713 FXUS66 KSGX 171150 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 350 AM PST Fri Jan 17 2025 .SYNOPSIS... Onshore flow will bring cooler conditions and a return of marine layer low clouds during the nights and mornings through Sunday. The lower elevations west of the mountains will also see higher humidities. Westerly winds will also strengthen today, becoming locally gusty in the mountains and deserts through tonight. Dry weather is expected next week with periods of Santa Ana winds. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... This morning...Marine layer low clouds and higher humidities have returned to areas west of the mountains, although the coverage is patchy it seems to be increasing. The low cloud coverage will likely increase tonight into Saturday morning. Surface pressure gradients have turned onshore, enough to support westerly winds locally gusting 30-45 mph in the favored locations of the mountains and adjacent desert slopes. The onshore flow will weaken through the weekend as the upper level low over the lower Colorado River basin continues to move east, disappearing into the mean flow around the base of the longwave trough which will continue to dominate the pattern over the interior of North America at least through early next week. Meanwhile, another closed upper low formed over the Pacific Northwest and retrograded into a position about 700 miles off the coast of Northern California. Numerical models are in good agreement into Monday with respect to the synoptic pattern, and they show the closed low off the NorCal coast moving southward to a position west of Point Conception by Monday morning. Beyond that, model solutions begin to diverge significantly injecting much uncertainty in the specifics of the forecast beyond Monday. beginning several days ago, the GFS was the first model platform to resolve the low off the NorCal coast and has been initializing reasonably well with the evolution of that system which will ultimately have some influence over our weather at least through early next week as it moves into the region. We can say with reasonable confidence that this weekend will be cool with daytime temperatures generally below seasonal averages, except in the mountains and high deserts where temps will be near or a few degrees above seasonal averages. The return of onshore flow will bring marine layer humidity, and low clouds during the nights and mornings. As the onshore pressure gradients weaken, the gusty winds in the mtn passes and on the desert slopes will also weaken so as to have little or no impact beyond tonight. After Sunday, the marine layer will have little influence beyond the immediate coast. For most of next week we will resume a pattern of dry conditions with periods of Santa Ana winds of uncertain timing and strength. Mostly due to its recent performance, the forecast currently leans toward the GFS solution which shows localized weak to moderate Santa Ana winds on Monday and Tuesday. Followed by even weaker winds for Wed and Thu. Friday has the potential for stronger Santa Ana winds due to stronger offshore surface pressure gradients coupled with upper level support. A warming trend will begin on Monday, likely peaking on Thursday when daytime high temps could be up to 10 degrees above seasonal averages west of the mountains. && .AVIATION... 171045Z...Very patchy low clouds based around 1500-2500ft MSL are present along the coasts and inland valleys this morning. Given the highly patchy nature of these clouds, any potential CIGs at a given location will be very brief. Any lingering clouds will clear out around 17-19z. Low clouds look to push back ashore Friday evening after 03z. Coverage looks to be similarly patchy though possibly somewhat greater coverage than this morning. Bases look to be similar as well, around 1800-2500 ft MSL. NWS San Diego is aware of limited communications of observations from KTRM since January 8th. We are working with the FAA to address this issue. && .MARINE... o hazardous marine conditions are expected through Tuesday. && .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...PG AVIATION/MARINE...Munyan
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