Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
109 FXCA62 TJSJ 081955 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 355 PM AST Wed May 8 2024 .SYNOPSIS...Showers and thunderstorms are expected to diminish during the evening hours. Although trough aloft is expected to move away, locally induced afternoon showers and thunderstorms are still expected Thursday and Friday. There is a significant flood risk across the local islands due to saturated soils and river running well above normal values. Any rainfall will result in sharp raises along small streams and rivers. Model guidance still suggests a mid- to upper-level ridge trying to build over the Northeast Caribbean from Saturday onward, promoting stable conditions. && .SHORT TERM...Tonight thru Friday... Another active day has been observed across the local islands, with showers and thunderstorms over portions of central Puerto Rico. Rainfall totals so far have ranged between 2 and 4 inches with isolated higher amounts. Several Flash Flood Warnings were issued. The organized convection affecting the local islands will decrease as the upper-level trough moves east and away from the forecast area. Although convergence aloft is expected by Thursday night, fast moisture erosion is not expected at this time. In fact, precipitable water values will exceed 2.0 inches on Thursday, decreasing just below 2.0 inches on Friday. As a result, available moisture will combine with sea breeze convergence and diurnal heating to result in additional showers and thunderstorms on Thursday and Friday afternoons. At lower levels, east-to-east southeast winds at 10 knots or less will prevail through the forecast cycle. Under this evolving pattern, showers and thunderstorms are expected to develop across the northwest quadrant and portions of the San Juan Metro Area each afternoon. Due to the already saturated soils and rivers running above average values, heavy rainfall will produce sharp rises of rivers and small streams. In addition, warmer temperatures and higher heat indices are likely under east- southeast winds. .LONG TERM...Sat thru Wed/issued 439 AM AST Wed May 8 2024/ Model guidance still suggests a mid- to upper-level ridge trying to build over the Northeast Caribbean from Saturday onward, promoting stable conditions and potentially bringing relief from the current weather conditions. Model guidance also suggests a return to the climatological moisture values over the region, with the advection of occasional patches of enhanced moisture. Additionally, we are more likely to observe variable weather conditions with a mixture of sunshine/clear skies and clouds daily. Also, the NASA aerosol optical thickness (AOT) guidance one more time suggests the arrival of an air mass with possible African dust particles by the upcoming weekend, which may extend into next week. Meanwhile, intense heating, local effects, and sea breeze fluctuations each day will drive strong afternoon convection. Furthermore, the above-normal warmer sea surface temperature will also enhance the typical nighttime warm air convection. These frequent nighttime showers could affect the windward sections and local waters of the U.S. Virgin Islands and Puerto Rico. Combining high moisture content with the typical upper 80s to low 90s maximum air temperatures will result in heat indices between 100-107 Fahrenheit or even higher, especially during the daily maximum heating, across the USVI and PR`s urban and coastal areas where no significant rain is observed. && .AVIATION... (18Z TAFS) SHRA/TSRA will continue to affect the area, causing mountain obscuration across the Cordillera Central gradually moving towards the coast through at least 23Z. Tempo MVFR to brief IFR conds are possible through 23z at TJBQ/TJSJ/TJPS. Winds will turn light and variable after 22z with VCSH expected for the USVI and TJSJ terminals. Another round of SHRA and TSRA expected tomorrow along the Cordillera Central after 17Z. Winds are expected from the ESE at 5-10 kts. && .MARINE... A surface high-pressure north of the islands will yield easterly winds through Friday, becoming more east-southeast during the weekend. Pulses of northeasterly swell will spread across the Atlantic waters and Caribbean Passages. Weather conditions across the local waters are expected to improve Thursday onwards. && .BEACH FORECAST... Expect increasing seas of 4 to 6 feet across most of the local beaches in the Atlantic. As a result a moderate risk of strong rip currents is expected across the north and east-facing beaches in PR/USVI. && .HYDROLOGY... There is a significant flood risk across the local islands due to saturated soils and river running well above normal values. Any rainfall will result in sharp raises along small streams and rivers as well as mudslides, pariculalry central Puerto Rico. Some rivers will likely overflow their banks. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...OMS AVIATION...CVS