Cooler Days and Warmer Nights This Summer in Midwest

Cooler than average daytime highs (first map, blue shaded areas) and warmer than average night-time lows (second map) this summer were common across much of the Midwest, according to data from the National Centers for Environmental Information.

It is typical in Illinois for this to happen in summer after a wet spring. We marched through summer with above-average soil moisture, streamflows, and lake levels. As a result, more of the sun’s energy went into evaporating this water instead of heating up the land surface and the atmosphere. The end results were higher humidity levels and lower high temperatures.

At night, the higher humidity levels kept temperatures from dropping as much. The old forecasting rule of thumb was to consider the dew-point temperature as the floor to night-time temperatures. Therefore, the higher dew-point temperatures led to higher night-time temperatures.*

Finally, all the extra humidity in the atmosphere turned into more rainfall for thunderstorms, maintaining the wetter conditions through at least July.

summer-max-map
Click to enlarge. Cooler than average daytime high temperatures this summer across the Midwest. Source: National Centers for Environmental Information (NCEI).

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Cool, Dry August and Sixth Wettest Summer in Illinois

August was cool across the state and dry in most places in Illinois, capping off a summer that was cool and wet.

Precipitation

The statewide average precipitation for August in Illinois was 2.95 inches, 0.64 inches below average.  However, this was followed by a very wet June with 9.44 inches, and a wet July with 4.84 inches. As a result, the summer precipitation total was 17.23 inches. That was 5.36 inches above average and the 6th wettest summer on record.

Here are the top ten wettest summer in Illinois. It was wetter than last summer and 2010, but nearly an inch away from the incredible summer of 1993.

Rank Year Total Departure % of Average
1 1993 18.51 6.64 156
2 1902 18.14 6.27 153
3 1981 17.62 5.75 148
4 1915 17.58 5.71 148
5 1958 17.53 5.66 148
6 2015 17.23 5.36 144
7 2010 16.24 4.37 137
8 1907 15.78 3.91 133
9 2000 15.26 3.39 129
10 2014 15.25 3.38 128

The precipitation for August was unevenly distributed, which is typical in the summer months. Amounts of 3 to 5 inches were common in northern Illinois and points east of St. Louis. Continue reading

Summer Weather Gives Way to Fall

If you look at the start to September, temperatures have run 3 degrees above average. And yesterday and today have been the hottest yet with temperatures the 80s and 90s across much of state. But that is about to change as a cold front pushes through later today. This weekend and next week will be cooler with highs in the 70s and low 80s.  I am looking forward to that.

Average daily highs in September in Illinois range from the low to mid 70s in northern Illinois to the low 80s in southern Illinois (map below, click to enlarge).

Illinois-tmax-09SEP-normals

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Wet August Wraps Up Cool, Wet Summer in Illinois

Highlights: The 12th wettest August in Illinois finishes out the 10th wettest summer on record. While August was slightly warmer than average, the summer was cooler than average. Here are the statistics.

August Statistics

The statewide average precipitation for August was 5.18 inches, 1.59 inches above average and the 12th wettest on record. The wettest area of the state was Cook County. The largest monthly total was from a CoCoRaHS site (IL-CK-100) in Cicero with 10.20 inches of precipitation.

This first map shows several areas across the state with amounts of 7 to 10 inches (oranges and reds), according to radar estimates. There were a few areas in the northwest and east-central Illinois with only 2 to 3 inches. The second map shows the departures from average, showing the many areas with 2 to 8 inches above average for the month.

IL-prcp-mpe-m2d-tot-20140901

 

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August Near-Normal But Summer Hot, Dry for Illinois

August

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.

Summer

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.

August precipitation for the last 30 days through the morning of August 31. Source: NWS. Click to enlarge.

Latest Seasonal Outlook

NOAA’s Climate Prediction Center released their June and June-August outlook yesterday (figure below). On the left hand side of the figure, they have far southern Illinois with an increased chance of above normal temperatures for June. For June-August, that area of increased chances expands and covers an area roughly south of I-70.  The rest of the state has “EC” or equal chances of above, below, or near-normal temperatures. In other words, they don’t see anything to push temperatures one way or another.

It’s the same story with precipitation – equal chances of above, below, or near-normal precipitation across the state for June and June-August. I think its fair to say that our current skill is pretty low in forecasting summer-time precipitation months in advance.

NOAA’s Climate Prediction Center outlook for this summer. Click to enlarge.

Summer – One of the Warmest and Wettest on Record

Summer

This summer was one of the warmest and wettest in Illinois history, based on preliminary data. The average statewide temperature for this summer (June-August) was 76.4 degrees, 2.7 degrees above normal and the seventh warmest summer on record. The average statewide rainfall was 16.7 inches, 5.2 inches above normal and the sixth wettest summer on record. Statewide records for Illinois extend back to 1895.

August

The average statewide temperature for August was 76.8 degrees, 3.2 degrees above normal. That puts it at the 13th warmest August on record.  August was on track to being even warmer but a late-month cool spell knocked it down a few notches in the ranking. August rainfall has been close to normal with a statewide average of 3.4 inches, just 0.3 inches below normal.

[This is an update of a post earlier in August, now removed to avoid confusion]