon Tuesday to midnight AST Thursday night for AMZ711.