A 2,000-mile swath of snow thanks to winter storm Gorgon

By Nick Wiltgen

Winter Storm Gorgon formed over the Northwest Sunday, and in a mere 48 hours it is expected to bring a swath of snow more than 2,000 miles long from the Cascades and Northern Rockies across the Midwest and into the Northeast through Tuesday.

This fast-moving system has already prompted winter storm watches, warnings, and advisories for parts of 15 states from Washington all the way to Ohio.

Let's get into what's happening now and what we expect next. Later, for you hardcore weather geeks, we'll get into the science behind why this is happening.

Snow Reports and Forecast Timeline

Snow has already begun over the Northwest and northern Rockies, but in the Cascade passes of western Washington, warmer air has changed snow over to rain. That has prompted avalanche warnings for the crest and western slopes of the Washington Cascades.

Snow-covered conditions have improved since Sunday evening on Interstate 90 over Snoqualmie Pass (see webcam).

Top snowfall totals as of 4 a.m. PST Monday included 17 inches at Jackson Hole Mountain in western Wyoming; 7 inches near Ski Bluewood in southeast Washington; and 15.7 inches near Pine Creek Pass in southeast Idaho.

The wind has been strong as well with a gust of 72 mph, with sustained winds of 41 mph, reported near Elk Mountain, Wyoming on Monday morning.

(MORE: Current Interactive Radar)

Monday: Snow lingers over the northern Rockies while streaking southeastward across parts of the northern and central Plains. Breezy conditions will combine with snow to reduce visibility. Wind chills below zero could create a life-threatening situation for anyone stranded on the road in open country (see Impact section below for more details).

(FORECAST: BillingsBismarckSioux City)

Monday night: Snow continues to zip east across the southern Great Lakes region and portions of the Ohio Valley, reaching the Appalachians of West Virginia and Pennsylvania late at night. Conditions will deteriorate quickly as the snow moves in with this fast-moving storm.

Tuesday: Gorgon will lose some of its punch as it moves into the Northeast, but some light snow is expected for the morning commute in Philadelphia, Baltimore and Washington, D.C.New York may see a dusting Tuesday morning.

Tuesday night: Gorgon zips eastward out to sea, but lake-effect snows should kick off in its wake across the entire Great Lakes region.

Snowfall Amounts and Impact

The heaviest snowfall will be in the northern Rockies, where the mountains will enhance snowfall by means of orographic lifting. Snowfall totals could exceed 2 feet in a few spots there.

East of the Rockies, Gorgon's heaviest snowfall is expected to target a swath from southeast South Dakota through a good chunk of Iowa and into parts of Illinois. Snowfall could top 6 inches in this corridor.

Driving conditions will become treacherous on parts of Interstates 29, 35, 80, 90 and 94 over the Upper Midwest on Monday. Snow may fall at a rate of 1 inch per hour, which is typically when road crews start to have a difficult time plowing fast enough to keep the roads clear. Worse, very cold temperatures will limit the effectiveness of road salt.

(FORECAST: Sioux FallsDes MoinesQuad Cities)

Gusty southerly winds of 20 to 30 mph with higher gusts will add to the travel dangers from southeast Montana through western and southern South Dakota, northeast Nebraska and western Iowa. The winds, along with the heavy snowfall rates, will create areas of blowing and drifting snow and very poor visibility; near-blizzard conditions are possible and may require major roads to be shut down for several hours Monday.

Furthermore, with temperatures struggling to crack the 10-degree mark in parts of the Missouri River valley, wind chills will hover around 10 below zero during the height of the storm. This makes it especially important to be properly dressed and prepared in case you become stranded on the open highway.

Fortunately, Gorgon will move out of the Plains quickly Monday night as it zips east of the Mississippi River. As it does, it will lose some of its punch. Snowfall amounts will taper off slightly over the southern Great Lakes and the Ohio Valley. Importantly, winds will be much lighter during the snowfall over those areas.

(FORECAST: ChicagoIndianapolisPittsburgh)

Only light snow is expected from Winter Storm Gorgon over the Northeast. That said, lake-effect snow falling in the wake of Winter Storm Frona before Gorgon's arrival, and to a lesser extent after Gorgon's departure Tuesday night, will add to the snow cover east and/or southeast of each of the five Great Lakes.

(FORECAST: ErieBuffaloSyracuse)

Some drivers along the I-95 corridor haven't had much experience with snow this season. For instance, Philadelphia has only recorded 0.1 inch of snow – a far cry from the 20.2 inches they'd had through Jan. 4 in the winter of 2013-14. All the major cities from Boston to Washington have had below-average snowfall so far this season.

On the other hand, the I-95 corridor in Maine has seen plenty of snow; Bangor and Caribou are each more than 20 inches above average for the season to date. Maine, however, can expect little or none of the white stuff from Winter Storm Gorgon.

The Science Behind Winter Storm Gorgon

Winter Storm Gorgon will be driven mostly by an energetic disturbance in the jet stream -- that fast-moving ribbon of air about 30,000 feet above the ground that usually blows in a general west-to-east direction across North America.

Disturbances in the jet stream can sometimes spin up massive low-pressure systems, but this one is not expected to do that. Instead, a weak low developed over western Montana late Sunday night and will zip southeast across the Central Plains Monday before moving east into the Ohio Valley and Mid-Atlantic Monday night into Tuesday. These systems are often referred to as "clippers" – though this one isn't strictly an "Alberta clipper" since it will be forming in Montana.

Along its journey, the center of low pressure may be difficult to pick out because it will remain very very weak. However, the disturbance in the jet stream will remain quite strong – and it will create strong rising motions in the mid-layers of the atmosphere, some 5,000 to 20,000 feet above the ground. It just so happens that this layer of air will be saturated with water vapor – that is to say, its humidity will be close to 100 percent – in the same area where the air rises quickly.

When air is saturated and it rises, clouds and precipitation form. In this case it will be plenty cold enough for the precipitation to form as snowflakes inside the clouds – and plenty cold enough for those snowflakes to reach the ground as snowflakes and to accumulate, as temperatures along the storm's path will be firmly below freezing.

In fact, east of the Rockies and Black Hills, this storm will probably produce nothing but snow, as it won't be warm enough for rain even along the southern edge of its path.

Winter Storm Gorgon has met The Weather Channel criteria for both population and land area affected. As of early Monday morning, winter storm warnings included over 4 million people from Washington to Illinois. Those warnings covered over 750,000 square kilometers.

For 2014-15, The Weather Channel names any winter storm that meets or is likely to meet National Weather Service warning criteria (winter storm warnings, blizzard warnings, ice storm warnings) over an area of at least 400,000 square kilometers OR a population of at least 2 million people. Gorgon happens to meet both criteria.

Stay with The Weather Channel and weather.com for the latest on Winter Storm Gorgon.

MORE ON WEATHER.COM: Winter Storm Gemini – Dec. 20-26, 2013 (last season's "G" storm)

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