Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
900 FXCA62 TJSJ 062051 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 439 PM AST Sat Jul 6 2024 .SYNOPSIS... A dry and warm pattern will continue tonight as a Saharan Air Layer (SAL) promotes hazy skies and limits radiative cooling. A noticeable break in the hot, hazy, dry air will arrive in the form of a tropical wave between Monday afternoon and Tuesday. Another pulse of SAL may move in by the middle of next week. Another tropical wave is forecast to reach the islands by the next weekend. && .SHORT TERM...Tonight through Monday... Calm, hot and hazy conditions prevailed today. Shower and overall cloud development was limited today. Only light showers advected into the area, steered by southeasterly winds, and grazed southwestern Puerto Rico and crossed Vieques, Culebra and easternmost Puerto Rico. Overall dry conditions with abundant saharan air, combined with southeasterly winds promoted hot conditions across the islands with most if not all low elevation stations reporting heat indices above 100 degrees with values above 110 degrees were reported in western, northern, eastern and southwestern Puerto Rico as well as in St. Croix. Current satellite derived precipitable water values indicate below normal to normal precipitable water (PWAT) values across the islands and local waters. A dry and warm pattern will continue tonight, with lows in the upper 70s to low 80s at lower elevations, as a Saharan Air Layer (SAL) promotes hazy skies and limits radiative cooling. A surface high pressure will continue to build across the central Atlantic and extend into the northeast Caribbean, promoting up to breezy easterlies during the period. These could help advect some limited showers across windward sectors. PWAT values should continue under the below normal to normal range, leaning more towards normal values, tomorrow where a trade wind perturbation can potentially bring an increase in showers. Some drier air will move across the region early Monday morning. This drier air will be short lived however as moisture from a tropical wave will start to gradually engulf the area and promote above normal PWAT values on Monday afternoon, increasing shower activity and giving us somewhat of a break from the saharan dust. .LONG TERM...Tuesday through Saturday... ./FROM PREV DISCUSSION...Issued at 511 AM AST Sat Jul 6 2024/ 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... (18z TAFs) Strong capping and plenty of Saharan dust have all but eliminated showers during the day. Weak moisture will enter the area arnd 07/10Z with sct SHRA in ern PR. E LLVL flow up to 12 kt with higher gusts in sea breezes will diminish aft 06/22Z. Weak land breezes up to 8 kt in PR but winds cont 8-12 kt in the USVI. Vsbys genly 7-12 SM. && .MARINE... 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 Monday afternoon into early Tuesday, increasing the chance of showers across the local waters. && .BEACH FORECAST... For tonight, there is a moderate risk of rip currents for southwestern PR, Culebra, Vieques and St. Croix with a low risk elsewhere. For tomorrow, the moderate risk spreads to the northern USVI as well as to northern, northeastern and southeastern PR due to an increase in easterly winds. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001- 003-005-007-008-010-011. Heat Advisory until 5 PM AST this afternoon for PRZ002-012-013. VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002. Heat Advisory until 5 PM AST this afternoon for VIZ001. AM...None. && $$ SHORT TERM....CVB LONG TERM.....MRR PUBLIC DESK...RVT