Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
889 FXCA62 TJSJ 230825 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 425 AM AST Sun Jun 23 2024 .SYNOPSIS... There is a Heat Advisory in effect for most urban areas and lower elevations of the islands. A weak upper-level trough and the available low-level moisture will promote shower and thunderstorm development across the area today. A tropical wave will increase shower activity on Monday. A strong Saharan Air Layer with a dense area of dust is expected to move over the islands by midweek. && .SHORT TERM...Today through Tuesday... Advective showers and isolated thunderstorms will continue to effect windward sections throughout the morning hours as relatively drier air filters into the region under a surface easterly to southeasterly flow. High pressure continues to dominate the Atlantic Basin and as it shifts towards the eastern Atlantic and a tropical wave moves into the eastern Caribbean, winds will begin to veer slightly remaining from a southeasterly direction through the short term. Afternoon convection later on today is expected to bring heavy thunderstorms across the western section of Puerto Rico, mainly in the center of the western part of the island as sea breezes keep most of the rain inland. Streamer formation downwind of El Yunque and the smaller islands may bring some good showers are well. Flooding is possible with the heaviest activity and thunderstorms capable of producing frequent lightning are possible. The tropical wave is forecast to move across the region on Monday afternoon, bringing a moist airmass that will elevate the chance of passing showers and overall cloudy, muggy conditions. Winds may increase to strong values at times, which may result in squally weather conditions especially across the southeastern sections of Puerto Rico and the USVI. A drier airmass with high concentrations of Saharan Dust is forecast to replace this airmass as the tropical wave exits to the northwest of the region on Tuesday morning. Hazy, dry conditions are forecast on Tuesday as a result of this. Monday night may see a good amount of thunderstorms as the interaction of the saharan dust causes somewhat of dryline effect on the moisture in front of it. Yet as mentioned, Tuesday appears to be quite dry with high maximum temperatures which may yield for a heat advisory or warning, stay tuned. .LONG TERM...Wednesday through Sunday... The NASA Dust Aerosol Optical Thickness model shows a dense Saharan Air Layer (SAL) lingering over the local islands through Thursday. In addition, there will be a mid-to upper-level ridge across the northeastern Caribbean, resulting in drier conditions aloft and inducing a very stable weather pattern with hazy skies, sunny mornings, and some afternoon showers across the western sections of Puerto Rico. Instability will slightly increase from late Thursday and Friday as an induced low-level trade wind perturbation streams across the local area. Moisture content will increase once again and isolated thunderstorms are expected to develop over portions of the waters and across the islands. Later in the period, as the SAL exits the region, the islands will be influenced by the easterly winds resulting from the interaction between a surface high pressure over the central Atlantic and an induced surface trough just northwest of the region over the Western Atlantic. This wind pattern will drag tropical moisture into the islands. According to global model guidance, PWAT values will fluctuate from 1.70 to 2.10 inches, just above climatology for the region. That moisture, along with diurnal heating, will promote the development of afternoon showers and isolated thunderstorms each day across the western interior sections of Puerto Rico. Winds will return from the southeast by Saturday into Sunday, increasing the potential to observe heat indices near 108 to 112F across the coastal and urban areas of the islands. && .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, mainly effecting 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 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 from late tonight through Monday. Increasing winds and showers with thunderstorms are expected through Monday across the local waters. Hazy skies are expected from Tuesday through Thursday due to Saharan dust. && .BEACH FORECAST... There is a low to moderate risk of rip currents across PR and the USVI. Showers and isolated thunderstorms are possible across the coastal areas of the USVI and eastern PR during the morning hours, and across the west/northwest coast of PR during the afternoon. The afternoon thunderstorms could produce frequent lightning and gusty winds. The weather conditions should improve during the night. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001-003-005-007-008-010>013. VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ002. AM...None. && $$ SHORT TERM...RC LONG TERM....LIS/DSR AVIATION...RC