AFDSJU Area Forecast Discussion National Weather Service San Juan PR 100 PM AST Thu Mar 12 2026 ...New SHORT TERM, AVIATION, MARINE, BEACH FORECAST, FIRE WEATHER... .KEY MESSAGES... Issued at 100 PM AST Thu Mar 12 2026 * Flooding in urban areas, on roads, and along small streams is possible this afternoon across the interior and western Puerto Rico due to shower activity and the possibility of isolated thunderstorms. * A drier trend is expected by late Friday into Saturday, although localized showers may still develop during the afternoon hours due to local effects. * Increasing winds are expected on Sunday into next week, leading to hazardous marine and beach conditions. * Across the U.S. Virgin Islands, occasional passing trade-wind showers will continue, with a drying trend expected by late Friday into Saturday. && .Short Term(This afternoon through Saturday)... Issued at 100 PM AST Thu Mar 12 2026 Today`s observed morning sounding at San Juan (TJSJ) shows low- level easterly winds of 15 to 25 knots and medium to high moisture content, with a precipitable water (PWAT) of 1.65 inches. However, a strong trade-wind inversion is present near the 850mb level, limiting instability (CAPE of 533 J/kg), which could inhibit thunderstorm development. Based on this setup, the available moisture content is expected to support passing showers that favor the USVI and eastern areas of PR through early this afternoon, before fueling afternoon showers across western and southwestern Puerto Rico. Hi-res NBM and ECMWF ensemble models show rainfall accumulations generally between 1.00 and 2.00 inches across the interior and western sections, while the USVI and eastern PR will see lighter amounts ranging from 0.25 to 0.50 inches. Having said that, this morning`s sounding also indicated a strong upper-level jet of 90 knots, and isolated thunderstorms cannot be ruled out with this afternoon`s shower activity, and locally higher rainfall amounts are possible. Leading to urban and small stream flooding. A noticeable drying trend will begin late Friday and continue on Saturday. Global guidance indicates a sharp drop in moisture, with precipitable water values falling below 1.25 inches and mid-level relative humidity (700mb-500mb) dropping below 20%. Low-level winds are also expected to decrease slightly during this time. Consequently, hi-res precipitation data show a reduction in overall shower coverage. However, localized afternoon showers will still produce roughly 0.50-1.50 inches over western Puerto Rico on Friday. At the same time, the U.S. Virgin Islands and eastern Puerto Rico remain mostly on the dry side of the model solutions, with minor rainfall accumulations. Regardless, the proximity of the upper jet segment can enhance early-morning showers across the USVI and eastern PR, before a drier air mass moves in later in the day. By Saturday, the drier airmass will prevail over the region, and stronger trades are expected to return. Promoting overall breezy and fair weather conditions across the islands, with limited shower development. Therefore, expect mostly sunny skies and stable conditions to start the weekend. && .LONG TERM(Sunday through Thursday)... Issued at 155 AM AST Thu Mar 12 2026 A broad surface high will move into the western Atlantic and continue building, promoting increased E to ESE winds. 925 mb wind speeds will be above normal to briefly 2 standard deviations above normal, resulting in breezy to locally windy conditions to start the next week. Unsecured items could blow around. A limited wind risk will persist during the long term period with an elevated wind risk possible on Sunday and Monday. Although an upper trough will move northwest of the islands on Monday and an upper jet over the area can provide ventilation, 500 mb temperatures will be warmer compared to the short term period, at -7 to -6 degrees Celsius, while 700 to 500 mb lapse rates will be at below normal to low end normal values for this time of the year. This will promote more stable conditions and limit vertical shower development. However, the breezy to windy E to ESE flow will steer showers and patches of moisture towards windward sectors with diurnal heating promoting afternoon convective showers over interior to northwestern PR, as well as lines of showers towards the USVI, Vieques and Culebra to E and ESE PR. Patches of moisture arriving towards the islands will maintain precipitable water (PWAT) values around normal values (with some variability related to the moist and dry patches), model guidance suggests that the most broad moisture patch will arrive late Sunday into Monday. 925 mb temperatures will remain at normal values but under a general warming trend with less cloud cover and the E to ESE flow. Patchy fog during the overnight hours at sectors of the interior. Low concentrations of saharan dust will also be steered over the region, particularly on Sunday and Monday. && .AVIATION... (18Z TAFS) Issued at 1157 AM AST Thu Mar 12 2026 Mainly VFR conditions are expected to prevail across all terminals through most of the fcst period. However, locally induced SHRA and possible iso TSTMs over the western interior of PR may lead to tempo MVFR conds thru 12/22z at TJPS/TJBQ, although VCTS is likely. Elsewhere, trade wind SHRA will move at times, causing brief periods of -RA and MVFR cigs. The 12/12z TJSJ sounding indicated east to east-southeast winds up to 23 kt blo 7000ft. && .MARINE... Issued at 100 PM AST Thu Mar 12 2026 As the pressure gradient weakens across the northeastern Caribbean, winds will remain gentle to moderate from the east to northeast with seas of 3 to 5 feet. These improving marine conditions will persist through today and into the early part of the weekend. However, another area of high pressure building across the northwestern Atlantic by late Saturday into early next week will tighten the pressure gradient. Winds are expected to increase (fresh to strong trades) Saturday night into Sunday, resulting in deteriorating marine conditions with rough and choppy seas across our regional waters and local Caribbean passages. Small Craft Advisory conditions are likely from late in the weekend into early next week, with seas building to around 6 to 8 feet. && .BEACH FORECAST... Issued at 100 PM AST Thu Mar 12 2026 A moderate risk of rip currents remains in place along the north- and east-facing beaches of Puerto Rico, Culebra, and St. Croix, meaning that life-threatening rip currents are possible in the surf zone tonight. A low risk remains elsewhere, however, keep in mind that life-threatening rip currents often occur in the vicinity of groins, jetties, reefs, and piers. At the moment, the forecast shows this same pattern persisting at least through Saturday night, followed by deteriorating conditions beginning Sunday as the moderate risk expands to most beaches and a high rip current risk likely develops by Monday onward, along with breezy to windy conditions. Residents and visitors are encouraged to monitor the forecast for any updates or adjustments. For additional information and location-specific rip current details, please visit: weather.gov/beach/sju. && .FIRE WEATHER... Issued at 100 PM AST Thu Mar 12 2026 The KBDI in Cabo Rojo slightly decreased to 660 compared to yesterday, but it is still above the critical fire weather threshold. Guanica is in 520 and Camp Santiago 374. Before noon, RH values decreased to low the low 50s in Cabo Rojo and even winds are lighter, some period of gusts up to 20 mph were registered. For the rest of the afternoon, RH values should increase as afternoon convection is expected across western PR, keeping the risk low. At this time an RFD will not be issued, but we encourage partners to continue monitoring conditions in the coming days, particularly on Saturday as a drier airmass is expected to arrive. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$