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Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
180 FXCA62 TJSJ 250926 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 526 AM AST Tue Jun 25 2024 .SYNOPSIS... As mentioned in previous discussions, a dense plume of Saharan dust is expected to arrive on today along with a dry airmass. These conditions will promote hazy skies and hot temperatures from Tuesday through Thursday. Significant rainfall activity is not anticipated due to the presence of the dust particles. && .SHORT TERM...Today through Thursday... Calm weather conditions prevailed overnight, with warmer-than-normal low temperatures across Puerto Rico and the U.S. Virgin Islands. In the satellite imagery, a mid-to-upper-level cloud deck was observed moving over the islands, limiting nighttime cooling overnight. The winds were mainly from the east-southeast at 10 mph or less. Additionally, the Saharan Air Layer began to produce hazy skies across the region. Due to the warmer-than-normal low temperatures, we can expect maximum temperatures to rise to near or above-normal values today and tomorrow. The arrival of a Saharan Air Layer will reduce the available moisture, but the high low-level humidity will still lead to warm and hot heat indices. This has prompted an Excessive Heat Warning for most of the coastal areas and western interior urban sections of Puerto Rico, Culebra, and all of the U.S. Virgin Islands. The rest of mainland PR and Vieques are under a Heat Advisory, indicating the potential hazards of these weather conditions. We cannot rule out some showers arriving while steering by the winds across the windward sections, but it should be minimal today. During the afternoon, some showers will develop across PR`s mountain and western sections. Due to similar weather conditions, an Excessive Heat Watch will be in effect tomorrow. Additionally, the hazy skies will remain across the islands throughout the short term. The heat spell may extend through at least Thursday when a trade wind perturbation will increase humidity, increasing the potential for showery weather and some isolated afternoon strong convection. && .LONG TERM...Friday through Tuesday... The inherited forecast remains unchanged. The latest model guidance continues to suggest a slight increase in instability through the end of the workweek into the early weekend as an induced low-level trade wind perturbation streams across the local area. This perturbation will allow the moisture content to increase through the mid-levels of the atmosphere, increasing the chance of isolated thunderstorms and showers. Precipitable water models show values ranging from normal to slightly above normal climatological levels. By the weekend, the surface high-pressure system will position across the Atlantic basin, dragging tropical moisture into the islands through next week. At this time, we anticipate that this moisture, along with the diurnal heating, will promote the development of afternoon showers and isolated thunderstorms each day, particularly across the western interior sections of Puerto Rico, as mostly easterly winds will dominate the forecast area. A slight change in weather conditions is foreseen as drier air will filter across Puerto Rico and the U.S. Virgin Islands on Monday, which may reduce the chance of showers. Nevertheless, a typical seasonal weather pattern of passing showers and afternoon convection is expected. && .AVIATION... (06Z TAFs) Hazy skies will limit VIS btwn 4 and 6 SM or less. The winds will remain mainly from the east to east-southeast at 10-20 kt with gusts between 20 and 30 kt. Some showers will develop across the interior and western PR between 25/18-23z. && .MARINE... A surface high pressure over the central Atlantic will promote moderate to fresh east-southeasterly winds. Trade wind showers will move across the regional waters from time to time. Hazy skies are expected through Thursday due to Saharan dust. && .BEACH FORECAST... There is a moderate risk of rip currents across PR and the U.S. Virgin Islands. A low risk is forecast for western PR. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning from 10 AM this morning to 6 PM AST this evening for PRZ001-003-005-007>012. Excessive Heat Watch from Wednesday morning through Wednesday afternoon for PRZ001-003-005-007-008-010>012. Heat Advisory from 10 AM this morning to 6 PM AST this evening for PRZ002-004-006-013. VI...Excessive Heat Warning from 10 AM this morning to 6 PM AST this evening for VIZ001-002. Excessive Heat Watch from Wednesday morning through Wednesday afternoon for VIZ002. AM...None. && $$ SHORT TERM...CAM LONG TERM....MMC AVIATION...CAM MARINE...CAM/MMC BEACH FORECAST...MMC