Extreme Fall Weather Event Gorilla Will Set Records

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The jet stream on Friday morning, 10/31/12

The jet stream on Friday morning, 10/31/12. Analysis courtesy of NOAA

The fold in the jet stream which became evident early in the week has now become more pronounced.

The result is the opening of a corridor from Canada through which will spill the coldest air of the fall season over much of the eastern United States.

And when it gets cold enough — it snows. Some places may experience the earliest measurable snowfall on record.

Decoded Science is naming this extreme weather event Gorilla.

Cold temperature records will fall from the Midwest to the Deep South. Earliest snow accumulations will occur in the Mississippi Valley and along the Atlantic coast.

Our Old Friend: The Polar Vortex

The polar vortex became a popular villain last winter, as a branch of the normally arctic-bound atmospheric gyre camped over central Canada with regular incursions into the central and eastern United Sates. The result was a plethora of foul weather phenomena: snow, wind, cold, and ice.

This week it was shades of last winter, as the vortex plunged south and is now centered over east-central Canada. A low pressure center will form along the mid-Atlantic coast and move northeastward — a nor’easter. If it stays close enough to the coast, it will bring wind and rain to New England, New York, and New Jersey. But even if the center of the storm remains well offshore, the north winds in its wake will drop temperatures sharply as Gorilla prowls the eastern half of the country.

Where Does Gorilla Get Its Strength?

Differential heating of the earth by the sun produces a temperature gradient from pole to equator. Some mechanism must redistribute the heat or the  temperature near the equator would become unbearably hot and the high latitudes would be much colder than they are now. This redistribution of heat is the job of the atmosphere, specifically storms.

The temperature gradient from high to low latitudes represents potential energy. An efficient way for the atmosphere to accomplish its goal of redistributing heat is to turn potential energy into kinetic energy of wind and have the winds blow north to south and vice-versa.

We can get an idea of the amount of potential energy that needs to be constantly converted to something else by looking at the result of the redistribution — the wind generated by cyclones. Much more research has been done on hurricanes, but the winds in the most powerful extra-tropical storms are comparable to those in a hurricane. A single hurricane releases about half the electrical output of the earth, approximately 10^12 watts.

No wonder we have so much foul weather; an enormous amount of energy has to be released.

The horizontal and vertical structure of an extra-tropical cyclone is complicated, but the essential facts are these:

  • Initially cold air lies next to warm air.
  • A wave in the jet stream begins the process of rotation.
  • The rotation works its way to the surface.
  • The cold air forces its way under the warm air until it completely wraps around the system and forces the warm air to take a new position above the cold.
  • The new potential energy state is lower than that at the start.
  • The lost potential energy appears as kinetic energy of wind.
  • The wind dissipates by frictional interaction with the ground.

Who Will Feel The Effects Of Gorilla?

Current watches and warnings courtesy of NOAA

Current watches and warnings courtesy of NOAA

Gorilla is the result of a very sharp dip in the jet stream. A storm will form and move northeastward. On the west side of the storm, north winds will transport unseasonably cold air far to the south, reaching southern Florida by Sunday.

Cold temperature records are likely to be set in the midwest, the south all the way to the Gulf Coast, and all of the eastern seaboard.

Locations where snow will accumulate:

  • Northern Maine will be close enough to the storm to get snow accumulation of half a foot.
  • Heavy snow will accumulate up to a foot in the Appalachians from Tennessee to New England above 2,000 feet.
  • Parts of the midwest and northeast will be cold enough to see some snow at lower elevations.
  • Great Lakes locations with a wind off the warm water will get some Lake-Effect Snow.

Is It Global Warming?

Climate-change deniers will crow about the cold weather, but in fact it is consistent with anthropogenic climate change. Most models of the atmosphere predict that the increase in greenhouse gases will create greater variability in the weather, both cold and hot — and more frequent storms.

Does Gorilla Have Friends?

The jet stream pattern is looking qualitatively like last winter’s. However, there are differences which might lead to a very different kind of meteorological winter this year. Much of the winter weather hinges on whether an El Niño develops. The National Weather Service puts the chance at 65%, but Decoded Science rates it only 35% likely. El Niños are correlated with warm weather over much of the United States and rain in California.

The ridge over northern Europe which has kept the weather mild is holding for now, but a southern branch of the jet stream has developed over southern Europe and this could portend a wholesale reconfiguration of the jet stream flow.

In the Pacific, the ridge over the eastern Pacific has been eroded by a series of jet stream waves associated with the recent storms on the northwest coast. Though water temperatures in the Gulf of Alaska remain warm, the ridge is not as persistent as it was.

The Decoded Science Winter Forecast

The atmosphere doesn’t seem to want to make up its mind which way it will go this winter. Decoded Science is making no winter forecast except to say it’s more likely than not to be somewhat like last year but not so pronounced. How’s that for meteorological equivocation?

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