Brrr-ace Yourself!!

We've had some cold weather in the last two weeks, but nothing like what will occur over the next several days. In the upper atmosphere a broad trough low pressure extends across the continental U.S.  and over the next several days that trough will deepen as a sharp ridge develops just off the west coast.

500 millibar map (~18,000 ft.) for 6:00 a.m.Tuesday, December 3

That will clear the way for cold air from the Arctic to plunge southward well into the U.S.  Snow cover is fairly extensive across Canada, but the southern extent of the snow is a little north of normal in the central and northern Plains. That will change over the next few days.

Snow on the ground for December 3, 2013.
Credit: hamweather.com

While the upper air system gets its act together, a complex low pressure system will be organizing over the central Rockies, producing precipitation eastward along and behind the leading edge of the cold air. Winter Storm Warnings are in effect from western Nevada through the central Colorado Rockies, and from the eastern two thirds of Montana across the Dakotas and Minnesota to northwestern Wisconsin.

Watches, warnings and advisories in effect as of 7:40 p.m. CST.
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Four to eight inches of snow is likely from southern Utah to western Lake Superior, with amounts possibly reaching 12 inches or more in the Colorado Rockies and along the west side of Lake Superior.  On Thursday and Friday a mix of wintry precipitation including freezing rain is possible from southern Oklahoma through Arkansas into western Kentucky and Tennessee.

By Saturday the leading edge of this cold air mass will push off the east and Gulf coasts, but another reinforcing surge of cold air will push through the the central U.S. early next week as  the closed upper low settles in over the Great Lakes.

Forecast 500 millibar map for Tuesday, December 10.

Maximum temperatures are expected to be below zero in the northern Plains northern Great Lakes by Saturday. It will warm slightly and then the reinforcing surge of cold air will bring maximum temperatures back down by Tuesday.

Forecast maximu, temperatures for Saturday, December 7 (left) and Tuesday, December 10 (right).

On Saturday minimum temperatures are expected to be well below zero in the northern Plains and Rockies, and below freezing from the Appalachians to the Sierra Nevadas

Forecast minimum temperatures for Saturday, December 7.

The outbreaks of very cold air over this much of the country in early December do happen, but not very often. Severe cold air outbreaks have occurred in December 1977, 1983, 1989, and 2000. December 1983 still ranks as the coldest December on record for the Midwest, with December 2000 the second coldest and December 1989 the third. The unusually cold weather the first half of December 1977 was followed by a very mild period, and December 1977 ranks as the 27th coldest. However, just after Christmas that year it turned cold again, and the winter of 1977-1978 ended up going into the record books as one of the most severe winters on record for the eastern two-thirds of the country.

So, button up and batten down. Be sure to keep abreast of the latest weather developments at your local NWS office web site. It looks like winter weather will be holding on for much of the country, especially the central U.S., through the middle of the month.

Temperture outlook for the period December 11-17, 2013.
Credit: NOAA/NWS Climate Prediction Center

CoCoRaHS observers, it's time to review snow measurement techniques!  See the training animations at the CoCoRaHS YouTube channel. They are a great refresher and it will take only around 20 minutes to view them all.