Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
733 FXCA62 TJSJ 181840 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 240 PM AST Fri Sep 18 2026 ...New SHORT TERM, AVIATION, MARINE, BEACH FORECAST... .KEY MESSAGES... Issued at 240 PM AST Fri Sep 18 2026 - Heat will be the main concern this weekend, with an increasing risk of heat-related illness across Puerto Rico and the U.S. Virgin Islands. The heat threat may persist through early next week. - Afternoon showers and thunderstorms are expected each day across interior and western Puerto Rico this weekend. Brief heavy rainfall could lead to ponding of water and localized flooding. - Decreasing winds will lead to amenable marine and rip current conditions throughout the rest of the forecast period. && .Short Term(This evening through Sunday)... Issued at 240 PM AST Fri Sep 18 2026 Fair weather has prevailed this morning. Moisture in the low to mid-levels (800-500mb) has decreased, due to drier air intrusion. As a result, shower and thunderstorm activity may be limited. However, any development fueled by lingering moisture from the tropical wave combined with local effects (i.e., daytime heating, orographic effects, and sea breeze convergence) will be more localized today, mainly over the west central into western portions of Puerto Rico. Activity over the eastern interior cannot be ruled out. This weather pattern should continue through the weekend. However, wind speeds are expected to decrease, which could lead to slightly higher rainfall accumulations from afternoon convection. As a result, an elevated excessive rainfall risk will be focused to the interior and western areas, while a limited risk is expected for surrounding areas, including some metropolitan and eastern interior areas, on Saturday due to the El Yunque streamer development. On Sunday, the elevated risk shifts to northwestern areas as winds are expected to shift to an ESE flow. Also, isolated landslides cannot be ruled out in areas of steep terrain due to high soil saturation levels from past days rainfall accumulations. For the weekend, the main hazard will likely be heat-related risks. Decreasing winds, low cloud cover during peak daytime hours, and seasonal to near above-normal moisture levels (around 2.0 to 2.15 inches) across the CWA will promote humid and hot conditions for the majority of the short-term period. Additionally, the high-pressure system to the north of the area shifting eastward could promote the aforementioned wind direction change to an ESE flow, which will likely drag warmer air into the islands. The excessive heat risk will remain limited the rest of afternoon but could reach a significant level through the weekend. A significant level of heat risk affects anyone without effective cooling and/or adequate hydration. In addition, impacts are possible in some health systems, heat-sensitive industries, and infrastructure. We recommend that individuals avoid outdoor activities during peak daytime heating hours and remain well- hydrated, particularly over the weekend. && .Long Term(Monday through Friday)... Issued at 213 AM AST Fri Sep 18 2026 A mid- to upper-level trough will linger over the Atlantic and northeastern Caribbean, supporting occasional induced surface perturbations throughout the period. Meanwhile, surface high pressure over the central Atlantic will maintain east to east- southeast winds and a generally wet advective pattern through much of the workweek. Passing showers will affect windward portions of Puerto Rico and the U.S. Virgin Islands, followed by afternoon showers and isolated thunderstorms over interior and western Puerto Rico. Convective development will shift according to the prevailing steering flow. However, the strongest afternoon activity will generally favor interior and western Puerto Rico, where a limited flooding rain threat may develop each afternoon. The flooding threat elsewhere across the islands will be reassessed as the period approaches. A warming trend will continue at least during the first half of the week. Near- to above-normal moisture, combined with limited ventilation across urban and coastal areas, could result in periods of elevated to locally significant heat risk. Guidance then indicates a drying trend during the latter part of the week as precipitable water decreases to near or below normal levels. The upper-level trough will also maintain strong vertical wind shear across the region, creating an unfavorable environment for tropical cyclone development. Local sea-breeze variations are expected during the day, with land-breeze influences possible overnight along coastal areas. && .AVIATION... (18Z TAFS) Issued at 240 PM AST Fri Sep 18 2026 Mostly VFR conds expected through the period. VCTS at TJBQ possible thru 18/23Z, with VCTS returning at TJPS/TJBQ aft 19/16-17Z. VCSH expected at TIST/TISX and briefly at PR terminals this evening/overnight. E-SE winds 13-15 kt with hir gusts, becoming light/VRB after 18/23Z, then increasing again after 19/13-15Z. && .MARINE... Issued at 240 PM AST Fri Sep 18 2026 A surface high pressure over the western into central Atlantic will promote gentle to moderate easterly winds across the local waters over the coming days, gradually weakening from Sunday onward. The high will then build across the eastern Atlantic, promoting a southeast wind flow through early next week. Seas will generally range from 2 to 4 feet, with occasional seas up to 5 feet. Tropical moisture will remain across the region through the weekend and into early next week, enhancing the potential for overnight showers and localized hazardous marine conditions, particularly across the western waters of Puerto Rico each afternoon due to isolated thunderstorms. && .BEACH FORECAST... Issued at 240 PM AST Fri Sep 18 2026 Tonight, a moderate risk for life-threatening rip currents is expected along the northwestern beaches of Puerto Rico, with a low risk persisting elsewhere. A moderate risk means that life- threatening rip currents are possible, and beachgoers are urged to exercise caution. From Saturday through at least Tuesday, a low risk will likely prevail across most beaches of Puerto Rico and the U.S. Virgin Islands. However, even with a low risk, life- threatening rip currents can occur near groins, jetties, reefs, and piers. By midweek, the moderate risk will likely return to some areas. Throughout the weekend, another beach hazard will be the heat. Beachgoers should remain hydrated and limit prolonged sun exposure. Additionally, gusty winds and lightning due to afternoon showers and isolated thunderstorms, particularly across the western beaches of Puerto Rico. Beachgoers should seek shelter whenever they hear thunder. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...JCP LONG TERM...CAM AVIATION...CVB MARINE/BEACH...YZR