Area Forecast Discussion
Issued by NWS San Diego, CA
Issued by NWS San Diego, CA
549 FXUS66 KSGX 121534 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 834 AM PDT Mon Aug 12 2024 .SYNOPSIS... Monsoonal moisture will decrease today, with a lingering slight chance of thunderstorms for the southern mountains and deserts this afternoon. This will be followed by dry and seasonal weather through the week as a trough to the north brings in cooler and drier air. The marine layer will provide more consistent low clouds and fog to coastal areas each night and morning. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Residual monsoonal moisture will hang in the southeastern part of our region one more day, and that will bring a slight chance of thunderstorms this afternoon, mainly in the mountains and Anza Borrego Desert. Low clouds and fog were on the patchy side this morning, tickling parts of the coast. An upper level trough of low pressure off the Pacific Northwest coast will complete its breakdown of our hot ridge of the weekend. That will bring temperatures back to normal today and it will also sweep eastward the lingering monsoonal moisture today and tonight. The moisture decreases during the day, so there will be a race between the increasing convective trigger mechanism (surface heating) and the decreasing moisture. Will the two trending forces overlap enough to produce a thunderstorm or two? Hence, the slight chance in southern and eastern parts of our region. The trough will boost onshore winds during the afternoons and nights today through Tuesday night. Strongest winds will blow through mountain passes into adjacent deserts, where top gusts could exceed 40 mph. With no more monsoon disruption, the marine layer will rebound so that coastal low clouds and fog will become more uniform, extensive and longer lasting. But the marine layer doesn`t appear to deepen enough for clouds/fog to extend beyond the westernmost valleys each night and morning. A coastal eddy could enhance the marine layer and cloud coverage Wednesday and Thursday. Counterintuitively, high pressure aloft actually strengthens those days, so inland temperatures will be a few degrees higher. Monsoon moisture appears to increase this coming weekend, leading to a very small but increasing chance of thunderstorms for mountains and deserts by Sunday or Monday. Not enough to make our forecast yet, but something to watch and fine tune in coming days. The low pressure to our northwest and a growing high pressure to our east seem to set up a stationary pattern with SoCal right in between the two. Some guidance indicates the trough dominating while other guidance suggests the ridge taking precedence. For now we have a fairly persistent forecast in our fair weather pattern. && .AVIATION... 121530Z...Coast/Valleys...Patchy low clouds with bases 400-800ft MSL were slow to build in this morning, briefly impacting coastal sites, and clearing is starting to occur. SCT-BKN high clouds with bases to 15000ft-20000ft MSL possible through the afternoon. Redevelopment of patchy low clouds at coastal sites expected again tonight as early as 06z, with bases to 1000ft MSL and localized VIS reductions down to 3-5SM for higher coastal terrain and valleys. Mountains/Deserts...Unrestricted VIS with FEW-SCT to 20000ft MSL through the evening. FEW Cu with bases to 12000ft MSL near the mountains this afternoon with iso TSRA (10%-15% chance) which may bring gusty and erratic winds. Mod up/downdrafts near the mountains this afternoon. Impacts from storms not expected at any TAF sites, and storms diminish by sunset. && .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...MM AVIATION/MARINE...Zuber