After months of exceptionally warm temperatures and drought, Illinois finally experienced temperatures and precipitation closer to normal in August.
The statewide average temperature for August was 73.5 degrees, just 0.1 degree below normal. That’s about as “normal” as you can get.
The statewide average precipitation for August was 3.4 inches, which is 95 percent of normal. In the map below, areas in yellow and orange had 3 to 5 inches, while the areas in red had more than 6 inches. Areas in northern and western Illinois in green received less than 2.5 inches. The largest rainfall total in the state was at Grayville in southeast Illinois with 10.69 inches. In second place was Hoopeston with 8.33 inches.
The statewide average temperature for the three summer months of June, July, and August was 76.1 degrees, 2.6 degrees above normal. It was the eighth warmest summer on record in Illinois. The warmest was 1936 at 78.6 degrees.
The statewide average precipitation for June-August was 6.64 inches, 5.21 inches below normal. It was the sixth driest summer on record in Illinois. The driest was 1988 with 6.17 inches.
Year to Date
The statewide average temperature for January-August was 59.0 degrees, 4.2 degrees above normal. It was the warmest January-August on record in Illinois. The second warmest was 1921 with 58.3 degrees.
The statewide average precipitation for January-August was 17.45 inches, 7.31 inches below normal. It was the fourth driest January-August on record in Illinois. The driest was 1936 with 14.95 inches, followed by 1988 with 17.12 inches, and 1934 with 17.41 inches.
Drought eases slightly in Illinois thanks to the rainfall and cooler temperatures of the last few weeks. The US Drought Monitor for August 28 shows improvement in northeast Illinois, especially Cook County.
While not yet reflected in the Drought Monitor, August has been a better month than July with more rain and milder temperatures. I’ll post the end of the month stuff on Friday. In the meantime, here are the latest departures from normal for the month so far (second figure). Parts of western and central Illinois as well as much of Illinois north of Interstate 80 have been below normal, areas in east-central and southern Illinois received above-normal precipitation for the month.
Of course, the real game-changer is yet to come – Tropical Storm Isaac. More on that later.
As of noon on Monday, August 27, the track of Hurricane Isaac could pass through Illinois on Saturday (first map). Of course, it won’t be a hurricane – just a tropical depression. Even so, large rainfall amounts are expected to fall in parts of Illinois and Missouri (second map).
Back in 2008, I wrote about the impact of Hurricanes Ike and Gustav as the remains of those two systems dumped an impressive amount of rainfall across Illinois.
Perhaps a better comparison is with the drought of 2005. Like this year, Illinois was faced with a severe drought. However, the remains of four tropical systems brought needed rainfall into the region, especially southern Illinois (third map). The four tropical systems were Tropical Storm Arlene in June, Hurricane Dennis in July, Hurricane Katrina in August, and Hurricane Rita in September. In fact, the total rainfall from these four systems ranged from almost 8 inches in southern Illinois to about an inch in northern Illinois (fourth map). You can read more about the impact of tropical storms in 2005 in this article and this report (starting page 47).
We had some decent rainfall amounts in much of the northern half of the state in the last 24 hours. In this image from the NWS Southern Region, green is good – meaning that between 1 and 2 inches fell in those areas. Areas in blue received 3/4 of an inch or less.
Here in Champaign, we had 2.07 inches at the official site while I had 2.15 inches at my house. This is the largest 24-hour rainfall we have had since April 26, 2011. Also, it brings our monthly total to 5.56 inches, which is 1.63 inches above normal for August.
The second figure shows the precipitation departures from normal for August so far. Areas in green and blue are above normal, areas in gray are near normal, and areas in yellow are still below normal. Parts of northern Illinois are below normal as well as a wedge extending from Quincy to Decatur.
However, if you are thinking the drought is fading, the last map shows the precipitation deficits since January 1, 2012. Large areas of Illinois are still 8 to 12 inches below normal (yellow) and a few areas in central and southern Illinois are 12 to 16 inches below normal (red). We have a long ways to go before this drought is over.