577 FXCA62 TJSJ 290900 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 500 AM AST Sat Aug 29 2020 .SYNOPSIS... A dry air mass with Saharan dust particulate will continue to limit shower activity and produce hazy skies across the islands through the weekend. This weather pattern will persist until the arrival of a tropical wave early next week, which NHC is monitoring and has a moderate chance of becoming a tropical cyclone. A TUTT-low will bring unstable weather conditions for the second part of the work-week next week. && .SHORT TERM...Today through Monday... A high pressure ridge is dominating the local pattern along with a pulse of Saharan dust over Puerto Rico and the U.S. Virgin islands. Therefore, dry conditions and hazy skies are expected to continue for the rest of the weekend. However, for this afternoon, isolated to scattered showers are possibles over southwestern Puerto Rico. The proximity of an upper-level low, low temp temperatures at 500mb (between -9 and -9.5 degrees C, according to the GFS) will increase the possibility of thunderstorm formation. The limiting factor for today's activity is the lack of moisture in the atmosphere. At this time, not significant rainfall accumulation is expected. Maximum temperatures will range between upper 80s to the low 90s with heat indices in the low and mid 100s. A similar weather pattern is forecast on Sunday. For Monday, a strong tropical wave is expected to cross the Lesser Antilles and its leading moisture will reach the islands. Ahead of the wave, a wind surge is forecast to bring low-level moisture and increasing winds across the regional waters and coastal areas. This moisture will increase the potential for locally induced afternoon convection across PR/USVI with a better chance to have urban and small stream flooding across the islands. .LONG TERM...Tuesday through Saturday... The axis of a tropical wave is forecast to move across the islands on Tuesday. Both global models are depicting this vigorous tropical wave, GFS/ECMWF. The National Hurricane Center (NHC) is currently monitoring this wave and has a medium chance of formation (40%) through the next five days. The weather picture for that day looks wet and unstable, and if the guidance is correct, expect an increasing potential for urban and small stream flooding across the islands. An upper-level trough moving westward, from the Central Atlantic into the Eastern Caribbean, will follow the aforementioned tropical wave by mid-week. This upper-level feature will swing aloft until position over the Hispaniola by Friday afternoon. The following unstable scenario is suggested by ECMWF/GFS increasing confidence in this solution. This instability with the available moisture, diurnal heating, and local effects could result in inclement weather, especially on Friday afternoon. The weather pattern for Wednesday and Saturday seems similar, where the lingering moisture will aid in the development of the typical September weather conditions with overnight/early morning passing showers across the windward sections in PR/USVI. Then, afternoon convection will develop across interior and west PR, as well as downwind from the U.S. Virgin Islands and El Yunque. However, a mid-to upper-level ridge will build from the east over the region by the end of the long-term period, next weekend. && .AVIATION... Mostly VFR conditions are expected to prevail through the forecast period. Although Saharan dust will continue to move into the region and result in hazy skies the visibility is expected to remain at P6SM. Winds will be out of the east-northeast between 10 to 15 knots. && .MARINE... Mariners can expect seas to remain below 5 feet through much of the forecast period. Surface high pressure will build across the Atlantic Ocean, promoting a moderate to locally fresh easterly wind flow. A tropical wave is forecast to cross the Lesser Antilles on Monday, moving across PR/USVI on Tuesday, and increasing the chance for showers and thunderstorms those days. Please, remain vigilant to the development of this tropical wave, because it could deteriorate even more the marine conditions. && .PRELIMINARY POINT TEMPS/POPS... SJU 88 81 90 80 / 20 20 20 30 STT 91 81 90 79 / 20 20 20 40 && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...LIS LONG TERM....CAM PUBLIC DESK...LIS