At least two National Weather Service (NWS) forecast offices Thursday morning had connected the looming blast of autumn-like air with the weather pattern blamed for brutally cold temperatures in January. The NWS office in Chicago even created a splashy graphic advertising a “July version” of the polar vortex.
But, midday, the NWS’ national forecasting hub in College Park – countered the polar vortex assertion in its extended forecast discussion (I’ve bold-faced the relevant text):
A DEEP UPPER LOW… NOT THE POLAR VORTEX AS ITS ORIGINS ARE FROM THE NE PACIFIC… WILL SWING THROUGH THE GREAT LAKES EARLY NEXT WEEK WITH AN IMPRESSIVE COLD SHOT OF AIR INTO THE CENTRAL AND THEN SOUTHERN PLAINS AND THE MIDWEST.
A memo was emailed from the NWS’ Central Region to local offices directing forecasters to cease use of the term according to Chris Vaccaro, director of NWS public affairs.
“Internally, WFOs [Weather Forecast Offices] that used the term polar vortex were gently reminded that with the term “polar vortex” comes a range of definitions and perceived impacts (however temperatures won’t be any where near the low temperatures experienced over the winter, nor would there be snow) which can distort the primary message of actual impacts that the public needs to know,” Vaccaro said in an email.
On its Web site, The Weather Channel also challenged use of the term polar vortex to describe next week’s weather. “[W]hile it’s a catchy term that gets a lot of attention, the polar vortex isn’t really to blame for the unusually cool weather that much of the nation is expected to see next week,” it writes.”[W]hat we’ll see next week is a trough, a dip in the jet stream that’s actually a lobe of the tropospheric circumpolar vortex — like a spoke on a wheel.”
The Weather Channel is instead using “polar air invasion” to describe the blast. As of Thursday, AccuWeather was using the polar vortex description.
Meteorologists have debated how the term should be used and what types of cold air outbreaks it applies to since it hit the mainstream this past January.
The American Meteorological Society’s “official” definition of polar vortex is both technical and subject to a range of practical interpretations. For the brave, here it is:
A planetary-scale mid- to high-latitude circumpolar cyclonic circulation, extending from the middle troposphere to the stratosphere. The Northern Hemisphere vortex often features two centers—one near Baffin Island and the other over northeast Siberia—with analogous circumpolar asymmetry atypical in the Southern Hemisphere. The westerly airflow is largely a manifestation of the thermal wind above the polar frontal zone of middle and subpolar latitudes. The vortex is strongest during the winter in the upper troposphere and stratosphere when the pole-to-equator temperature gradient is strongest. The stratosphere component of the circulation may be referred to separately as the “polar stratospheric vortex.” In summer, the strongest westerly circulation is largely confined to the troposphere, and the polar stratospheric vortex reverses in the upper stratosphere because of solar heating during the polar day.
A good analogy I’ve encountered to understand the vortex is to think of it as a chain-linked fence containing frigid air at high altitudes around the North Pole (and South Pole), with dogs running around inside of it. When a dog finds a weakness in the fence and escapes and dashes off to the south, that’s when we have these cold air outbreaks or “so-called” polar vortex events in the mid-latitudes.
Many meteorologists reject calling next week’s cold air outbreak a true polar vortex event because the deep pool of cool air feeding into the north central U.S. is not originating from the polar latitudes or within that Arctic “fence”, but rather from the northeast Pacific.
Irrespective of the origins of the cold air mass, the weather map early next week will look an awful lot like mid-January, hence the label I used yesterday: a “poor man’s polar vortex“…