After a wet start to the 2014 growing season, we have seen a significant drop in rainfall across parts of northern and central Illinois in the last few weeks. Here is the 30-day rainfall as a percent of average. Areas in the orange are 25 to 75 percent of average while the areas in red are less than 25 percent of average. There are reports of soil moisture running low in some areas. On the other hand, southern Illinois has received above-average rainfall in the last 30 days.
Besides the switch from too wet to too dry in northern and central Illinois, and too much rain in southern Illinois, the other issue is that temperatures have been running about 4 degrees below average for the past 30 days. We are getting some heat this week. However, the longer-term forecasts indicate a return to cooler temperatures and more rain after this week through September 1.
If you look at the last 90 days the heavier rains in June and early July masks the recent dryness (map below). In fact, at the 90 day time scale rainfall in Illinois is generally at or above long-term average (1981-2010), as denoted by the grays and greens. This is one of the challenges of drought monitoring – sorting out short-term dryness versus long-term wetness or vice versa.
I will admit it – ever since I was a kid I have been fascinated/obsessed with maps. It is probably why I ended up in a career in climatology since we use maps all the time.
Here are maps from the National Climatic Data Center showing how cool July was across the Midwest. The regions are called “climate divisions” and there are nine of them in Illinois. The ones in the darkest blue had their coolest July on record, with records going back to 1895. Click on the map for a larger version which shows the numbers more clearly.
You can read more about what happened in the US in July.
July Temperature Ranking. Click to enlarge. Source National Climatic Data Center.
And here are the temperature departures from the 1981-2010 average.
July Temperature Departures. Click to enlarge. Source National Climatic Data Center.
Here is the US map for July, showing that while the Midwest was cool, the western US was experiencing record warmth.
Click to enlarge. Source National Climatic Data Center
This temperature pattern across the United States is the result of a ridge of high pressure over the western US and a trough of low pressure over the central US throughout most of July.
As impressive as this cool summer has been in Illinois, the experience here and across the Midwest stands in stark contrast to the West Coast. Here is the temperature contrast from the last full month – July (map below – click to enlarge). While the Corn Belt was 3 to 6 degrees below average in July, the West Coast and parts of the Rockies have been running 3 to 6 degrees, or more, above average.
The contrast between Chicago and Portland (OR), two of my favorite cities, show how different things have been. For July, the average high in Chicago was 79.8 degrees and the average low was 60.9 degrees. The average monthly temperature of 70.4 degrees meant that Chicago was 3.6 degrees below average.
On the other hand, the average high for July in Portland was 83.8 degrees and the average low was 59.8 degrees. As a result, their average monthly temperature of 71.8 degrees meant that Portland was 2.6 degrees above average.
Furthermore, while Chicago barely reached 90 degrees on one day in July, Portland reached or exceeded 90 degrees 7 days in July, including a reading of 99 on July 1.
The long-term average temperature for July in Chicago is 74 degrees and in Portland is 69 degrees.
It is no surprise that the NWS forecasts indicate that the heat will remain in the West for the next two weeks. However, it looks like Illinois has a good chance of seeing above-average temperatures for a change in the August 16-24 period.
The statewide average temperature for August so far is 72 degrees, 2 degrees below average. This follows on the heels of the cool July. The NWS forecast show that the mild temperatures will continue this upcoming week with highs in the upper 70s and low 80s in northern Illinois to the low to mid 80s in central and southern Illinois. The 6-10 and 8-14 day forecasts that extend out to August 21 point towards a continuation of cooler-than-average conditions.
All in all, it should be great weather for the Illinois State Fair. I can remember many years of the State Fair being hot and humid with your choice of either dust or mud. It’s a wonder the butter cow did not melt.
The map of observed precipitation below from the NWS shows that rainfall has been widespread and fairly heavy in western and southern Illinois with amounts ranging from 1 inch (green) to 5 inches (red). It is lighter and more variable in northern and eastern Illinois, ranging from 0.1 inches (light blue) to 2 inches (dark green). Much of that heavy rain to the east of St. Louis fell in a part of Illinois that was dry in July.
The statewide temperature in Illinois for the January-July period this year stands at 47.5 degrees, 3.9 degrees below average and the 5th coldest January-July on record for the state. It is no surprise why this would be the case. The bar graph below shows the monthly temperature departures from average (1981-2010) for 2014 in Illinois. The much-below-average temperatures of January, February, and March standout as well as for July.
This graph is for monthly precipitation departures from average for 2014 in Illinois. Precipitation seems to be doing a good job of alternating between above and below-average months. In fact, the statewide precipitation departure to date is very close to zero.
Here are the precipitation departures from average across Illinois. The areas in the lightest shade of green are 0 to 2 inches above average. The areas in the lightest tan are 0 to 2 inches below average. There is a band of wetter conditions in the northeast and a band of drier conditions close to Interstate 70.
The statewide average temperature for July was 70.3 degrees in Illinois, which ties the record cool July of 70.3 degrees set back in 2009 and 5 degrees below average.
Below is the plot of July temperatures for Illinois since 1895. The green dots are the actual temperature for each year, while the red and blue shading indicate periods of warmer or cooler temperatures. July 1936 (82.8 degrees) is the warmest July on record, followed closely by July 2012 (81.8 degrees). In the bottom right hand corner, are July 2009 and 2014. So in six years, we have experienced the 2nd warmest and twice the coldest July on record.
As this plot indicates, the observed range in July monthly temperatures in Illinois is 12.5 degrees. On another note, the July 2014 average temperature is based on preliminary data so it is very likely that we will break the tie with 2009 as more data arrives.
As we approach the end of July the statewide average temperature in Illinois is 70.6 degrees, which currently puts it in second place for the coldest July on record. I will post more on this at the end of the week.
Here is how the previous top 10 coldest July temperatures for Illinois looked and what happened in the following August (table below). In 8 out of the 10 cases, the following August was colder than average. However, two of those “colder” August’s were marginally so (1924 and 1996). The one spectacular reversal was in 1947, where August was 7.2 degrees above average after the 3rd coldest July. Therefore there is a historical tendency for cooler weather to prevail into August.
|Temperature (degrees F)
We are using the 1981-2010 average for August (73.6 degrees) as the benchmark for this comparison. Statewide records go back to 1895.
The NWS forecasts are pointing towards a colder than average start to August, based on the latest 6-10 and 8-14 day forecasts.