Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
566 FXCA62 TJSJ 220814 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 414 AM AST Wed May 22 2024 .SYNOPSIS... Unsettled, squally weather is expected to prevail through the end of the workweek. The risk for flooding, rapid river rises, and mudslides will be elevated. Conditions will gradually improve by the weekend, but still with the risk for locally strong rains in the afternoons for western and northern Puerto Rico. Some Saharan dust is expected early next week, as well as an elevated heat risk. && .SHORT TERM...Today through Friday... A significant increase of passing showers were observed across the Caribbean and Atlantic waters as seen on the radar overnight. It appears the trend towards unstable conditions are beginning as we approach wetter days for the rest of the week into the weekend. This morning an increase of passing showers across southern and eastern Puerto Rico and also across the USVI is expected as a moist airmass begins to lift and spread throughout the local islands. This is associated with an approaching deep-layered trough that is currently situated between eastern Cuba and Hispanola which will increase instability across the local area into the weekend. As this feature induces a surface perturbation south of Hispanola late tonight, winds are expected to strengthen over the western coastal waters through Thursday which can bring squally weather conditions. Precipitable water content is likely to reach above 2.4 inches will significantly enhance the rainfall activity across the region through this time. Thunderstorms capable of producing strong winds, heavy downpours, and frequent lightning remain very possible during today through Friday. Currently models are suggesting the heaviest congestion of rainfall to be across the interior to eastern sections of Puerto Rico, however elsewhere plenty of rain remains possible. The mid-level jet segment of this trough begins to move northward into the western Atlantic Thursday night, which is projected to then cause a surface low to form north of the region by Friday night. This will cause surface winds to veer through Thursday into Friday becoming generally southerly by the end of the week. A situation like this would bring a good chance for afternoon convection along northern Puerto Rico, however the influence of a thick cloud layer may disrupt diurnal heating. Nonetheless, conditions will remain unstable through this time for Puerto Rico and the Virgin Islands with an increased risk of flooding. .LONG TERM...Saturday through Wednesday... The islands will continue to be under the influence of a moist low to mid weak southerly flow associated with a trough centered well north of Puerto Rico by this time. Precipitable water values will remain above the climatological value for late May, meaning that the potential for showers will persist. On Sunday, the trade winds will begin to recover and will gain an easterly component, at about 10 to 12 knots. A mid to upper level ridge centered to the east of the Leeward islands will begin to push small pockets of drier air, along with Saharan dust, but the moisture channel just west of Puerto Rico will also push back, reaching the islands at times. Under this environment, it is likely that passing showers will continue to form around the Virgin Islands and eastern Puerto Rico, while strong afternoon convection is anticipated for the interior and western Puerto Rico each afternoon. In fact, the Galvez-Davison index indicate isolated to scattered thunderstorms from Saturday through Tuesday. The risk of flooding and mudslides will remain elevated for these areas. Temperatures will once again warm up to the low 90s, with heat indices above 102 degrees, mainly for the coastal and urban areas of the islands. && .AVIATION... (06z TAFS) VFR conditions will continue, however a rapid increase of shower activity will bring bring periods of reduced VIS and low ceilings. Chance of TSRA will increase through the day, worsening during the afternoon with this activity impacting mostly all terminals. Winds will blow from SE at 8-15 kts, with stronger wind gusts and sea breezes variations. && .MARINE... The interaction of an induced surface trough to the west and a weakening surface high pressure to the east will promote moderate east-southeast winds through today. As the surface trough moves closer to the region, expect increasing shower and thunderstorm activity across the local waters through early in the weekend. In addition, the proximity of an amplifying mid to upper level trough just west of the region will enhance thunderstorm development as well as increase winds and the potential for squally conditions across the Mona Passage and Atlantic waters late tonight through at least Friday. Winds are forecast to diminish once again by Friday night. && .BEACH FORECAST... Surf heights 4 to 5 increased the rip current risk for eastern St. Croix. Elsewhere, the rip current risk is low. && .HYDROLOGY... Periods of heavy showers and thunderstorms are anticipated, especially today through Friday. The USGS streamflows show that most rivers are running near normal to above normal. Additionally, the experimental Puerto Rico Landslide Forecast Network (Slides-PR) are showing high soil saturation levels. With prolonged episodes of heavy rain, the risk for flooding, rapid river rises, water surges and mudslides will be elevated. && .CLIMATE... The heat spell that has been affecting the islands should appease today and tomorrow due to abundant cloudiness brought by the deep- layered trough. So far, for the San Juan Area climate site (LMM Airport), eight daily warm-minimum temperatures, and three daily maximum records have been broken for May. To date, it is also the second warmest May in history (84.0F), after 2020 (84.1). The month is also running 3.96 inches wetter than normal. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...RC LONG TERM....ERG