AFDSJU Area Forecast Discussion National Weather Service San Juan PR 433 AM AST Wed Dec 31 2025 ...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST... .KEY MESSAGES... Issued at 424 AM AST Wed Dec 31 2025 * The forecast for today " New Year`s Eve" into Thursday calls for a more unsettled pattern as an upper- level trough and associated frontal boundary approach from the west. * A more unsettled pattern late in the week, with increasing moisture and instability aloft for tomorrow * For today,life-threatening rip currents will continue to prevail across the north-facing beaches of Puerto Rico, Culebra, and the northern U.S. Virgin Islands. Improving conditions are forecast from tomorrow. * For the U.S. Virgin Islands, some showers are forecast, however the well developed showers will remain west of the islands. && .Short Term(Today through Friday)... Issued at 424 AM AST Wed Dec 31 2025 Today through Friday... The short-term forecast remains on track, beginning with relatively stable and pleasant conditions today, followed by a gradual transition toward a more unsettled pattern late Wednesday night through Friday as a deepening polar trough crosses the northeastern Caribbean. For today, mostly fair weather is expected across much of the forecast area. The low-level flow will remain from the east- southeast, resulting in mostly light to moderate winds. Only isolated brief showers are expected, mainly across portions of western Puerto Rico during the afternoon due to local and diurnal effects. From tonight onwards, a deepening polar trough will advance eastward across the region, leading to increasingly favorable upper-level dynamics. Model guidance shows strengthening ventilation aloft associated with a 60-80 kt jet at 250 mb moving over the area. In addition, mid-level cold air advection will lower 500 mb temperatures to around -9 to -10 C, particularly on Thursday (New Years Day), resulting in steeper lapse rates and enhanced instability aloft. At the low to mid levels, tropical moisture continues to be drawn northward, with GOES-19 PWAT imagery already showing higher moisture values west of Puerto Rico. Precipitable water values are forecast to increase to near or slightly above climatological normals, while 700-500 mb relative humidity rises to well above two standard deviations above normal. Despite this, the latest model guidance and ECMWF Extreme Forecast Index indicate that the highest moisture content and most organized convective activity will remain primarily west and north of the forecast area; over Hispaniola, the Mona Passage, and the Atlantic waters. Puerto Rico and the U.S. Virgin Islands are expected to remain on the eastern and southeastern periphery of the most active region. Additionally, persistent dry air intrusions in the 850-700 mb layer noted in several model cycles may somewhat inhibit convective development locally. As a result, widespread thunderstorm activity is not anticipated. However, sufficiently low-level moisture combined with increasing instability aloft and favorable upper-level dynamics could still lead to localized shower activity and isolated thunderstorms, particularly when enhanced by local island effects. From Thursday into Friday, shower coverage is expected to increase somewhat, particularly during the afternoon and evening hours across western and interior sections of Puerto Rico, driven by diurnal heating, sea breeze convergence, and local effects. Isolated showers and thunderstorms may also develop during the overnight and early morning hours over the surrounding waters, occasionally brushing coastal areas, especially along the Mona Passage and Atlantic waters. Activity across the U.S. Virgin Islands is expected to remain more limited and mainly in the form of passing showers. Overall, the short-term period will feature a transition from todays relatively stable conditions toward a more unsettled pattern late in the week, with increasing moisture and instability aloft. While the greatest impacts are expected to remain over areas west and north of the forecast region, localized heavy rainfall remains possible. Any area experiencing prolonged rainfall could see limited to moderate flooding, including urban and small stream flooding, which may create hazardous travel conditions during the holiday period. && .Long Term(Saturday through Wednesday)... Issued at 424 AM AST Wed Dec 31 2025 From Saturday into Sunday, weather conditions will remain dependent on a high-pressure system building over the central Atlantic. On Saturday, surface winds will remain from the east, veering on Sunday more from the southeast to south, and eventually from the west with the approach of a pre-frontal boundary late. At the upper levels, conditions will rapidly improve as the subsidence side of the upper-level trough moves eastward, allowing a more stable pattern to dominate. At 500 mb, a ridge will move in, allowing warmer temperatures of 6 to -5 degrees. Although stable conditions will persist at the mid to upper levels, at the surface, as surface winds vary, shallow patches of moisture will support some morning showers across the eastern sections, with afternoon showers developing across the interior and western Puerto Rico. According to the global model guidance, precipitable water values will remain within the climatological normals, ranging from 1.5 to 1.7 inches. These surface conditions and the local mountain effect might aid some very short-lived showers at the peak of the afternoon on Saturday and Sunday. Conditions will rapidly deteriorate from late Monday into Tuesday as a deep mid- to upper-level trough moves across the forecast area. This mid- to upper-level feature will enhance very unstable weather conditions across the islands, with colder mid-level temperatures ranging from 9 to 10 degrees. In addition to these very favorable conditions for thunderstorm activity, the GDI index also shows an increase in the potential for scattered showers late Monday into Tuesday. Furthermore, 850500 mb relative humidity values will spike well above climatological norms, surpassing two standard deviations by Tuesday. Although changes are highly likely, residents and visitors are urged to stay tuned for further updates, especially given the Three Kings Day celebration on Tuesday. && .AVIATION... (06Z TAFS) Issued at 424 AM AST Wed Dec 31 2025 Mainly VFR conds exp across all TAF sites thru the fcst pd. SHRA/VCSH psbl at times, mainly across western PR terminals. VCTS included at TJBQ late tonight as convective activity over the western waters may approach the area. Widespread TSRA not anticipated. Sfc winds light and vrb overnight, becoming E-SE 812 kt aft 1315Z with higher gusts and sea breeze variations. && .MARINE... Issued at 424 AM AST Wed Dec 31 2025 A weakening northerly swell will continue to result in brief hazardous marine conditions, especially across offshore waters during the day, improving in the night hours. A surface high pressure system moving eastward from the western into the central Atlantic, interacting with the remnants of an old frontal boundary, will maintain a light east-southeasterly wind flow across the region. As the frontal boundary approaches, increased moisture will increase the potential for showers and isolated thunderstorms, as well as localized hazardous marine conditions. During the period, pulses of long-period northerly swell will continue to propagate across the Atlantic waters and Caribbean passages through the middle of the week. Small craft operators are urged to exercise caution, particularly across the exposed Atlantic waters and Caribbean passages. && .BEACH FORECAST... Issued at 424 AM AST Wed Dec 31 2025 For the rest of the day, energy from the weakening northerly swell will continue to pose a high risk of rip currents across northern coastal areas of Puerto Rico and Culebra until 6 PM this evening. For the southern coastal areas and the western side of Puerto Rico, including Vieques, and the U.S. Virgin Islands, there will be a moderate rip current. Although conditions are forecast to slightly improve, continuous pulses of energy from further swells will remain near the coast, resulting in hazardous coastal conditions. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...High Rip Current Risk through this afternoon for PRZ001-002-005- 008-012. VI...High Rip Current Risk until 6 AM AST early this morning for VIZ001. AM...None. && $$