Massive Heat Bubble

On the evening of July 17, an expansive 500 mb subtropical upper-level anticyclone was centered near the Kansas and Nebraska border and nearly stretched from border to border in the center part of the United States (Figure 1). The strength and placement of this feature is unusual, and in fact, standard deviations of +2.5 were common near the centroid of the anticyclone (Figure 2). When strong anticyclones like this develop, they are accompanied by unseasonably warm temperatures during the day and at night along with high moisture content. The combination of these factors creates a potentially dangerous situation because the heat can add serious stress on the human body. Accordingly, several heat-related watches, warnings, and advisories were in effect for 16 states (Figure 3) last evening. What compounds matters is that a lot of the heat is expected to be concentrated in urban areas, including Chicago, Milwaukee, and Minneapolis where many people do not have air conditioning or cannot afford to run the air conditioner.

Figure 1. 18 July 2011 00Z 500 mb height analysis

Figure 2. 18 July 2011 00Z 500 mb height standardized anomalies.


Figure 3. Watch, warnings, and advisories for July 17.

It surprises many people to learn that heat kills more people on average that tornadoes and lightning combined. Granted, these statistics are slightly skewed by the 1995 heat wave in the Midwest that killed hundreds of people, but it proves that heat is a very underrated killer.

Kunkel, K. E., S. A. Changnon, B. C. Reinke, and R. W. Arritt, 1996: The July 1995 heat wave in the Midwest: A climatic perspective and critical weather factors. Bull. Amer. Meteor. Soc., 77, 1507–1518.