Category: Climate Monitoring

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Mild October Had a Chilly End

The preliminary statewide average October temperature was 56.7 degrees, 1.9 degrees above the 1991–2020 average and tied for the 31st warmest on record going back to 1895. The preliminary statewide total October precipitation was 3.46 inches, 0.20 inches above the 1991–2020 average and the 41st wettest on record.

Data are provisional and may change slightly over time.

Mild October Temperatures with Some Extremes Thrown In

For my money, October is the best weather month of the year. The summer heat mellows out and mixes with the first real shots of chilly air. We had both summer- and winter-like weather last month, but a little more of the former than the latter. As the daily temperature departures from normal in Mt. Vernon show, most October days had above average temperatures, including a few days in the final week of the month that were 15 to 20 degrees above normal (Figure 1).

Figure 1. Daily October average temperature departures in Mt. Vernon.

October average temperatures ranged from the low 50s in northern Illinois to the low 60s in southern Illinois, between 1 and 2 degrees above normal in most places (Figure 2). Most of the state saw high temperatures in the upper 80s or low 90s in early October, including 94 in Quincy and 90 in Hoopeston. The brief taste of winter in the last few days of the month brought widespread low temperatures in the low 20s, including 21 degrees in Mt. Vernon and 23 in Bloomington.

The warm periods last month broke 31 daily high maximum temperature records and an incredible 66 daily high minimum temperature records. Aledo in Mercer County broke its all-time October high temperature records last month with a high temperature of 93 degrees on October 3. The cold end to the month broke 18 daily low maximum temperature records and 37 daily low minimum temperature records. 

Figure 2. Maps of (left) October average temperature and (right) October average temperature departures from normal.

Overall, the preliminary statewide average October temperature was 56.7 degrees, 1.9 degrees above the 1991–2020 average and tied for the 31st warmest on record going back to 1895.

Dry Start and Wet End to October

The month-end precipitation totals across the state do not tell the entire story of October precipitation. The first two-thirds of the month were somewhat to very dry across the state, as most places were 1 to 2 inches drier than normal through October 24. More active weather brought multiple rounds of rain–and some snow–to Illinois, raising month-end totals near or above the 30-year normals. As Figure 3 shows, Peoria picked up more rain in the last 7 days of October than in the first 24 days, and the month ended just slightly wetter than normal in Peoria. The dry start to the month was ideal for fall harvest, while the wetter end of the month helped improve dry conditions that prevailed in August and September.

Figure 3. Plot of October daily precipitation accumulation in Peoria (shaded area) versus the normal daily accumulation (black line).

October precipitation ranged from nearly 6 inches in northeast Illinois to less than 2.5 inches in the St. Louis Metro East. Most of the state north of Interstate 70 was 1 to 2 inches wetter than normal, while areas farther south were just slightly drier, up to 2 inches drier than normal (Figure 4).

Figure 4. Maps show (left) October total precipitation and (right) October precipitation departure from average.

Last month wasn’t an extremely wet or extremely dry month anywhere in the state; however, the wetter conditions in the northern half of the state helped reduce drought extent from 23 percent of the state on October 3 to 7 percent on October 31. Small areas of western and southern Illinois remain in moderate drought due to longer-term precipitation deficits. For example, the first 10 months of the year have had the third lowest precipitation total in Quincy with 20.17 inches, around 10 inches below average. While topsoil moisture in western Illinois has improved from the wetter end of October, deeper layer moisture and water table levels remain less than ideal because of the long-term dryness.

Overall, the preliminary statewide total October precipitation was 3.46 inches, 0.20 inches above the 1991–2020 average and the 41st wettest on record.

A Spooky, Winter-y Halloween

Like all the horror film characters who inevitably run back into the haunted house, mild October temperatures lulled all of us into a false sense of security. The intense cold, wind, and for some folks, snow on Halloween was more shocking than the sound of a revving chainsaw. Halloween nighttime temperatures dipped into the low 20s and high teens across much of the state, including 18-degree lows in Monmouth and Rochelle (Figure 5). The daytime high temperatures on Halloween were the coldest on record in several spots across the state, including at Chicago’s Midway airport (37 degrees) and Mt. Vernon (43 degrees). The low temperatures on Halloween night also set records in many places, including in Kankakee (24 degrees) and Olney (22 degrees). In fact, this year was the coldest Halloween in Olney since observations began there in 1896.

Halloween snow in northern and central Illinois is not necessarily a rarity but happens once every 4 to 6 years. Measurable snow, with totals exceeding 0.1 inches, was recorded in much of the state north of Interstate 74, with totals as high as 1.5 inches in Mundelein and 0.9 inches at O’Hare (Figure 5). For reference, the average first measurable snow comes in the third or fourth week of November for most of northern and central Illinois, so this year’s event came about two to three weeks early. It’s important to note that an early snowfall does not mean we will necessarily have a very snowy winter… but this State Climatologist can hope.

Figure 5. Maps of (left) nighttime low temperatures and (right) total snowfall on Halloween. Maps are from the Lincoln National Weather Service Office: https://www.weather.gov/ilx/illinois-daily.

Outlooks

November doesn’t get the love it deserves, because it is so often associated with cloudy, blustery weather. But November typically gives us a great mix of pleasant and not-so-pleasant weather and portends the beautiful winter season in Illinois. The latest Climate Prediction Center outlooks lean into El Niño with higher chances of below normal November precipitation. November temperature outlooks are more mixed, with equal chances of warmer and colder weather this month (Figure 6).

Figure 6. Maps of (left) temperature and (right) precipitation outlooks for November.

Meanwhile, NOAA leans even more heavily into El Niño for climatological winter (December–February) outlooks (Figure 7). The outlooks show highest chances of above normal temperatures in winter, with mostly equal chances of above or below normal precipitation.

Figure 7. Maps of (left) temperature and (right) precipitation outlooks for winter (December–February).
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Dry and Mild Start to Fall

The preliminary statewide average September temperature was 66.8 degrees, 0.6 degrees above the 1991–2020 average and the 57th warmest on record going back to 1895. The preliminary statewide average total September precipitation was 2.66 inches, 0.53 inches below the 1991–2020 average and the 41st driest on record statewide.    

Data are provisional and may change slightly over time.

A Mostly Mild September

September followed August’s lead and brought mostly mild temperatures across the state. As the temperature departures from Jacksonville in Figure 1 show, the only prolonged warm period last month was in the third week, when temperatures were 5 to 15 degrees above normal. A cold front swept through Illinois around the fall equinox and brought much cooler, fall-like weather to wrap up the first month of climatological fall.

Figure 1. Daily September average temperature departures in Jacksonville.

During the hot week in the third week of September, high temperatures reached into the mid- to upper 90s across the state. Among the very high temperatures was 99 degrees observed in Carbondale on Sept. 21, tied with 1940 and 1955 for the second latest 99+ degree temperature on record in Carbondale.

However, the cold front that moved through the state around Sept. 21 brought much cooler, drier air into the region. Nighttime low temperatures in the final week of the month regularly dipped into the upper 30s and low 40s. A few stations even reached the 32-degree mark in the last week of September, including the 19th earliest first fall freeze on record in Normal.

September average temperatures ranged from the low 60s in northern Illinois to the low 70s in southern, within 1 degree of normal statewide (Figure 2). The warmest place in the state last month was Bean Ridge in Alexander County with an average September temperature of 72.8 degrees. The coolest place was Stockton in Jo Daviess County with an average September temperature of 63.0 degrees.

Figure 2. Maps of (left) September average temperature and (right) September average temperature departures from normal.

Daily high maximum temperature records were broken last month at 27 stations, and daily high minimum temperature records were broken at 5 stations. Daily low maximum temperature records were broken last month at 4 stations, and daily low minimum temperature records were also broken at 4 stations.  

Overall, the preliminary statewide average September temperature was 66.8 degrees, 0.6 degrees above the 1991–2020 average and the 57th warmest on record going back to 1895.

 Drier Start to Fall

Early to mid-September brought multiple rounds of isolated, heavy precipitation to parts of northern, western, and east-central Illinois. However, significant rainfall eluded most of southern and northeastern Illinois in the first two-thirds of the month. Meanwhile, the final 10 days of the month were cooler and very dry statewide. The result was a much drier than normal September for most of southern and northeastern Illinois and near normal to slightly wetter than normal conditions in parts of northern and west-central Illinois (Figure 3). Specifically, total precipitation last month ranged from over 6 inches in parts of western and northern Illinois to less than 1 inch in far southern Illinois. Smithland Lock and Dam in Pope County recorded only 0.60 inches of rain last month, making it the fourth driest September since 1980 there.

Among the driest points in the state in September was Morton in Tazewell County, which only had 0.58 inches of rain. In contrast, the wettest point in the state in September was Warsaw in Hancock County, with 7.51 inches. One silver lining of the dry weather in the latter half of September was that it allowed a smooth start to fall harvest. However, deteriorating conditions in southern and central Illinois have resulted in an increased fire risk and concerns of low levels in the Mississippi and Illinois Rivers.     

Figure 3. Maps show (left) total September precipitation and (right) September precipitation departure from normal.                                                           

Overall, the preliminary statewide average total September precipitation was 2.66 inches, 0.53 inches below the 1991–2020 average and the 41st driest on record statewide.

The 2021-2022 Water Year

The transition from summer to fall is typically our hydrologically driest time of the year as a result of higher evaporative demand and human demand for water in summer. In contrast, the transition from winter to spring is typically our hydrologically wettest time of the year. Therefore, to best capture the seasonal variability of water availability and water balance, we denote a “water year,” which runs from October 1 to September 30. Figure 4 shows total precipitation and precipitation as a percent of normal for the 2021-2022 water year (October 1, 2021–September 30, 2022). Total water year precipitation ranged from nearly 60 inches in southern Illinois to less than 30 inches in far western Illinois. In all, water year precipitation was between 75 and 125 percent of normal statewide.

Figure 4. Maps show total precipitation and precipitation percent of normal for the 2021-2022 water year.

Outlooks

Climate Prediction Center outlooks for October lean toward warmer and drier than normal conditions statewide (Figure 5). Similarly, outlooks for the last three months of the year also lean toward warmer than normal conditions, with near normal rainfall (Figure 6).

Figure 5. Maps show (left) temperature and (right) precipitation outlooks for October.
Figure 6. Climate Prediction Center (left) temperature and (right) precipitation outlooks for October through December.
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A Cool Start to Fall

September was slightly cooler and wetter than average across Illinois. The preliminary statewide average September temperature was 65.4 degrees, 0.8 degrees below the 30-year normal, and tied for the 45th coolest on record. Preliminary statewide average total precipitation for August was 3.39 inches, 0.16 inches more than the 30-year normal, and the 58th wettest on record.

Data are provisional and may change slightly over time

September Temperatures

A cold front moved through the Midwest in late August, bringing Illinois its first real taste of fall air. Below average temperatures remained in place for most of September, resulting in an overall statewide average temperature of 65.4 degrees, 0.8 degrees below the 30-year normal.

The maps below show September average temperatures and their departure from the long-term average. September temperatures ranged from the low 60s in northern Illinois to the low 70s in far southern Illinois. All but the very northeastern corner of the state was cooler than average, including parts of Putman and LaSalle Counties, which were nearly 3 degrees below the long-term September average temperature.

September Temperatures

A cold front moved through the Midwest in late August, bringing Illinois its first real taste of fall air. Below average temperatures remained in place for most of September, resulting in an overall statewide average temperature of 65.4 degrees, 0.8 degrees below the 30-year normal.

The maps below show September average temperatures and their departure from the long-term average. September temperatures ranged from the low 60s in northern Illinois to the low 70s in far southern Illinois. All but the very northeastern corner of the state was cooler than average, including parts of Putman and LaSalle Counties, which were nearly 3 degrees below the long-term September average temperature.

Although temperature departures were largest in northern Illinois, the southern half of the state experienced unusually cold weather during the third week of September. The map below shows observed minimum temperatures on September 20. The station in Cairo in Alexander County observed 37 degrees that morning, which broke the all-time September minimum temperature record in Cairo set in 1989.

Western U.S. Wildfire Smoke

On several otherwise cloud-free days last month, the sky over Illinois appeared an odd milky-white color. This was caused by smoke from wildfires in the western U.S. that had moved across the country. The figure below shows a satellite estimate of vertically integrated smoke across the U.S. on September 17. Areas with relatively high smoke content in the atmosphere are shown in red and pink. Although the smoke created an eerie appearance in the sky, it did not pose a health risk to Illinoisans. The smoke though did likely, although modestly, suppress our daytime high temperatures last month as it reduced the amount of sunlight reaching the surface and heating the afternoon air.

Southern Illinois’ Turn for Drought

September statewide total precipitation was 3.39 inches, approximately 0.16 inches above the long-term average. However, much like the previous three months, large precipitation gradients created rainfall winners and losers. The maps below show total September precipitation and September precipitation percent of normal.

Total September rainfall ranged from over 9 inches in northwest Illinois to less than a half inch in eastern Illinois. Most areas of northern Illinois experienced between 125 percent and 300 percent of normal September precipitation, while areas of southern Illinois experienced between 10 percent and 90 percent of normal.

On one end of the extreme, the station at the Quad Cities airport in Moline experienced its 10th wettest September with a total of 6.25 inches. This followed the driest August on record in Moline, with only 0.15 inches of total August precipitation. This is contrasted by the station in Paris in Edgar County, which observed only 0.34 inches of total precipitation in September, making it the third driest September on record in Paris. This came after the second wettest July on record in Paris, with over 10 inches of precipitation.

However, Belleville in St. Clair County wins the prize of most variable rainfall over the past few months with the second wettest July on record, followed by the wettest August on record, followed by the seventh driest September on record.

The rainfall in northern Illinois was welcome, and alleviated drought conditions that had persisted there since early August. However, the very dry conditions in central and southern Illinois resulted in an expansion of moderate drought and abnormally dry conditions in the most recent, October 1 edition of the U.S. Drought Monitor (map below).

Outlooks

The first few days of October have continued the cool weather in September. However, outlooks from 6 to 10 days out to 3 months are indicating the highest odds for warmer than normal conditions.

The 8- to 14-day outlooks below indicate strongly elevated odds of warmer and drier than normal conditions in the second week of October across the state. Although this will not help alleviate ongoing drought in central and southern Illinois, it will help crop dry down as we enter the peak harvest season.

Looking farther out, the week three to four outlooks are similar with warmer and drier than normal conditions prevailing for the latter part of October.

As we begin to look toward climatological winter, the three-month outlooks for November, December, and January are still tilted toward warmer than normal conditions. Precipitation outlooks for the same three-month period show an equal chance of wetter and drier than normal weather. 

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Cool, Warm, Wet, Dry, and a Derecho: A Wild August Ends Climatological Summer

August was slightly cooler and much drier than average across Illinois. The preliminary statewide average August temperature was 72.7 degrees, 0.9 degrees below the 30-year normal and the 45th coolest on record. Preliminary statewide average total precipitation for August was 2.01 inches, 1.58 inches below than the 30-year normal and the 15th driest on record.

Data are provisional and may change slightly over time

August Temperatures

Following the warmer than average months of June and July, August began much cooler than average. The below average temperatures persisted through the third week of the month in response to a persistent atmospheric trough over the central U.S. The map below shows temperatures were between 1 and 4 degrees below average through the first three weeks of August.

Between August 1 and 21, 43 daily low maximum temperature and 6 daily low minimum temperature records were broken across the state. This included a 70-degree high temperature in Salem in Marion County, which broke the previous record by 10 degrees.

As the ridge in the western U.S. broke down in the third week of August, heat spread east, and Illinois temperatures switched to considerably above average. As the map below shows, temperatures in the week of August 22 were 1 to 10 degrees above average with the highest departures in northern Illinois.

During this fourth week of August, 10 daily high maximum temperature and 6 daily high minimum temperature records were broken. Mount Carroll in Carroll County broke or tied their daily high maximum temperature records on three consecutive days between August 25 and 27. Most stations observed daily high temperatures in the 90s this week, including five consecutive 90+-degree days in Rockford. This was the longest such streak in Rockford in August since 2011. The station at Chicago’s O’Hare Airport observed 10 days with a high temperature at or above 90 degrees last month. This is tied for the sixth most 90+-degree days at O’Hare, going back to 1959, and the most since 2012. The climatological average August frequency of 90+-degree days at O’Hare is 4. This is compared to the station in Carbondale that typically experiences 11 90+-degree days in August, but only observed 4 this last month.

Although the fourth week of August was unusually warm in northern Illinois, temperatures were closer to average in southern Illinois. The plot below shows daily temperature departures in Rosiclare in Hardin County. The southeast Illinois climate division, containing Rosiclare, experienced its eighth coolest August on record.

The statewide August temperature was 72.7 degrees, nearly 1 degree below the 30-year normal. The maps below show that average temperatures were in the mid- to high 70s across the state last month, very close to the long-term average in northern Illinois, and between 1 and 4 degrees below average in southern Illinois.

The maps below show the climatological summer (June–August) 2020 maximum, average, and minimum temperature departures from average. June and July this year were both in the top 30 warmest months on record, resulting in an overall warmer than average summer in northern Illinois. However, the cooler August pushed summer temperatures within a degree of the long-term average in most of southern and south-central Illinois.

August Derecho

August was not without its fair share of severe weather. On August 10, a strong mesoscale convective system moved across the Upper Midwest. The system intensified in the eastern Dakotas and caused a derecho–a widespread, long-lived windstorm–that impacted areas of Iowa, Illinois, Wisconsin, and Indiana. A derecho is characterized by strong straight-line winds that can exceed 75 mph and often affect areas between 250 and 500 miles. Dr. Marshall Shepherd at the University of Georgia provides an excellent description in his piece in Forbes: https://www.forbes.com/sites/marshallshepherd/2020/08/10/what-is-a-derecho/#44cb250c3b8e.

The derecho on August 10 produced observed winds exceeding 100 mph and estimated (from damage) wind gusts up to 140 mph across east-central Iowa. Based on initial reports, the derecho damaged between 6 and 10 million acres of crops across Iowa and northern Illinois. In addition, the winds caused significant damage and destruction in residential and urban areas. The city of Cedar Rapids, Iowa was hit particularly hard. The local newspaper reported estimates of over 20,000 trees downed in Cedar Rapids alone (https://www.thegazette.com/subject/news/derecho-by-the-digits-numbers-help-tell-the-story-of-the-storm-20200830), causing hundreds of thousands to lose power and remain without power for several days. The storm also resulted in four fatalities, three in Iowa and one in Indiana.

Along with the derecho, the storm produced 15 confirmed tornadoes in the Chicagoland area. The figure below is from the Chicago National Weather Service, showing the tracks of these tornadoes, including a couple that moved through the city of Chicago.

More research is necessary to better understand the environment that produces a derecho, and the corresponding warning of these events and risks they pose. One paper by Guastini and Bosart in Monthly Weather Review (https://journals.ametsoc.org/mwr/article/144/4/1363/72372) found northern Illinois experiences a derecho once every two years. However, not all derecho events are as large, long-lived, and intense as the event earlier last month.

Southern Illinois Remains Wet

Statewide August total precipitation was 2.01 inches, 1.58 inches below the 30-year normal and the 15th driest on record. However, like the varying temperatures, the southern and northern halves of the state experienced two very different August precipitation patterns. The maps below show August total precipitation and departures from average across the state. August totals ranged from less than a quarter of an inch in northwest Illinois to over 8 inches in southwest Illinois. In general, the northern half of the state experienced 1 to 4 inches below average, while most of southern Illinois experienced a 1 to 3 inches above average rainfall last month.

We can contrast the two halves of the state by comparing total rainfall in the Quad Cites with that in the St. Louis Metro East. The station at the Quad Cities Airport in Moline observed just 0.15 inches of total rain in August, which was less than half the previous low August total record of 0.35 inches in 1971 (see plot below).

As the Quad Cities experienced their driest August on record, the station at the SIU Research Farm in Belleville observed their wettest at over 10 inches of total rainfall last month. Of particular note was a strong thunderstorm that moved through the St. Louis area on August 12, producing heavy rainfall for the Metro East area. The station at Scott Air Force Base near Belleville recorded 5.36 inches in only three hours from this storm. The heavy rain produced flash flooding across the area, including multiple hangars on the base that were flooded. According to new estimates from the Illinois State Water Survey’s Bulletin 75 (https://www.ideals.illinois.edu/handle/2142/106653), this was approximately a 125-year rainfall event.

Northern Illinois Drought

July was slightly drier than average across most of northern Illinois. The first week of August was somewhat wet across the state; however, for most areas of northern and central Illinois, more rain fell in the first week of August than in the last three weeks of the month. The rainfall deficit was somewhat offset by below average temperatures during the first few weeks of the month. However, as heat began to set in and the northern half of the state experienced multiple, consecutive 90-degree days, the lack of moisture became quite apparent.

The plot below shows the daily accumulated difference between precipitation and reference evapotranspiration–an indicator of atmospheric evaporative demand–at the Illinois Climate Network Monmouth station. Looking at dry conditions through this lens provides a water balance perspective. The Monmouth record shows a positive water balance at the beginning of the month due to precipitation. However, the subsequent lack of rainfall after August 5 results in a negative water balance that is accelerated in the final weeks of the month. The station in Monmouth ended the month with an over 4-inch moisture deficit.  

In response to pervasive dry conditions in northern Illinois, most of the area is considered abnormally dry in the August 25 edition of the U.S. Drought Monitor (below). There are also pockets of moderate drought in western and northeast Illinois in response to agricultural and ecological impacts of the dryness.

Climatological summer (June–August) precipitation patterns are like those in August, with contrasting conditions in northern and southern Illinois. The northern half of the state finished summer with between 1 and 4 inches below average precipitation, while southern Illinois was 1 to 6 inches wetter than average this last season. The official summer season rankings will be released later this month, but it is worth mentioning that statewide total summer precipitation has only been below the 30-year normal 3 out of the last 10 years (2017, 2013, and 2012).

Outlooks

The late August heat will likely be replaced by cooler than average conditions throughout September. The Climate Prediction Center’s 8- to 14-day outlook and 1-month outlook both indicate strongly elevated odds of below normal temperatures.

Precipitation outlooks are mixed. The 8- to 14-day outlook indicates weakly elevated odds of wetter than normal conditions in the eastern half of the state to start September, with near normal precipitation elsewhere. The one-month September outlook indicates weakly elevated odds of drier than normal conditions in northern Illinois, but equal odds of above and below normal precipitation elsewhere.

Cooler weather in September will help to temper ongoing drought in northern and central Illinois. However, September is one of the drier months in Illinois, and given the outlooks, it is unlikely that dry conditions will be completely alleviated.