December 9, 2010
Why So Cold? Blame the Greenland Block
The Greenland Block is a very strong area of high pressure located over the country of Greenland.
The block does what you may think it does - it creates an atmospheric traffic jam.
Air currents want to move west to east (in the northern hemisphere) but when the Greenland Block is in place it is has to navigate around the block. So air currents either flow up and around the block or dig south.
In the graphic above, the block is designated by a ridge of high pressure with the jet stream buckling northward up and around the high pressure area.
On either side of the ridge, the jet stream buckles southward creating two troughs - one located over the central and eastern United States and another over western and central Europe.
As the trough digs south, arctic air is no longer locked in the...well...arctic. It is free to spill away from the cold dungeon.
The cold air surges southward and depending on how far south the jet stream digs, is sometimes capable of reaching typically mild or warm areas such as south Texas, the Deep South and Florida.
Over Europe, the cold air originates out of Siberia and spill south and west overwhelming much of the continent.
Meanwhile, at the surface the traffic jam is played out a bit differently but the conclusion is the same.
Let's use the recent arctic blast over the eastern 2/3 of the United States as an example.
A large area of low pressure is incapable of moving west to east with the block in place and either loiters or retrogrades back to the west - hanging out over eastern Canada.
With a counter-clockwise flow around low pressure systems, a persistent northerly flow is triggered. This northerly wind flow captures the very cold air that is bottled up over the arctic and transports it southward.
The Greenland Block was so strong that the atmospheric traffic jam was capable of existing for not just days but weeks.
This allowed for the cold northerly flow to continue nearly uninterrupted day in and day out - transporting fresh bouts of arctic air into the Deep South including Florida.
Finally, the trough that has been in place over the eastern 2/3 of the United States will begin to lift and the jet stream will shift northward allowing milder air from the south penetrate northward.
This will give way to a United States that will enjoy more seasonable temperatures after a very long record cold stretch of days.
by Tim Ballisty - Weather.com
Posted by AG-ER Team at 12/09/2010