Very Low & Heavy Snow Friday into Saturday

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Fast, fairly strong/potent shortwave that has developed along the strong 100 – 120kt kt northerly jet blasting down the western U.S. coast will continue to spread precipitation throughout northern California through the rest of the overnight hours ahead of the cold front, with showers lingering behind the front into Saturday.

Currently as of 6:30pm, the front was just passing through the northwest coast, where snow levels are around 500 – 1000ft.

Back edge to the cold frontal precipitation band noted slipping passed the north coast.

Back edge to the cold frontal precipitation band noted slipping passed the north coast.

The system itself is a 1012mb surface low which is now centered over the CA/OR border, supporting a band of strong vertical motion through northern California, which in combination with a narrow plume of moisture offshore will yield some moderate to heavy rainfall along the main frontal precipitation band.

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The decent pressure gradient between Medford and Sacramento will yield a swath of 30 – 45kt 850mb winds from Red Bluff to Stockton, eastward banked up into the foothills. Bands of heavier precipitation could drive down some 30 – 40mph gusts, which combined with snow, where it falls, will create very low visibility on top of already hard travel.

Forecast 850mb winds for around 10pm.

Forecast 850mb winds for around 10pm.

Based on radar/satellite trends in combination with mesoscale analysis and HRRR model trends, it appears some convective activity is possible along the back edge of the front. Mesoscale analysis indicates surface CAPE values of 200 – 300j/kg along the back edge of the front, which combined with very strong vertical motions and cold air aloft is indeed supportive of some convective precipitation to perhaps even some thunderstorms, generally south of Santa Rosa and north of Santa Cruz, east to the foothills of the sierra. HRRR supports this thinking well, introducing some fairly strong convection, perhaps lightning discharge capable, from the SFO area east to the Sacramento area and south to perhaps the Stockton area. Even if there is no lightning, heavy rainfall and perhaps sleet or hail could mix in with this convection.

00z HRRR forecast radar reflectivity valid for 11pm, indicating some convection possibly including thunderstorms from SFO to STO.

00z HRRR forecast radar reflectivity valid for 11pm, indicating some convection possibly including thunderstorms from SFO to STO.

Snow levels are already low. Up in the north valley, snow is falling in Redding, with a cold rain in Red Bluff. In the northeast foothills, snow levels are about 1000ft, with 1000ft snow levels all the way down into the motherload. Against the eastern side of the valley, temperatures have been falling into the low 40s as of 6:30pm, and should start creeping into the upper 30s shortly. By after around 8pm snow should be falling in Chico and perhaps Oroville. Between 8 and 10pm snow may mix into areas such as Beale AFB, Loomis, Rocklin, and possibly Roseville by around midnight. These areas have the best chance at seeing any accumulations, though as far west as Sacramento snow may mix in.

Snowfall rates at times in the foothills and mountains could top 2″ per hour, locally upwards of 3″ per hour given the strength of upsloping winds and strong vertical motion, which is very heavy when it occurs for a several hour period.

Above 500ft accumulations are most likely, and with QPFs (liquid precipitation totals through the event), between .25 and .75″ across much of the region south of Redding with locally upwards of 1.5″ in liquid over the mountains, heavy accumulations at that above around 1000 – 1500ft. Based on liquid precipitation forecasts and snow ratios between 1:18 and 1:24, 3 – 6″ of snow is likely between 800 and 1500ft, with 5 – 10″ above 1500 – 2000ft. 7 – 14″ seems reasonable between 2000 – 3000ft, with 1 – 2.5ft over 4000ft.

Dec 4th Snow Map v2

Showers will develop behind the front through the afternoon Saturday, with snow likely continuing with no stop for much of the sierra and parts of the foothills through at least the morning Saturday. Some valley showers can’t be ruled out as well as the low itself slides directly overhead.

An extremely cold night is in order Saturday night/Sunday morning and again Sunday night/Monday morning as the cold northerly flow behind the system takes over. I’ll go into detail on this some more later Saturday, as the freeze itself looks to be very significant.

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