Residents of the Northern Plains are accustomed to major weather extremes throughout the course of a year. Temperatures can range from 50 below to over 100 in this part of the country. This past winter certainly gave the region plenty of low temperature extremes as unabated blasts of arctic air pummelled the region. Meanwhile, this summer continues to lack in the category of extreme heat, as 90-degree heat has been fleeting at best.
In a typical summer, Minneapolis, MN will reach a high of 90 or above 14 times. As of August 28th, the temperature has only reached into the 90s two times. Meanwhile, Chanhassen and Rochester, MN have still not experienced the first 90-degree day of the summer. Rochester is on track to tie the record for the lowest maximum high temperature for a year. The highest the mercury has risen in Rochester this year is 87, which occurred on August 24th. Many other towns across the Northern Plains are in a similar situation. Rapid City, SD is well known for its extended bouts of summer heat, as temperatures typically soar into the 90s thirty times during the course of a summer. As of August 28th, the temperature has only reached 90 or above 7 times. The lack of any sustained summer heat has also been felt down in Norfolk, NE. According to the averages, a resident of Norfolk can expect the high temperatures to reach into the 90s thirty-two times a year. This year, that has only happened 10 times, making for one of the cooler summers they have seen in a while.
This evident lack of heat over the Northern Plains has been driven by two main factors this summer. The first contributing factor is the wetter than normal conditions that have occurred periodically over the region. A cloudy and rainy pattern will obviously reduce the potential of any 90-degree days, but even after a rainy pattern subsides, it can continue to be a difficult feat. This is related to the effects of soil moisture and evapotranspiration. Evaporation is the process where energy is used to convert water into water vapor (converting any liquid into a gas). When soil moisture is high, heightened levels of evapotranspiration will reduce the amount of energy the sun can use to warm the air temperature. This taxing of the sun's energy makes it more difficult for 90-degree heat to occur. In arid regions, nearly all of the sun's energy is able to focus on heating up the air temperature instead of being compromised by evaporative processes. This is why highs in the 90s and 100s are common occurrences in the deserts of the world.
The other important factor that has contributed to the lack of 90-degree weather is related to the position of the jet stream. During the summer, the polar jet stream acts as a boundary between the cool Canadian air masses to the north and the hot and oftentimes humid air masses that reside to the south. When the jet stream dives southward, these cooler Canadian-based air masses are able to penetrate further south into the Northern Plains. July and August are the hottest months of the summer, due to the tendency for the jet stream to remain north into Canada. This has not been the case this year, as the jet stream has been diving south out of Canada more often than usual. This tendency has led to more Canadian cool fronts heading south across the region, thereby keeping those hot air masses further to the south.
If you have been missing the heat, it hasn't been hard to find over parts of the central Plains. Kansas has continued to bake in the heat this summer, with Wichita, KS having seen 56 days of 90-degree heat already. They will likely end up getting close to or surpassing their average of 63 ninety-degree days a year. But that is far from the prize for having the highest average of 90-degree days. That prize goes to the lucky folks in Yuma, AZ, who can expect 175 days of 90-degree heat each year.
