Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
508 FXCA62 TJSJ 010832 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 432 AM AST Tue Jul 1 2025 .KEY MESSAGES... * Saharan dust will gradually decrease today, but high concentrations of dust are again expected this weekend, just after the passage of a weak tropical wave. * Winds will be breezy, with choppy seas and a moderate rip current risk anticipated. * High heat indices, above 102 degrees are expected each day for all the coastal and urban areas of Puerto Rico and the Virgin Islands. && .SHORT TERM...Today through Thursday... Since midnight, the highest radar estimated rainfall accumulations over land ranged from 0.10 to 0.20 inches, mainly over Isabela, Aguadilla, Vieques, St. Thomas, St. John and eastern PR. Lows have ranged from the upper 60s at higher elevations of Puerto Rico to the upper 70s and low 80s at lower elevations across the islands. Current satellite-derived precipitable water (PWAT) values indicate up to around 1.3 to (locally) 1.5 inches over the area. A mid-level ridge over the islands, continues to promote dry air between 850 and 700 mb. At the surface, high pressure over the central Atlantic will continue to promote breezy conditions. A weak tropical wave will move well south of the islands today, over the Caribbean Sea. Saharan dust concentrations will continue to decrease, with lingering low amounts persisting through the period. The mid-level ridge will hold through at least Tuesday, maintaining a trade wind inversion around 850 mb. This will limit widespread rainfall development, with only isolated afternoon convection possible over W-SW Puerto Rico (under ENE steering flow) today and over W-NW Puerto Rico (under ESE steering flow) tomorrow and Thursday. A surge of moisture (a weak surface disturbance) will arrive late tonight into tomorrow and then on Thursday peripheral moisture from a nearby tropical wave will cause precipitable water values to reach around 1.5 to 1.80 inches. However, patches of drier air will also filter in. The diurnal pattern of passing morning/overnight showers and afternoon showers and isolated thunderstorms over W-NW PR, where a limited flooding risk will remain, will continue. Tomorrow, the mid-level ridge will begin to give way to a mid-to-upper-level low northeast of the area that will deepen and approach the region. However, relative humidity in the 700-500 mb layer will remain below normal, possibly limiting widespread convection. A limited heat threat will remain across urban and coastal areas during the late morning to afternoon hours. .LONG TERM...Friday through Tuesday... A weak tropical wave will approach the islands on Friday. The bulk of the wave will remain over the Caribbean Sea, but enough moisture should trigger some showers over the Virgin Islands and Puerto Rico, with stronger convection developing across western Puerto Rico in the afternoon. Just like the last wave that moved over the region, this one also comes with a lot of Saharan dust, so skies will be hazy late on Friday and on Saturday. Later on the weekend and early next week, a series of surface troughs will also move into the region, but they will also be embedded in Saharan dust. The pattern aloft will not favor deep convection, but occasional showers will move into the islands at times, with additional showers and isolated thunderstorms each afternoon across the west. The winds will be driven by a surface high pressure over the eastern Atlantic, at 10 to 16 kts. It will be hot too, as it is expected for July, with heat indices above 102 degrees along all coastal and urban areas in the region. && .AVIATION... (06Z TAFS) Mainly VFR conditions expected. Saharan dust will continue to decrease today. ENE steering flow up to around 15 to 20 kts with higher gusts will steer VCSH towards the terminals. Afternoon convection forecast to concentrate west of TJPS`S VCTY. Winds decreasing at around 01/22Z && .MARINE... Mariners can expect choppy seas across the local Atlantic and Caribbean waters due persistent moderate to fresh easterly winds. Small Craft Should Exercise Caution over most local waters. A weak tropical wave is forecast to move over the local waters on Friday, increasing shower and thunderstorm chances. Saharan dust will follow the tropical wave, with hazy skies returning on Saturday. && .BEACH... Breezy conditions will maintain a moderate rip current risk for most of the beaches of Puerto Rico and the Virgin islands. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...MRR LONG TERM....ERG