Freeze warning for extremely cold temps and low snow levels tonight and Monday morning

February 18, 2018 – A cold weather system brings breezy winds and snow over the mountains and foothills Sunday into Monday, impacting travel. This will be followed by near record cold temperatures with a possible widespread freeze.

Winter Weather Advisory until 4 PM PST Monday for West Slope Northern Sierra Nevada.

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Freeze Warning from 2 AM Monday to 8 AM PST Wednesday.

Discussion

Radar shows showers spreading into the mountains of Shasta and western Plumas counties this morning. This will increase and spread down into the northern Sierra later this morning and early afternoon. This is a bit slower than projected, so have adjusted forecast. Models have trended a little wetter than in previous runs, though. Snow showers over the Sierra are expected to bring slippery roads and possible travel delays. Use caution if traveling in the mountains today, and even in the upper foothills tonight.

Temperatures in Valley are rapidly increasing in mostly sunny skies. Have increased forecast afternoon highs a few degrees for Valley locations, with upper 50s to lower 60s expected.

Increased northeast winds over the northern San Joaquin Valley a little for today into early evening, generally around 15 to 20 mph, with gusts to 30 mph.

Previous Discussion

The eastern Pacific ridge has begun to retrograde westward as a shortwave trough slides southward along the Pacific Northwest. Skies remain mostly clear across interior Northern CA, though a few light radar returns are being detected just north of our CWA. South winds / onshore flow has begun to pick up this morning across much of the region ahead of the cold front. These winds are keeping the boundary layer well-mixed – temperatures are running several degrees warmer than 24 hours ago.

Precipitation associated with the aforementioned trough will begin to spread across the northern mountains this morning, and down through the Sierra through the course of the day. The system taps into very little moisture, so it will difficult to manage more than a sprinkle or flurries across lower elevations. A few inches of light, fluffy snow are expected across the higher elevations of the Sierra later today, with periodic showers continuing into Monday.

Snow levels are projected to lower rapidly with this system, and a dusting of snow will be possible across the foothills. Snow accumulations are not expected to be impressive, but this holiday weekend traditionally sees a high volume of traffic over the mountains and snow could cause exacerbated impacts.

What will likely be the coldest air of the winter season will begin to impact portions of the region tonight. For lower elevations, the coldest temperatures tonight will likely be across the northern Valley, where skies clear and winds slacken. Skies should clear further Monday night into Tuesday morning, and currently appears to be the coldest night of this event. Widespread freeze conditions may continue Tuesday night.

NAEFS ensembles are characterizing this event as roughly 2-3 standard deviations colder than normal, and with a 5-10 year recurrence interval. With forecast lows on the order of 10-20 degrees colder than normal, record minimum temperatures will be possible across Valley climate sites.

Widespread frost is expected. Agricultural impacts could be significant, due to an early bloom from unusually mild weather in recent weeks. People should begin planning now on ways to protect sensitive plants, outdoor pets, and outdoor pipes.

Extended discussion (Thursday through Sunday)

Series of cold shortwave lows continue to drop through the Pacific Northwest and Norcal through the extended period. Extended models in good agreement at this time in dropping one of these troughs through Norcal on Thursday bringing light rain to much or all of the CWA. This cold system is not expected to contain a lot of moisture so precipitation amounts are expected to be minimal but snow levels will be quite low at between 1000 and 2000 feet. If current timing holds then Friday looks dry under a brief period of northerly flow between systems. There should be enough sun on Friday to bring daytime highs from a little below normal to a little above normal. Next system on track to drop into the Pacific Northwest and Norcal late Saturday. This system looks to be a little more shallow so best threat of precipitation will be the northern mountains and Sierra Cascade crest. Overnight lows will remain on the cool side through the extended period but valley lows should stay above freezing while daytime highs over next weekend should come in around normal.