Extreme Weather Event Artichoke: Many Petals And A Heart Of Cold


Home / Extreme Weather Event Artichoke: Many Petals And A Heart Of Cold
The jet stream on November 16, 2014. Analysis courtesy of NOAA

This is what the jet stream on November 16, 2014 looks like. Analysis courtesy of NOAA

The push of Arctic air into the eastern US that Decoded Science named Artichoke will break records for its duration.

This weather event is a single fortnight-long deep freeze exacerbated by the transition of Super-Typhoon Nuri into an extratropical low pressure center over the Bering Straits.

The Weather Channel has broken Artichoke into separate storms, but they are all part of the atmospheric gyre that is a lobe of the polar vortex which has settled over eastern North America.

When cold air is established at the ground, its density makes it hard to dislodge. As warm air tries to return, it is forced to ride over the colder, denser air.

Rising air produces precipitation, so it is not surprising that at the periphery and in the interior of Artichoke, snow and rain are falling.

The Cutoff Low And The Omega Block

The jet stream normally flows from west to east, and within it shorter waves ripple along, causing high and low pressure areas to advance eastward. Occasionally — more often in recent years — the system breaks down: A closed low or high pressure area stops the eastward propagation of the waves. These blocking patterns keep weather systems in place for extended periods of time.

Low pressure has been persistent in the north-central Pacific and high pressure over the eastern Pacific and western US. The next downstream low pressure swirl is in the eastern US. When the Pacific low became more pronounced as Nuri was absorbed, so did the high pressure off the west coast (an omega block) and the low in the eastern US — Artichoke was born.

Air Trajectories

The temperature of the air is largely a function of where it comes from. Land areas absorb and emit radiation. Since the land surface is a poor conductor, the heat and cold remain at the surface and can be transferred to the atmosphere. Sea surface temperatures tend to be much more moderate, because water is a good conductor and heat is more efficiently distributed to the deeper ocean.

The land area of northern Canada and Alaska, receiving little sunlight, becomes very cold in the winter. The structure of high and low pressure areas associated with Artichoke has created a direct flow of air from northern Canada to the central US. The cold has been centered on the northern plains, but has spread over all of the eastern US except for the extreme south. The next push from the north should even reach that area in the next couple of days.

The Heart Of Artichoke

Artichoke’s main artery is the river of cold air from northern Canada, down the east slopes of the Rockies and plains into the United States. Pulses of fresh Canadian air reinforce the cold air pool, and spread it east and south.

Where warm air tries to dislodge Artichoke’s cold, snow and rain occur as the warm air rides up over the cold. Lifted air cools as the pressure drops; cooler air holds less moisture than warm; and if the air cools to the dew point, clouds and rain or snow form.

One Petal of Artichoke: Weather Channel’s Winter Storm Bozeman

The Weather Channel has inexplicably named their second winter storm after a city in Montana. The stigmatizing of an unassuming town and the separate naming of what is really just a piece of Artichoke was ill-advised.

‘Bozeman’ is a pulse of energy from the Pacific that is interacting with the cold air in place over the continent, and which spilled through the Columbia River Gorge to the west coast. As Winter Storm Bozeman reaches the east slopes of the Rockies, light snow will fall through the central plains and then the midwest.

A few spots could receive as much as six inches of snow, but in most places only an inch or two will fall. However, with the temperatures so cold, the snow will remain on the ground, and this will lead to a feedback effect on temperatures.

Albedo And Its Effect On Temperature

The albedo of a surface is the percentage of radiation that it reflects. Land and ocean surfaces have varying albedos; snow-covered ground has the highest.

The widespread snow in the plains and midwest will cause the ground to reflect a large amount of the incoming sunshine; the result will be that Artichoke will be a little colder and last a little longer than it would without the snow cover.

More of Artichoke’s Petals

Precipitation could occur anywhere in the cold air, as warm air from the Gulf of Mexico and Atlantic Ocean try to displace the cold. The cold air on land adjacent to the warm air over the ocean creates a high potential energy state. There is a very good chance for a major coastal or ocean storm late this week, as the potential energy is converted to kinetic energy (wind). The storm could be along the Atlantic coast — or it could be far enough offshore to spare land areas of any effects except high waves at the seashore.

Artichoke’s Lake-Effect Snow

A lake-effect snowstorm approaches. Photo courtesy of NOAA

This is what it looks like when a lake-effect snowstorm approaches. Photo courtesy of NOAA

As the cold air settles over the Great Lakes, bands of lake-effect snow will form wherever the wind blows from water to land. The warm water evaporates into the air, and as the moisture-laden air hits the cold land, it cools below the dew point. As a result, the water vapor condenses. Heavy snow, often measured in feet, falls in places near the shore.

At this time of year, with the water still very warm, thundersnow is possible, as the instability of the rising warm air produces a thunderstorm.

Some Of Artichoke’s Records

Many cities set records for daily minimum temperature last week. A couple recorded the lowest temperature for any November day and some for longevity of cold temperatures. Here’s a sampling:

  • Burlington, Colorado: Lowest November temperature — minus ten.
  • Casper, Wyoming: Lowest November temperature — minus twenty-seven.
  • Kansas City, Missouri: Longest November period of below-freezing temperature — five days and counting.

More records will surely fall with the coming reinforcement of the cold air and its intrusion further south and east.

Does Artichoke Herald a Cold Winter?

If past is prologue, the cold November means little as a predictor of winter weather. Though some sources find a correlation, it is weak.

That said, Decoded Science reiterates its observation that the ridge in the jet stream over the eastern Pacific and western US has been resilient and long-lasting. This feature has anchored the polar vortex in central and eastern Canada. Betting against a continuation of this feature has been a losing proposition for nearly a year, and Decoded Science is still predicting that it will remain in place for this winter, but not be as pronounced as it was last winter — and is now with Artichoke.

Is It Global Warming?

It might seem nonsensical to ask if record cold is a result of global warming, but in fact most models of the result of increasing greenhouse gases in the atmosphere agree that as the temperature rises, so will the occurrence of extreme events — both warmer and colder than normal. Artichoke doesn’t prove global warming — no single weather event does. But Artichoke is consistent with the theoretical science of climate change.

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