Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
548 FXCA62 TJSJ 220756 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 356 AM AST Sat Jun 22 2024 .SYNOPSIS... A summer-like pattern will persist for the weekend across the islands, with passing showers early in the day, and afternoon convection for the interior and western Puerto Rico each day. A tropical wave will approach the area on Monday, followed by Saharan dust. Hot temperatures will prevail for the forecast period. && .SHORT TERM...Saturday through Monday... Surface ridging continues to dominate the Atlantic Basin, allowing moderate easterly flow to prevail across the lower levels through Sunday. This trade wind pattern will stream humid air masses across the region, allowing windward showers and afternoon convection to occur. Under a weak ridge across the mid to upper- levels, local effects and diurnal heating will provide enough instability to bring heavy rainfall during the afternoon hours for western Puerto Rico where flooding and strong thunderstorms remain possible. On Sunday, lower level winds start to veer, becoming southeasterly as the surface high in the Atlantic becomes more centered at about 45W longitude and as a tropical wave begins to move into eastern Caribbean Basin. Weather conditions are likely to deteriorate Sunday Afternoon onward as this tropical wave moves through the area on Monday. Expect a rapid increase of passing showers Sunday night into Monday morning, then heavy convection during Monday afternoon for multiple sections of the islands. Breezy conditions are forecast to occur as well, which will bring squally weather at times with strong thunderstorms. Northwestern Puerto Rico may see the heaviest rainfall during Monday afternoon, however southeastern PR may see some impressive thunderstorms capable of bringing lots of rain. .LONG TERM...Tuesday through Saturday... After the tropical wave departs, a surface high pressure will take control of the atmospheric conditions. First, this feature will drive the trade winds from the southeast, and will tighten the pressure gradient, with surface speeds around 20 knots. Also, a trade wind cap inversion will form at around 800 mb, trapping all the available moisture below this level. Moderate to strong concentrations of Saharan dust may arrive too, with hazy skies and little precipitation expected through at least early Thursday. This conditions, however, will make temperature soar, with very hot conditions expected to return these days. By Thursday, as an induced surface trough (more evident in the 700 mb pressure field) approaches, the winds are expected to shift from the northeast at these heights, and the trade wind cap will weaken. Instability should be enough to fire up showers and thunderstorms for the interior and western portions of Puerto Rico, with an elevated risk for urban and small stream flooding, and lightning. Also, by the end of the week, an upper level trough will approach the Leeward Islands. This trough is not expected to affect the local islands directly, but it will allow for better shower and thunderstorm production each afternoon across the usual interior and western Puerto Rico. Shower activity will become more frequent too for eastern Puerto Rico and for the Virgin Islands. Saharan dust will not completely go away, but concentrations are expected to be lower. This low concentrations of dust could contribute to enhance the frequency of the lightning activity within the thunderstorms. && .AVIATION... (06Z TAFs) Mainly VFR conditions expected to prevail through the forecast period with periods of reduced VIS and low ceilings possible during the afternoon. SHRA will continue across the region, impacting the USVI terminals and TJSJ through the day. Widespread SHRA and isolated TSRA expected after 15Z, effecting also TJPS and TJBQ. Winds will be out of the E at 14-18 kts, with gusts around 25 kts from 13-22z. && .MARINE... A surface high pressure over the central Atlantic will promote moderate to locally fresh easterly winds through the rest of the week. Trade wind showers will move across the regional waters from time to time. The next tropical wave will approach the local islands by the beginning of the next workweek. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...RC LONG TERM....ERG