872 FXCA62 TJSJ 250822 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 422 AM AST Thu Sep 25 2025 .KEY MESSAGES... * The wet pattern will persist during the day as trailing moisture from the tropical wave (Invest 94L) continues to affect the local islands. The additional rainfall accumulations will exacerbate flooding, especially across eastern sections of Puerto Rico. * For the U.S. Virgin Islands, additional shower activity is expected for the rest of the morning, enhancing urban flooding in already affected areas. * Tropical moisture from the tropical wave (Invest 94L) is expected to move out of the region by Saturday, resulting in a more typical pattern with variable days: warm mornings and showers in the afternoons. * For the long term, a wet pattern is forecast from Monday into Tuesday as moisture from the tail of tropical system Humberto moves into the region. * Seas will remain up to 6 feet today across the offshore Atlantic waters and a moderate risk of rip current will remain for the northern and western coastal areas. && .SHORT TERM...Today through Saturday... Overnight, the vigorous tropical wave (AL94) continued to promote widespread heavy showers and thunderstorms across the regional waters. By early night, a strong band of showers developed in a southeast-to-northwest orientation over St. Thomas. By that time, the wave was positioned just south to southwest of Puerto Rico, inducing a southeasterly flow over the region. This advected lines of strong showers into the southern coastal areas of Puerto Rico, prompting the issuance of a Flood Advisory for the southern coastal municipalities. Elevated streamflows persisted, with rivers across the eastern third of Puerto Rico running at or near record highs. A Flood Warning was issued for Rio Blanco due to overflowing banks, which rendered some roads along the river impassable. As the wave continues its west-northwestward movement, winds are expected to gradually shift more southerly, advecting additional deep tropical moisture over the region. Hi-Res model guidance suggests Thursday will maintain variable weather conditions, with morning convection mainly affecting the southern and interior portions of Puerto Rico and the smaller islands. During the afternoon, additional convection may develop across the northern slopes of Puerto Rico; however, the extent of this activity will depend heavily on cloud coverage during the day. Extensive cloud cover, as suggested by the high-resolution guidance, could limit instability and suppress afternoon thunderstorm development. Nonetheless, tropical waves often behave erratically in terms of shower and thunderstorm activity, and therefore, close monitoring is warranted. Looking ahead to Friday and Saturday, model guidance indicates notable changes compared to previous cycles. Now that Tropical Storm Humberto has developed northeast of the region, its forecasted track has separated slightly from AL94. This shift allows drier air to filter into the region much earlier than previously expected, reducing the interaction between the two moisture plumes. Both the GFS and ECMWF suggest a significant drop in mid-level relative humidity (700-500 mb) from values above 90% to below 40%, approaching below to near normal climatological levels. As a result, widespread shower and thunderstorm activity will likely diminish compared to earlier expectations. Still, localized afternoon convection remains possible, especially where daytime heating interacts with lingering moisture. Despite this drier trend, uncertainty remains high regarding the eventual evolution and interaction between AL94 and Tropical Storm Humberto. Should the systems shift track or slow down, moisture availability and convective potential could increase again. Therefore, residents and local authorities should remain alert for rapid changes in forecast conditions. With saturated soils and elevated rivers, the threshold for flooding and landslide impacts remains low, and even isolated strong thunderstorms could pose significant hazards. From Friday onward, the prevailing southerly flow combined with ample low-level moisture will maintain above-normal temperatures and elevated heat indices, especially across coastal and urban areas. Periods of reduced cloud cover could further enhance daytime heating, resulting in oppressive and potentially hazardous heat conditions. .LONG TERM...Sunday through Thursday.... From Sunday (September 30) through early Tuesday, the local weather pattern across Puerto Rico and the U.S. Virgin Islands will be influenced by two tropical systems situated over the Atlantic. The combination of these two systems will maintain a southeasterly wind flow across the islands. On Sunday, both global model guidance (GFS & ECMWF) show a less wet area moving into the region, with precipitable water values dropping to around 1.70 inches (near the 25th percentile). This brief window of reduced moisture presence will rapidly end as Hurricane Humberto progresses farther into the western Atlantic, leading to an increase in tropical moisture and the tail of the tropical system over the islands by Tuesday and into early Wednesday. During this period, mid-level lapse rates between 700 and 500 mb will support stable conditions aloft due to an upper-level ridge. Thus, although moisture levels increase, instability will be limited, and afternoon convection on Tuesday will be driven primarily by local effects. The bulk of the showers is forecast for the interior and northwestern sections, as well as the San Juan Metro area. By Wednesday, weather conditions become more typical, with PWAT values between 1.70 and 1.9 inches and relative humidity in the 700–500 mb layer climbing above 60%, allowing for variable weather of warm mornings with heat indices rising up to 108 degrees and active evenings with thunderstorms and gusty winds from 12 to 5 PM. By Thursday, conditions will evolve again as an upper- level low moves northwestward across the Caribbean, increasing instability over the region. Despite slightly lower moisture, with PWAT values still around the climatological median, the presence of instability will be sufficient to support scattered convection, particularly during the afternoon hours across the interior and northwestern Puerto Rico. Meanwhile, the synoptic pattern at the surface will favor a persistent east- southeasterly wind flow, with breezy conditions as suggest by the 925 mb wind speeds in the model guidance, largely driven by the interaction between a tropical system moving over the central Atlantic. This will sustain a warm trend in the morning with an increase in heat indices followed by afternoon thunderstorm development as the week progresses. && .AVIATION... (06Z TAFS) A vigorous tropical wave (AL94) will continue to bring SHRA/TSRA across through 24/15z, with TEMPO MVFR/IFR conds possible at times due to low CIGS/VIS, especially at TJSJ/TJPS/TIST/TISX. TSRA activity will gradually shift to the interior and northern PR by aftn, potentially affecting TJSJ with PROB30 for reduced VIS/CIGS. Elsewhere, VFR conds should prevail outside of convection. Sfc winds SE 10-15 kt, bcmg more S aft 24/15Z, with sea breeze variations at coastal terminals. Gusts up to 25-30 kt psbl in/near SHRA/TSRA. && .MARINE... A surface high pressure over the Western Atlantic and a tropical wave moving across the region will enhance a moderate southeasterly wind flow across the region. The actual moisture from the tropical wave (Invest 94L) will continue to result in localized hazardous marine conditions near the thunderstorm activity along the local waters. As a result, mariners are urged to exercise caution along the exposed local waters due to seas up to 6 feet, especially across the offshore Atlantic waters. Improving marine conditions are forecast for the upcoming weekend with seas up to 5 feet. && .BEACH FORECAST... There is a moderate risk of rip current along the northern and western coastal areas of Puerto Rico, including Culebra. For the rest of the coastal areas, the risk will remain low. Similar coastal conditions are expected to persist from Friday into Saturday. Thunderstorms producing frequent cloud-to-ground or water lightning will affect the local beaches. If you hear thunder, please seek shelter by going indoors or moving to a safe location. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM & AVIATION...CVB LONG TERM, MARINE & BEACH FORECAST....LIS