Tuesday, January 14, 2014

The Polar Vortex & More

The first major North American weather event of 2014 was the cold Arctic outbreak in the first week of January.  The term polar vortex became the buzz word (in the media) for the cold outbreak, though it was much more.  As a previous post notes this was a pattern extending back to late October of 2013.

The term polar vortex has been around since the 1940s.  From the American Meteorological Society Glossary of Meteorology:

Polar vortex - (Also called polar cyclone, polar low, circumpolar whirl.) The planetary-scale cyclonic circulation, centered generally in the polar regions, extending from the middle troposphere to the stratosphere.

The westerly airflow is largely a manifestation of the thermal wind above the polar frontal zone of middle and subpolar latitudes. The vortex is strongest in winter when the pole-to-equator temperature gradient is strongest. In the Northern Hemisphere, the vortex has two centers in the mean, one near Baffin Island and the other over northeast Siberia.

I prefer to use the term Arctic outbreak to describe what happened in early January.  The polar vortex was involved, but the overall pattern was much more than the polar vortex.  The winds in the upper atmosphere plunged from northwestern Canada into the southeastern U.S. due to an amplification of the ridge of high pressure off the West coast and the deep trough over the eastern part of North America.

NASA put together a movie of temperature observations from NASA's Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua spacecraft.  The Atmospheric Infrared Sounder, AIRS, in conjunction with the Advanced Microwave Sounding Unit, AMSU, sense emitted infrared and microwave radiation from the Earth to provide a three-dimensional look at Earth's weather and climate. Working in tandem, the two instruments make simultaneous observations all the way down to the Earth's surface, even in the presence of heavy clouds. With more than 2,000 channels sensing different regions of the atmosphere, the system creates a global, three-dimensional map of atmospheric temperature and humidity, cloud amounts and heights, greenhouse gas concentrations, and many other atmospheric phenomena. The AIRS and AMSU fly onboard NASA's Aqua spacecraft and are managed by the Jet Propulsion Laboratory, Pasadena, California, under contract to NASA. JPL is a division of the California Institute of Technology in Pasadena.

The temperatures shown are at a pressure of 850 hectopascals (hPa, also known as 850 mbs) which is at an altitude of 4500 to 5000 feet over much of the U.S. It begins on December 1, 2013 and runs through January 7, 2014.  The most obvious feature of the movie is the tongue of cold air moving out of Canada and southward to cover much of the eastern United States during early January 2014.

This is a picture of the last day of the movie, Tuesday, January 7, 2014.  The dark blue areas start at -12° C and become purple at -24° C.  This was the farthest extent south and the surface temperature dropped to 13° F in Columbia, SC, with a wind chill temperature of 3° F.  Click on the image for a larger view.  Image Credit: NASA.

The pattern de-amplified toward last weekend allowing temperatures to moderate across the U.S.  Warm, moist air began flowing from the Gulf of Mexico which became the fuel for severe weather across the Southeast Friday and Saturday.  However, the ridge of high pressure amplified over the weekend and now the situation is similar to the Arctic outbreak earlier in the month.

The flow pattern at 500 mbs for 00z January 14, 2014.  Note the high pressure ridge along the West coast and the deep trough over the U.S.  This represents the amplified part of the pattern that has persisted since late October.  It signals more cold air intrusion to the U.S.  Click on the image for a larger view.  Image Credit: WLTX-TV.

This time it will be cold, but not as cold since the air mass is not as intense.  Temperatures in northern and central Canada have been in the range of -20° F to -35° F, whereas about 10 days ago they were in the range of -35° F to -55° F.

Selected temperatures across Canada for 1 a.m. EST, Tuesday, January 14, 2014.  Click on the image for a larger view.  Image Credit: WLTX-TV.

Last week saw the bitter cold grip the eastern half of the country.  This week it will be cold in the East, but unseasonably warm in parts of the Southwest.  This is a result of the high pressure ridge off the West coast.  The drought in California and much of the Southwest is a direct result of this ridge.

The impacts in California are seen here is a CBS report by Ben Tracy:

Note for Apple users.  The story uses Flash, but you can access the story here.

It appears that this weather pattern of amplification and de-amplification will continue for the remainder of January.  Thus some wild swings in temperature are still possible.  It will be interesting when we get to February.  If the pattern persists, it could increase chances for snow in South Carolina.  February is the snowiest month of the year.  Go snow-lovers!