Dry and milder weather with night and morning Valley fog and stratus expected over Christmas weekend. An extended period of significant wet weather with high snow levels next week. Gusty winds with Storm #1 on Tuesday.
We are going to see quiet weather over the Holiday weekend as upper level ridging builds over the west. We will see fog during the morning hours over much of the Sacramento and northern San Joaquin Valley. The fog will be dense from around Chico south. With a strong substance inversion around 850 mb the fog will likely rise into a stratus layer for a time in the afternoon. It does look like the stratus will break up some mid-afternoon into the evening but there is some uncertainty on if that will happen.
High temperatures will be mild but in areas that stay in the stratus layer will remain cooler. Similar conditions are expected for Christmas Day but the fog may not be as dense in areas that remain under a stratus layer.
The first of a series of troughs will move into the region late Monday into Tuesday. This trough will have the most moisture with it as the Integrated Water Vapor Transport (IVT) pushes up to 750+ kg/ms and PWATs push to 1-2″. An intensifying low level jet will pull in this deep moisture ahead of the cold front Tuesday. Moderate to heavy precip pushes in Monday night with the warm front and will continue through Tuesday afternoon as the cold front works south.
850 mb winds amplify to 50-70 knots ahead of the front and we are going to see strong winds across the region into the afternoon. Right now wind gusts of 35-55 mph can be expected across the Valley with the highest gusts in the northern Sacramento Valley and 50-75 mph winds over the mountains.
Some weak instability builds in right behind the cold front and that may bring a thunderstorm or two during the afternoon mainly from around Marysville north to Redding east into the foothills. CAPE looks very limited and the thunderstorm threat remains isolated.
Activity will become more showery over the Valley by the evening Tuesday but precip will continue over the mountains as zonal/up slope flow continues. With a good stream of deep Pacific moisture we will see high snow levels with the onset of the precip with them 7500-8500 feet.
Cold air will begin to build in Tuesday afternoon and we will see snow levels start to fall falling to 5000-6000 feet Wednesday. 14-40″ of snow is expected above 6500 feet with a few inches above 5500 feet through Wednesday afternoon.
QPF totals through Wednesday afternoon are looking to be 1.50-3″ in the Valley, 2.50-4.50″ in the foothills, and 4-6″ over the mountains. We are expecting a period of moderate to heavy rain but convective rain is looking very limited, this should keep rain rates below burn scar thresholds.
Significant flooding from snow melt is not expected to be an issue at this point since river and reservoir levels are low. However, urban and small stream flooding could be possible on Tuesday, especially in the mountains and foothills.
Extended Discussion (Wednesday through Saturday)
We are going to remain in an active weather pattern for the extended period as long wave troughing holds over the West. We will see multiple short waves with more widespread precip but there is some disagreement in the ensembles with timing and placement.
It does look like we will see a short wave trough Wednesday night into Thursday with another one Friday. More widespread precip is expected over the New Year’s weekend but once again there is differences in timing.
Snow levels will lower on Wednesday falling to 4500-5500 feet. They come up some on Thursday to 5000-6000 feet. Mountain travel will likely be difficult at times through the weekend.