Snow Cover Expanding Southward Over the U.S.

It has taken a while, but the storm track finally shifted south, spreading snow far into the U.S.  Below is the depth of snow, on the ground, as of 6:00 am today (the 28th), courtesy of NOAA's Environmental Visualization Lab. What's especially interesting is that the swaths of snow, from the two major snowstorms in the past week, can clearly be seen. One extends from Nebraska to Michigan (the storm from December 19th and 20th). The other extends from Texas and Arkansas, across Ohio, to Maine. The darkest blue color indicates 20 inches.

There is an interesting meteorology lesson behind this as well. Low pressure systems usually come in families, with each low separated by a couple to even several days. When the first low moves along the leading edge of arctic air, and puts down a swath of heavy snow, such as what happened here on the 19th, it pulls the arctic air further southward in its wake. The second storm ends up tracking further south, along the strongest temperature gradient. That can be seen in the image below. In fact, this is going to happen again. The third major storm will track across the Gulf Coast this weekend, exiting the North Carolina coast into the Atlantic. This will extend the snow cover all the way to the coast of the Northeast, including Philadelphia, New York City, and Boston, which saw little or no snow with the second system.

Unfortunately, this process does not occur in a nice stepped progression, where everyone measures snowfall. The image shows that parts of Kansas, Oklahoma, and Missouri, up into Illinois, were "skipped" and missed out. Those gray areas indicate bare ground.

This snow cover lays the foundation for much colder arctic air, as we have experienced in the past week. As chunks of bitter cold plunge out of the Arctic, if there is bare ground, the magnitude of the cold lessens over time. Once this snow cover is established, the arctic air does not modify as much, resulting in much colder temperatures.



This page was composed by the staff at the National Weather Service in Hastings, Nebraska.

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