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Late autumn cold wave; Midwest 2004 - 12.23.04

Doc Weather missed the recent Midwest cold by not shifting from autumn thinking to winter thinking.

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Fig.1

The recent cold surge in the Midwest was off of Doc Weather's radar screen because he didn't shift early enough to a winter pattern for reading his charts. In figure 1 we see the 45�° jet curves from the western pair of eclipse points. These are over the eastern Pacific. They were under a strong high- pressure influence from Jupiter and later Saturn over the Pacific. High-pressure is designated with blue lines. On the continent, we can see the 72�° jet curves from these same points situated over the western third of the United States. These areas were also under the Jupiter/Saturn high-pressure influence.

From the eastern pair of points we can see the 72�° jet curves crossing the western pair of curves. It can be seen that these two jet curves are split in their values. The western most jet curve is aspected to low-pressure (red) and the eastern most jet curve is aspected to high-pressure (blue). In the summer or fall a placement like this is most likely to result in the type of chart seen in figure 1. The high-pressure areas in the eastern Pacific and the high-pressure area over the continent most often would link up with each other causing a broad high to form at a low latitude in the west and spread out over the southern sections of the continent peaking in the High Plains. This would force any low-pressure to the north for a zonal weather pattern bringing cold and rains to the eastern third of the country. This is the gist of the chart posted in Doc Weather's December outlook. This chart would most likely have been accurate for late summer and early fall. Unfortunately Doc Weather didn't shift from this type of setting into a winter setting early enough to pick up the cold waves and snow.

Fig.2

In figure 2 the corrected chart is given that shows how the pattern unfolded for the second and third weeks of December. The high over the eastern Pacific was very strong. It pushed up into the ocean off of California and the polar jet was forced up into Alaska.. What was unusual was that the continental high-pressure didn't block from a southerly position typical of late fall. When the high-pressure 72�° jet curve (thick blue arrow) from the eastern pair of eclipse points was stimulated to high-pressure this mingled with the strong high-pressure from the western pair of 72�° jet curves and the resulting high shot northward following the Canadian Rockies and tracked northwestward into Alaska. This happened on December 14th to 16th. From there it blocked any passage to the east for cold arctic air. This set up the cold pattern for the following week. The only path left available for the cold air to travel southward was the 72�° jet curve from the eastern pair (heavy red curve) that was under the influence of a low-pressure aspect. This is the chosen path that the cold air took to enter into the continent. From this gateway it turned eastward over the High Plains. At the same time the low-pressure values on the eastern 45�° jet curve over the East Coast enhanced the Hudson Bay low. North American Climate Patterns This feature is a dominant element in the early winter patterns for the eastern Corn Belt and the East Coast. The strengthened low grew into a vast circulation including part of the northern High Plains. The cold jet from the northwest dropped into the Plains as it curved east and spread a narrow band of deep snow from Missouri to Ohio. The descending cold brought with it the potential for snow and what would have been a late fall rainstorm was a very early winter snowstorm. We can expect more from this pattern this winter. It was a surprise for it to set up so early on the last week of autumn.

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