161 FXUS66 KSGX 140950 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 250 AM PDT Mon Apr 14 2025 .SYNOPSIS... Little change in temperatures today followed by more significant cooling Tuesday through Friday. By Friday, high temperatures will be 10-20 degrees below normal. The marine layer will deepen through the week, filling in across the coastal basin by Wednesday. Patchy drizzle is possible west of the mountains each morning Tuesday through Thursday, followed by a 15-40 percent chance of precipitation Thursday night through early Saturday. Warmer and drier early next week. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... The 00Z KNKX sounding shows the marine inversion based near 1800 ft MSL, down from around 2200 ft yesterday morning. Marine layer low clouds have filled in over the coastal areas and western valleys, and will continue progress into the western and southern portions of the Inland Empire through the morning. Otherwise there were scattered high clouds moving north over the region ahead of an upper level low around 275 mi to our southwest. This low will open up and become a negatively tilted trough as it moves into So Cal and Arizona tonight. Overall not much change in temperatures today with highs near to slightly below normal west of the mountains but still 5-10 degrees above normal in the mountains and deserts. The passing wave tonight will cause the marine layer to deepen, potentially producing some patchy drizzle Tuesday morning. The path of the trough axis and associated vort max has trended further east, bringing less moisture and dynamics to So Cal, so precipitation chances over the mountains have dropped below 15% for tonight. Otherwise cooler Tuesday with breezy onshore winds below the passes Tuesday evening as another closed low develops off the coast. This low will progress westwards into Wednesday, opening up and moving into CA on Thursday as it phases with another trough dropping south through the Pac NW. This will maintain the cooling trend and generate stronger southwest to west winds across the desert mountain slopes and below the passes each afternoon and evening. The marine layer deepens significantly, bringing more late night and morning drizzle to areas west of the mountains. Some high-res guidance even shows legitimate showers from the marine layer on Thursday morning as it deepens to around 5000 ft. There is still some uncertainty with the evolution of the upper pattern late in the week. About 40% of the ensemble members maintain a large low progressing south into CA/NV, another 23% keep more of an inside track through NV/UT, and another 37% with a less amplified wave and an inland track. The track of this low has big implications on virtually every aspect of the forecast: temperatures, snow levels, precipitation amounts, thunderstorm chances, and wind. By Saturday around 45% of the members have the closed low somewhere over interior So Cal/southern NV, while another 36% maintain the inside slider track. If the low moves closer to us, expect cooler weather, higher precipitation amounts, and stronger onshore flow. The inside track will produce less cooling, lower precipitation amounts, and weaker winds. Depending on how far inside it tracks, we could end up with no precipitation outside of whatever drizzle/light showers the marine layer manages to produce. For now, the forecast maintains a 15-40% chance of precipitation Thursday night and Friday for all areas expect the lower deserts, with showers tapering off from north to south Friday evening into Saturday morning. && .AVIATION... 140930Z...Coasts/Valleys...Low clouds based 1000-1500 ft MSL and tops up to 2000 feet MSL will continue to spread into the Inland Empire. VIS reductions limited to the coastal slopes of the mountains and foothills. Low clouds scatter out in the inland valleys first after 15-16z, with the coasts clearing out by 18-19z. Slightly higher based low clouds will redevelop this evening and push inland after 03z. The coastal basin should fill in completely before 12z Tuesday morning. Elsewhere...SCT-BKN high clouds above 20000 feet MSL. Local west wind gusts 25-35 kt over the mountains into the passes and deserts return after 22z today and last through the evening. && .MARINE... No hazardous marine conditions are expected through Friday. && .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...SS AVIATION/MARINE...KW