Winter Storm Decima to Bring a Cross-Country Mess of Snow and Ice to the West, Midwest and Northeast Into the Weekend

December 13,2016
Winter Storm Decima – pronounced DEH-si-mah – is poised to deliver another cross-country mess of snow, strong winds and some ice, impacting a large swath of the nation into the upcoming weekend.
(MORE: The Science Behind Naming Winter Storms | 2016-2017 Winter Storm Names)
Decima follows on the heels of Winter Storm Caly, which blanketed the U.S. with wintry weather from coast-to-coast last Thursday into Monday.
(MORE: Arctic Cold Dominates Northern States)
Decima will get its start as a southward plunge of the jet stream, originating from the Gulf of Alaska, begins to plow into the western United States.

Cold air is in place across the Midwest and East with high pressure in store. A storm system sweeping into the western states then moves eastward into the cold air, making wintry weather possible.

Winter storm warnings have been posted from southern Washington state into Oregon, northern California's Siskiyous and parts of Idaho.
Winter storm watches span from California's Sierra Nevada into portions of Montana, Wyoming and South Dakota.

Current Winter Alerts

Energy in that jet-stream dip will eventually spawn an area of surface low pressure in the Plains, which will track northeastward with time through the central and eastern states. Moisture from that system will overlap with cold air to its north and from high pressure in the eastern states to make conditions ripe for the potential of snow, sleet and freezing rain in a large area.
(MORE: Winter Storm Central)
Below is a general overview of the timing and impacts from Decima at this time. Keep in mind, however, that parts of the forecast are still a few days out in time, so revisions may be necessary in the coming days.

Wednesday-Thursday

Wednesday's Forecast

• Snow will spread into the West Wednesday from parts of Oregon to the Rockies of Colorado. 
• The larger-scale jet-stream dip will then plow into the West Coast starting Thursday. Snow is expected from California's Sierra Nevada north and eastward into Oregon, southeast Washington, Idaho, Montana, northern Nevada, Utah and the Colorado Rockies.
• Some initial cold air trapped west of the Cascades could set up parts of the Portland and Seattle metro areas with more snow late Wednesday into Thursday.
• Light snow begins to spread into the northern Plains Thursday.
• Cities: Boise, Idaho | Butte, Montana | Jackson Hole, Wyoming | Tahoe City, California

Friday

Friday's Forecast

• Decima will expand its reach from the northern Plains into the upper Midwest and Great Lakes Thursday night through Friday.
• Snow, possibly heavy in spots, and strong winds are possible in those areas, with the strength of the winds dependent on how intense a developing area of surface low pressure becomes. Blowing snow could create low visibility and dangerous travel conditions.
• As milder air overruns cold air near the surface of the earth, some freezing rain or sleet could affect the central Plains and mid-Mississippi Valley.
• Snow will also continue in the Rockies, Great Basin and southern Sierra, and could be heavy in spots.
• Cities: Green Bay, Wisconsin | Minneapolis | Rapid City, South Dakota | Salt Lake City

(MORE: Extended Forecast Maps)

Friday Night-Saturday

Saturday's Forecast

• Moisture from Decima will overrun cold air from a retreating area of high pressure in the East. This would result in some sleet or freezing rain affecting parts of the Ohio Valley, Appalachians, adjacent piedmont and mid-Atlantic states.
• Meanwhile, snow will spread east from the upper Midwest into parts of the Northeast.
• A second area of snow will stretch farther west into the central Plains and Front Range of the Rockies.
• Cities: Buffalo, New York | Milwaukee | Pittsburgh | Denver

How Much Snow, Ice?

West

• The heaviest accumulations are likely to be in the Oregon Cascades (and adjacent eastern foothills), Sierra, high country of central Idaho, Tetons and Wind River range of Wyoming and, eventually by the weekend, parts of the Rockies of Colorado and Wasatch of Utah.
• Some accumulations are also expected in lower elevations, such as the Willamette Valley of Oregon, Snake River Plain of Idaho, the Salt Lake Valley, the High Plains of Montana and Wyoming, and Front Range of Colorado.
• In parts of the Willamette Valley, some sleet or freezing rain may mix in, at times.

West Snowfall Forecast

Central/East

• While it is too soon for precise snowfall forecasts, a swath of at least 6 inches of accumulating snow is possible from the Plains of South Dakota and northern Nebraska through the Great Lakes and parts of the interior Northeast Friday into the weekend.
• Some accumulations of ice and sleet are possible in parts of the mid-Mississippi Valley, Ohio Valley, Appalachians, adjacent piedmont and Northeast Friday into Saturday. This does not appear to be a major ice storm, but some icing of trees and overpasses may occur.
• Some lighter snow accumulations are possible in the cold air as the storm exits from the central Plains to the Ohio Valley and Northeast Saturday and Sunday.

Snowfall Outlook

Once again, there will be changes to this forecast in the days ahead, and more details will follow. Check back with weather.com for updates.
Incidentally, Decima was one of the Fates in Roman mythology.

PHOTOS: Winter Storm Caly (December 2016)