Area Forecast Discussion
Issued by NWS San Diego, CA
Issued by NWS San Diego, CA
396 FXUS66 KSGX 242042 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 142 PM PDT Sun Aug 24 2025 .SYNOPSIS... Daily chances for showers and thunderstorms will continue through the middle of the week as the monsoonal weather pattern continues across the mountains and deserts. Valleys west of the mountains may see a storm or two, but otherwise, all areas will remain humid through the early part of the week. Cooler weather will occur by Tuesday through Friday. A weak trough offshore will help to lower chances for monsoonal storm activity across the mountains and deserts by the end of the week with less humid conditions. The marine layer will be of similar depth over the next few days with patchy fog near the coast, pushing further inland by the end of the week. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Clouds continue to build over the mountains this afternoon with weak thunderstorms forming across southeastern San Diego County. Thunderstorm activity will continue to increase this afternoon over the SD/RIV Co mountains, locally up into the eastern SBD mts. Rain rates will generally be under one half inch across these areas with desert areas seeing another chance to see light rainfall. Areas west of the mountains are seeing another warm and sticky afternoon as dew points reach near 70 degrees for western San Diego and Orange Counties. The marine layer will slightly deepen tonight, where most places along the coast will see morning clouds and fog by Monday. The monsoonal moisture influx will peak on Monday as PWATs surge above 1.5 inches across the lower deserts. Atmospheric sounding profiles show a fairly deep layer of moisture between 300-700mb and adequate instability to have scattered thunderstorms occur across the mountains and deserts. Hi-res guidance shows the greatest chance (70-80%) of storms to occur will be from the Mexican border north to Big Bear, with chances closer to 40-60% across the deserts. Confidence in how heavy the rainfall will be is moderate as models depict areas seeing rates near 0.50-0.75 inches per hour by Monday afternoon. These storms may be accompanied by heavy downpours and minor flooding, small hail and gusty winds. The flow aloft is fairly southerly, so storms will not drift too far west off the mountains, but adjacent valleys have a smaller chance (15-30%) to see any rainfall. These same areas will see another chance for storm activity by Tuesday, but chances are expected to be slightly lower (50-65%) as moisture decreases with PWATs closer to 1.25" across the lower deserts. Hourly rain rates therefore, will lower closer to one half an inch or less; chances also decrease to see rainfall across the far western valleys. Areas west of the mountains will remain on the humid side through Tuesday as monsoonal moisture hangs on. The good news will be a weak trough off the California coast will begin to form as the high aloft weakens. This will allow for high temperatures to cool a few more degrees closer to average. High temperatures will remain very similar Tuesday through Friday. By Wednesday, monsoonal moisture and instability will continue to decrease, leaving around a 15-30% chance of thunderstorms across the mountains and deserts. The trough`s influence becomes more pronounced as 500 mb winds, that are now southerly, begin to turn westerly. This will help to bring in drier air aloft and stabilize the atmosphere by Thursday into the weekend. Storm chances across the mountains will be minimal by this time period, with less humid weather expected for all areas. Ensemble models show the high across the desert southwest may restrengthen some by next weekend, which may bring slightly warmer weather. Looking further out into early next week, a trough across the Pacific may begin to influence the weather across California, which would bring near to below average temperatures and dry weather for our area. && .AVIATION... 242100Z...Coast/Valleys...VFR conditions with some mid and high clouds at/above 15,000 ft MSL through this evening. Patchy low clouds based 500-800 ft will slowly develop overnight tonight, after 08z Monday. Fog will redevelop in inland valleys and higher coastal terrain up to 10 miles inland with VIS locally below 1SM. VIS improves around 15z and clearing 16-17z. Mountains/Deserts...SHRA/TSRA have developed over the mountains. 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. 25-35% chance a SHRA/TSRA could drift over or develop over the low deserts through 02z this evening. Higher chance and greater coverage of +SHRA/TSRA over the mountains and deserts Monday afternoon after 19z. 45-55% chance of SHRA/TSRA over the lower deserts with more widespread impactful conditions - erratic winds, hail and CG lightning - Monday afternoon. && .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...APR AVIATION/MARINE...Westerink