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Area Forecast Discussion (AFD)
879 FXCA62 TJSJ 040554 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 154 AM AST Sat Jul 4 2026 ...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST, FIRE WEATHER... .KEY MESSAGES... Issued at 136 AM AST Sat Jul 4 2026 * Elevated fire weather conditions are expected across the southern coastal plains of Puerto Rico through the holiday weekend. Dry, breezy conditions will favor the rapid spread of any fires that develop. * High concentrations of Saharan dust will affect Puerto Rico and the U.S. Virgin Islands through the holiday weekend, resulting in hazy skies, reduced visibility, and poor air quality. * Hot and humid conditions will persist today across Puerto Rico and the U.S. Virgin Islands, increasing the risk of heat- related illnesses. * A moderate risk of rip currents will continue through the weekend across several local beaches. Life-threatening rip currents are possible. && .Short Term(Today through Monday)... Issued at 136 AM AST Sat Jul 4 2026 Overnight, mostly cloudy skies prevailed across the local islands as pockets of trade wind showers moved inland, affecting the eastern half of Puerto Rico. Rainfall was generally light, with no significant impacts. Temperatures remained warm, with lows in the lower 80s across urban and coastal areas and in the 70s across the interior. Winds were generally light and variable at times. Meanwhile, trace concentrations of Saharan dust have already begun filtering across the U.S. Virgin Islands, meaning that the Saharan Air Layer will eventually spread across the local islands today. We forecast that a typical summertime trade wind pattern will persist across Puerto Rico and the U.S. Virgin Islands through Monday as a broad surface high pressure over the central Atlantic maintains moderate east-southeast winds. This pattern will favor passing showers across the U.S. Virgin Islands and eastern Puerto Rico during the overnight and morning hours, followed by isolated afternoon showers over the interior and western sections of Puerto Rico. Although a slight increase in moisture is expected Sunday into Monday, a strengthening mid-level ridge and pockets of drier air will continue to limit the vertical growth of any shower that develops. As a result, rainfall amounts should generally remain light to moderate, with localized ponding possible where showers repeatedly affect the same area, but the overall flooding threat will remain below limited criteria. The primary weather hazard through the weekend will be the arrival of a Saharan Air Layer (SAL). Moderate to locally high concentrations of Saharan dust will overspread the islands today before gradually decreasing to moderate concentrations as we head into Monday. The dust will result in hazy skies, reductions in visibility, and poor air quality, especially for sensitive groups. The drier air associated with the SAL will also help suppress shower coverage and intensity despite the increase in available moisture later in the period. Above-normal temperatures will persist through Monday under a stable weather pattern. Combined with periods of sunshine, the Saharan Air Layer will contribute to warmer overnight conditions by limiting radiational cooling, resulting in little relief from the daytime heat. Although heat index values are expected to approach Heat Advisory criteria across many coastal and urban areas of Puerto Rico and the U.S. Virgin Islands this afternoon, we decided to not issue it at this time as the available widespread cloud cover could delay the onset of peak heating. Residents and visitors are encouraged to stay informed of the latest forecasts and any changes to heat-related headlines as conditions evolve throughout the day. Regardless, warm and muggy conditions will persist across the islands due to above-normal temperatures and the presence of Saharan dust. Those planning outdoor activities should stay hydrated, take frequent breaks in the shade or air conditioning, and limit prolonged exposure during the hottest part of the day. && .Long Term(Tuesday through Saturday)... Issued at 136 AM AST Sat Jul 4 2026 Mostly dry and stable conditions are expected in the long-term forecast, though the flooding threat will increase on Wednesday. A surface high pressure building north of the region will maintain east-east-to-southeast winds throughout the forecast period, with breezy to locally windy conditions across coastal areas of the islands. A dense plume of SAL will linger on Tuesday, with moderate concentrations filtering into the region and limiting shower activity. Concentrations will diminish early Wednesday, while low and mid-level moisture should increase as a tropical wave approaching the Caribbean Basin moves across the local area. From the latest model guidance, PWAT values may range between 1.8 and 2.0 inches, with a low to medium chance of reaching 2.25 inches, well above the climatological normal. Additionally, ensemble members of the Grand Ensemble are tending toward wetter conditions, with low variability among global models (PWAT difference of approximately 0.2 inches). In terms of instability, the vicinity of an upper-level low will maintain cooler-than-normal mid-level temperatures (between -8 and -9 degrees Celsius), while model guidance points out an enhancement in vorticity across the CWA, indicating upward motion favorable for cloud growth. Additionally, the Galvez-Davison Index (GDI) suggests the potential for isolated to scattered thunderstorms that may produce heavy rainfall. The most likely scenario could be showers and isolated thunderstorms moving early Wednesday morning across the U.S. Virgin Islands and eastern exposed portions of Puerto Rico, followed by afternoon convection over northwestern portions of Puerto Rico. Additionally, local island streamers could bring shower and thunderstorm activity over eastern Puerto Rico, including the San Juan Metropolitan Area. Given the expected conditions, rainfall accumulations could lead to ponding of water over roadways and poorly drained areas, along with urban and small stream flooding over the aforementioned areas. The flooding threat may vary, as winds will remain strong, making showers more progressive rather than stationary. Additionally, the tropical wave may be surrounded by SAL, which may weaken the system; changes to the forecast will be introduced if necessary. As the tropical wave continues its way across the Caribbean Basin, a drier air mass will filter into the region by Thursday, along with another plume of Saharan Dust. From the latest model guidance, PWAT values should drop to below normal values (between 1.2 and 1.4 inches), with mid- level moisture content plummeting to 20 - 30 %. Shower activity can still be expected, but it should remain limited, with no flooding threat expected across the CWA for the rest of the forecast period. Model guidance continues to suggest seasonal 925 mb temperatures, though combined with the available moisture, heat indices may reach 100 degrees Fahrenheit across urban and coastal areas of the islands. Additionally, the presence of SAL may inhibit nighttime cooling, with warmer minimum temperatures. Hence, the heat threat will remain elevated throughout the forecast period. && .AVIATION... (06Z TAFS) Issued at 136 AM AST Sat Jul 4 2026 VFR conditions across all terminals during the next 24 hours. ESE winds around 15 to 20 kts with gusts near 24 to 28 kts. HZ will affect all TAF site, reducing VIS after 04/12Z. && .MARINE... Issued at 136 AM AST Sat Jul 4 2026 Surface high pressure over the central Atlantic will continue to promote moderate to locally fresh easterly winds across the regional waters through the next several days. Seas will generally range between 3 and 5 feet, with locally choppy conditions and occasional 6-foot seas across the offshore Atlantic and Caribbean waters. A Saharan Air Layer will bring hazy skies through the weekend, while isolated afternoon showers and thunderstorms may develop over the northwestern coastal waters of Puerto Rico, producing locally higher winds and seas. && .BEACH FORECAST... Issued at 136 AM AST Sat Jul 4 2026 A moderate risk of life-threatening rip currents will develop today along portions of the southern and southeastern beaches of Puerto Rico, Vieques, Culebra, and the U.S. Virgin Islands. The moderate risk will gradually expand to additional east, north, and south facing beaches through the weekend as easterly winds strengthen and breaking waves become more frequent. Isolated afternoon thunderstorms may develop across portions of Puerto Rico today. If thunder is heard, seek shelter immediately. Hot and humid conditions will create dangerous heat levels at the beaches. Stay hydrated, seek shade often, and limit prolonged sun exposure. && .FIRE WEATHER... Issued at 136 AM AST Sat Jul 4 2026 Elevated fire weather conditions are expected across the southern coastal plains of Puerto Rico through the holiday weekend. A combination of drier air associated with a Saharan Air Layer, limited rainfall, above-normal temperatures, and stronger east- southerly winds will promote additional drying of fuels. Wind speeds will also be sufficient to support the rapid spread of any fires that develop. With increased outdoor activities and fireworks expected during the 4th of July holiday weekend, the risk of human-caused fire starts will be higher than normal. Residents and visitors are urged to use extreme caution when handling fireworks or any open flames, particularly near dry vegetation. Even a small spark could quickly ignite a grass or brush fire under the prevailing weather conditions. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM/AVIATION/FIRE WEATHER...MMC LONG TERM...MNG MARINE/BEACH FORECAST...ICP
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