Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
295 FXCA62 TJSJ 230929 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 529 AM AST Mon Sep 23 2024 .SYNOPSIS...Light winds will continue well into the week with showers and thunderstorms each day, but generally less on Wednesday. A low risk of rip currents will be seen Monday and Tuesday due to light winds. Winds will increase somewhat on Wednesday through Friday when a mild tropical wave moves through the area. && .SHORT TERM...Today through Wednesday... Overnight, mostly calm weather conditions prevailed across Puerto Rico and the U.S. Virgin Islands. However, isolated to scattered showers and thunderstorms developed over the waters and the U.S. Virgin Islands Passage. Temperatures remained in the upper 60s to lower 70s in the higher terrain and in the upper 70s to lower 80s in the lower terrain (urban/coastal areas). The winds were relatively light and variable across the islands. As for today, citizens and visitors in Puerto Rico, Vieques, Culebra, Saint Thomas, Saint John, and Saint Croix should experience tranquil weather conditions through the morning hours. Nonetheless, as the morning and day progresses, the heat indices will increase, exacerbating the risk of heat illness due to the high humidity and high temperatures. By the afternoon, we are forecasting the development of showers across Puerto Rico, which will allow for cooling where the rainfall occurs. This rainfall activity will be associated with typical afternoon patterns and the presence of an upper-level trough moving from the western Atlantic. The upper- level trough will position itself to the north of the islands, upward vertical motion and better moisture content through Tuesday. Therefore, showers and thunderstorms will increase in coverage once again, particularly on Tuesday afternoon. Weak steering winds will also lead to higher rainfall accumulations over the same areas, increasing the potential for flooding and mudslides in steep terrain, mainly along the Cordillera Central. Across the USVI, a similar weather pattern is expected to continue over the next few days, with diurnally induced afternoon showers developing downwind of the islands. High temperatures in the upper 80s to lower 90s will persist across the lower elevations of the islands during the morning and early afternoon hours. Therefore, excessive heat conditions can be anticipated throughout the short-term period. .LONG TERM...Thursday through Monday... High pressure in the lower levels will be found just north of the area and this will induce a more typical easterly surface flow. That high pressure will move westward and a mild tropical wave will move through the U.S. Virgin Islands, Puerto Rico and most of the eastern Caribbean south of the area Friday afternoon and Friday night. On Saturday a shearline will approach the islands from the north but remain north of 20 north. This will maintain easterly flow and given that the moisture does not really diminish we will continue to see typical shower and thunderstorm formation Saturday and Sunday. On Monday flow becomes southerly and very light and temperatures will begin to climb a few degrees. This will lead to stronger showers and thunderstorms in central and western Puerto Rico. At upper levels a weak TUTT low will be found just south of Puerto Rico on Thursday. When it moves west, a second TUTT will dive in from the northeast and will create a environment only somewhat favorable to active convection given that 500 mb temperatures will rise during the period--especially after the tropical wave passes. Strong high pressure at upper levels will hold over Cuba and then move over southern Florida adding to the stability over the weekend and into Monday. With light southerly surface winds beginning then, we will see a warming trend. && .AVIATION... (06Z TAFs) VFR conditions across all terminals. Clds and SHRA are expected to dvlp across the interior sections of PR, followed by SHRA/TSRA with mtn obscurations and MVFR conds over the interior through 23/21Z. TJSJ and TJBQ will experience brief MVFR conds, particularly after 18Z. Winds less than 10 kt with a few higher gusts along the coasts and near TSRA till 23/22Z. && .MARINE...An active low to our north will bring in 4-5 foot swell THursday through Sunday. There may also be a northeasterly component due to tropical storm formation--which the National Hurricane Center is now giving a 70 percent chance of formation during the next 7 days. && .BEACH FORECAST...After a period of low risk on Monday and Tuesday rip current risk will increase on north and northeast facing beaches Wednesday through Friday. Rip current risk could be high on Friday due to a 5 foot swell reaching the area from the north northeast. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>004-007- 013. Excessive Heat Warning until 5 PM AST this afternoon for PRZ005- 008-010-011. VI...Heat Advisory until 5 PM AST this afternoon for VIZ001. Excessive Heat Warning until 5 PM AST this afternoon for VIZ002. AM...None. && $$ SHORT TERM/AVIATION...MMC LONG TERM/MARINE....WS