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November 25, 2019 – Breezy north winds across the Central Valley today. A significant winter storm is forecast to arrive Tuesday bringing heavy snow, rain, and strong winds to the area with major travel impacts extending into the Thanksgiving holiday. Much cooler temperatures are in store for the week.

Discussion

Mostly clear skies cover the region early this morning as the dry cold front enters the far northern portion of the state bringing the first signal of a change to more active weather this week. The front will sweep southward this morning with breezy northerly winds increasing in the wake of its passage. Gusts of 35-45 mph look likely through the Central Valley from late morning through the afternoon, and with dry vegetation elevated fire weather concerns are expected. High temperatures today will be around 5-15 degrees cooler compared to Sunday.

A brief period of quieter weather is expected tonight as today’s system slides to the southeast and the stronger Tuesday system approaches. Winds will subside through the Central Valley by late this evening as the core of stronger winds aloft shifts to the south, but enough gradient lingers for breeziness to continue along the western edge of the Sacramento Valley and across the northern San Joaquin Valley. Overnight temperatures will be chilly (upper 20s and lower 30s) over the north end of the valley and along its eastern edge where lighter winds will be present.

A strong, unseasonably cold weather system will bring the first major winter storm of the season to Northern California later Tuesday through Thanksgiving. Strong southerly winds and heavy mountain snow is expected with snow levels lowering into the foothill elevations. Hazardous travel conditions are likely with major travel impacts expected for holiday travelers Tuesday night into Wednesday across all of the northern Sierra passes and over Interstate 5 to the north of Redding.

Strongest southerly winds expected Tuesday night as the “bomb cyclone” reaches the coast near the OR/CA border. This will intensify the MFR-SAC gradient with inflow into the low center. Appears likely the forecast gradient will exceed 20 mbs which is stronger than the Jan 4, 2008 and Dec 13, 2015 wind events affecting Norcal, although there are synoptic differences (it’s looking at this time like the surface low will “bottom out” off the coast and will be filling as it moves inland near the OR/CA border). At least this puts this low center in some sort of relative terms.

A band of 55+ kts 925 mbs NAM winds are forecast Tuesday evening, down slightly from the 60 kts from the day run. But still, we should be looking at a high end advisory, or at least a low end, High Wind event. Stay tuned…all the leaves that blow south today will head back north on Tuesday night!

This storm will be a major snow producer for the northern Sierra with snow levels lowering into the foothills. Several feet (1-4 ft) of snow accumulation is expected which will put a serious wrench in any holiday travel planned across the mountains Tuesday and Wednesday. Heaviest snow in the northern Sierra is likely Tuesday night into Wednesday morning (hourly snow accumulation rates of 2-3 inches) as upslope flow ramps up and mid-level lapse rates steepen. Snow will keep on accumulating Wednesday and Wednesday night, though likely at a slower rate.

Still looking at low snow levels for the northern Sacramento Valley where we can’t rule out some evaporative cooling lowering snow levels to perhaps the valley floor around Redding. What makes this a difficult forecast is the strength of the southerly winds that are forecast. At times, winds not quite this strong will ride up the cold, denser air instead of mixing it out, and we get an evaporative snow event (discussed in WRH Tech Attachment, 96-10). However, winds forecast this strong Tuesday evening would offer a challenge to the denser air and could mix or scour it completely out in the North Valley. Due to topography and surface roughness north of Redding, the frictional effects of topography sufficiently reducing the wind speeds would not suffice to mix out the denser air. Thus, it looks very likely (60-70%) that elevations as low as the foothills north of Redding will get snow Tuesday night. Areas north of Shasta Lake in the Sacramento River canyon will likely see several feet of snowfall accumulation.

Cold and unsettled pattern continues Thanksgiving, though showers will likely be tapering off from the north.

Extended Discussion (Friday through Monday)

The deep upper level low pressure system finally begins moving east Friday. Current ensembles show mountain precipitation dissipating a bit faster than previous forecasts. Light mountain snow showers will linger over the Central Sierra south of Highway 50 through Friday afternoon with little additional accumulation expected. Saturday brings a brief period of dry weather before ensembles suggest another weather system will move towards the West Coast by Sunday, bringing another chance of precipitation to the area. Confidence is low on details of this system at the moment, so check back for forecast updates.

Cold temperatures remain across the forecast area Friday into the weekend in the wake of the significant cold air advection behind the trough. Daytime temperatures Friday will be in the upper 40s for the Valley and teens to 30s for the foothills and mountains. Overnight lows will be very cold Friday night with widespread Valley temperatures below freezing and mountain temperatures in the single digits to the 20s. Expect temperatures to rise a couple degrees each day through the weekend.