Frequency Of 500 mb Closed Lows

David C. asked: Are these cold lows becoming more common in the SW? It doesn't seem like they happen much in the east, just here...

Basically the answer is yes, they are more frequent in the Southwest, but have been for many years. So, I can't comment on whether there is a trend. The figures below are from Bell and Bosart (1989).

The figures show that the formation of closed, 500 mb lows is more frequent over the Southwest than over other parts of the continental U.S. for all seasons except summer. Their results are for a study of 15 years, but are similar to other findings for longer periods. I examined several articles, but could not find much in the way of explanations for the observed frequency distributions. If anyone can steer me to a good reference, I'd appreciate it.

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Ref: Bell, Gerald D., and Lance Bosart, 1989: A 15-year climatology of Northern Hemisphere 500 mb closed cyclone and anticyclone centers. Mon. Wea. Rev., 117, 2142-2164. From their Abstract:

... The occurrence of closed cyclone centers is maximized north of and within the main belt of westerlies extending from northeast Asia to the Gulf of Alaska near 50°N, and extending from eastern Canada and the extreme northeast United States to southeast of Greenland and west of the United Kingdom. Their occurrence is also maximized south of the main belt of westerlies in a band extending from the east-central Atlantic Ocean across southern Europe to the Caspian Sea and central Asia. Important regional features include cool season maxima over the Mediterranean basin and the southwestern United States, and year-round maxima south of the jet stream extending from the eastern Pacific into the southwestern United States...