US January outlook: Will milder weather return or will the polar vortex come into play?

Following snow and cold in 49 states during the first 10 days of January, cold air will retreat in parts of the United States but hold on with stormy consequences in others for the rest of the month.

The upcoming pattern for the remainder of the month will have some people in the South shedding winter coats, while others in the northern tier will be changing up their outerwear on a daily basis.

In many parts of the nation, early January lived up to its wintry reputation.

Temperatures plummeted to minus 30 F with AccuWeather RealFeel® Temperatures as low as minus 40 in parts of Montana, North Dakota and Minnesota. Actual temperatures plunged to nearly minus 50 in parts of Colorado. Northern California was hit with drought-busting rain and flooding, while yards of snow buried the high country of the Sierra Nevada, Cascades and Rockies.

At one point this past weekend, snow was on the ground and freezing temperatures existed in every state except Florida, thanks to a snowstorm that swept from the Northwest to the Southern states and then to the coastal Northeast.

Arctic air will retreat from much of the southern half of the nation during the middle of the month. Highs most days in Atlanta will be in the 60s. The normal high is in the lower 50s.

"The main branch of the jet stream will retreat to near the border of Canada and the U.S.," according to AccuWeather Senior Meteorologist Brett Anderson.

The jet stream is a high-speed river of air at the level in the atmosphere where jet aircraft cruise. South of the jet stream, the weather is generally warm. North of the jet stream, the weather is generally cold.

"There will still be some dips in the jet stream, which will allow pockets of arctic cold air to periodically sink southward into parts of the nation, including across the Upper Midwest and Northeast," Anderson said.

During the middle 10 days of January, highs in Chicago will generally range from the 20s to the 40s. In New York City, highs most days will be in the 30s and 40s with an exception here and there. For both cities, it will neither be bitterly cold nor warm for very long. Normal highs are near freezing in Chicago and in the upper 30s for New York City.

Sometimes, where the boundary of cold versus mild air sets up, episodes of snow, ice and rain can occur.

One such weather battlefield will set up from the southern Plains to portions of the Midwest and the Northeast from late this week into early next week.

In the Southwest, a storm will manufacture cold air and unleash wintry precipitation for a time during the middle of the month.

The formation of the storm over the interior Southwest will allow the weather to dry out for several days for weather-weary areas of California, Oregon and Nevada.

Around Los Angeles, high temperatures during the balance of the month will generally range from the middle 60s to the lower 70s, or within a few degrees of average. Likewise, in Seattle, highs will trend to near average, which is in the 40s.

The overall pattern first appears to be mild for much of the nation during the latter part of the month, unless the polar vortex comes into play.

"There are some indications that the polar vortex may weaken enough to allow a southward discharge of arctic air prior to the end of the month," according to AccuWeather Lead Long-Range Meteorologist Paul Pastelok.

"Even in cases when there is certainty about the weakening of the polar vortex, you never know for sure where the discharge of cold air will be directed, such as western versus eastern U.S.," Pastelok said.

If the polar vortex remains strong, then it will keep the arctic air locked up in the Arctic and more places in the lower 48 states may trend warmer rather than colder late in the month.

"We believe the eastern part of the U.S. will trend colder and stormier again toward the end of the month, but the question is how much," Pastelok said.

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