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Area Forecast Discussion (AFD)
836 FXCA62 TJSJ 271727 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 127 PM AST Sun Sep 27 2026 ...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST... .KEY MESSAGES... Issued at 108 PM AST Sun Sep 27 2026 - Afternoon showers and thunderstorms will pose an elevated flood risk today across the interior to northwestern Puerto Rico. Flood threats will diminish to limited on Monday and Tuesday, primarily presenting a risk of roadway ponding. - A Heat Advisory remains in effect through 4 PM AST today for Puerto Rico and the U.S. Virgin Islands. Heat risks will ease starting Monday as easterly trade winds strengthen. - A moderate risk of rip currents continues along northwestern to eastern beaches of Puerto Rico and the northern U.S. Virgin Islands. A high risk of rip currents is possible on Monday due to a small northerly swell. && .Short Term(This evening through Tuesday)... Issued at 108 PM AST Sun Sep 27 2026 Partly to mostly sunny skies prevailed this morning across the local area, with isolated passing showers in portions of the U.S. Virgin Islands (USVI) and eastern Puerto Rico leaving minimal accumulations. This resulted in an elevated heat threat today, with heat indices over 100 degrees Fahrenheit across many coastal areas and urban areas of Puerto Rico and the USVI, with some locations exceeding 108 degrees. The Heat Advisory remains in effect through at least 4 PM AST this afternoon. Those spending time outdoors should stay hydrated, take frequent breaks in shaded or air- conditioned areas, and limit strenuous activities during the hottest hours. Some heat relief is being observed in areas experiencing shower activity. Compared to previous days, the latest 27/12z TJSJ sounding showed a slightly stronger low-level steering flow, with a mean wind of 11 knots from the surface to 3 km. Precipitable water remains within seasonal levels around 1.75 inches, although slightly higher moisture content (between 1.8 and 1.9 inches) is expected this afternoon and tonight as moisture embedded in the trade winds (mainly induced by an upper-level trough just east of the local area) enters the CWA from the east. This moisture, along with diurnal heating and local effects, is promoting shower activity across the interior and western to northwestern quadrants of Puerto Rico, where an elevated flood risk will continue through the afternoon. Isolated strong thunderstorms cannot be ruled out. Streamers will continue to develop downwind from El Yunque, Vieques, Culebra, and the USVI. This will favor the potential for showers and possibly isolated thunderstorms across portions of northeastern Puerto Rico and the northern half of the San Juan metropolitan area, where a limited flood risk exists. As more moisture pools into the region, more frequent passing showers are expected to reach portions of eastern Puerto Rico and the USVI before a drier air mass arrives overnight into Monday morning. On Monday, a more seasonal weather pattern is expected with normal to below-normal precipitable water. Expect typical morning passing showers across the windward areas of Puerto Rico and the USVI, followed by afternoon convection across western interior, western, and northwestern Puerto Rico. Streamers could promote shower activity downwind from El Yunque into northeastern PR and downwind from the USVI. However, overall drier air and a stronger east- southeasterly steering flow should maintain a limited flood risk for Monday compared to today. Partly to mostly sunny skies with limited shower coverage should prevail otherwise. By Monday night into Tuesday, winds will shift to a more easterly direction and strengthen, bringing higher moisture content (still within seasonal values) into the local area. This will promote slightly more variable weather conditions across the forecast area, with more frequent passing showers reaching the USVI and eastern and northeastern Puerto Rico. Ponding of water on roadways will be possible where the more frequent activity occurs. In the afternoon, showers and isolated thunderstorms will develop across the interior into western Puerto Rico, as well as downwind from El Yunque and the smaller islands due to streamers. However, due to the stronger steering flow, flood risk is expected to remain limited, with ponding of water on roadways in areas of heavier activity. The elevated heat threat could diminish after today as winds increase in speed and shift more easterly over the next few days. This will favor a more limited heat risk from Monday onward, bringing slight relief from the hot conditions experienced over the last few days. && .Long Term(Wednesday through next Saturday)... Issued at 208 AM AST Sun Sep 27 2026 Near a seasonal weather pattern is expected to persist in the long- term forecast, increasing the flooding threat by the end of the week. A surface high pressure building over the western Atlantic may linger throughout the forecast period, leading to an E-ESE windflow, and breezy conditions across coastal areas of the islands. According to the deterministic guidance of the GFS and ECMWF, PWAT values should range between 1.4 and 1.6, unusual for this time of the year. Moisture content in the low levels should increase by Friday, with PWATs reaching normal values (1.6 - 1.8 inches). In the mid to upper levels, the presence of a TUTT east of the CWA and a ridge across the central Caribbean will vary the weather conditions across the islands. Based on the latest model guidance, the ridge and the subsidence side of trough over the region should increase stability aloft and limit deeper convection, particularly on Wednesday and Thursday. By Friday into Saturday, the ridge may weaken, as the upper-level trough migrated westward, positioning over the CWA and introducing instability. According to the latest Galvez-Davison Index (GDI) solution, the likelihood of isolated to scattered thunderstorms should increase by Friday and Saturday afternoon. Under this scenario, conditions may become favorable for long- lasting showers and thunderstorms in the afternoons. In general, the combination of local effects, sea breeze convergence, and daytime heating should trigger the afternoon convection and local island streamers each day, with the best flooding chance across interior and western northwestern Puerto Rico. On Wednesday and Thursday, rainfall accumulations could lead with mainly ponding of water over roadways and poorly drained areas, while the flooding threat should increase on Friday and Saturday. Under a E-ESE windflow, seasonal to slightly warmer conditions can be expected in the long-term. With the available moisture, heat indices may exceed 100 F, particularly across urban and low-lying areas. Therefore, the heat threat should remain limited to elevated throughout the forecast period, with warmer conditions on Friday. && .AVIATION... (18Z TAFS) Issued at 108 PM AST Sun Sep 27 2026 Mainly VFR conditions are expected to prevail across all terminals during the next 24 hours. However, locally induced afternoon TSRA/SHRA in and around TJBQ/TJSJ may cause tempo MVFR to brief IFR conds thru 27/22z. The 27/12z TJSJ sounding indicated ESE winds up to 18 knots blo FL100. && .MARINE... Issued at 108 PM AST Sun Sep 27 2026 Light to moderate east to southeast winds will prevail through tonight, and strengthen to around 15 knots through midweek, as they turn more from the east to northeast. A small northerly swell will continue to spread across the Atlantic waters and passages, combining with the increasing trade winds to produce choppy conditions. Seas will build up to 6 feet by Monday, particularly across the Atlantic waters. Periods of showers and thunderstorms will continue during the afternoon/evening hours, producing locally higher winds and seas, lightning, and reduced visibility. && .BEACH FORECAST... Issued at 108 PM AST Sun Sep 27 2026 A moderate risk of rip currents will continue today along the west through northeastern beaches of Puerto Rico, as well as Culebra and the U.S. Virgin Islands. Conditions will become more hazardous Monday as a northerly swell builds and winds increase, with a high risk of rip currents likely along portions of the northwestern through northern coast of Puerto Rico. Moderate risk conditions will continue elsewhere across exposed Atlantic beaches. Additional hazards include heat and afternoon thunderstorms, particularly across western Puerto Rico. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory until 4 PM AST this afternoon for PRZ001>013. VI...Heat Advisory until 4 PM AST this afternoon for VIZ001-002. AM...None. && $$ SHORT TERM...KML LONG TERM....MNG AVIATION/MARINE/BEACH FORECAST...DSR
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