Area Forecast Discussion
Issued by NWS San Diego, CA
Issued by NWS San Diego, CA
808 FXUS66 KSGX 240928 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 228 AM PDT Sun Aug 24 2025 .SYNOPSIS... High temperatures will remain near or a little above average through Monday. High temperatures will cool to a few degrees below average for Tuesday through Friday. Significant monsoonal moisture will remain in place through Tuesday with chances for afternoon and early evening showers and thunderstorms each day, mainly for the mountains and deserts. Drying will begin on Wednesday and continue into next weekend with decreasing thunderstorms chances each day. Night and morning low clouds will mostly be limited to coastal areas through the weekend, but could return to the western valleys for the latter half of next week. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Today through tonight... A subtropical ridge has expanded over the region, and allowed for high pressure to build in quite strong. The center of the high pressure is located over northern Arizona, and this has allowed the clockwise flow to advect in some mid-level monsoonal moisture over SOCAL. Due to this, humidity has increased, keeping temperatures somewhat modified and slightly cooler over the deserts. There will be lack of mid to upper level cloud coverage this morning, unlike yesterday, which will result in temperatures being slightly warmer for the deserts. Given the convection earlier in the day, the disruption to the marine layer has prevented it from formation this morning, and therefore this will likely struggle to fill in. The most likely locations of going broken to overcast this morning will be along the coastal ares of southern San Diego County, and perhaps for several hours at SAN, or in the vicinity. Showers and thunderstorms will develop again over the mountains, and possibly over the deserts, especially adjacent to the east-facing slopes of the mountains. Water vapor does reveal that there will be less moisture to work with in the mid levels this afternoon, so coverage should be quite sparse and mainly just over the higher terrain (with orographic lifting as the forcing). Given the steering flow being primarily out of the south, any showers/storms that do develop are likely not going to get into the inland valleys. The marine layer is going to be similar tonight, although there should be better coverage as onshore flow is enhanced throughout the day tomorrow. There could also be some areas of patchy fog along some of the coast for Orange and San Diego counties, as well as over the inner coastal waters and within some of the inland valleys. Monday through next weekend... Beginning Monday, the ridge is finally going to begin to break down slightly with better influence from the trough upstream. The will begin a gradual cooldown which will continue through the middle of the week, as the trough begins to propagate ever so slowly towards the region with lower heights aloft advancing in from the northwest. There will still be residual monsoonal moisture moving in some the southeast around the flow of the high pressure to the northeast, which will keep the threat of afternoon showers and thunderstorms still alive over the mountains, and possibly the deserts, at least through Wednesday...with even a lesser chance of the inland areas (less than a 20 percent chance). As the troughing continues to slowly advance towards the region, the tightening of the pressure gradient will allow for afternoon gusty winds to increase over the higher elevations and within the areas prone to strong and gusty/gap flow winds. By Thursday, there should be enough drier mid-level flow between 700-500mb due to the flow becoming more southwesterly that this should help inhibit anymore afternoon convection, and this should remain the same through next weekend, with similar temperatures. The marine layer will also become slightly thicker and elevated throughout the course of the week. && .AVIATION... 240900Z...Coast/Valleys...Patchy low clouds currently in far southern San Diego Co based 500-800 ft MSL will continue to develop along parts of the coast and spread locally 5-10 mi inland this morning, with a 50-60% chance of CIGS at KSAN and KCRQ 11-16Z. Chance of CIGS at KSNA is less than 20%. Vis 3-6 SM in BR/HZ where low clouds develop, locally 2 SM or less over higher coastal terrain. Clouds and vis restrictions clearing 15-17Z, then redeveloping early Monday morning after 25/09Z. Otherwise variable mid and high cloud cover at/above 15,000 ft MSL through this evening. Mountains/Deserts...25-45% chance of SHRA/TSRA over the mountains and deserts 19-02Z this afternoon. CB bases near 12 kft with tops to 35 kft. SHRA/TSRA may produce local VIS of 1-3 SM, gusty and erratic winds, cloud-to-ground lightning, and small hail. Otherwise variable mid and high clouds at/above 15,000 ft MSL through this evening. && .MARINE... No hazardous marine conditions are expected through Thursday. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...None. && $$ PUBLIC...Stewey AVIATION/MARINE...CSP