What first comes to mind when you hear "December"? Unless you're in Florida, Southern California or Hawaii, we would guess "cold" or "snow" would be one of the first five words to pop in your head.
Well, the atmosphere is poised to deliver a fresh blast of Arctic air and more snow just after the calendar turns. Let's jump into the details starting with more snow.
Model forecast animation of the upper-level trough, or dip in the jet stream, responsible for the developing Western snowstorm early in the week.
Snow Forecast: Skier's Delight
Snow levels will drop Sunday night as a Pacific cold front slides into the Pacific Northwest, bringing snow to progressively lower elevations within the Cascades, Bitterroots, and Tetons through Monday.
Given the trajectory of the attendant upper-level trough diving more toward the south and southeast into the West, cold Arctic air will be pulled not only down the Front Range of the Rockies, but also into the Great Basin and even into the I-5 corridor of western Washington and western Oregon. Thus, a few snow showers are possible even in these low elevations starting Monday.
Before the cold air arrives, heavy rainfall could lead to some localized flooding problems, particularly in western Washington where flood watches have been issued.
The snowfall will create significant impacts to travel through the mountain passes including Sherman Pass in northeast Washington and Stevens and Snoqualmie Passes in the Cascades. Several inches of snow will be likely in the passes with well over a foot at higher elevations of the Cascades through Bitterroots through Monday. Seattle and Portland could see some snow mix in with rain as early as Monday, mainly on the higher hills.
(MAP: 48-Hour Snow Forecast)
Tuesday into Wednesday, the deep upper-trough and cold front continue plowing south, spreading snow into the Sierra Nevada, Wasatch and Rockies. While this is happening, another broad area of potentially heavy snow may develop behind the Arctic front in the High Plains of Montana and Wyoming, spreading into parts of the Northern Plains and Upper Midwest.
With the Arctic front still on a southward plunge and the aforementioned jet stream dip, another wintry mess of snow, sleet, and freezing rain is possible late this week or this weekend in the southern and central Rockies and Plains, potentially extending east into the mid-Mississippi and Ohio Valleys.
There is still considerable uncertainty about the timing and details of this potential bout of wintry weather later in the week, so check back with us as we refine the forecast and are able to bring more details such as timing, precipitation type and precipitation amounts into focus.
What's much more certain now is the potency of this cold plunge.
Here is the general timing for the Arctic cold front, by region:
- Monday: Northwest, northern Great Basin, Montana
- Tuesday: California, rest of Great Basin, Wyoming
- Wednesday: Desert Southwest, northern and central Plains, Colorado, New Mexico
- Thursday-Friday: Upper Midwest, mid-Mississippi Valley, parts of Southern Plains
Keep in mind the coldest air will lag the arrival of the cold front as laid out in the timing above.
Now, let's lay out the magnitude of the cold by region.
- Pacific Northwest: Highs will be stuck in the 30s from Tuesday through much of next week in western Washington and western Oregon. Lows will be in the 20s, with some teens. Some daily record lows may be threatened.
- California: This will easily be the coldest air of the season, with highs barely reaching into the low 50s in the Bay Area and struggling to rise out of the upper 50s in L.A. and San Diego by mid-to-late week. Lows in the 20s are possible in the Central Valley. Some morning lows near or below freezing are also possible in parts of the high deserts of Southern California and interior parts of the Bay Area such as Santa Rosa and Napa by mid-to-late week. Some daily record lows may be threatened in Northern California.
- Great Basin: Highs in the 20s or low 30s will be common by mid-week. Lows in the teens, single digits, or locally subzero will settle in.
- Desert Southwest: Highs may not rise out of the 50s in Phoenix and may struggle to climb out of the 40s in Las Vegas by Thursday. A freeze is possible in the Vegas Valley late in the week.
- Northern/Central Rockies & Plains: Highs in the single digits and teens will spread from Montana Tuesday to the Northern Plains Wednesday and Thursday. A few spots may not rise above zero during the day. Subzero morning lows will spread from the northern Rockies into the Dakotas and northwest Minnesota by Thursday. A few spots, including Yellowstone National Park and Stanley, Idaho, could see temperatures drop into the twenties below zero.
MORE: Coldest Places on Earth
An old man looks out on a Greenland glacier. (Flickr/Goran Ingman)