A Cool Spell Since July 24

From July 24 through August 14, Detroit Metropolitan Airport (DTW) has seen just one day of above-normal daily average temperatures (August 7).  During this stretch, the average temperature at DTW was 68.4 degrees, or 4.5 degrees below the normal values for that stretch of time, making it the 9th coolest such period on record.  The following graph shows the daily temperature range through this period: 

Click image to enlarge

 What's behind the recent cool weather?  The main culprit appears to be a persistent upper-level low pressure trough centered over southern Canada.  To understand why upper-level troughs are associated with cool weather, let's think about two identical columns of air.  If one column of air is heated, it will expand, and so the top of the cool column will be located below the top of the warm column.  So lower heights at a certain pressure level imply a lower temperature through the column of air that extends up to that pressure level.  Looking at the height anomalies (departure from normal) averaged from July 24 to August 12, this trough can be seen as an area of below-normal heights at 300 hPa (an upper level of the atmosphere) extending across much of southern Canada and the Upper Midwest:


Looking at the average temperature anomalies at a lower level of the atmosphere (850 hPa or about 5,000 feet), we can see that temperatures were well below-normal over much of the northern U.S. during the same time frame:


This cool airmass translated into cooler than average temperatures across much of the Upper Midwest, including Detroit, for July 24 through August 14.  However, despite the recent cool spell, the average temperature at DTW for the summer so far (June 1-August 14) has actually ranked as the 44th warmest such period out of 140 periods dating back to 1874.  This actually ranks this summer so far in the top third warmest summers at DTW!

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