January 14, 2020 – A pair of systems will move through this week with the first one continuing to bring showers to the region this morning. The following stronger system is expected to impact the region late Wednesday into early Friday bringing heavy snow to the mountains and upper foothill elevations.
Winter Storm Watch now in effect from Thursday morning through late Thursday night. Heavy snow possible above about 2500 feet. Plan on difficult travel conditions. Total snow accumulations of 4 to 8 inches, with localized amounts up to a foot, are possible across the upper foothill elevations.
Discussion
Upper trough over the area continues to generate showers across the region early this morning. Radar indicates most of the activity occurring over the northern Sierra Nevada with light showers over the Central Valley, most numerous between Sacramento and Modesto as the weakening front slides south. Snow level radars indicating the snow level is presently around 2-3k ft over the northern mountains, and around 4-5k ft across the northern Sierra.
Still seeing some southerly wind gusts in the 20-30 mph range. These will likely subside early this morning as gradients drop off. Current temperatures are milder compared to 24 hours ago and range from the 20s and 30s in the mountains to the 40s and lower 50s elsewhere.
The current system moves out into the northern Rockies later today. Most of the current ongoing shower activity is expected to taper off by the afternoon as weak short-wave ridging moves in, however, some light precipitation will likely linger across the higher terrain of Shasta County. Additional snowfall accumulation of a few inches can be expected into midday across the mountains while QPF over the valley will likely be only a few hundredths of an inch at most.
Consensus in the short-range models is to slow the onset of the next upstream system, now moving through the Gulf of Alaska, by about 6-12 hours. Most of the significant impacts, as far as heavier precipitation and gusty winds, are expected later Wednesday into early Friday. Have pushed back the timing of the winter storm watches, but little change to forecast QPF or snow amounts from the previous forecast.
In addition to the rainfall and wind, isolated thunderstorms will be possible on Thursday in the unstable post-frontal environment. They should not be widespread given limited vertical wind shear. Conditions to gradually improve by Thursday night as the frontal system moves into the Desert Southwest.
Extended Discussion (Saturday through Tuesday)
Another Pacific frontal system progged to move into Interior NorCal Saturday. Models indicate weakening of the baroclinic zone as the front moves inland. Bulk of precip looks to be north of I-80 with highest QPF over the Coastal and Western Shasta mountains. Lingering showers Sunday morning, mainly over the mountains, otherwise drier weather for the second half of the weekend under short wave ridging.
Forecast uncertainty increases early next week as oper GFS and GEFS support a more progressive pattern with a pair of extra- tropical cyclones moving through Mon/Tue. ECMWF, EPS, and GEPS support more ridging Mon, with weaker system moving through Tuesday.