Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
517 FXCA62 TJSJ 240920 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 520 AM AST Tue Jun 24 2025 .KEY MESSAGES... * Moderate concentrations of Saharan Dust are expected mainly tomorrow and Thursday, with a slightly decrease in concentrations on Wednesday. Hazy skies and poor air quality are expected. * Daily afternoon showers and isolated thunderstorms are expected, particularly over northwestern Puerto Rico due to local effects. * By the end of the week into the weekend, a TUTT-low is forecast to develop just north/northeast of the region, and another tropical wave is expected to enter the eastern Caribbean. && .SHORT TERM...Today through Thursday... Overnight, a combination of an upper-level trough and a tropical wave brought periods of clouds and showers across Puerto Rico and the U.S. Virgin Islands. Scattered to numerous showers were detected by Doppler radar moving over local waters and into windward areas, favored by east-southeast winds at 10-15 mph. Minimum temperatures ranged from the 60s in the mountains to the mid and upper 70s along the coast, with the moist and unstable conditions supporting continued shower activity into the early morning hours. Today, a drier air mass will move into the region following the passage of the tropical wave. However, moisture from the east and southeast trade winds will still cause showers across Puerto Rico and the U.S. Virgin Islands, resulting in moderate to heavy rain early this morning. In the afternoon, limited convection is expected, with showers and isolated thunderstorms mainly over western Puerto Rico due to local effects. The flood threat in these areas will be limited to elevated. By this afternoon, the Saharan Air Layer is expected to bring hazy skies, reduced visibility, and degraded air quality. Tomorrow, we expect drier conditions with continued moderate Saharan dust concentrations and warm temperatures, which may prompt the issuance of Heat Advisories. By Thursday, the lingering effects of a weak Saharan Air Layer (SAL) will create hazy skies and generally dry weather across the region. Most islands will experience fair conditions, but western Puerto Rico may see isolated afternoon showers and thunderstorms due to daytime heating. A surface high-pressure system over the central Atlantic will sustain breezy conditions, and a dry air mass above 850 mb will suppress widespread convection. Overall, expect warm, hazy, and relatively dry conditions, with some localized afternoon rainfall in the interior and western areas. .LONG TERM...Friday through Tuesday... The inherited forecast remains on track, with a TUTT-low expected to develop just north to northeast of the region between Friday and Saturday. During this period, an induced surface trough will move across the area, shifting winds from the east-northeast. This shift will favor the development of afternoon showers, primarily across the southwestern quadrant of Puerto Rico. Across the U.S. Virgin Islands and eastern Puerto Rico, shower activity is expected to increase mainly during the nighttime hours. Despite this, precipitable water values are forecast to remain near climatological normals, and 500 mb temperatures will range from the 50th to 75th percentile. These factors suggest that while some moisture and instability will be present, conditions will not be particularly favorable for deep convective development or widespread thunderstorm activity. On Saturday, lingering moisture and the onset of east-southeasterly winds behind the trough will once again support afternoon shower development, especially across the northwestern quadrant of Puerto Rico and downwind of El Yunque. A tropical wave is then expected to quickly follow, entering the eastern Caribbean between late Saturday and Sunday morning. This feature will enhance moisture and instability, likely increasing the areal coverage of showers and thunderstorms across portions of the islands and surrounding local waters. && .AVIATION... (06Z TAFS) Expect VFR conds at all TAF sites during the period, although some SHRA will move across the windward locations and surrounding waters. Then, hazy skies will persist due to the SAL, especially aft 24/15z. VCTS will develop near JBQ around 24/17z-23z, and some SHRA/+SHRA will impact the eastern locations of PR and the USVI early this morning. Expect ESE winds around 10-15 kt thru 24/13Z, then winds from the E-ESE 15-25 kt with local sea breezes with higher gusts. && .MARINE... A subtropical Atlantic ridge extending southward into the northern Caribbean continues to promote moderate to fresh easterly winds, resulting in choppy seas over portions of the local Atlantic and Caribbean waters, and local passages. Therefore, small craft should exercise caution. Past tropical wave over the Caribbean Sea will continue to leave cloudiness and some shower activity across the across the local waters. && .BEACH FORECAST... Wave heights of around 3 to 4 feet and a moderate risk of rip currents are expected along most beaches of Puerto Rico, including Vieques, Culebra, and the U.S. Virgin Islands. A moderate risk indicates that life-threatening rip currents are possible in the surf zone. As a result, beachgoers are encouraged to exercise caution. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM/AVIATION...CAM LONG TERM....MMC