The Synoptic Picture - December 22, 2004

 

 

Surface Weather Maps

The American Meteorology Society (AMS) describes a synoptic chart as "any chart or map on which data and analysis are presented that describe the state of the atmosphere over a large area at a given moment in time". Most meteorologists consider synoptic analysis to include those weather features that range in size from 1,000 to 10,000 kilometers (621 to 6,213 miles). Larger features are considered to be on the planetary scale while smaller ones are classified as mesoscale. Mesoscale features are further subdivided into:

  • Mesoscale-α are those features that are 200 to 1,000 kilometers (124 to 621 miles)

  • Mesoscale-β are those features that are 20 to 200 kilometers (12.4 to 124 miles)

  • Mesoscale-γ are those features that are 2 to 20 kilometers (1.2 to 12.4)

The chart below provides a synoptic analysis of the weather conditions affecting the continental United States on the morning of December 21, 2004, one day before the record-breaking winter storm. An area of low pressure was centered over Lake Superior while its associated cold front (shaded in blue with periodic blue triangles) swung south into northern Missouri and Kansas before curving to the northwest through Nebraska.

 

Surface Weather Map as of 7:00am Eastern on December 21, 2004 from the Hydrometeorological Prediction Center.

The areas of precipitation (most likely snow) associated with this low pressure system are shaded in green. A second area of low pressure, although not well organized, stretched from western Texas, through Oklahoma and into northeastern Arkansas. This area of low pressure became better organized and moved rapidly northeasterly along the approaching cold front in the two days following December 21, 2004.

The two light blue dashed lines snaking across the country are the 0°F and the 32°F isotherm (a line drawn through points of equal temperature). A majority of the United States was under the influence of below freezing temperatures. Temperatures below 0°F were advancing into the northern plains behind the fast-moving cold front.

 

The last feature of significance on the synoptic chart for December 24th is the area of high pressure centered over the southeastern United States. Although it may not be readily apparent, flow from this high pressure system played an important part of this historic storm.

 

Next: The Daily Forecast

 

© 2005-2006 Mark A. Thornton