Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
592 FXCA62 TJSJ 280736 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 336 AM AST Tue May 28 2024 .SYNOPSIS... Marginally unstable weather conditions are expected to persist across the region through the next several days. A southeasterly wind flow will continue to bring plenty of tropical moisture from the warmer Caribbean waters, enhancing the potential for strong showers and thunderstorms over the interior and northwestern Puerto Rico each afternoon.A relatively drier airmass is expected to filter into the region by the end of the workweek, promoting stable weather conditions and warm to hot temperatures. && .SHORT TERM...Today through Thursday... Overnight observations from satellite and radar have indicated persistent variable to mostly cloudy skies across the region. Scattered showers with isolated thunderstorms and areas of persistent light rains were observed across the local waters, with some moving over the US Virgin Islands, Vieques, Culebra, and Eastern Puerto Rico. So far, the highest rainfall totals of up to three quarters of an inch were observed over western Saint Thomas. Overnight low temperatures ranged from the upper 60s across higher elevations to the lower 80s across eastern and southern Puerto Rico and the local islands. Winds were generally light and variable in Puerto Rico, influenced by land breezes, and from the east-northeast across the US Virgin Islands. As the tropical wave moves away from the region, moisture levels will gradually decrease over the next few days. However, persistent moisture will keep precipitable water (PWAT) values well above the climatological thresholds, exceeding 2.0 inches through at least late Wednesday night. By Thursday, PWAT values are expected to drop to more typical levels, around 1.8 inches. Meanwhile, the upper- level trough will start lifting northward and weakening. Still, it will remain close enough to hold a mid-to-upper-level ridge to the south-southeast, maintaining marginally unstable conditions. Jet dynamics aloft and 500 mb temperatures between -6 to -7 degrees Celsius support this instability. As the ridge moves closer, conditions will likely become less conducive to deep convective development by midweek due to the entrainment of drier air and higher-than-normal 500 mb temperatures. East to east-southeast winds at 5-15 mph will continue, resulting in a relatively weak steering flow. Weather conditions will gradually transition to a more typical seasonal shower pattern, driven by diurnal heating and local effects, with enhanced afternoon convective development over the interior and west to northwestern Puerto Rico each day. Afternoon convection is also possible elsewhere, and similar to the potential impacts from overnight and early morning showers affecting eastern Puerto Rico and the U.S. Virgin Islands, the expected flooding risk is minimal. Recent GOES-16 Geostationary Lightning Mapper (GLM) data, along with the Galvez-Davidson Index and the latest model guidance, indicate the potential for scattered thunderstorms, some capable of producing heavy rain or scattered shallow convection with isolated thunderstorms. This scenario raises concerns about elevated flooding risks in urban areas, roads, small streams, and washes. Given the current soil saturation levels between 90-100%, isolated flash floods and mudslides in steep terrain are also possible. Although the intensity and spread of the expected activity may decrease, the flood risk will persist throughout the forecast period. Despite persistent cloud cover and high rain probabilities, warmer- than-normal conditions will continue. Coastal and urban areas are expected to have lows in the lower 80s, while higher elevations may see lows in the upper 60s to around 70 degrees. In contrast, highs will range from the lower 90s in lower elevations to the upper 70s in higher elevations. With ample moisture present, heat indices may exceed 108 degrees Fahrenheit in many coastal and urban areas, with some locations experiencing brief periods of 112 degrees or higher. These warm conditions pose a significant health risk, prompting the issuance of Excessive Heat Warnings/Advisories. Residents and visitors should take necessary precautions, particularly those engaging in outdoor activities. .LONG TERM...Friday through Tuesday... A mid-to-upper level ridge moving from the south-southeast into the local area will promote relatively drier weather conditions and warmer temperatures. Nonetheless, light wind flow from the southeast in combination with daytime heating and local effects will enhance showers and thunderstorm development across the northwestern portions of Puerto Rico each afternoon. Despite of the presence of the ridge, tropical moisture being pulled from the warm Caribbean Waters will approach the local area by late Saturday. The latest Precipitable Water Content Analysis suggest values from 1.9 to 2.45 through the period. Due to the expected conditions, there is a limited to elevated risk of urban and small stream flooding and isolated flash flooding mainly for central, northern and northwestern Puerto Rico. By Sunday into Tuesday a weak surface trough is forecast to develop west of our area, increasing once again the potential for moderate to heavy rainfall across the islands. Warmer temperatures are expected to persist through the entire period with values between the upper 80s to low 90s along the coastal and urban areas of the islands. Heat indices are likely to surpass the mid 100s, resulting in excessive heat conditions for the islands. && .AVIATION... (06z TAFs) SHRA/TSRA will result in brief MVFR conds at TJSJ/USVI terminals through 28/14Z. Afternoon SHRA/TSRA will generate MVFR to brief IFR conditions across TJSJ/TJBQ between 28/16-22Z, followed by VCTS after 28/22Z. VCSH possible across USVI terminals throughout the period. Light and variable winds across PR terminals and E-ENE winds at 5-10 knots across USVI terminals. Winds will increase to 10-15 knots after 28/14Z, with sea breeze variations and higher gusts near SHRA/TSRA. && .MARINE... A moderate east to southeast wind flow will prevail across the region over the next few days. Showers and isolated thunderstorms associated with the trailing moisture of a departing tropical wave and a surface trough east of us will continue to affect the regional waters and local passages overnight. So far, the next tropical wave, located in the central tropical Atlantic near 47W, is forecast to approach the regional waters by mid-week, with its bulk of moisture well to the south across the Caribbean Sea. && .BEACH FORECAST... There is a low risk of rip currents across all beaches of Puerto Rico and the U.S. Virgin Islands tonight. Moderate to heavy showers and isolated thunderstorms are possible each afternoon mainly across the northwestern waters of Puerto Rico. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning from 10 AM this morning to 6 PM AST Wednesday for PRZ001-005-008. Heat Advisory from 10 AM this morning to 6 PM AST Wednesday for PRZ002-003-007-010>013. VI...Heat Advisory from 10 AM this morning to 6 PM AST Wednesday for VIZ001-002. AM...None. && $$ SHORT TERM...ICP LONG TERM....GRS AVIATION...ICP MARINE...CAM