FXCA62 TJSJ 100852 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 452 AM AST Fri May 10 2024 .SYNOPSIS...Elevated to locally Significant Rainfall Risk today across the region due to afternoon convection enhanced by unstable conditions and saturated soils. Yesterday's flooding event continues to affect areas near Rio Grande de Manati, Rio Cibuco, Rio Guanajibo and Rio Culebrinas, where Flood Warnings are in effect. Patchy fog continues to affect areas of the interior and light to moderate showers are currently affecting eastern municipalities. The upper-level trough will continue to weaken as it moves away from the region. Simultaneously, an upper- level jet streak will spread across the northeastern Caribbean, further enabling the development of deep convection through at least Saturday night. A shift towards southerly winds this weekend will prompt a warmer trend, further boosting afternoon convective development. This can also promote limited to locally elevated heat risk as we head into the weekend. Lingering saharan dust will persist through at least early next week. && .SHORT TERM...Today through Sunday... Overnight satellite and radar observations have indicated the development of showers over the local waters, with some moving across the local islands and eastern Puerto Rico. Rainfall totals have peaked at approximately 2 inches between Naguabo and Ceiba, and around a quarter of an inch in St Croix. Despite clearer conditions elsewhere, excessive runoff from previous heavy rainfall has caused Rio Guanajibo, Rio Culebrinas, Rio Grande de Manati, and Rio Cibuco to remain above the action to flood stage, leading to continued Flood Warnings for neighboring municipalities. From around 8 PM AST, satellite-derived data has also indicated persistent foggy conditions extending from Orocovis east-southeastward to Cayey. Coastal areas have experienced temperatures ranging from the mid-70s to around 80 degrees, while higher elevations have recorded temperatures in the mid-to-upper 60s. Winds have generally been light and variable, influenced by land breezes. The upper-level trough will continue to weaken as it lifts out to the northeast, away from the region. Simultaneously, an upper-level jet streak (70-90 knots) will spread across the northeastern Caribbean, further enabling the development of deep convection through at least Saturday night, with 500 mbar temperatures falling to around -7 degrees by Saturday evening. Consequently, there is a potential for isolated to locally scattered thunderstorms, as indicated by the Galvez-Davison Index. According to the latest model guidance, the most active period is expected during peak diurnal heating this afternoon and again on Saturday afternoon. Nevertheless, anticipate prolonged stormy conditions over the local waters tonight and again over Saturday night, with some activity affecting Puerto Rico's eastern half, Vieques, Culebra, and the U.S. Virgin Islands. Although conditions may become less favorable for convective development on Sunday, afternoon showers with isolated thunderstorm development are still likely. Surface winds will persist at light to moderate speeds, ranging from 5 to 15 mph, with easterly winds gradually transitioning to south- southeast by Saturday and Sunday. This shift towards southerly winds will prompt a warmer trend, further boosting afternoon convective development. As the prevailing steering flow influences the distribution of showers, anticipate the highest rainfall totals to transition from the interior today to the northern sections of Puerto Rico during the weekend. Furthermore, a southwesterly component at the 700 mbar level will likely drive some afternoon activity towards the San Juan metropolitan area and northeastern Puerto Rico. Given the persistently weak steering flow, moderate to locally heavy rainfall from slow-moving showers over already saturated soils will sustain an elevated flooding risk, exacerbating existing flood situations, and disrupting ongoing recovery and cleanup efforts. Temperatures may soar into the upper 80s to mid-90s across coastal and other urban areas, while higher elevations could see highs in the upper 70s to mid-80s. With plenty of moisture across the region, heat indices could exceed 102 degrees Fahrenheit, potentially reaching around 108-110 degrees in localized coastal areas of northern Puerto Rico over the weekend. .LONG TERM...Monday through Friday... No major changes to the long term forecast. Model guidance to suggest a mid- to upper-level ridge building over the region by the start of the workweek, potentially promoting more typical weather with afternoon convection due to diurnal and local effects. Precipitable water (PWAT) values are forecast to generally stay at normal values between 1.5 to near 2.0 inches, through the workweek as patches of moisture advect into the region. ESE winds to start the period will back to become more easterly by late Monday through the rest of the period as a surface high moves into the western Atlantic. Most available moisture should reach up to 850 to 800 mb for the period. The NASA aerosol optical thickness (AOT) guidance continues to suggests the presence of an air mass with Saharan dust particles lingering to start the period, this could extend through the period. Limited Heat Risk (heat indices from 102 to 107 degrees Fahrenheit) or even higher, especially during the daily maximum heating, will persists across several lower elevation sectors of the islands where no significant rain is observed. This diurnal heating, local effects and sea breeze variations will continue to promote up to strong afternoon convection, steered by east-southeast winds on Monday, backing to become more easterly winds for the rest of the workweek, promoting convective activity mainly over sectors of the interior to western half of the Puerto Rico and downwind of the local islands. Above-normal sea surface temperatures can also promote nighttime showers across windward sectors of the islands, patchy fog also forecast for areas of the interior during the early morning hours. && .AVIATION... (06Z TAFS) SHRA may result in reduced visibilities and lower ceilings across the USVI terminals and TJSJ through 10/14Z. Afternoon SHRA/TSRA development will prompt VCSH/VCTS and, thus, possible MVFR conditions, mainly across PR terminals between 10/16-22Z. The potential for VCTS and showers will likely increase after 10/22Z for the USVI and TJSJ sites. Light and variable low-level winds, increasing to 10-15 knots and influenced by sea breeze variations between 10/14-22Z. Light and variable winds will return after 10/22Z. && .MARINE...A surface high-pressure across the Atlantic will yield light to moderate easterly winds through today, becoming more east to southeast during the weekend. Pulses of northeasterly swell will continue to spread across the Atlantic waters and Caribbean Passages for the next few days. Weather conditions across the local waters are expected to gradually improve towards the weekend. && .BEACH FORECAST...A High Rip Current risk is in effect for the northern coastline of Puerto Rico, from Aguadilla to Fajardo, through this afternoon. A Moderate Risk of Rip Currents is in effect today for Aguada, Rincon, Culebra and most of the northern USVI's coastlines and for the eastern half of St. Croix. Low risk elsewhere. The risk of rip currents are forecast to gradually diminish and an overall low risk of rip currents is forecast next weekend. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...High Rip Current Risk through this afternoon for PRZ001-002-005- 008. VI...None. AM...None. && $$ SHORT TERM...ICP LONG TERM....MRR