Why No Upper 90's?

Well, earlier this week forecast models were suggesting high temperatures in the middle and upper 90s. We did reach 94° on Thursday, but we only reached 89° on Friday. So why exactly is that the case? During patterns like the one that the United States is seeing this week (the "Ring of Fire" setup), forecast models struggle with determining where exactly the pipeline of storms (usually MCS's) will set up. In other words, the models aren't very good at figuring out just how expansive the hottest of the weather will be, and consequently where the best zone for storms will be. Sometimes the day of models still can't figure out where the storms will and will not fire later that day.

That holds true this week here in Illinois and Wisconsin. Earlier in the week models were putting all their marbles into the mostly sunny and extremely warm pot. As the week progressed, they slowly added increasing chances for rain and storms, but never really nailed down when the storms would hit specifically on any given day.

They did not really perform well on showing Thursday evening's strong line of storms moving into the area from Wisconsin. Some models didn't even show rain for Thursday evening. So, now we understand meteorologists oftentimes get headaches when trying to use forecast models to make their forecasts during this type of pattern.

And it was exactly that line of storms and all of the heavy rain we saw from it that disallowed temperatures to soar Friday. Some locations saw nearly 2" of rainfall, with nearly everyone eclipsing the 1" mark. The ground absorbed all of that rain, and when the sun came up Friday, it all began to be pulled out of the ground via evaporation. That added water to the atmosphere near the surface. Water has a high specific heat, or the amount of heat energy necessary to increase the temperature of a given mass of a particular substance by some amount.

Since water was added to the air via evaporation and plant transpiration, the air's specific heat increased, making it harder for solar heating to effectively heat the surface. Without the rain, the soil would have remained dry, thus making the air easier to heat. We easily could have seen middle and upper 90s if we did not see the rain!