December 19, 2018 – Patchy valley fog this morning, otherwise dry today. light rain spreading across the north state Thursday through Friday. Dry Saturday with another round of light precipitation across the north state Sunday. Heavier precipitation possible Monday.
Upper level high pressure ridge over the west coast bringing dry conditions over the north state this morning. Some high cloud cover is spilling over the ridge and across the CWA but areas of fog have still developed throughout the central valley. Cloud cover has brought up temperatures a few degrees this morning so early morning temperatures are up a bit. Fair skies and a warming airmass will bring warmer temperatures today with highs 10 to 15 degrees above normal.
A weak Pacific frontal band is forecast to move into northwest California Thursday morning. High clouds will begin spilling in overnight but valley fog is likely again Thursday morning. If current forecast timing holds, any precipitation will stay out of most of forecast area until after the evening commute. Most of the CWA will see light precipitation Thursday night or Friday morning. Snow levels are expected to be below pass levels but precipitation amounts are not anticipated to be high enough for more than an inch or two of snow so impacts with this system should be minimal. A secondary shortwave trough may bring a few showers Friday afternoon but most areas will see drying Friday afternoon and evening.
Upper ridge builds temporarily over the west coast for a dry day Saturday with daytime highs remaining above normal. Dry period will be brief, however, with next weak Pacific frontal band spreading light precipitation over the northern half of the CWA Saturday night.
Extended Discussion (Sunday through Wednesday)
The active pattern across the Pacific will continue allowing for additional precipitation chances into next week. Beginning with the initial shortwave arriving early Sunday morning, models have come into better agreement favoring a drier scenario. The exception would be the more northern counties which reside closer to better lifting mechanisms. Moving into Sunday night and Monday (Christmas Eve), a much more powerful system is forecast to impact the region. It should take a more northwest to southeast path across Northern California which is not the most favorable for heavy rainfall across Valley locations. Based on the latest forecast, a quarter to half inch of rainfall would be expected over such regions with mountain snow over the Sierra Nevada and southern Cascade ranges. While snow levels are still subject to change this far out in time, it seems reasonable to believe mountain passes will be impacted on Christmas Eve.
Looking toward Christmas Day and beyond, the model guidance consistency starts to break down with much uncertainty noted. A broad upper ridge sets up somewhere between 140-130W longitude which would support a longwave trough in the vicinity of the West Coast. This could prove to bring additional systems into California by mid-week and beyond. However, any adjustment in the configuration of this ridge/trough pattern will relocate the prevailing storm track. Until then, expect a cooler air mass to filter in beyond Christmas Day. Highs may stay in the low 50s across the Valley with 40s in the foothills and near freezing temperatures in mountain locales.