The weather has turned so violent in the eastern United States that The Weather Channel has simultaneously-named storms Neptune and Octavia.
Decoded Science prefers to think of all that’s going on as an interconnected web of weather related to an unyielding polar vortex. We’re naming the event Yeti.
Weather will be weather and sometimes it will snow, rain, blow, and freeze. But the persistence of the pattern this winter defies any comparison with past weather, and has to be taken seriously as a warning of a fundamental change in the workings of the atmosphere.
The Polar Vortex Has Moved
The polar vortex is a circumpolar low pressure center normally aligned so that its most powerful manifestation is a jet stream across Canada. Occasional dips into the US are normally transient.
In recent winters, particularly the last two, the vortex has split into two and sometimes three centers, and they have taken residence in preferred locations. For the past two winters, one of those locations is south-central and southeastern Canada and the adjacent areas of the United States.
Dangerous Weather Event Dill Transitions To Yeti
Decoded Science recognized the persistent polar vortex over North America and named it Extreme Weather Event Dill. The primary risk at the time was from ice, but subsequently snow, wind, and cold have become major factors. We are renaming the expanded event Yeti.
The major meteorological phenomenon connected with Yeti is an unusually strong jet stream. Beneath the jet lies the greatest temperature contrast and thus the largest amount of potential energy. The energy is converted to wind via an atmospheric overturning that also produces precipitation.
Another Blizzard For Bost0n
As low pressure has developed offshore New England during the night, the wind has increased. With the new definition of blizzard requiring only frequent gusts to 35 miles per hour instead of sustained winds of that speed, Boston will easily reach the threshold of wind, reduced visibility (1/4 mile or less in falling or blowing snow), and duration (3 hours or longer).
What Makes Wind?
Wind is produced by a pressure gradient (change in pressure with distance), so as the pressure drops in the center of the storm, the wind increases.
Nature tries to restore the pressure balance by moving air from the area of high pressure to where it’s lower, but there’s a snag in the plan. The rotation of the earth introduces a force (the Coriolis force) which turns the wind at a right angle, so that it blows with low pressure on its left and high pressure on its right (in the northern hemisphere). Hence the well-known counterclockwise flow around a low pressure center.
How Much Snow With This Storm?
Boston has already broken its February snowfall record, and will add at least another half foot. Thundersnow squalls are occurring this morning with embedded very heavy localized snow, so it is possible that local accumulations will be closer to a foot.
Yeti’s Southern Border
Cyclonic activity generally takes place on the eastern side of a jet stream trough (dip). When the eastern edge of the trough lines up with the Atlantic coast, the potential energy available due to the contrast of cold continental air with warm oceanic air can lead to explosive cyclonic development — a nor-easter.
On the southern boundary of the trough, a different phenomenon occurs. Warm air from the Gulf of Mexico rides up over the cold air at the surface. The gentle lifting (compared to the violent lifting in a cyclone) produces steady precipitation.
When the atmosphere has a particular vertical temperature structure, the precipitation falls as freezing rain.
- The air at ground level has to be below freezing in order to freeze surfaces on which the rain falls.
- The air above the surface layer has to be above freezing.
- The warm layer has to be thick enough to melt snow falling into it.
- The cold layer near the surface has to be shallow enough so that the rain falling into it remains liquid (supercooled) until it hits the ground.
If the warm layer is too thin, the snowflakes will survive to the ground. If the cold layer is too thick, the raindrops will freeze and fall as sleet.
Where Will The Precipitation Fall As Freezing Rain?
A significant band of freezing rain will accompany Yeti as the cold air pushes through the deep south. Accumulating ice, heavy enough to break branches and cause power outages as tree limbs fall on power lines, could occur from Oklahoma through southern Missouri, Arkansas, southern Tennessee, Alabama, and Mississippi today, and move into Georgia and the Carolinas tonight and Monday, possibly lingering into Tuesday.
Yeti’s Enduring Legacy: Killing Cold
Because of its longevity, Yeti may ultimately be remembered more for its cold than snow, wind, or ice. Some highlights:
- Boston has not recorded a daily high temperature above normal since January 25, and is forecast to do so only a handful of times before the end of March.
- Chicago’s high temperatures will average 26 degrees below normal for the next five days.
- Atlanta, where the average high is 56, will be below freezing all day Thursday.
At zero degrees, frostbite can occur within five minutes. Longer exposures produce hypothermia and death.
Did Global Warming Cause Yeti?
No single weather occurrence can positively be linked to global warming. However, an increase in extreme weather of all kinds is consistent with most models of the future weather given an increase in greenhouse gases.
How Long Will Yeti Last?
There is no indication that Yeti will leave the building soon, though the weather may not be as wickedly windy, savagely snowy, or immobilizingly icy later in the week. Still, temperatures are forecast to stay well below normal in the entire eastern half of the US for at least ten days, and more storms are inevitable.
Decoded Science will probably run out of alliterative phrases and The Weather Channel may run out of names before Yeti departs.
Decoding Science. One article at a time.