Strong system arrives late today bringing moderate rain, thunderstorm chances, gusty winds, heavy mountain snow and significant mountain travel impacts through mid-week.
Discussion
A powerful Pacific storm system will approach the West Coast Monday, with high Pacific moisture quickly overspreading Northern California Monday night, bringing yet another round of moderate lower elevation rain and heavy mountain snow.
The heaviest rainfall is expected Tuesday. Very heavy snow is forecast for higher elevations of the Coastal Range and Sierra Monday night- Tuesday, with totals of several feet possible.
In addition to the precipitation, gusty winds are also likely late Monday over the Sacramento Valley with gusts up to 50 mph possible, downed trees and scattered power outages will be possible.
Temperatures over northern/central California will remain unseasonably cool through the week with Valley highs generally in the 50’s to near 60 degrees.
Snow will overspread the NorCal ranges tonight into early Tuesday as a precipitable water plume (0.50-0.75″ or +1 to +2 sigma) surges into the area in advance of the cold front and back around the wrapped occlusion.
Snow levels will rise to around 3000-4000ft when the heavier snowfall arrives before falling back 1000-2000ft lower post-FROPA as the snow starts to ease back.
The system, though anomalous with 500mb heights near -3 sigma, will remain progressive and continue to sink southward then southeastward paralleling the northern/central CA coast Wed into early Thursday as it then opens up into a strong trough by the end of Thursday morning as the upper jet stretches northeastward across the Great Basin.
This will focus most of the heaviest snowfall over the Coast Range, Southern Cascade ranges and through the Sierra where 3-day totals will likely exceed 1-2ft (with >4ft at the highest elevations).
Snowfall rates will be intense at times with 1-3″/hr rates likely per the HREF probs and WPC snowband tool.
Winter Storm Warning is in effect for Coastal Range, Shasta County mountains, Lassen, and West Slope Northern Sierra from late Monday through Wednesday.
Multiple feet of snow are forecast in the mountains through Wednesday. Difficult to impossible mountain travel is expected.
Confidence is medium for snow over northern Sacramento Valley, including the city of Redding for Monday night through Tuesday morning and have upgraded to a winter storm warning. Snow over this area will be very dependent on low level wind field set up and how far northward south wind field progresses northward Monday night.
The late night diurnal start time along with the heaviest precip timing favors lower snow accumulation, but as always smaller scale micro- climate features have the greatest influence in snow over the North Sac Valley.
The upper level polar cutoff low begins the day Tuesday morning centered off the Oregon coast, and tracks southeastward to be centered over the northern California coast by Wednesday morning.
A 100 kt westerly upper level jet streak wrapping around the low will direct a plume of Pacific moisture and precipitation into California as it tracks south. PWATs will approach 0.75-1.00″, which is about +1.5-2 sigma above the climatological normal and IVT peaks at about 500-600 kg/m*s along central California.
The attendant surface low will follow a similar track as the upper level cutoff low through this period, making it a vertically stacked low, that will result in largely unidirectional flow throughout the column south of the low.
Since the vertically stacked low has polar origins, it will have plenty of cold air and a lack of plentiful Pacific moisture with it.
Thus, this rainfall event is not expected to be as intense as the atmospheric rivers impacting the state in recent weeks. Storm total amounts of 0.50-2.00 inches of rain are forecast for the Valley, with 1.00-3.00 inches in the foothills through Wednesday.
Slick roads, ponding of water on roadways, rises on creeks and streams, and increased travel time will be possible.
Thunderstorms will also be possible with this weather system Tuesday and Wednesday afternoon and evenings. The better chances will be on Wednesday, from Marysville southward. Any thunderstorms that develop will be capable of producing dangerous lightning, small accumulating hail, gusty winds and heavy rain.
Precipitation will become more showery by mid-day Wednesday and taper off overnight.
Extended Discussion (Friday through Monday)
Longwave troughing remains along the West Coast through the extended forecast period as cold upper low from GOA drops into it. This will result in continued unsettled weather with a chance of showers, best chances over the mountains, and below normal temperatures. Snow levels will generally be around 3000 to 4500 feet.