000 FXCA62 TJSJ 131629 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 1229 PM AST Mon May 13 2024 .UPDATE... Based on the latest observations, the Heat Advisory was extended to southwestern Puerto Rico, Vieques, Culebra, St. Thomas and St. John. The heat indices surpassed 108 degrees in Lajas, and was close in the other areas. Hot conditions are expected to prevail through late this afternoon. && .PREV DISCUSSION... /issued 441 AM AST Mon May 13 2024/ SYNOPSIS... Stable conditions during the morning hours and a southeasterly component in the wind flow will promote warmer conditions across the northern coastal areas of Puerto Rico. Warmer conditions are forecast to begin around 10 AM, which will be more significant for the coastal areas. Given that, a heat advisory was issued for the northern coastal municipalities. Residents are urged to stay hydrated and follow the local law enforcement recommendations. Typical weather is forecast for the short term, with mostly stable weather conditions in the morning, followed by afternoon showers across mountain sections. An upper-level trough is forecast to increase shower activity late in the upcoming weekend. SHORT TERM...Today through Wednesday... Overnight observations from satellite and radar have indicated mainly clear to partly cloudy skies, with a few isolated showers remaining over the local waters. From around 11 PM AST and continuing through the night, satellite-derived data indicated persistent foggy conditions along the Sierra Cayey and Sierra Luquillo. Overnight low temperatures ranged from the upper 60s across higher elevations to the lower 80s across northeastern Puerto Rico and the local islands. Winds were generally light and variable, influenced by land breezes. Expect drier-than-normal weather conditions during the short-term forecast period, with heightened rains favoring a typical seasonal pattern each afternoon. As the jet streak moves northward and away from the area, a mid-level high pressure system will start extending into the northeastern Caribbean, gradually establishing a trade wind cap inversion and enhancing drier air entrainment. Consequently, anticipate a reduction in moisture levels, with precipitable water values dropping from typical to below typical thresholds or from approximately 1.8 inches to around 1.5 inches through Tuesday evening, as moisture becomes confined below the 700 bar layers. From Tuesday night onward, precipitable water values will rise again to typical moisture levels of around 1.6-1.8 inches. Light concentrations of Saharan dust will persist throughout the forecast period. Despite conditions becoming less conducive to convective development and fluctuations in moisture levels, afternoon showers with possible isolated thunderstorms are still probable, even during the driest periods on Tuesday. As a result of diminishing area coverage and intensity, the risk level for excessive rainfall hazards will decrease from elevated today, with urban and small stream flooding likely, to limited on Tuesday. While there is a limited risk on Wednesday, the potential for flooding could increase with future forecast updates as moisture levels return to normal thresholds. As light winds shift from east-southeasterly today, afternoon activity will concentrate on central to western sections of Puerto Rico. However, as winds become more easterly and speeds increase to 10-15 mph on Tuesday and Wednesday, activity will likely focus on far western sections of the island. Due to a southeasterly component in the prevailing wind pattern, temperatures along the coast and in urban areas today could reach the upper 80s to mid-90s. Conversely, higher elevations might see highs ranging from the upper 70s to mid-80s. With plenty of moisture throughout the region, heat indices could surpass 102 degrees Fahrenheit in most coastal and urban areas, with some sections likely experiencing extended periods of 108 degrees or higher across western, northern, and eastern Puerto Rico and Saint Croix. Consequently, a Heat Advisory has been issued, effective from 10 AM to 5 PM AST this afternoon. Although model guidance indicates slightly cooler 925 mb temperatures in the following days, warmer conditions will persist. LONG TERM...Thursday through Monday... A typical weather pattern will be present for most of the long term. On Thursday, at the surface, east-southeast winds promoted from a surface ridge extending across all the central Atlantic will continue to inject the islands with tropical moisture. According to the late model guidance, PWAT values from Thursday to Friday will remain near the climatological normals. Both model guidances (GFS and ECMWF) at the upper levels agreed on an upper- level trough just in the region. Although the present moisture will keep high chances for general shower activity, the stability aloft provided for the convergent side of the TUTT will permit only localized showers resulting from the local and diurnal effect across the interior sections, the San Juan metro area, and the northwestern quadrant of Puerto Rico. As the upper feature moves out, moisture availability will increase even more by Saturday into Sunday when surface winds turn again from the southeast in response to a building high pressure just over the northwestern Atlantic. This influx of moisture and the warm temperatures over the land areas in the afternoon hours will enhance periods of moderate to heavy showers with isolated thunderstorms. Given the moderate southeasterly wind pattern, the showers will be mostly over the interior and northwestern sections; according to the Galvez-Davison Index (GDI-GFS), the shower activity is forecast to remain very localized. Unfavorable weather conditions are forecast for the upcoming week as upper-level conditions change. By late Sunday into Monday, an upper-level trough will move just over the region, leading to divergence aloft and colder temperatures at 500 MB, enhancing vertical development for shower activity. Uncertainty remains present due to the forecast period. However, the islands can expect increased episodes of widespread shower activity in the afternoon. AVIATION... (06Z TAFS) Mainly VFR conditions across most local terminals during the next 24 hours. Afternoon SHRA/TSRA may result in brief MVFR conditions, with periods of reduced visibility and lower ceilings, across TJSJ/TJBQ between 13/16-23Z. Light to calm and variable winds increasing to 8-12 knots and turning more from the east with sea breeze variations between 13/13-23Z. MARINE... Surface high pressure extending over the northeastern Atlantic into the Central Atlantic will continue to promote east- southeasterly winds today. Therefore, winds will remain at 15 knots or less, with some gusty winds near the afternoon showers across some coastal sections. Seas are forecast to remain up to 4 feet across most local waters and even lower across the coastal areas. From late Tuesday night onward, pulses of northeasterly swell will spread across the regional waters.Afternoon convection may result in thunderstorms across the coastal sections of north and western PR each day. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory until 5 PM AST this afternoon for PRZ001-002-005- 008-010>013. VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002. AM...None. && $$ ERG/GRS/YZR