Outlook for Summer, Above Normal Temps

The NWS released their outlooks for the month of July and the 3-month period of July-September.

Illinois and much of the US has an increased chance of warmer-than-normal temperatures for both July and July-September (first column in the figure). Illinois has equal chances of being above, below, and near-normal on rainfall – a virtual coin toss. That is not a surprise on the rainfall. Most of our summertime rain is guided by local conditions and fast-changing weather patterns.

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Historical Trends for July

Temperatures: Historically in Illinois, daytime temperatures in July have become milder over time (first graph) while nighttime temperatures have become warmer over time (second graph). While the decreased heating during the day may be welcome, the increased warming at night can be problematic for humans, animals, and plants as they rest. The green line is a smoothed curve to help the eye see the underlying pattern of change from the variability of individual years.

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July Daytime
graph2
July Nighttime

Rainfall: July rainfall was low from the 1910s through the 1940s, before becoming much wetter since 1940. There is still a considerable amount of year to year variability. The red line is a smoothed curve to help the eye see the underlying pattern of change from the variability of individual years.

Even in July, the sun only has so much energy. It can either go into evaporation from the ground and transpiration from plants, causing higher humidity. Or, if the surface is dry, more of the energy can go into heating up the land surface. In other words, higher daytime temperatures and dry weather go hand in hand, which is what happened from 1910 to 1940. The reverse has happened in recent decades – more rain and lower daytime temperatures. By the same token, more rain in July means more humidity which restricts how cool it can get at night.

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