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.
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.
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.
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
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 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).
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.
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.
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.
July was warmer and wetter than average across Illinois, continuing a warmer than average summer season. The preliminary statewide average July temperature was 77.2 degrees, 1.8 degrees above the 30-year normal and tied for the 29th warmest on record. Preliminary statewide average total precipitation for July was 5.79 inches, 1.71 inches wetter than the 30-year normal and the 12th wettest on record.
Data are provisional and may change slightly over time
July Heat, Humidity
June was the 20th warmest on record statewide, and above average temperatures persisted throughout the month of July. The figure below shows daily maximum and minimum temperatures between June 1 and July 30 in Champaign. The Champaign COOP station recorded 36 consecutive days with a daily maximum temperature above 80 degrees, a streak that ended on July 30. The longest streak of 80+ degree days on record in Champaign is 74, between June 19 and August 31, 1959.
To show the persistence of warm weather last month, the plot below shows daily average temperature departures from normal in Rockford. Only five July days exhibited a daily average temperature below the 30-year normal, and those days were only 2 to 3 degrees below the daily normal.
The elevated temperatures last month were accompanied by abundant humidity because of persistent atmospheric flow off from the Gulf of Mexico and actively transpiring crops. The heat index combines the effects of temperature and humidity to better represent how humans feel hot, humid weather. The daily average heat index value in Belleville this last month was 87.2 degrees, which was the second highest July average heat index value on record, after 2011.
Overall, July temperatures ranged from the mid-70s in northern Illinois to the mid-80s in southern Illinois, between 1 and 5 degrees above average. The preliminary statewide average July temperature was 77.2 degrees, 1.8 degrees above the 30-year normal and the 29th warmest on record. Last month, one daily high maximum temperature record and two daily high minimum temperature records were broken.
The abundant heat and humidity made for a very active severe weather month in July. The NOAA Storm Prediction Center reported 10 tornado, 22 hail, and 154 wind reports in Illinois during July. This follows a very inactive severe weather month in June, with only 1 tornado and 10 hail reports statewide. Although areas of the state needed the rain that accompanied severe thunderstorms last month, several producers reported widespread crop damage from hail and strong winds.
Rain for All, Too Much for Some
As the two maps below show, June was very dry for areas of central and southeast Illinois. For example, Morton in Tazewell County received only 0.50 inches of rainfall in June, approximately 15 percent of normal. In contrast, July was wetter than normal for most of the state, and areas of central and southeast Illinois received 4 to 6 inches more than normal July rainfall.
The station at the Peoria airport recorded 5.15 inches in just six hours on July 15, marking the highest six-hour rainfall total on record in Peoria, with the record going back to 1948. This event is approximately a 50-year event based on estimates from the Illinois State Water Survey’s newly release Bulletin 75 study (https://www.ideals.illinois.edu/handle/2142/106653). This means there is a 2% chance of exceeding this six-hour total in Peoria each year.
Twenty-four-hour rainfall totals from this event exceeded 6 inches in parts of Tazewell and Woodford Counties, resulting in widespread flash flooding in agricultural and residential areas. The station in Minonk in Woodford County, for example, recorded 6.50 on July 15. This day, along with a 9-inch rainfall observation in September of last year, are the only two days in Minonk’s 125-year record with more than 6 inches of rainfall observed.
Heavy rain in central and southeast Illinois alleviated acute dryness that continued from June and the first half of July. The maps below show U.S. Drought Monitor conditions as of July 14 and 28, showing the reduction in moderate drought (D1) and abnormally dry (D0) categories in central and southeast Illinois in response to the rain.
July total precipitation ranged from over 13 inches in central and eastern Illinois to less than 2 inches in the northeast. Overall, statewide July total precipitation was 5.79 inches, 1.71 inches wetter than the 30-year normal and the 12th wettest on record. Single-day precipitation total records were broken at 88 stations across the state last month. Five of those stations broke the July all-time daily precipitation records. Last month was also the wettest July on record at Casey in Clark County, Minonk in Woodford County, and Clay City in Clay County; all three stations have records exceeding 70 years.
Temperatures have been quite moderate at the start of August. However, short-term 8- to 14-day outlooks from the NOAA Climate Prediction Center suggest the heat may return later this month, with elevated odds of above normal temperatures in the second week of August. The state is also covered with slightly elevated odds of wetter than normal conditions over the same time period.
Despite the short-term outlook, one-month outlooks indicate elevated odds of below normal temperatures for August as an entire month, with the strongest odds of near-normal precipitation.