Major Arctic Blast This Week

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After a fairly warm last several weeks or even couple of months of Fall, the current pattern will undergo a major flip, with our large ridge sliding off to the west, allowing a pool of very cold arctic air to plunge into the western and central U.S. beginning Monday afternoon into the evening as the leading edge of the arctic airmass sends a tight thermal gradient/cold front slides through northern California. Strong dynamics at play along with a narrow band of decent moisture (.9 to 1.1″ precipitable water values) during the frontal passage may yield some showers Monday afternoon into the evening, and possibly some terrain enhanced convection along the eastern side of the valley and/or in the foothills.

Dec 1st

Snow levels start out around 4000 – 5000ft Monday afternoon, but drop to around 2000ft by later in the night, though showers may be winding down. Showers may remain possible into the the afternoon Tuesday in the higher sierra and to the east of the sierra.

Things dry out Tuesday night as moisture becomes almost non-existent as cold, dry air settles over the western and western-interior U.S. Once the moisture content in the atmosphere drops, temperatures will to. Beginning Tuesday, high temperatures in the valley will generally be in the low to mid 50s, with temperatures cooling further through the week as colder and colder air filters southwest from the Great Basin. By late in the week, valley highs may only be in the 40s… which even for arctic blasts deep in winter is quite cold.

On Tuesday, while the Pacific jet is still directly overhead, a 120kt jet streak will be blasting northern California, with winds at 850mb up around 30 – 40kts along the coastal mountains and western Sacramento valley. With there being some weak to moderate negative vertical velocity (or downward motion), it would seem possible that the valley and coastal mountains could see some surface winds around 15 – 20mph sustained, and perhaps some 30 – 40mph gusts. This would make for a notable wind chill, with some valley wind chill temperatures being in the 30s and low 40s even during the day.

Dec 1st wind

To give you an idea of the cooling over time, on Tuesday 850mb (approximately 5000ft) temperatures are forecast around -2c to -3c, then by Friday around -4 or -7c. Compare those temperatures to the +14 – +16c 850mb temperatures we have this weekend! Higher up in the atmosphere, around 500mb (approx. 18,000ft), temperatures are forecast around -25 to -30c.

00z NAM's 850mb temperatures and 500mb heights valid for Wednesday afternoon.

00z NAM’s 850mb temperatures and 500mb heights valid for Wednesday afternoon.

Overnight lows in the valley beginning Tuesday night will dip into the 20s, with low 20s for some valley locations possible later in the week. Could be some low high temperature records to be broken this week, and it looks very likely many record low low temperatures could be broken through the week.

Dec 1st

Temperatures will remain cold through at least early next week as reinforcing waves of cold drop into the Great Basin and trickle over the sierra.

There are some long range indications of a wetter change to the pattern next week, but that’s way too far out to put any real thoughts on that yet.

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