If there’s anything a Bostonian would less like to hear than a forecast of 25 degrees and snow, it would be ten degrees and snow. But that’s what’s coming.
Decoded Science has named the cold spell encompassing most of the northeast United States Extreme Weather Event Iceberg Lettuce. We think the title is more appropriate for the current bout of frigid temperatures than for a vegetable.
Polar Vortex To Blame — Again
The weather pattern has settled in to a near replica of last year’s, with a relatively minor shift that has major implications for millions of people.
The trough (a pronounced dip) in the jet stream that covered central and eastern Canada last year is now farther east — mainly over extreme eastern North America. This shift of about 500 miles has spared the midwest from the worst of the winter, brought springtime in February to the intermountain west and the Rockies, and produced the coldest temperatures in several seasons in the northeast.
A Persistent Weather Pattern
The trend that has accompanied warmer climates is for the weather to get stuck in place. What you see has always been what you get, but now it’s also what you’re gonna get. Europe has been warm for a year and a half and eastern North America has been below normal for that time.
The extreme patterns have their greatest impact near the time of maximum and minimum temperatures — mid-winter and mid-summer. Mother Nature sees her windows of opportunity for all-time records and tries to squeeze through.
The Forgotten Word: Trajectory
Just as a human being will acquire the blonde hair, ruddy skin, or hazel eyes of an ancestor, so the subdivisions of air that meteorologists call ‘parcels’ display their place of origin — their ancestry. The coldest air normally arrives in New England directly from the Yukon, where it has been chilled by a radiating earth’s surface. This time, the air has actually originated in Siberia. The path of the air from one place to another is called the trajectory.
The balance of energy at any spot on the earth is the profit or loss difference between incoming sunshine and outgoing microwave radiation. In northern Canada and Siberia, conditions are perfect for major red ink on the ledger. In the dead of winter, there is almost no sunlight, and most of what little there is is reflected back to space by the snowy surface.
Meanwhile the earth is always radiating at a nearly constant rate.
When an air mass spends some time over the frozen tundra, the temperature can fall to 50 below zero or lower, even in an age of global warming.
When the pressure pattern becomes favorable for Southward Ho, the arctic air spills into the US, normally into the plains and midwest, but this year farther east over the upper midwest and northeast.
The New Normal Jet Stream And The Polar Vortex
The average position of the polar vortex is over extreme northern Canada and the Arctic. It is basically a cold low pressure system centered at the north pole.
But in recent years, higher pressures, the result of higher temperatures over the pole, have nudged the polar vortex southward and caused it to become distorted. The vortex has split, sometimes into a bipolar configuration, sometimes several polar vortices, displaced to the south of their normal positions.
Last winter, one such vortex took up residence over central and southern Canada, much to the dismay of residents of the midwestern US.
This year’s pattern features the vortex farther east, so that New England is bearing the brunt of the onslaught.
There are indications that the successive blasts of cold air associated with Iceberg Lettuce will move west, so that by the middle of next week, the plains and midwest will be experiencing low temperatures near the daily records.
Who Will Suffer The Deepest Freeze?
Along the Canadian border, temperatures will fall below zero for many nights in a row. Farther south, the repeated thrusts of cold air will finally reach the Gulf coast by early next week.
The coldest temperatures will occur after the cold air is fully established, where the sky is clear and the wind has subsided. These conditions lead to the maximum ‘radiational cooling’ as the outgoing radiation cools the ground which cools the adjacent air. With no wind to mix the layers, very cold air hugs the surface.
The forecast indicates that the most favorable conditions for radiational cooling will take place Monday morning in the eastern Great Lakes and Ohio Valley.
Another leaf of Iceberg Lettuce will impact the plains midweek.
How Long Will Iceberg Lettuce Last?
Current long-range forecasts for the jet stream show a slight warmup for New England early next week; but then the cold air returns, perhaps with even more frigid conditions and record-braking temperatures over a wider area including the midwest and deep south.
The extreme persistence of the weather patterns in recent years suggests that it could be a long winter for residents of the eastern US.
Meanwhile Out In the Rockies…..
What goes up must come down, and this is never more true than when it is applied to the jet stream. As it dips in one place, it creates a ridge in another. In this case, the Rocky mountains and the intermountain west are the beneficiaries, with temperatures running 20 degrees above normal on a daily basis in places such as Denver and Salt Lake City.
When Will Iceberg Lettuce Finally Wilt?
The current long-range forecasts show some moderation in the extreme cold, but the overall pattern of warm in the west and cold in the east will persist for at least ten days.
Spring will surely come — but not this week in the eastern United States.