Weather chatter to start December was squarely focused on the potential for a cold and snowy pattern to take hold in the eastern United States. One week later, confidence in this idea has slipped, as computer models project mild weather into the middle of the month.
The key to a cold, snowy pattern is the evolution of a high-pressure zone in the North Atlantic region, or “Greenland block,” which is also known as a negative North Atlantic oscillation (NAO). The presence of a Greenland block impedes weather systems from running into it and forces the jet stream, along which storms track, to dive south over eastern North America.
The Greenland block has developed, and become quite strong, but so far that hasn’t allowed much cold air to spill into the eastern United States, primarily because the configuration of weather systems in western North America has prevented it. Across the pond, the negative NAO has helped deliver colder air to Europe, including snow and ice to parts of the United Kingdom.
For the eastern United States, there are few signs that the pattern will flip to cold and stormy over the next week. However, a pair of weak storm systems, diverted south by the Greenland block, could bring some minor wintry weather to parts of the Northeast. The first is slated to track across the Mid-Atlantic late Friday into Saturday. Despite limited chilly air, some wet snowflakes are likely to fall, especially in higher elevations of Pennsylvania. A second system may pass late Sunday and early Monday, bringing light snow or mixed wintry precipitation to New York and New England.
Despite the presence of the Greenland block, an active jet stream over the Pacific Ocean continues to send storm systems into the western United States. This inherently makes it much trickier to get cold and snow in the Northeast, as the region will often sit in a warmer flow from the south-southwest in a response to western storminess.
Computer models project that one of these western systems will develop into a major storm over the central United States next week. It could bring near-blizzard conditions to the northern Plains in addition to severe weather in the South. It may also be a catalyst for colder weather in the eastern half of the nation in its wake.
“We won’t know for sure how much of a pattern change we’ll get until the big storm that will be impacting the northern Plains and Upper Midwest is resolved,” wrote Wes Junker, Capital Weather Gang’s winter weather expert, in a text message.
Around that time, partly in response to the central U.S. storm, high pressure is expected to build over the Gulf of Alaska and surrounding areas. The presence of high pressure there reliably helps steer cold air into the Lower 48, and particularly the East.
“That suggests by the 18th we should start getting some colder air here,” Junker said. “The change in the pattern in the Pacific potentially is a significant one if it verifies.”
Stepping into a colder pattern
Pattern changes are notoriously difficult to accurately predict — especially more than a week in advance. This is part of the reason for caution when you come across any specific predictions for snowstorms or intense cold snaps more than a few days in advance.
“Models tend to prematurely predict pattern changes, and I think that this is a good example of that,” wrote Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research, in a text message.
Cohen noted that Europe responds more quickly to the negative phase of the NAO compared with the eastern United States. In that sense, he expects cold air to continue to affect Europe before taking aim at the Lower 48.
As for the chances for significant snow in the eastern United States, Cohen said that “the payoff usually comes at the end of the pattern.” He added, “The best chance for a big snowfall is when the NAO is transitioning from negative to positive.”
The Mid-Atlantic and Northeast still appear on track to turn more wintry during the second half of the month. As Christmas approaches, if you’re rooting for snow, waiting is often the hardest part.