Why Has This Summer Been So Cool?

People across Southern Wisconsin have noticed that this summer has been rather cool. Milwaukee finished the month of June with average temperatures 1.2 degrees below normal, and average high temperatures 2.6 degrees below normal. Madison had average high temperatures 1.3 degrees below normal in June. Meanwhile, July has started remarkably cool. You can read about that in one of our other top news stories. But what has caused the unusually cool summer?

In general, large air masses will follow the configuration of the upper level jet stream. Thus far this summer, the jet stream has been aligned in a pattern that has a ridge across the Western United States, and a trough across the Eastern United States. A ridge will push the jet stream further north and allow warm air to reach higher latitudes. Meanwhile, a trough will push the jet stream further south and enable cool Canadian air to dip to lower latitudes. A graphical depiction of the general weather setup this summer is shown below (note that smoothing and estimation are used):

This type of pattern can happen in any given summer, but what has been unusual this year is the persistence of the pattern. The western ridge and eastern trough upper level flow pattern has been entrenched across North America since essentially mid-June, approximately when temperatures began to noticeably cool around Southern Wisconsin.

You can ascertain these different weather features in the graph pictured below. It basically shows the average height of the 500mb pressure level at our latitude (around 42-43N) over the past few months. As you move from top to bottom, you are moving forward in time. The dark black line indicates the longitude of Wisconsin. Areas to the left of the black line are to the west of Wisconsin, and areas to the right of the black line are to the east of Wisconsin. Around mid-June, the pattern we've been stuck in developed, with a ridge centered roughly to the west of Wisconsin and a trough centered roughly to the east of Wisconsin.

The last significant trough to impact the area moved through in early to mid June. Other weaker troughs have moved from west to east across the Northern US since then, but they were either too weak or small to show up in the above analysis. With the stubborn ridge to our west, this type of pattern is referred to by meteorologists as a blocking pattern, where overall large scale changes are negligible over long periods of time.

The yellow and green blips that show up just to the right (east) of the black line (Wisconsin) in July are very large and potent troughs that developed over the Northeast US. The general trough pattern over the Northeast US has persisted for all of July, but about once a week it has been reinforced by another shot of energy from Canada. When this occurs, the weather in Southern Wisconsin has responded with cloudy days, high temperatures in the 60s, and isolated rain showers - something that seems more like September or October than July. Below is a MODIS satellite image (courtesy of CIMSS/SSEC) from July 18th that shows how extensive the cloud cover was with one such shot of cooler air from Canada:

In summary, the cause of the cold weather has been a persistent trough in the Northeast US with a persistent ridge in the Western US. This pattern hasn't changed or modified much since the middle of June. Therefore, it's not surprising that this summer has been cooler than normal in Southern Wisconsin, and we are still experiencing one of the coldest starts to July on record.

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