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December 11, 2020 – Cooler with a pair of storms arriving tonight and continuing into early next week. This will bring rain and mountain snow, causing travel difficulties.

Discussion

We are seeing quiet conditions across the area early this morning with some high clouds beginning to build in. Morning lows will be quite chilly and we can expect areas of frost by daybreak. Our weather will become more active as we head into tonight and over the weekend. A short wave trough will track into the PacNW dig into the Great Basin this evening into the overnight. This will kick off some showers during the evening and overnight across the area but the stronger forcing will be associated with a deeper trough over the Pacific. This trough will bring warm air advection to the region after 21z and will also push moisture back into NorCal with our PWats going from generally under 0.25″ to 0.50-1.10″. The strongest forcing will be overnight night and we can expected widespread showers across the area.

While warm advection will bring rising snow levels, entrenched cold air and evaporative cooling will allow for some low level snow for northern Shasta County this evening into early Saturday. Some wet snow accumulations could reach I-5. Snow levels over the Sierra will be a bit higher in the 3500-4000 foot range rising to above 6000 feet by late morning Saturday. The majority of the accumulating snow for the Sierra will be above 6000 feet but some light accumulation will be possible down to around 4500 feet. Snow totals over Shasta County are looking to be 1-5″ with 4-8″ over the Sierra, locally higher amounts over the peaks.

A Winter Weather Advisory will go in to effect for Shasta County above 2500 feet at 1 PM and remain in effect until 7 AM tomorrow morning. A Winter Weather Advisory will also go in to effect for the Sierra above 6000 feet at 4 PM this evening and remain in effect until 10 AM tomorrow morning.

Mainly dry conditions settle in by Saturday afternoon and continue into Saturday evening as short wave riding builds in. The Pacific trough will push east Saturday night pushing a cold front into NorCal reaching northern areas early Sunday morning and pushing through the rest of the area during the day Sunday. widespread showers can be expected along and ahead of the front. Snow levels ahead of the front will be hovering around pass level and will fall to 4000-5500 feet Sunday night. Most shower activity will diminish Sunday evening but showers will linger in the Sierra into Monday as the trough axis works through.

Short wave riding builds in for the second half of Monday. This will bring quiet conditions with decreasing clouds. Highs throughout the period will be near average and we can expected warmer overnight lows after this morning.

Extended discussion (Tuesday through Friday)

Reasonable certainty at the start of the EFP as a Pacific frontal system approaches the W Coast of NOAM on Tue. Divergent model solutions as the system moves inland leads to some uncertainty. While the GFS weakens the system, giving NorCal little to no precip, the ECMWF maintains the low pressure center longer than the GFS and brings cooler air and a little more precip into NorCal through much of Wed. Eventually, this system weakens/leaves the area allowing some ridging and precip-free weather, but for how long?

The GFS brings in another cool trof and precip on Thu, while the ECMWF maintains a dry and milder ridge. The 5H Cluster tool indicates the W Coast pattern will primarily be dominated by the Pacific ridge and hence the drier solutions. Cluster 4 on Day 6 (Thu) would suggest the cooler/wetter scenario as this one is heavily weighted to the GFS. The GFS Prob of IVT tool indicates there is plume of moisture then, but both plume ensembles (GFS/ECMWF) have the plume centered N of 40N, which suggests some sort of ridging would prevail, and not a digging system.