A More Favorable Pattern for a Snowstorm in the East May Be Ahead

Linda Lam
Published: January 9,2016
An expected change in the large-scale weather pattern over North America and the higher latitudes of the North Atlantic Ocean appear to be increasing the odds for an impactful winter storm in the East beginning late in the new week ahead.
There is broad support in computer forecast model guidance for colder-than-average conditions across portions of the East at times over the next two weeks. That forecast is of relative high confidence.
There are also indications of an increase in storminess, though whether a Southern or East Coast snowstorm actually forms is of much lower confidence, which is a typical conundrum facing meteorologists this time of year.
The result could be snow returning to portions of the U.S. which have not seen much in the way of snowfall so far this season.
One of the factors to examine to determine if the pattern is favorable for an East Coast snowstorm is the Arctic Oscillation (AO). When the AO is in a negative phase, winds around the North Pole become weaker, colder air can push southward, and storminess usually increases in the mid-latitudes (the region south of the Arctic Circle and north of the tropics).
(MORE: Arctic Blast Ahead)
The AO index has been mostly positive since mid-October, contributing to the lack of very cold conditions and lack of East Coast snowstorms early in the winter season. However, the AO has become negative, and may continue to be so for the next 1-2 weeks, if not longer.

The graph above shows the forecast for the NAO index to become negative in mid-January.

The North Atlantic Oscillation (NAO), which is closely related to the AO, is another important factor which can signal the likelihood of an Eastern winter storm. The NAO is a measurement of the differences in atmospheric pressure between the Icelandic low (an area of low pressure over northern Iceland) and the Azores high (an area of high pressure located near the Azores). Differences in the location and strength of these two factors plays an important role in the direction and strength of the jet stream and consequently storm tracks across the North Atlantic.
The negative phase of the NAO reflects above-normal atmospheric pressure across the high latitudes of the North Atlantic (Greenland, Iceland, and adjacent regions) and below-normal atmospheric pressure over the central North Atlantic, the eastern U.S. and western Europe, according to the National Oceanic and Atmospheric Administration (NOAA). The positive phase is the opposite. The differences in the jet stream that result from the NAO being positive or negative create changes in temperatures and precipitation patterns in eastern Canada and the eastern U.S., as well as in western and central Europe.
(MORE: Winter Storm Central)
The NAO had been positive since October before turning negative within the last few days. It is forecast to become even more negative as a large low-pressure system east of Bermuda moves east-southeast into the region that typically hosts the Azores high, potentially becoming subtropical in the process.
During a negative phase of the NAO in the winter months, below-normal temperatures typically occur in the East, and there is an increased risk of snowstorms.
Given the expected upcoming changes in both the AO and NAO, the next two weeks could become more favorable for a signficant East Coast winter storm. Mid-to-late January is typically the coldest time of year for much of the Midwest and Northeast, so any pattern favoring below-normal temperatures would mean ample below-freezing air would be in place to keep precipitation in frozen form should a major storm develop.
(MORE: Coldest Time of Year)

The above graphic shows the ingredients needed for a possible East Coast snow event.

This pattern would then allow for more of what meteorologists call "atmospheric blocking." In the case of a negative NAO, the "block" often takes the form of a strong bubble of high pressure near Greenland. This "blocks" the typical west-to-east flow of the jet stream, forcing it to meander northward and southward instead. This results in disruptions of the normal eastward progression of weather disturbances. Instead of cold air draining from west to east across Canada, the cold air would be forced deep into the Lower 48 States, sometimes locking in place for a period of a week or more.
With cold air in place, an active subtropical, or southern-branch, jet stream (enchanced by the current record-tying El Niño, by the way) in the southern U.S. could bring enough moisture and lift in the atmosphere to produce wintry precipitation there as long as the pattern is in place.
Then, if this southern jet stream's energy can couple with northern-branch jet stream energy, low pressure could intensify, wrapping Atlantic moisture into the cold air, resulting in an East Coast snowstorm.
(MAPS: 10-day Forecast)
For now, the details of where and when a potential snowstorm may develop in the East is very unclear. But the big picture of an active more winter-like pattern in the South and Eas looks likely.
Check back to weather.com for forecast updates as the overall pattern looks to become more interesting and potentially impactful in the Eastern U.S.
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