Weather in the U.S. this week has been a study in stark contrasts: While chilly air has plunged the Northeast into temperatures up to 30°F below normal, the Plains have heated up like a torch, with temperatures more than 30°F above normal.

That heat has extended well into central Canada, and has played a role in the intense wildfire in Fort McMurray, Alberta.


There are many weather terms meteorologists discuss among themselves when describing patterns that produce high-impact weather. Many of those have bled into the public lexicon, like “derecho” and “polar vortex.” The next one, implicated in this week’s wild weather, might be “omega block.”

A block, as it sounds, is an obstruction in the general west-to-east winds in the upper atmosphere, and it slows the progression of storms. The omega block — which takes its name from the Greek letter, omega (Ω), and mirrors its shape — allows large amounts of warm air to penetrate northward in the middle of the block and very chilly air southward on the eastern and western sides.

The chart below illustrates the steering winds over North America on Thursday afternoon. Note the surge of warm colors in the center of the U.S. and Canada. Colder colors are on either side.  The intensity of the colors indicate just how far above (or below) normal that flow is.

Forecast steering winds Thursday afternoon across North America

Forecast steering winds Thursday afternoon across North America.


This is the reason the heat has surged into Canada, worsening the ongoing fire in Fort McMurray. (Climate change also played a role in setting the stage for earlier and more intense fires in the region.) Similarly, it is the reason that the Northeast U.S. has been so chilly.

Omega blocks are fairly common in spring, as the jet stream begins to weaken and migrate northward for its summer residence. Like slower moving water near the side of a riverbank, as that flow slows down and moves away, it leaves behind spinning swirls. In the atmosphere, those swirls become blocks.

While blocks are a normal part of weather, there is some tentative evidence that blocking may become more common with climate change. The warming Arctic may be the key driver and is a reminder that what happens in the Arctic does not stay in the Arctic.

Fundamentally, the speed of the jet stream depends on the contrast between the temperature in the tropics and the Arctic. A larger temperature difference yields a faster jet stream. If the Arctic warms more quickly than the tropics, which has been observed, that temperature difference decreases and the jet stream slows. And as it does during its annual summer retreat, the slower jet stream tends to meander, leaving behind these large swirls responsible for atmospheric blocking.

At the forefront of this research is Jennifer Francis at Rutgers University. And while the strength of this relationship that she has highlighted is still the cause of much discussion in scientific circles, a new study in the Journal of Climate lends support to the idea.

Extremes of heat and cold are not the only feature of blocks. They can aid in the formation of large coastal storms, like nor’easters. From the climate angle, a warming world leads to more evaporation of water, so there is more moisture available for precipitation inside of such storms. Paradoxically, this can also lead to snowstorms with larger accumulations.

As an example, Washington, D.C.’s Capital Weather Gang looked up the top 10 snowstorms on record in five cities: Washington, D.C., Baltimore, Philadelphia, New York, and Boston.

top snowstorms on record

Records in each of those cities go back to the late 1800s. They found that in each city, at least 50 percent of the top 10 snowstorms have occurred since 1990, meaning the deeper snowstorms are coming more recently. The links between blocking and intense snowstorms in the Northeast will likely continue to be an active area of research.

But whether or not “omega block” becomes as vogue a term as “derecho” or “polar vortex,” only time will tell.