Warm and Dry September Extended Summer

The preliminary statewide average September temperature was 69.4 degrees, 2.6 degrees above the 1991–2020 average, and tied for the 21st warmest on record going back to 1895. The preliminary statewide average total September precipitation was 2.38 inches, 0.97 inches below the 1991–2020 average and tied for the 36th driest on record going back to 1895.  

Data are provisional and may change slightly over time

September was hot, but less humid

The cooldown that started the month of September gave us a taste of fall, but it was fleeting. Most of September passed with above normal temperatures across the state, and as Figure 1 shows from Peoria, temperatures on the hottest days were 5 to 15 degrees above normal.

Figure 1. Daily September average temperature departures at Peoria.

Warmer weather in September broke 15high daily maximum temperature records and 4high daily minimum temperature records across the state, including a record-breaking 77-degree minimum temperature in Alton on September 19. In contrast, only two low daily maximum temperature records and two low daily minimum temperature records were broken last month. The highest maximum temperature recorded last month was 94 degrees in Alton, while the lowest minimum temperature recorded last month was 36 degrees in Galena.

Overall, the September average temperature ranged from the mid- to high 60s in northern and north-central Illinois to the low 70s in southern Illinois. Last month was 3 to 5 degrees warmer than normal in northern Illinois and within 1 degree of normal in southern Illinois (Figure 2).

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

Despite the higher temperatures last month, the unusually high humidity across the state in July and August disappeared in September. Figure 3 shows the September average vapor pressure deficit, a representation of humidity, in Springfield. The above average vapor pressure deficit last month illustrates less humid conditions compared to Septembers in 2018, 2019, and 2020.

Figure 3. September average vapor pressure deficit in Springfield between 1948 and 2021.

Overall, the preliminary statewide average September temperature was 69.4 degrees, 2.6 degrees above the 1991–2020 average and tied for the 21st warmest on record going back to 1895.

Split Precipitation Pattern… Again

southern and central Illinois and dry in northern Illinois. September’s precipitation totals followed suit. Last month’s precipitation totals ranged from less than an inch northwest Illinois to over 5 inches in east-central and southern Illinois. September was 2 to 3 inches drier than normal throughout northern Illinois and near normal to about 1 inch wetter than normal in central and southern Illinois (Figure 4).

September was one of the drier months of the warm season, so despite only 0.53 inches of rain recorded last month, it was still only the seventh driest September on record in Rockford. However, dry conditions last month added to very dry spring and summer seasons in northern Illinois. The 2020-2021 water year, which runs from October 1 to September 30, was the third driest on record in Rockford. Total water year precipitation in Rockford was 21.69 inches, approximately 60 percent of average and 4 inches less than the 2011-2012 water year.

Overall, the preliminary statewide average total September precipitation was 2.38 inches, 0.97 inches below the 1991–2020 average and tied for 36th driest on record going back to 1895. 

Figure 4. Maps of (left) total September precipitation, (middle) precipitation departure from normal, and (right) precipitation percent of normal.

Northern Illinois Drought

Very dry conditions last month added to existing precipitation deficits in northern Illinois. In response, the latest U.S. Drought Monitor expanded severe and moderate drought across the northern third of the state and expanded extreme drought in McHenry County (Figure 5). The percentage of the state in severe drought (8.78%) is the highest since fall of 2013.

Agricultural and ecological impacts have mostly run their course by this time of the year. However, the dry conditions have left substantial soil moisture deficits across northern Illinois, and a near to wetter than normal winter will be necessary to recharge dry soils. A healthy, persistent snowpack across northern Illinois would also go a long way to help improve soil moisture conditions as we move into next spring.

Figure 5. The U.S. Drought Monitor as of September 28th.

Outlooks

The gap between the 30-year normal September average temperature and October average temperature is the largest of any two months statewide, at 11.5 degrees. This means that as we look ahead, irrespective of the outlook, temperatures will decrease as we move into October. 

With that said, the most recent Climate Prediction Center outlooks for the entire month of October show strongly elevated odds of above normal temperatures sticking around, though with slightly higher odds of wetter than normal conditions for the month as a whole (Figure 6).

Figure 6. Climate Prediction Center temperature (left) and precipitation (right) outlooks for the month of October.

 

Hot and Stormy End to Summer

The preliminary statewide average August temperature was 75.6 degrees, 2 degrees above the 1991–2020 average and tied for the 20th warmest on record going back to 1895. The preliminary statewide average total August precipitation was 3.76 inches, 0.20 inches above the 1991–2020 average and the 50th wettest on record going back to 1895.  

Data are provisional and may change slightly over time

August Heat & Humidity

Apart from the first few days of the month, August temperatures were largely above normal across Illinois. As Figure 1 shows, all but 5 days in August had above normal temperatures in Elgin, and several days in the last week of August exhibited temperatures that were 10 to 15 degrees above normal.

Figure 1. Daily August average temperature departures in Elgin.

Overall, the August average temperature ranged from the low 70s in northern and north-central Illinois to the high 70s in southern Illinois. Last month was 2 to 4 degrees warmer than normal in northern Illinois and within 1 degree of normal in southern Illinois. However, as the maps in the bottom panel of Figure 2 show, daily August minimum temperatures had larger deviations from normal than daily maximum temperatures. In fact, in far southeast Illinois August minimum temperatures were 2 to 3 degrees above normal, while maximum temperatures were 1 to 2 degrees below normal.

Figure 2. Maps of (top left) average August temperature, (top right) August temperature departures from normal, (bottom left) August minimum temperature departures from normal, and (bottom right) August maximum temperature departures from normal.

Unusually high nighttime low temperatures last month were partly caused by persistently high humidity. One useful measure of humidity is the dewpoint temperature, which is the air temperature at which the air is saturated. Higher dewpoint temperatures indicate higher humidity, and humans begin to feel uncomfortable as dewpoints exceed 65 degrees.

The figure below shows the hourly frequency of dewpoint temperature observations exceeding 65 degrees at Champaign last month. Frequencies from last month (orange bars) are compared with average frequencies over the past 40 years (blue bars). August dewpoint temperatures exceeded 65 degrees more frequently than usual in each of the 24 hours of the day in Champaign, but the largest anomalies were in the early morning between 1 and 5 a.m. For example, well over half of the 3 a.m. dewpoints exceeded 65 degrees in Champaign last month, compared to around 30 percent of days on average. August in Champaign had the 2nd highest frequency of 3 a.m. dewpoint temperature observations.

Figure 3. Bar plot of frequency of August hourly dewpoint temperatures in Champaign this year (orange bars) compared to the 40-year average (blue bars).

Overall, the preliminary statewide average August temperature was 75.6 degrees, 2 degrees above the 1991–2020 average and tied for 20th warmest on record going back to 1895.

Climatological Summer Temperatures

August brings a wrap-up of climatological summer. Summer average temperatures ranged from the high 60s in southern Illinois to the mid- to high 70s in southern Illinois. This summer was 1 to 3 degrees warmer than average in the northern one-third of the state, and within 1 degree of normal south of Interstate 74. Similar to August, we saw large differences between minimum and maximum temperature departures. Summer minimum temperatures were 1 to 4 degrees above normal statewide, while summer maximum temperatures were more than 1 degree above normal only north of Interstate 80.

Figure 4. Maps of (top left) average summer temperature, (top right) summer temperature departures from normal, (bottom left) summer minimum temperature departures from normal, and (bottom right) summer maximum temperature departures from normal.

Wet, Active Weather in Northern Illinois

August precipitation totals ranged from less than an inch in parts of northwest and western Illinois to over 11 inches in north-central Illinois. Last month was 1 to 3 inches drier than normal throughout most of the northern two-thirds of the state and was between 1 and 5 inches wetter than normal in southern Illinois.

Figure 5. Maps of (left) total August precipitation and (right) precipitation departure from normal.

Because of the heat and humidity, August precipitation was accompanied by frequent severe weather events including several tornadoes, hail, and strong winds. Seven confirmed tornadoes touched down in northern Illinois on August 9 alone, including three EF-1 tornadoes. The five National Weather Service offices serving Illinois issued a combined 18 tornado warnings last month, the second most for August on record going back to 1986. Additionally, there were 93 severe thunderstorm warnings issued in Illinois last month, tied for the fourth most on record for August.

On August 12, a series of thunderstorms moved over the same area of north-central Illinois for hours, a phenomenon called training. The storms produced 4 to 5 inches of rain in just 6 hours across parts of McLean, Champaign, and Ford Counties in central Illinois. The epicenter of the heavy rainfall was Gibson City in Ford County, which received 10 to 12 inches in less than 6 hours on August 12. Consequently, Gibson City suffered from destructive urban flooding that displaced residents and flooded roads and buildings. 

Figure 6. 24-hour precipitation totals from thunderstorms on August 12th across north-central Illinois.

Overall, the preliminary statewide average total August precipitation was 3.76 inches, 0.20 inches above the 1991–2020 average and the 50th wettest on record going back to 1895.  

August ended a mostly wetter than normal summer for Illinois. Total summer precipitation ranged from around 5 inches in northwest Illinois to over 25 inches in parts of southeast and central Illinois. Most of the northwest corner of the state was 3 to 5 inches drier than normal this summer, while parts of central and south-central Illinois were 9 to 12 inches wetter than normal.

Figure 7. Maps of (left) summer total precipitation and (right) precipitation departure from normal.

Outlooks

The fall season has begun with cooler and less humid weather across Illinois, much to the relief of us all. As crops continue to mature and we inch toward sweaters and pumpkin-flavored drinks, we look to what the season has in store.

Climate Prediction Center outlooks for the second week of September show strongly elevated odds of below normal temperatures sticking around, with slightly elevated chances of below normal precipitation statewide.

Figure 8. Climate Prediction Center outlooks for the second week of September. Temperature on the left and precipitation on the right.

Although outlooks for the entire month of September do not show any expectations as to wetter, drier, hotter, or colder than normal, the 3-month outlooks for September through November lean toward warmer than average conditions across the Midwest.

Figure 9. Climate Prediction Center outlooks for the second week of September. Temperature on the left and precipitation on the right.

July Brought High Humidity and Above Average Rainfall

The preliminary statewide average July temperature was 74.6 degrees, 0.8 degrees below the 1991–2020 average and tied for the 45th coldest on record going back to 1895. The preliminary statewide average total July precipitation was 5.37 inches, 1.31 inches above the 1991–2020 average and tied for the 18th wettest on record going back to 1895.  

Data are provisional and may change slightly over time

Cool but Humid July Weather

Apart from a late-month heat wave, temperatures in July were mostly below the 1991–2020 normal. Figure 1 shows daily average July temperature departures in Springfield. Most places observed a single-digit number of days with a high temperature above 90 degrees. For example, Carbondale observed only 9 days in July with a high temperature above 90 degrees, the 10th fewest on record there going back to the early 1900s. Comparatively, Carbondale recorded 29 days with a high above 90 degrees in 2012.  

Figure 1. Daily average July temperature departure from average in Springfield.

Most of central Illinois was between 1 and 2 degrees below normal for the month of July, while northern and southern Illinois were less than 1 degree below normal for the month (Fig. 2). Last month, 45 daily low maximum temperature records were broken across the state, including a 67-degree high temperature on July 11 in Kewanee. Additionally, three daily low minimum temperature records were broken in July. Rock Island was the lone station with a broken high maximum temperature record in July, with a 99-degree high on July 28, and Barrington was the only station with a broken high minimum temperature record last month, with a 72-degree low on July 24.

Figure 2. July average temperature (left) and average temperature departure from normal (right). Departures are from the 1991–2020 normal.

The cooler July temperatures maintained growing degree day (GDD) accumulation departures from the end of June. The maps below in Figure 3 show total GDD accumulation and departures from the 1991–2020 normal since April 1. As of the start of August, growing degree day accumulation is ahead of normal in northern Illinois and slightly to much below normal in southern Illinois.

Figure 3. Maps show total base 50-degree modified growing degree day accumulation (left) and growing degree day departures from the 1991–2020 normal (right) since April 1.

Overall, the preliminary statewide average July temperature was 74.6 degrees, 0.8 degrees below the 1991–2020 average and tied for the 45th coldest on record going back to 1895.

July Humidity

Continental-scale atmospheric circulation and the high pressure over the north-central Atlantic Ocean maintained the movement of moisture and humidity out of the Gulf of Mexico into the Midwest all month. The result was a very humid July, even by recent climate standards. Stations in both Jacksonville and Taylorville recorded dew point temperatures of 86 degrees in the afternoon of July 28, which was the highest dew point observed at both stations on their 15+ year records.

Another way of representing humidity is the vapor pressure deficit (VPD). The potential amount of humidity in the air is dependent on temperature, and VPD is a measure of how much humidity is in the air relative to how much could potentially be in the air at that temperature. Therefore, a low VPD indicates a humidity level that is very close to the maximum potential humidity at that temperature, and when VPD nears zero, the air approaches saturation. The plot below (Fig. 4) shows last month’s average VPD at Scott Air Force Base near Belleville was the lowest (i.e., most humid) on record going back to 1938.

Figure 4. July average vapor pressure deficit at Scott Air Force Base between 1938 and 2021. The dashed, black line shows the station’s long-term July VPD trend.

“Rinse and Repeat” Rainfall Patterns in July

High humidity and southerly flow off the Gulf of Mexico helped continue to fuel scattered thunderstorms across the state in July. Combined with a handful of mesoscale systems that moved across the Midwest last month, most of Illinois experienced a near to wetter than average July.

The maps in Fig. 5 show total July precipitation ranged from less than 2 inches in the northeast part of the state to over 10 inches in south-central Illinois. Long-running precipitation deficits in northern Illinois grew as July rainfall was 2 to 3 inches below normal there, within 1 inch of normal in most of central Illinois, and 2 to 5 inches above normal in southern Illinois.

Figure 5. Maps of (left) July total precipitation and (right) July precipitation departure from 1991-2020 normal.

Last month was the second wettest on record in Salem in Marion County (9.01 inches) going back to 1915, and the seventh wettest on record in Du Quoin in Perry County (7.78 inches) going back to 1942. In contrast, Chicago observed only 1.9 inches in July, which was the 27th driest on record there going back to 1871. Chicago has recorded 15.63 inches of rain since the start of the year, making the first 7 months of 2021 the 20th driest on record and the driest since 2005.

Drought Continues in Northern Illinois

Below normal temperatures throughout July helped alleviate stress from high evaporation rates in drought-stricken northern Illinois. However, most of northeast Illinois was unable to make progress reducing long-running precipitation deficits last month. The latest version of the U.S. Drought Monitor released on July 29 showed most of northern Illinois from Jo Daviess County to Cook County remained in moderate drought, and parts or most of Boone, McHenry, and Lake Counties were in severe drought (Fig. 6).

Figure 6. The U.S. Drought Monitor map, released on July 27.

Lake and McHenry Counties in particular have been in at least severe drought for 11 consecutive weeks, going back to late May. Soil moisture and streamflow in this area remain well below average for this time of the year, and several reports of ecological and agricultural impacts–including trees dropping leaves and corn showing signs of drought stress–have come from the area.

Overall, the preliminary statewide average total July precipitation was 5.37 inches, 1.31 inches above the 1991–2020 average and tied for the 18th wettest on record going back to 1895.  

Outlooks

Moderate temperatures and dry weather ended July and started the month of August. However, outlooks from the Climate Prediction Center indicate the potential for a transition to more summer-like conditions. Outlooks show strongly elevated odds of warmer than normal conditions and elevated odds of wetter than normal conditions in the second week of August (Fig. 7).   

Figure 7. Climate Prediction Center Outlooks of (left) temperature and (right) precipitation for the second week of August

A Bit of Everything to Start Summer

June took a page out of March’s playbook—came in like a lamb and went out like a lion. Overall, the preliminary statewide average June temperature was 73.8 degrees, 1.6 degrees above the 1991–2020 average and tied for the 25th warmest on record going back to 1895. The preliminary statewide average total June precipitation was 5.33 inches, 0.68 inches above the 1991–2020 average and tied for the 30th wettest on record going back to 1895.  

Data are provisional and may change slightly over time

Hot Start, Cooler End to June

Drier soils in the northern two-thirds of the state helped elevate temperatures in the first half of June. Figure 1 shows daily temperature departures and daily average temperature this past month in Joliet. Most of northern and central Illinois experienced temperatures that were between 5 and 15 degrees warmer than normal in the first half of June. Rockford recorded 13 days with a high temperature at or above 90 degrees,  the fourth most for any June on record there back to 1893.

Figure 1. Daily temperatures in Joliet (black line) and daily temperature departures from the 1991 to 2020 normals (red and blue bars).

While northern Illinois stayed hot and dry in the first two weeks of June, temperatures in southern Illinois remained near to slightly cooler than normal (Figure 2). Temperatures in the latter half of the month were within 1 degree of normal north of Interstate 64, and between 1 and 2 degrees above normal in southern Illinois.

Figure 2. Average temperature departures between (left) June 1 and 15 and (right) June 16 and 30. Departures are from the 1991–2020 normal.

The warm start to June broke 31 daily high maximum temperature records and 15 daily high minimum temperature records. The subsequent cooldown in the third week of June broke 12 daily low minimum temperature records across the state.


Following a slow start to the growing season from cooler April and May weather, June temperatures helped accumulate growing degree days (GDD). The maps below in Figure 3 show total GDD accumulation and departures from the 1991–2020 normal since April 1. As of the start of July, growing degree day accumulation is ahead of normal in northern Illinois and slightly to much below normal in southern Illinois.

Figure 3. Maps show total base 50-degree modified growing degree day accumulation (left) and growing degree day departures from the 1991–2020 normal (right) since April 1.

Overall, June average temperatures ranged from the low 70s in northern and central Illinois to the mid- to high-70s in southern Illinois, between 1 and 5 degrees warmer than normal across the state (Figure 4). The preliminary statewide average June temperature was 73.8 degrees, 1.6 degrees above the 1991–2020 average and tied for 25th warmest on record going back to 1895.

Figure 4. June average temperature (left) and departure from the 1981-2010 normal (right).

Wild End to Otherwise Dry June

The first two-thirds of June was very dry across Illinois (Figure 5). In fact, the period between June 1 and 20 was the sixth and seventh driest on record in the northwest and northeast Illinois climate divisions, respectively. As the large atmospheric ridge established over the Pacific Northwest, most of Illinois found itself on the edge of a stationary front that produced several rounds of heavy rain from the St. Louis Metro East to Chicagoland. Due to the very wet last 7 to 10 days of the month, June ended wetter than average in all but the northwest and southwest climate divisions, and it was the sixth wettest on record in the east-central division.

Figure 5. Climate division rankings of total precipitation from (left) June 1 through the 20, and (right) the entire month of June. Lower values indicate wetter conditions. Data source: Iowa Environmental Mesonet.

Rain in the final week of June was particularly heavy along the Interstate 55 corridor between Bloomington-Normal and Chicago. Areas of McLean and Livingston Counties observed 8 to 10 inches of rain in just 4 days (Figure 6), resulting in serious flooding in Bloomington, flooding on and the temporary shutdown of Interstates 55 and 74, and standing water in fields across central and northeast Illinois.


CoCoRaHS observers in both south Bloomington and Heyworth in McLean County observed over 10 inches of rain in just 3 days between June 25 and 27. According to the recently published Bulletin 75, these totals exceeded the 1% annual probability or “100-year” rainfall event by 1.5 inches.

Figure 6. Total precipitation at weather stations across central Illinois between June 25 and 28. Source: National Weather Service, Lincoln, IL.

Total June rainfall ranged from less than 3 inches in far northwest and southeast Illinois to over 12 inches in central and east-central Illinois (Figure 7). Most areas between the St. Louis Metro East and Chicago areas were between 1 to 5 inches wetter than normal, while southern and northwest Illinois were 1 to 3 inches drier than normal. Overall, the preliminary statewide average total June precipitation was 5.33 inches, 0.68 inches above the 1991–2020 average and tied for the 30th wettest on record going back to 1895.

Figure 7. Maps show (left) total June precipitation and (right) total June precipitation departure from 1991–2020 normal.

Drought Continues in Northern Illinois

The dry, hot start to June culminated in the U.S. Drought Monitor introducing extreme drought in northeast Illinois in mid-June in response to very dry soils, low streamflow in major streams, and increasingly concerning crop stress. Persistent heavy rains in the Chicagoland area in late June were crucial to improving drought conditions and alleviating crop and water stress across the region. The most recent (July 1) edition of the U.S. Drought Monitor showed most of the southern and central Chicagoland metro area remained abnormally dry to near normal, which represented a significant improvement since mid-month (Figure 8). However, areas north of Interstate 88 and west of Interstate 39 did not receive as much rain in the final week of June, and drought conditions have worsened, particularly from Winnebago to Jo Daviess Counties. In fact, June was the fifth driest on record in Stockton in Jo Daviess County back to 1944, and the driest since 2012.

Figure 8. U.S. Drought Monitor map, updated through June 29.

We will continue to monitor evolving conditions across Illinois and hope to see continued improvement in hydrology and ecology throughout northeast Illinois. Concurrently, the northwest corner continues to deteriorate as precipitation deficits increase. July is the climatologically warmest month in Illinois, and high temperatures can intensify dry conditions. Therefore, the northwest corner of the state will need near to above normal rainfall in July to curb worsening drought conditions.

Outlooks

We’re in a bit of a transition in weather patterns to start July. Climate Prediction Center outlooks for the second week of the month show elevated odds of warmer and wetter than normal conditions statewide (Figure 9). Meanwhile, longer-term outlooks for the entire month of July do not show much of a strong signal in either temperature or precipitation (Figure 10). Illinois appears to be on a boundary between warm, dry conditions to our north and northwest and cooler, wetter conditions to our south and southeast. Climate prediction more than two weeks out in the summer is very difficult here in the Midwest, so we will have to see how shorter-term outlooks and forecasts evolve as we move deeper into the summer.

Figure 9. Climate Prediction Center Outlooks of (left) temperature and (right) precipitation for the second week of July.
Figure 10. Climate Prediction Center Outlooks of (left) temperature and (right) precipitation for the entire month of July.