Why did we get so much snow, so fast this season? A polar vortex

Snow season has gotten off to a fast start across swaths of the northern United States this year.

Fueled in part by an unusually early disruption to the polar vortex — which is now easing — 18 states and D.C. have already experienced more snow than average into December.

States such as Iowa, Illinois and Indiana experienced their snowiest start to the season since at least 2008, with Michigan, Ohio, Kentucky, Pennsylvania, West Virginia, Virginia, Maryland, Delaware, D.C., New Jersey and Vermont ranking in the top three snowiest over that same period.

In some of these places, snowfall has been more than quadruple what is typical through mid-December.

But as the Midwest and Northeast saw storm after storm, much of the West has been mild and lacked snow. States including Oregon, Nevada, Wyoming, Idaho, Utah and Colorado have seen their least snowy or second least snowy start to a season in almost two decades.

Where snowfall has been unusual

Snowfall has generally been above-average in a zone extending from the Dakotas to the Mid-Atlantic coast, northward into New England.

This includes Minneapolis (13 inches), Des Moines (19 inches), St. Louis (7 inches), Chicago (17 inches), Indianapolis (10 inches), Detroit (7 inches), Pittsburgh (9 inches), Buffalo (20 inches), D.C. (3 inches), Philadelphia (4 inches) and New York (4 inches). Across this corridor, snowfall has been two to five times the season-to-date average.

The snowiest place east of the Rocky Mountains has been New Hampshire’s White Mountains, where an estimated 158 inches have fallen since October.

Pockets of North Carolina have also had unusual amounts of snow. Last Sunday, a frigid air mass flowing over the relatively warm waters of the Albemarle Sound produced a very unusual band of sound-effect snow, which extended into the Outer Banks.

“This is easily the most intense lake or sound effect snow band I’ve ever seen in North Carolina,” wrote meteorologist Eric Webb.

But in the West, the story has been quite different. The Pacific Northwest has had very little snow compared to normal, with a recent destructive atmospheric river sending unseasonable warmth into the region, raising snow levels toward the tallest mountain peaks.

Snowfall has been well below-average across California’s Sierra Mountains and also historically low in Oregon, Nevada, Wyoming, Idaho, Utah and Colorado — places that depend on it for drinking water, irrigation and electricity.

Why snowfall patterns have been strange

An early-season disruption to the polar vortex played an important role in unleashing very cold air compared to what’s typical in December. Temperature records that stood for more than a century were threatened or broken on multiple occasions.

An anomaly in the stratosphere was centered near the North Pole and Alaska. It became connected to, or coupled with, the atmospheric layer below, called the troposphere. That’s the layer of the atmosphere where weather happens.

This anomalous configuration caused a large dip in the jet stream across eastern North America — allowing the hemisphere’s most frigid air to spill southward across Canada and the central and eastern United States.

On the other side of this jet stream dip was a ridge of high pressure — one that caused unseasonably warm conditions across northeastern Russia, western Alaska and along most of the West Coast, except for California’s Central Valley.

Changing snow patterns

Despite plenty of early season snow across the eastern half of the country, Mother Nature has different plans for the holidays.

Atmospheric rivers will slam the West in the weeks ahead, with increasing snowfall for the Pacific Northwest — where blizzard warnings are in effect — as well as the northern Rockies. The Sierra and other parts of the Intermountain West will probably also get snow before the end of the month — reducing snow shortfalls there.

However, warmer air pulsing across the country will probably reduce odds for a white Christmas in many central and eastern states.

In addition to the western mountains, the best chances for a snow-covered Christmas will likely be found along the far northern tier of states, from the northern Plains to northern New England.

Despite this predicted lull in eastern snow, January or February is often the snowiest month of the year — unless milder temperatures continue and subdue storms, as has frequently occurred in recent winters.