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,
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.
For Wednesday morning through Wednesday night for Lytle Creek on the coastal slopes of the eastern San Gabriel Mountains, the chance for 3 inches or more of rainfall has increased to 71 percent with a 35 percent chance for 5 inches or more and an 11 percent chance for 6 inches or more. For Wednesday morning through Thursday night, the chance for 8 inches or more of rainfall at Lytle Creek has increased to 13 percent. Chances for heavy rainfall continue to increase as model trends go upward.
Probabilities for seeing rainfall totals over 2 inches Tuesday through Thursday night are listed below:
OC/IE: 60-90% chance SD Co Mts/Coast/Valleys: 45-75% chance Deserts: 15%-25% (lower deserts), 40-55% chance (high desert)
Probabilities for seeing rainfall totals over 2 inches Tuesday through Thursday night are listed below:
OC/IE: 50-70% chance, 25-35% chance (eastern IE) SD Co Mts/Coast/Valleys: 20-50% chance Deserts: 5%-10% (lower deserts), 15-25% chance (high desert)
NBM deterministic rainfall for next week through Thursday night for northern Orange and southwestern San Bernardino Counties ranges from 3.5 to 4 inches near the coast to 6 to 9 inches on the coastal slopes of the mountains to the west of the Cajon Pass. Amounts decrease to the south to 1.5 to 2 inches near the coast of central and southern San Diego County to 2 to 3 inches for the mountains of central and southern San Diego County. For the high desert, 2 to 3 inches is expected with 0.5 to 1.5 inches for the lower deserts.
Most of the precipitation across southwestern California through Christmas Thursday is expected to fall as rain with snow levels remaining above 7000 feet. Snow levels will drop some by later next week, but exact levels and locations
191630z, Coast/Western Valleys, Locally dense fog mainly vcnty KSNA will lift and clear by 17-18Z. Areas of low clouds and fog will gather and spread inland after 02Z, eventually into western valleys overnight. Bases 300-600 feet MSL with vis 2-5SM, locally down to 1/4SM on the coastal mesas, higher coastal terrain, and western valleys. Scatter out Saturday 17-19Z.
Otherwise, FEW-SCT high clouds with unrestricted VIS through Saturday morning.
No hazardous marine conditions are expected through Tuesday morning, then gusty south winds developing Tuesday evening with a Pacific storm. Conditions could become hazardous to small craft at times Tuesday night and Wednesday.
Ca, Dense Fog Advisory until 10 AM PST this morning for Orange County Coastal Areas-Orange County Inland Areas.
PZ, None.
