Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
801 FXCA62 TJSJ 070803 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 403 AM AST Sun Jul 7 2024 .SYNOPSIS... The available moisture associated with a trade wind perturbation and maximum air temperatures in the 90s will promote hazardous heat index values across the Puerto Rico and US Virgin Islands coastal areas, where an Excessive Heat Warning and Heat Advisory are in effect. A relatively stable atmosphere will prevail throughout the week. However, the chance of rain will increase due to a couple of tropical waves, the first arriving on Tuesday and the second by the weekend. Between and around those waves (especially on Tuesday and Wednesday), air masses with important suspended dust particles from the African Deserts could inhibit widespread rain activity and promote hazy skies. && .SHORT TERM....Tonight through Tuesday... Overnight, Puerto Rico and the U.S. Virgin Islands experienced predominantly calm weather conditions, with hazy skies prevailing due to the Saharan Air Layer. Like previous days, the prevailing Saharan Air Layer prevented significant nighttime cooling, resulting in warmer-than-normal low temperatures. For that reason, official weather stations reported temperatures in the low 80s along the coastal/urban areas, while across the higher elevations, they were in the 70s. Wind-wise, they were mostly out of the east and stayed below 10 mph. The latest model guidance suggests a surface high-pressure system building over the central Atlantic and extending into the northeast Caribbean, leading to breezy easterly winds and occasional clouds and showers, especially in the windward sections of the U.S. Virgin Islands and Puerto Rico. Today, the forecast leans towards hazy skies with the introduction of a trade wind perturbation that can bring a slight increase in showery weather. Nonetheless, we anticipate little cloud growth across the islands, with some rainfall and isolated thunderstorm activity by late afternoon due to suspended dust particles. Precipitable water models suggest normal to below-normal climatological levels for this time of year through at least the beginning of the workweek. The forecast for Monday leans towards calm weather throughout the day and a gradual increase in moisture associated with an approaching tropical wave by late Monday. On Tuesday, a weak tropical wave with its associated wind surge will reach the islands. At this time, we do not anticipate significant rainfall activity nor cloud growth as an extensive mass of Saharan Air Layer will move across Puerto Rico and the U.S. Virgin Islands. The Dust Aerosol Optical Thickness model suggests concentration values around 0.55, and to be considered a slight to moderate event, it needs to exceed 0.20. Therefore, it was added to the forecast and we want to inform citizens and tourists about this upcoming slight to moderate dust event that could affect people with respiratory illness. .LONG TERM...WEDNESDAY THROUGH SUNDAY... Relatively stable atmospheric conditions will prevail throughout most of the forecast, with above-normal heights at mid- and upper- level, warmer-than-normal temperatures at 500 MB, and relatively stable lapse rates at mid- and low levels. Additionally, the 925 MB temperatures suggested warmer-than-normal temperatures, which may indicate a warm-to-hot spell during those days in the long term. The NASA Dust Extinction guidance suggests the presence of significant suspended dust particles from the African deserts, especially on Wednesday, with some traces remaining on Thursday. This dry air mass will surround the trailing moisture from a tropical wave, which may limit widespread rain activity but create hazy skies. However, be prepared for the possibility of some strong thunderstorms, especially on Wednesday afternoon, as steep lapse rates between 850-700 MB may be possible during those days due to the passing wave. However, the bulk of moisture and the African Jet-stream maxima may be located off to the south of the islands, across the southeastern Caribbean. Once again, model guidance suggests a surface high pressure anchored across the North Atlantic Ocean that could promote breezy easterlies across the islands throughout the second part of the week. Under this wind flow, we may observe the arrival of dry slots and pockets of moisture each day, resulting in a mix of clouds and sunshine or clear skies (during the nighttime) and the arrival of quick passing showers. Model guidance suggests a tropical wave arriving by the upcoming weekend. If the dry air mass surrounding it does not negatively impact it, we may see a wet and unstable pattern with a better chance of organizing convection on Saturday or Sunday. && .AVIATION... (06Z TAFS) VFR conds will prevail across all terminals. However, brief MVFR conds are possible after 07/17z near TJBQ. Hazy skies will remain possible as a SAL prevails in the region. The winds will remain from the E at 5 to 10kt through 07/12 and increase to 15-20 kt with gusts between 20 and 30 kt near TSRA and sea breeze variations after 14z. && .MARINE... A surface high pressure anchored across the Atlantic Ocean will tighten the pressure gradient across the Northeast Caribbean, promoting moderate to fresh easterly winds with choppy seas. A tropical wave will cross the Lesser Antilles late Monday night and approach the islands on Tuesday with residual moisture until Wednesday. && .BEACH FORECAST... The CariCOOS buoy network reported seas around 3 feet (from the northeast with a 5-second period in the Atlantic and from the southeast with a 6-second period in the Caribbean) and winds around 13 knots out from the east. The water temperatures were in the mid-80s, as were the air temperatures above the water. Winds will increase at 15 to 20 knots during the day, producing choppy seas between 4 and 5 feet or higher. Beachgoers, the risk of rip currents will be moderate in the north and southwest of PR, Culebra, and St Croix. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning from 10 AM this morning to 5 PM AST this afternoon for PRZ001-003-005-007-008-010-011. Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ012. VI...Excessive Heat Warning from 10 AM this morning to 5 PM AST this afternoon for VIZ002. Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ001. AM...None. && $$ SHORT TERM/AVIATION...MMC SYNOPSIS/LONG TERM/MARINE/BEACH FORECAST....CAM