© 2019 Terry Swails

IS THIS THE NEW NORM? IT SEEMS THAT WAY....

July 24, 2019

The continental U.S. is in the midst of the wettest 12 months in 124 years of records. In spots precipitation totals are more than 20 inches above normal. Here's the totals since October 1st, 2018. Some parts of the central Midwest have seen over 50".

These are the departures over that period...many 12-20".

Below you can see the shallow groundwater wetness percentile 

As you would expect, soil moisture is ranked in the top 90 percent or greater over much of the corn belt.

While dew points have dropped significantly the past few days thanks to a major push of cool Canadian air, the rich soil moisture from all the rain and evapotranspiration from maturing crops led to last weeks extreme heat indices approaching 115 degrees. The primary contributor wasn't so much the heat (highs 90-95) as it was the dew points which exceeded 80 degrees.

 

Over the past decade these 80 degree dew points have become increasingly frequent. Much of my early career in meteorology it was a rare occurrence to experience a dew point so high. The last 5 or 6 years its become an annual event with some years seeing as many as 3 or 4 days at that level, especially in July. It's resulted in a dramatic increase in heat indices greater than 100 degrees and significant rain events not nearly as common in previous decades. 

 

Steve Gottschalk a long time weather observer out of Lowden in Cedar county, sent me these numbers from his daily records. From 1993-1999 we had 3.8 days with heat indexes of 100 or higher. From 2000-2009 we had 6.5 days with heat indexes of 100 or higher. From 2010 up through the present time we had 12.7 days. We may add to this yet. This covers the season from June - September. I use a motorized sling psychrometer to take my humidity and dew point readings.For the month of July since 2000 we average 5.5 days with heat indexes of 100 or higher. Since 2010 the value jumps up 7 days. The take away is that in just the past decade the number of days with a +100 degree heat index has doubled from 6.5 to 12.7 and counting. This is no doubt due to the increased amount of water vapor most likely due to climate change.

 

The additional water vapor and improved agricultural practices have combined to produce fewer and fewer 100 degree days since the 1950s. The last time we hit the century mark here in Cedar Rapids was August 31, 2013. In the graphic below generated from the Iowa Mesonet, you can see the dramatic decrease in 95 degree highs. The 30 year trailing average (green line) has dropped form nearly 14 days a year in 1940 to about 2.8 in 2018. Since 1958 only 4 years have seen more than 10 days with highs 95 or above. Between 1910 and 1940 there were 22 with as many as 35 days in 1936. Is this the new norm? I think so.

What the evidence points to is less in the way of extreme temperatures but but more in the way of extreme heat indices due to the higher water vapor. A 90 degree high and an 80 degree dew point can feel far worse than a 100 degree when there's low dew points.

 

The other thing I've noticed is that these wicked heat indices seem centered on mid to late July when temperatures are at climatalogical highs and corn is mature enough to sweat. While all vegetation adds to water vapor, corn is particularly good at 'sweating', making for some very sticky July days across parts of the U.S. Midwest -- home to more than 90 million acres of corn.

Below you can see in Cedar Rapids how only 1 day in June had a heat index more than 90 or more. Notice the dramatic increase in July when summer finally came to life. A little pay back there.

Going forward the days where temperatures hit 100 are going to be few and far between if current trends and climate models are correct. On the other hand extreme heat will increase on a steady basis. A study by the Union of Concerned Scientists shows how the number of extreme heat days could expand during the period 2036 to 2065 if nothing alters the amount of water vapor. It's not pretty and would alter agriculture and our ability to produce food in a significant way. Something to think seriously about.

For now, the weather has returned to a cooler more collected state with the next warm-up not slated until the end of the week. Enjoy the break and by all means roll weather...TS 

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