Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
919 FXCA62 TJSJ 222034 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 434 PM AST Wed May 22 2024 .SYNOPSIS... Unstable weather conditions are expected to persist until the upcoming weekend, as a deep-layered trough approaches the Hispaniola. Moderate to locally heavy showers are expected to affect the region. Therefore, there is an elevated risk of urban and small stream flooding, rapid river rises, and mudslides. Weather conditions are expected to improve by the weekend with the arrival of drier air and Saharan dust particles. && .SHORT TERM...Tonight through Friday... Bands of moderate to locally heavy showers have been observed across the local region throughout the day. However, the bulk of scattered to numerous showers have stayed over the Mona Passage. Doppler radar estimated between 1.0 to 1.5 inches of rain mainly across the eastern half of Puerto Rico. Despite the cloud coverage across the region, daytime temperatures were observed in the mid to upper 80s along the coastal and urban areas of Puerto Rico and the U.S. Virgin Islands. Heat indices reached the low 100s in these sectors. Rainfall activity will increase overnight over the eastern and southern portions of Puerto Rico and the U.S. Virgin Islands. This activity could lead to minor flooding in flood prone areas. As a deep-layered trough located west of Hispanola approaches the local area, instability is forecast to increase and persist through the upcoming weekend. By Thursday, showers and isolated thunderstorms are expected to continue affecting the area. Therefore, there is an elevated risk of urban and stream flooding, rapid river rises and mudslides for the entire region. In terms of heat, we will continue under a vast cloud coverage inhibiting those temperatures to rise. However, expect daytime temperatures to remain in the mid 80s through the rest of the period. .LONG TERM...Saturday through Wednesday... PREV DISCUSSION.../issued 414 AM AST Wed May 22 2024/ 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... (18z TAFS) SHRA/TSRA en route from the Leeward Islands will spread across the USVI and eastern/southern PR terminals through the rest of the evening and overnight hours. Therefore, MVFR to brief IFR conds are possible across TIST/TISX/TJPS/TJSJ. Additionally, a deep layered trough over Hispaniola will continue to cause TSRA across the Mona Passage through the forecast period, impacting mainly TJBQ with VCTS/VCSH. BKN/OVC lyrs expected thru FL120. Low-level winds ESE up to 21 kt blo FL080, bcmg S/SW abv and increasing with height. && .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 located south of the Dominican Republic will enhance thunderstorm development as well as increase winds and the potential for squally conditions across the Mona Passage and Atlantic waters from 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. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...RAM LONG TERM....GRS AVIATION...DSR PUBLIC DESK...LIS