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Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
314 FXCA62 TJSJ 060911 CCA AFDSJU Area Forecast Discussion...CORRECTED National Weather Service San Juan PR 511 AM AST Sat Jul 6 2024 .SYNOPSIS... A Saharan Air Layer (SAL) will promote hazy skies and limit longwave radiational cooling, promoting warmer-than-normal low temperatures today and tomorrow. Combining the available moisture 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. The next tropical wave will arrive between Monday afternoon and Tuesday. Another pulse of SAL may move in by the middle of next week. Another tropical wave will swing by the islands around the following weekend. && .SHORT TERM...Today through Monday... During the overnight hours, Puerto Rico and the U.S. Virgin Islands experienced mostly calm weather conditions, with some tradewind showers in the southern and eastern municipalities of Puerto Rico. The showers were not significant, and accumulations were below 0.30 inches. Hazy skies were observed as a Saharan Air Layer prevailed. Satellite observations also suggested partly cloudy skies across the islands. The combination of cloudiness and hazy skies prevented significant nighttime cooling, resulting in warmer-than- normal low temperatures. Temperature-wise, they remain in the low 80s along the coastal/urban areas, while across the higher elevations, they remain in the 70s. Official weather stations reported southeasterly winds of less than 10 mph throughout the night. Todays forecast leans towards hazy skies and dry air dominating the area, with the possibility of some rainfall and isolated thunderstorm activity by late afternoon and evening due to suspended dust particles. These weather conditions should prevail through at least Sunday. Model guidance suggests a surface high-pressure system building across 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. An increase in moisture is expected on Sunday as a trade wind perturbation moves through the region, potentially bringing showery weather. Precipitable water models suggest normal to below-normal climatological levels for this time of year through the beginning of the workweek. The forecast for Monday leans towards calm weather throughout the day, with Saharan dust particles reaching the area by late Monday and a gradual increase in moisture associated with an approaching tropical wave. .LONG TERM...Tuesday through Saturday... Model guidance suggests the lingering moisture of a tropical wave on Tuesday, pooling total precipitable water (TPW) between 2.0 and 2.30 inches across PR and the USVI`s region. Additionally, we observed steep lapse rates at the 850-700 MB layer, and the African Easterly Jet maxima are moving south of the islands on Tuesday and Wednesday. Under this weather pattern, scattered to numerous showers and thunderstorms may develop across the region each day. We noted that model guidance suggests surges of SAL between the available moisture, which may also cause hazy skies. If thunderstorms develop, the interaction with the suspended dust particles could enhance them. A surface high pressure anchored across the North Atlantic Ocean will promote breezy easterlies across the islands throughout the week. This wind flow will promote a mixture between the arrival of dry slots and pockets of moisture each day. This means that we may observe a mix of clouds and sunshine or clear skies (during the nighttime) with the arrival of quick passing showers advect by the winds. Although we may observe periods of rainy conditions, the interaction of plenty of low-level moisture, above-normal sea surface temperatures, and maximum summer heat temperatures could result in an extended warm-to-hot spell during the upcoming week if we do not observe prolonged periods of rain activity. GFS suggests another tropical wave moving across the northeast Caribbean around next weekend, which could promote unsettled weather conditions. && .AVIATION... VFR conds will prevail across all terminals. However, brief MVFR conds are possible after 06/17z in TJBQ and TJSJ. Hazy skies will remain possible as a SAL prevails in the region. The winds will remain from the ESE at 5 to 10kt through 06/12 and increase to 15-20 kt with gusts between 20 and 30 kt near TSRA and sea breeze variations after 12z. && .MARINE... A surface high pressure across the Atlantic Ocean will tighten the local pressure gradient, promoting choppy seas across the islands through the next few days. Seas will range between 3 and 5 feet across most of the coastal waters. Winds will be mainly from the east to southeast at 10 to 15 knots, increasing between 15 and 20 knots later in the day. In summary, a drier airmass with Saharan Dust will continue to move over the region, limiting shower development. A strengthening surface high pressure over the central Atlantic will extend into the northeast Caribbean through the weekend, leading to moderate to fresh easterly winds. The next tropical wave will be approaching the islands by late Monday night into Tuesday, increasing the chance of showers across the local waters. && .BEACH FORECAST... There is a moderate risk of rip currents for southwest PR, Vieques, Culebra and St Croix today, becoming moderate for most of the beaches tomorrow due to the increase in the easterly winds. && .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 PRZ002-012-013. 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...MMC LONG TERM....CAM