AFDSJU Area Forecast Discussion National Weather Service San Juan PR 440 PM AST Wed Jan 29 2025 .SYNOPSIS... Afternoon convection may linger through the evening hours due to an upper level ridge and light steering winds. This can lead to minor flooding. A weak Saharan Air Layer will gradually fill in tonight and last through Thursday. Increasing trades and the arrival of low-level trough will bring showers across the islands from Thursday night through Friday morning. Breezy conditions will prevail through the weekend, with passing showers moving from time to time during the night and early morning hours under an advective weather pattern. && .SHORT TERM...Tonight through Friday... Radar and satellite imagery this morning showed a persistent band of isolated to scattered showers and increased cloud cover over eastern Puerto Rico, which continued into the early afternoon. By 11 AM AST, locally induced showers began to develop over the south-central hills, gradually spreading westward and northward across Puerto Rico as the day progressed. Since midnight, radar-estimated rainfall totals have reached around three-quarters of an inch to an inch in eastern coastal municipalities, while surface rain gauge observations have remained below a quarter of an inch. Light and variable winds, influenced in part by sea breezes, prevailed throughout the day, with speeds reaching 10-12 mph. Surface observations showed slightly warmer daytime highs, ranging from the mid-70s in the higher elevations of the Cordillera Central to the lower 90s in parts of the San Juan metropolitan area and eastern St. Croix. A pre-frontal trough and its associated col region will sustain light and variable winds tonight, with moderate to fresh northeasterly winds returning on Thursday and fresh to locally strong easterly winds developing on Friday as the approaching frontal boundary and a strengthening surface high influence the region. Meanwhile, the mid-level ridge will gradually weaken and shift eastward as a cutoff low moves southward, eventually settling and amplifying over the Lesser Antilles by Friday. This pattern will maintain a weak and elevated trade wind cap inversion (~600 mb layer), which, combined with increased instability, will significantly raise precipitable water (PWAT) valuespartly due to lingering moisture from the frontal boundary and enhanced low-level convergencereaching up to 1.8-2.0 inches by Thursday night. Additionally, low concentrations of Saharan dust, steered by easterly winds and embedded within the moisture bands of the old frontal boundary, will arrive by Thursday, reducing visibility and impacting air quality. Lingering showers from today`s slow-moving, locally induced afternoon convection will continue to affect the region, primarily central and western Puerto Rico, through this evening. Overnight, the focus will shift to windward areas, including the local islands, as approaching moisture bands support isolated to scattered trade wind showers. Based on the latest model guidance, shower activity is expected to increase as moisture levels rise, coinciding with periods of high instability, particularly from Thursday night into Friday. The prevailing wind pattern will contribute to slightly warmer-than- normal conditions on Thursday, followed by a cooling and more seasonal trend as winds shift from the northeast beginning Friday. Forecast confidence remains moderate, especially for the latter part of the workweek, as model guidance adjusts peak moisture timing. However, the risk of excessive rainfall remains limited each day, with the highest impacts dependent on the prevailing steering flow. Additionally, non-thunderstorm winds and any associated wind-related risks are expected to return on Thursday. For more details, visit the Experimental Graphical Hazard Weather Outlook at https://www.weather.gov/erh/ghwo?wfo=sju. .LONG TERM...Saturday trough Wednesday...From Prev Discussion A broad surface high-pressure system over the central Atlantic will maintain a moderate easterly trade wind flow across the region on Saturday. Meanwhile, a strong upper-level trough developing just east of the islands will weaken the mid to upper- level ridge, reducing stability in the atmosphere. This strong upper-level future will reflect at the surface and cloudiness and moisture across the Atlantic will reach the local islands dragged by the easterly winds during the period. On Sunday, as the surface high-pressure system shifts eastward, the pressure gradient will become stronger, causing an increase in wind speeds across the region. This change will accelerate the easterly trade winds, bringing moisture inland and enhancing local effects, resulting in shower activity across the interior and eastern slopes sections of Puerto Rico. PWAT values will rise to approximately 1.5 inches. Combined to that moisture, global model guidance suggest an cold trend at the 500 MB temperatures by Friday into Saturday, providing space for the development of isolated thunderstorms. Expect stronger trade wind showers, some of which could bring brief heavy rain and gusty winds, especially in the overnight and early morning hours. From Monday through Wednesday, the easterly trade winds will continue to bring patches of moist air into the region, leading to partly cloudy skies and scattered showers. These showers will mainly affect coastal and windward areas during the night and early morning hours. By the afternoon, some localized convection could develop over western and interior Puerto Rico, mainly due to daytime heating and sea breeze interactions. However, as the mid- level ridge builds back in, it will help limit thunderstorm activity by creating a more stable atmosphere. Even so, brief heavy downpours, ponding of water on roads, and occasional gusty winds remain possible with the strongest showers. && .AVIATION... (18z TAFs) VFR conditions will prevail across all TAF sites. However, showers may result in brief MVFR conditions, with VCSH to -RA and SCT-BKN ceilings possible through 29/23Z across all sites and after 30/04Z for TJSJ and USVI terminals. East winds at 8 to 12 kt, becoming light and variable after 29/22Z, then increasing to 10 to 15 kt after 30/13Z. && .MARINE... A surface high pressure across the central Atlantic and a pre-frontal trough from a frontal boundary well at north of the region will result in light to locally moderate easterly winds across the local waters. Another strong high pressure will promote fresh to locally strong winds by the latter part of the week. Increased winds and a weak northerly swell will likely deteriorate marine conditions for small craft from Friday onwards. && .BEACH FORECAST... A low to moderate risk of rip currents will persist across the northern and eastern beaches of PR, Culebra, and St. Croix for the next few days. A small northwesterly swell and choppy wind- driven waves will increase the risk of rip currents during the weekend. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$