How This Weather Pattern Could Hinder Your Thanksgiving Travel

Why Does the Greenland Block Happen?

As the U.K. Met Office explains, a "block" is an area of high pressure that sticks in one area, delaying the usual eastward movement of pressure systems. In this case, the "Greenland Block" is stuck over the island to the northeast of Canada and the U.S.

"It's kind of like if you were to stick a rock in the middle of a river," Kyle Mattingly, a graduate student in geography at the University of Georgia in Athens, explained to Popular Science.

Mattingly says the Greenland Block develops when storms push warm air northward. When it reaches the jet stream, the air circulation mounts over Greenland. Then, the air pressure climbs and the jet stream is pushed south.

That causes a chain reaction, leading to cold temperatures in the eastern U.S. and Western Europe. But the polar lands under the block are warmed and wind is slowed or goes in a new direction.

"That typical pattern, where you have cold to the north and warm to the south and strong westerly winds between, reverses," Stephen Baxter, a meteorologist at the National Oceanic and Atmospheric Administration's Climate Prediction Center, explained to Popular Science.

Will You Have to Change Your Thanksgiving Travel Plans?

The Greenland block doesn't mean you have to change your Thanksgiving plans. Since it is happening now instead of later in the winter season, like January or February, the chances of this creating a major snowstorm are slim. During those months, there is a greater chance of the Greenland block creating conditions for a nor'easter.

"Right now the pattern is not such that we would favor a big storm off the Eastern seaboard, but there is more uncertainty over the East Coast," Stephen Baxter, meteorologist at the National Oceanic and Atmospheric Administration's Climate Prediction Center, told Popular Science. "If we were in January or February, meteorologists would be very excited about the potential for some kind of nor'easter-type storm."

So if you were hoping for a white Thanksgiving, you might be disappointed.

However, if you live in the Great Lakes snowbelts, Ariondacks and Northern New England, you are expected to see snow early next week.

Does the Greenland Block Explain Why It Was So Cold This Week?

As Popular Science notes, most of the mid-Atlantic and New England states, as well as Canada, will see unseasonably cold temperatures in the days before Thanksgiving. Most days will be between five and 10 degrees colder than usual, with other days reaching highs eight to 16 degrees below average.

Baxter explained to Popular Science that the block could stay in place for a week or even the rest of November.

"You have colder conditions kind of locked in place from southeast Canada to the eastern U.S.," Baxter said.

Although the Greenland Block can have an impact for weeks at a time, it was not the reason for the recent cold temperatures we saw on the East Coast and Midwest. That was caused by a block over the North Pacific, which pushed cold air to the Northeast.

Baxter said a Greenland Block can come and go throughout the winter, as it has in the past, but it doesn't look like that is going to happen in 2017-2018.

Interestingly, the Greenland Block can develop in the summer, as it did in July 2012. That year, the block caused the record-breaking melting of the Greenland Ice Sheet.

How Did the Greenland Block Influence Hurricane Sandy?

In October 2012, a Greenland Block played a key role in pushing Hurricane Sandy up the Northeast and into a unique, westward turn, explains Popular Science. At first, the storm was moving in a normal pattern for an October storm, but then the Greenland Block influenced its chance.

"They can't go through it so they have to go around it, and in this case Sandy ended up being steered due west into New Jersey," Kyle Mattingly, a graduate student in geography at the University of Georgia in Athens, explained.

Mattingly and his colleagues published a paper in 2014 about the influence of the Greenland Block on Sandy. They found that the Greenland Block was extreme that year.

"The fact that this anomalous westward turn of Sandy happened during such an intense, almost unprecedented Greenland blocking episode was definitely something of note," he told Popular Science.

Will We See Another Extreme Greenland Block Like 2012?

Mattingly told Popular Science that it's difficult to predict if we'll ever see a Greenland Block as extreme as the 2012 one again. He said climate models have trouble predicting the North Atlantic jet stream, which means they still have trouble predicting Greenland Blocks today.

Since the Arctic is warming quicker than anywhere else in the world, researchers think Greenland Blocks will happen more often. This would also hasten the thawing of the Greenland ice sheet.

In contrast, Mattingly suggests that climate chance could push the jet stream more north, which would create less chances for a Greenland Block to develop.

"If there is more blocking in the future, we expect there to be more unusual hurricane tracks," Mattingly told Popular Science. "But the million-dollar question is whether blocking is actually going to increase or decrease with climate change."

Photo: NOAA