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Why is cold air returning to the East after an unusually warm start to spring?

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Following a very warm start to spring over the eastern United States, why is cold air returning in April, when temperatures typically trend upward?

During much of March and long stretches of the winter for that matter, arctic air had been locked up across central and northern Canada.

"Persistent high pressure aloft near the Atlantic coast prevented cold air from moving southward out of Canada and into the eastern United States," according to AccuWeather Senior Meteorologist Brett Anderson.

The pattern allowed a west to east flow of air to dominate across the nation much of the time.

The vast majority of the nation had temperatures averaging 2-6 degrees Fahrenheit above normal during March.

In parts of the Plains, Midwest and South, temperatures were 6-8 degrees above average.

"Starting this weekend and into part of next week, high pressure aloft will shift to the western United States," Anderson said.

The polar vortex will also shift its position a bit farther south and will open the doorway for cold air to be scooped southward and into the Upper Midwest and Northeast.

"April sunshine, a lack of snowcover and many ice-free areas on water bodies along the way will modify that air as it move southward," according to AccuWeather Senior Meteorologist Brett Anderson.

"However, the cold, dry nature of the air, combined with gusty winds will pack a sting at times."

Some lake-effect snow and snow showers will occur.

"There will also be a couple of clipper storms that move through with areas of general, steady snow," Anderson said.

Since the anchor for the cold air will hover near Hudson Bay and pivot, the cold blasts will be limited mostly to the northern tier and will be brief.

Two to three shots of arctic air will rotate through the northern tier states into next week with exponentially less extreme cold farther south, where progressively more mild air from the Pacific Ocean will be mixed in, AccuWeather Lead Long-Range Meteorologist Paul Pastelok said.

The first push of arctic air will be the most robust due to gusty winds and will slash temperatures by 10-30 degrees from one day to the next, from Friday to Saturday around the Great Lakes and from Saturday to Sunday in the Northeast.

As the arctic air approaches and sweeps in, winds could be strong enough to cause sporadic power outages.

The waves of arctic air will bring one or more days with highs in the 20s from the upper Great Lakes to northern New York state and northern New England. Nighttime lows in this swath will range from the single digits and lower teens.

Farther south, highs on multiple days will be lower to middle 40s from Chicago to Detroit, Pittsburgh and New York City.

AccuWeather RealFeel® Temperatures will be 10-20 degrees lower than the actual temperature at times.

During the pattern, temperatures can dip to below freezing on one or more nights as far south as Philadelphia and Washington, D.C.

The pattern with the waves of arctic air will break down during the middle of April and will allow more seasonable Pacific air to take over and break the stagnation.

However, typical of April, there may be a back-and-forth fight going on in the atmosphere for a time.

While days with strong April sunshine will bring warmth on occasion, episodes of clouds, rain and flow of air off chilly waters will contribute to below-average temperatures at times.