Weather models and forecasters alike are homing in on mid-December for a change in the weather pattern in the Eastern United States, potentially one leading toward colder and snowier conditions.
The key player in this pattern change is a phenomenon known as the “Greenland block.” It is a large high-pressure zone over Greenland that impedes weather systems from running into it and forces the jet stream, along which storms track, to dive south over eastern North America. That opens the gates for Arctic air to pour southward.
The Greenland block is indicated by the negative phase of the North Atlantic Oscillation (NAO) — which is a measure of the pressure difference between the high and mid-latitudes.
“The long-range models are forecasting a strong negative NAO, which usually helps push the westerlies and storm tracks along the East Coast farther south than normal,” said Wes Junker, Capital Weather Gang’s winter weather expert.
Indeed, ensemble modeling — which averages many atmospheric simulations together — is showing one of the most powerful negative NAOs in December since 2010, which was the second December in a row featuring powerful Greenland blocking. Both of those months also featured major East Coast snowstorms.
While the main effects of any pattern change are still at least 10 to 14 days away, the process gets underway much sooner than that.
High pressure is already building in a region from Scandinavia toward the Kara Sea north of Russia. This is typically a precursor to a Greenland block. True to form, long-range modeling is showing this region of high pressure strengthening before redeveloping westward over time.
By the end of the first week of December, powerful high pressure should cover much of Greenland.
“Similar retrograding blocks in the past have been associated with periods of colder and wintry weather in the Eastern U.S.,” meteorologist John Homenuk wrote on Twitter.
The initial signs of a Greenland block-induced pattern change could be colder weather arriving near the end of the first week of December. After that, the odds of an East Coast winter storm could increase.
“The forecast pattern on the models suggests that the latter half of December will average colder than normal,” Junker said. “The pattern also is a better-than-average one for seeing snow,” he wrote.
Above-average December snow in the Mid-Atlantic is a rarity of late. The last time Washington topped its monthly average of 1.7 inches was 2017, when 1.9 inches fell. Before that, we have to go back to December 2010, which had 2.1 inches.
In the early 2000s, it felt like December snow was a given in the D.C. area. Accumulating snow fell multiple times on Dec. 5, and the region saw back-to-back snowy Decembers in 2002 and 2003. Washington’s biggest December snowfall on record, named Snowpocalypse, occurred in 2009.
That storm came during a period of unusually powerful Greenland blocking, as have many of the region’s top snows, including the blockbuster January 2016 blizzard “Snowzilla.” A Greenland block is not a requirement for a large snowstorm, but it’s often a signal for high-end potential.
Greenland blocks can also come and go without delivering major snowfalls to the area, and sometimes lead to near misses. The mega Greenland block in December 2010 created a massive snowstorm that blasted a region from the Mid-Atlantic beaches to Boston on Boxing Day. But the D.C. area saw only flurries, after being teased with winter storm warnings.
“Even with a better-than-average pattern, getting a snowstorm in December is always an uphill battle,” Junker said. “We’re heading toward Christmas, so we can always hope.”