Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
639 FXCA62 TJSJ 222056 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 456 PM AST Sat Jun 22 2024 .SYNOPSIS... Showery weather conditions will prevail through the rest of the afternoon and early evening. Weather conditions will start to deteriorate by late Sunday with the arrival of a tropical wave. Expect an increase in showers and thunderstorms until at least Tuesday. && .SHORT TERM...Tonight through Monday... Relatively calm conditions prevailed during the morning hours across the local islands. By mid-morning the cloud coverage began to increase across the Cordillera Central, resulting eventually in the development of strong showers and thunderstorms. The strongest activity was seen from Adjuntas to Cabo Rojo, where Doppler radar estimated rainfall accumulations between 1.0 to 3.0 inches. The San Juan streamer also formed, leaving rainfall accumulations of 2.0 inches in the Guaynabo area. Moving into eastern Puerto Rico, late afternoon convective activity was observed over Humacao and Yabucoa. Flood Advisories were issued for the mentioned sectors. For the rest of the afternoon, the shower activity should move over the Mona Passage and dissipating before sunset. However, trailing showers over the Anegada Passage could reach the eastern portions of Puerto Rico and the U.S. Virgin Islands leaving minor rainfall accumulations. As the surface high pressure moves towards the central Atlantic, winds will start to veer from the east to southeast by Sunday morning. A tropical wave will approach the local area from the east on Sunday afternoon, increasing instability and the potential for convective activity through Monday. Squally weather conditions are anticipated, along with moderate winds with higher gusts embedded with the heaviest showers. By Monday, the stronger activity is expected across the western and northwestern portions of Puerto Rico during the afternoon hours. Despite the expected weather, daytime temperatures will remain warm to hot across the region, with highs in the range between the upper 80s to lower 90s along the coastal and urban sectors, and in the mid 80s in the mountains. Heat indices are expected to be elevated as well, with a limited heat risk anticipated. .LONG TERM...Tuesday through Saturday... ./FROM PREV DISCUSSION...Issued at 356 AM AST Sat Jun 22 2024/ 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... (18Z TAFs) SHRA/TSRA over mainland PR should end around sunset. Thereafter, mainly VFR conditions should prevail across all terminals through Sunday morning. Afternoon SHRA/TSRA expected to develop once again across the area terminals after 23/16z. HZ due to Saharan dust expected later this evening through Sunday, but VSBY should remain P6SM. Low-level winds from the east at 10-15 kt, bcmg light and variable overnight. && .MARINE... A surface high pressure over the central Atlantic will promote light to moderate easterly winds through the rest of the weekend. 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. Increasing winds and thunderstorm activity is expected from late Sunday through Monday. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...LIS LONG TERM....GRS AVIATION...DSR PUBLIC DESK...MRR