Cool Fall Temperatures Continued in October

October was much cooler and slightly drier than average across Illinois. The preliminary statewide average October temperature was 51.8 degrees, 2.6 degrees below the 30-year normal, and tied for the 19th coolest on record. Preliminary statewide average total precipitation for August was 3.03 inches, 0.21 inches less than the 30-year normal, and the 54th wettest on record.

Data are provisional and may change slightly over time

October Temperatures

Cool weather persisted from the end of September to the first week of October across the state. As the plot below of daily temperature departures from normal in Moline shows, the second week of October was 5 to 10 degrees warmer than average across the state. However, this was followed by a predominantly cooler than average second half of the month.

Eleven daily high maximum temperature records and eight daily high minimum temperature records were broken last month. Concurrently, 66 daily low maximum temperature records and 19 daily low minimum temperature records fell in October. The cool, cloudy day of October 27 broke the month’s low maximum temperature records at seven locations in Illinois. This included the 34-degree high in Kewanee in Henry County, which broke the previous October low maximum temperature record of 36 degrees, set just last year on Halloween.

The maps below show October average temperatures and their departure from the long-term average. October average temperatures ranged from the high 40s in northwest Illinois to the high 50s in southern Illinois, between 2 and 5 degrees below average. The preliminary statewide average October temperature was 51.8 degrees, 2.6 degrees below the 30-year normal and tied for the 19th coolest on record. This follows the cooler than normal months of August and September statewide.

Harvest Fires

The combination of high temperatures, very low humidity, and strong winds created a significant fire risk during the second week of October. The plot below shows a daily mean surface vapor pressure deficit, a measure of atmospheric humidity, at Springfield between January 1 and October 18 of this year (red line) compared to all previous years (gray shaded area). A vapor pressure deficit has been linked to wildfire risk, with higher vapor pressure deficit values indicating a higher risk of wildfire outbreak.

The very high values of vapor pressure deficit in Springfield, near record values on October 14, indicated a warm, dry atmosphere. Existing drought and crops drying down in fields provided ample, dry fuel for wildfire potential. Indeed, several small- to medium-sized field fires broke out across central and south-central Illinois during the second week of October.

First Snow of the Season

The map below shows the climatological earliest first measurable snowfall on record at stations across the state. The dates range from early to mid-October in northern Illinois to mid-November in southern Illinois.

The map below shows the climatological earliest first measurable snowfall on record at stations across the state. The dates range from early to mid-October in northern Illinois to mid-November in southern Illinois.

A Wet End to the Month in Southern Illinois

The first half of October was very dry for the southern half of the state, continuing a very dry September. However, around the middle of the month the atmosphere moved into a pattern more conducive to bringing precipitation into southern Illinois. Areas that received less than a tenth of an inch of rainfall in previous weeks saw widespread 4- to 9-inch precipitation totals during the final two weeks of October.

Belleville, for example, recorded its third wettest October with a total of 7.38 inches, following its wettest August and its second wettest July on record. At the end of October, Belleville was only 4 inches away from its wettest year on record, which was 2008 with 56.83 inches of rain.

Drought Continues in Central Illinois

In contrast to the wet end of October in southern Illinois, much of central Illinois ended yet another month with below average precipitation. The maps below show 30- and 180-day precipitation deficits across the state. Most of central and western Illinois received between 1 and 3 inches below average precipitation in October, adding to existing precipitation deficits from dry months in August and September.

Total six-month precipitation deficits in central Illinois exceed 7 inches in Fulton and Mason Counties and 8 inches in parts of Logan and Macon Counties. At Mount Pulaski in Logan County, for example, the June 1 to November 1 precipitation deficit of 8.47 inches is the third largest on record, smaller only than 1988 and 1893. However, it should be noted that Mount Pulaski and the surrounding areas of Logan and Macon Counties received more precipitation in the first four months of this year than in all of 1988. Therefore, drought conditions, although severe, are not nearly to the extent of those in 1988.

The most recent, October 27th version of the U.S. Drought Monitor showed continued widespread moderate drought across central Illinois in response to the rainfall deficits and below normal soil moisture and streamflow.

Outlooks

Pleasant weather is in store for the state during the first week of November. However, the Climate Prediction Center’s 8- to 14-day outlooks indicate an imminent pattern change in the second week of November, with odds moving back to below average temperatures, especially to the northwest, and above average precipitation statewide.

The most recent one-month outlooks for the entire month of November continue to show a strong La Niña influence, with elevated odds of warmer than normal conditions throughout the central U.S. and drier than normal weather in the south.

Finally, the three-month outlooks for climatological winter, December–February, show equal chances of warmer or colder than normal conditions in Illinois. However, the odds increase for wetter than normal conditions later in winter.

La Niña Conditions for Fall & Winter

A couple of weeks ago the NOAA Climate Prediction Center issued a La Niña advisory, which indicates that La Niña conditions were observed during the month of August and have a 75% chance to continue through climatological winter (December – February). Information on the advisory and a more diagnostic discussion can be found here: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml.

La Niña refers to one phase of the El Niño-Southern Oscillation or ENSO, the interaction between the ocean and atmosphere in the tropical Pacific that results in variations in sea surface temperatures, air temperatures, and precipitation. Because of the importance of the Pacific Ocean in the global climate, ENSO not only affects tropical climates but also climates of regions outside of the tropics, including here in the Midwest.

The following websites contain more deailed information on La Niña and ENSO more generally.

NOAA Climate.gov: https://www.climate.gov/enso

Columbia University: https://iridl.ldeo.columbia.edu/maproom/ENSO/ENSO_Info.html

North Carolina Climate Office: https://climate.ncsu.edu/climate/patterns/enso

Typical La Niña Winter

Ocean-atmosphere interactions modulated by ENSO can modify atmospheric circulation and affect remote climates, including the Midwest. These impacts are typically largest in climatological winter. The graphic below shows the typical La Niña impacts to winter climate across North America. La Niña winters tend to be wetter than average in the eastern Midwest, particularly in the Ohio Valley, whereas they tend to be colder than average in the northwest Midwest.

Typical La Niña impacts on winter weather in North America. Taken from NOAA Climate.gov: https://www.climate.gov/news/features/featured-images/how-el-ni%C3%B1o-and-la-ni%C3%B1a-affect-winter-jet-stream-and-us-climate.

The maps below show a different perspective on the impacts of La Niña on Midwest Winter climate. Specifically, the maps show the correlation between temperature/precipitation and the Southern Oscillation Index, a common indicator of ENSO conditions that is based on the observed sea level pressure differences between Tahiti and Darwin, Australia. Positive SOI values typically coincide with La Niña conditions. The top map shows correlations between SOI and winter air temperature across the U.S. The areas shaded in green exhibit a negative correlation between SOI and temperature, meaning that La Niña winters in these areas tend to be colder than average. The bottom map shows correlations between SOI and winter precipitation across the U.S. In this case, the eastern half of Illinois is shaded yellow, indicating a positive correlation. This suggests that La Niña winters tend to be wetter than average in northern and eastern Illinois.

The previous maps imply La Niña winters in Illinois tend to be colder and wetter. However, ENSO is only one of many features that influence winter weather in Illinois. Other impactful features include sea surface temperature and sea level pressure in the north Pacific, temperatures and sea ice extent in the Arctic, and long-term trends in both winter temperature and precipitation. The maps below are taken from NOAA’s Climate.gov website and show winter temperature and precipitation anomalies across the U.S. during every La Niña since 1950. Although most La Niña winters tend to be wetter than average in Illinois, we have also experienced drier than average La Niña winters as well.

Fall to Winter Outlooks

ENSO is an important part of seasonal climate forecasting, and the latest outlooks from the Climate Prediction Center are influenced by ongoing La Niña, while also showing influence from long-term trends and current soil moisture conditions. One-month outlooks show elevated odds of both warmer than drier than normal conditions in October.

The three-month outlooks show continued elevated chances of warmer than normal conditions, but equal chance of above, below, and normal precipitation for the period October through December.

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.

Looking Ahead to Fall – Recent Temperature Changes and First Freeze Dates

As we approach the end of climatological summer, I have received several questions regarding the fall, harvest, and potential of an early freeze. In this post I try to address some of those concerns based on recent outlooks and both long-term and recent historical data.

Fall Outlooks

The most recent 3-month outlook from the Climate Prediction Center (CPC) was released on July 16 and covers August through October. Those outlooks show elevated odds of above normal temperatures over these three months, with equal chances of above normal, normal, and below normal precipitation (see maps below).

Much of the temperature outlook is based on the persistence of warmer than average weather from this summer, as well as sea surface temperatures and corresponding atmospheric conditions in the tropical and extratropical Pacific and Atlantic Oceans. However, another important contribution to these outlooks is from decadal trends in fall temperatures.

Fall Temperature Changes

Statewide, average temperature in climatological fall (September to November) has increased at a rate of 0.08 degrees (Fahrenheit) per decade between 1895 and 2019. However, fall warming has accelerated in recent decades. Over the past 30 years, the statewide average rate of fall temperature changes is 0.6 degrees per decade.

The plot below shows statewide monthly average temperature in September, October, and November between 1990 and 2019, with trends fitted to show recent temperature changes. September and October temperature have both increased over the past 30 years, whereas November temperature has decreased slightly. The largest magnitude of change is in September; over the past 30 years September average temperature in Illinois has increased at a rate of 1.5 degrees per decade.

As the map below shows, all 102 Illinois counties have experienced a positive (i.e., warming) trend in September temperature over the past 30 years. From a different perspective, the figure next to the county map shows the distribution of all Illinois county temperature trends for each calendar month between 1990 and 2019.The boxplots show the range of temperature trends across all 102 counties for each calendar month. The boxplots clearly show that positive 30-year trends are largest in September, and all counties have a September temperature trend greater than or equal to 1.2 degrees per decade.

The plot below shows statewide September average temperatures between 2000 and 2019, depicted as a departure from the current (1981-2010) normal. The statewide average September temperature has been higher than the 30-year normal in 13 of the past 20 years, and each of the past five years. The average September temperature departure from the normal over the past five years is 4.2 degrees, and in four of the past five years, the September statewide average temperature has been closer to the August 30-year normal than the September normal.

The figures and maps above agree with anecdotal evidence I’ve heard from folks around the state who have noticed recent Septembers have been unusually warm. This was particularly noteworthy last year, when a very warm September statewide was critical to help mature late-planted crops. However, higher September temperatures also increase atmospheric evaporative demand, which can intensify dry conditions. The plot below shows total September evaporative demand, represented by reference evapotranspiration (ET), estimated from measurements at the Illinois Climate Network station in East Peoria between 1989 and 2019. Reference ET in East Peoria has increased by 0.28 inches per decade between 1989 and 2019. All 19 Illinois Climate Network stations show similar evaporative demand trends over this time period.

 

The importance of temperature and evaporative demand for drought played out in southern Illinois last year. Most counties south of Interstate 64 received 10 to 50 percent of normal September precipitation that combined with very high September temperatures to rapidly dry soil. The U.S. Drought Monitor maps below show the evolution of what could be called a “flash drought” last fall in southern Illinois, when many counties deteriorated from near normal conditions to severe drought in only three weeks. It is likely the drought in southern Illinois would not have intensified as quickly without the elevated September temperatures and high evaporative demand.

A consistent conclusion of recent climate assessments, including the 4th National Climate Assessment (https://nca2018.globalchange.gov/chapter/21/), is that increasing temperatures will exacerbate drought conditions, similar to what we witnessed last year in southern Illinois.

Fall Freeze Dates

Despite recent fall warming, studies have not consitently found a significant decrease in the frequency of very early fall freeze events in the central U.S. This may seem counterintuitive, but it provides a good example of the separation of variability and change. Daily, monthly, and inter-annual weather and climate variability exists on top of longer term change. Therefore, an early fall freeze is still very possible despite positive fall temperature trends. Thankfully, crops this year are much further along in their maturity than last year, so irrespective of fall freeze, there is overall less risk of crop damage. Still, as September approaches, it is a good time to review climatological fall freeze dates in Illinois.

The maps below were created by the State Climatologist Office for a discussion on fall freeze dates last year (https://stateclimatologist.web.illinois.edu/2019/09/24/illinois-first-fall-freeze-climatology/). The maps show the climatological earliest and latest first fall freeze dates, the median date, and the dates which represent 1 in 10 and 9 in 10-year events. The climatological period is 1979 and 2018. Although 2019 data are not included, all areas of the state experienced their first fall freeze event within days of the climatological median, so the exclusion of 2019 will not drastically affect the numbers.

The earliest fall freeze dates over the past 40 years range from late September in northwest and central Illinois to early October in southern and eastern Illinois. The tenth percentile (i.e., 1 in 10 years) first fall freeze dates range from early October in northern and central areas to late October in southern Illinois. Median first fall freeze dates range from mid- to late October in northwest and central Illinois to early November in southern and northeastern Illinois.

The 90th percentile first fall freeze dates (i.e., 9 in 10 years) range from early November in northwest and central areas to mid- to late November in southern areas. One way to think about the 9 in 10-year maps is that 90 percent of all years in the climatological record have recorded the first fall freeze before that date. Finally, the latest first fall freeze dates range from mid- to late November in northwest Illinois to mid-December in southern Illinois.