High pressure will begin to weaken over the area into the weekend, which will continue to provide warm conditions. Patchy fog will continue near the coast and western valleys into early next week. An area of low pressure will push an atmospheric river over the region around the Christmas holiday, where there is an increased risk of heavy rain and flooding. The area of low pressure may stick around into the latter next week, leading to more chances for rain and high elevation snow.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Update: The low clouds and fog currently stretched along the coastal areas of San Diego County, and within portions of the inland valleys, is continuing to lift. This will allow for visibilities to continue to improve for the coastal areas, and cause these clouds to penetrate further eastward throughout the night. These clouds will also continue to fill into the IE by later tonight and into tomorrow morning as well. The marine layer will continue to lift and thicken throughout the weekend as the region begins to become more influenced by the broad area of troughing over the E Pacific Ocean, which is slowly advancing towards the region. In general, there has been little change in the forecast, which still shows the main brunt of the showers associated with the first initial shortwave to begin to move into the coastal areas of Orange County by as early as Tuesday morning, and then the heaviest precipitation will likely occur on Christmas Eve (Wednesday), as the main frontal boundary with this system moves overhead. There could also be some patchy areas of drizzle as the marine layer continues to thicken up ahead of this trough on Monday. There will also be a bit of a stalling out of the progression of this trough, and then a secondary low will likely continue in Christmas on Thursday, with a colder core transitioning over SOCAL, which will help to also lower snow levels (with possibly Big Bear getting in on some snow). After Christmas, there is some deviation with the deterministic models on how long the duration of this trough remains in place over E Pac waters, with a few of the members keeping the chance of precipitation going into the following weekend, while others have mostly everything clearing out by Friday. We will continue to evaluate this in the upcoming days.
, Previous Discussion (1029 AM Friday),
A 1023mb area of high pressure to the southwest will continue to dominate our weather pattern through the weekend. Visible satellite shows dense fog persistent over the waters and parts of the coastline this morning. Intermittent fog will continue throughout the day near the beaches before clouds move around 5-10 miles inland where the foggiest conditions will occur on elevated coastal terrain. As the ridge of high pressure slowly breaks down, a slight increase in onshore flow will occur, cooling the coast and valleys a few degrees from yesterday. The mountains and deserts will see similar temperatures with highs in the 70s/80s across the deserts. The cooling will slowly continue into the weekend and Monday as the marine layer deepens, though highs will remain above average. Highs will be 5-10 degrees above normal west of the mountains and 10-15 degrees above normal in the mountains and deserts.
Two features to the north will lead to a big change in the weather as we head into the Christmas holiday. An atmospheric river seen from the Hawaiian Islands into Northern California and an area of low pressure over the eastern Gulf of Alaska will begin to push southward by Monday and Tuesday of next week. The trough of low pressure will aid to push the atmospheric river southward, orienting the system in a more north/south direction. This will first provide heavy rain to parts of the Central Coast late Monday night into Tuesday and push southward into our region late Tuesday into Wednesday. As a series of shortwaves pass the trough's axis, rapid deepening of the low will occur. Confidence on timing continues to increase with the heaviest rain falling from the atmospheric river on Wednesday (Christmas Eve Day). The wind forecast has increased as models show higher chances for wind gusts mainly across the immediate coast and mountain areas to gust over 30 MPH at times as the AR passes by on Wednesday.
A secondary shortwave in the trough may provide another moderate to heavier round of precipitation on Christmas Day. The track of the large low pressure system still remains uncertain. Some models show a slower moving system, which would give us more chances of rain into next weekend, while others show a faster and not as wet solution. Model ensemble trends show a slight increase over the past few days, so rain chances continue into the forecast for later next week.
200530z, Coast/Western Valleys, Low clouds and fog spreading inland at this hour impacting coastal TAF sites with CIG and VIS restrictions. Low clouds likely to reach 15-20 miles inland by sunrise and could approach KONT around 12Z but confidence is not high. Cloud bases will be around 300 ft MSL vcnty KSNA and about 1000 ft MSL vcnty KSAN, rising after sunrise. Local VIS restrictions blo 1SM vcnty KSNA, improving after 15Z. Expect partial clearing to the coast 17Z-19Z.
Otherwise, FEW-SCT high clouds with unrestricted VIS through Saturday morning.
Areas of fog with reduced visibility below 1 nautical mile through Saturday morning. Following that, no hazardous marine conditions are expected through Tuesday morning. Increasing south winds develop late Tuesday in advance of a Pacific storm. Conditions could become hazardous to small craft Tuesday night through Wednesday.
Ca, None. PZ, None.
