Author: Trent Ford

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25th Anniversary of the 1995 Chicago Heat Wave

This week marks the 25th anniversary of the July 1995 heat wave in Chicago, one of the worst weather-related disasters in Illinois history. The excessive heat and humidity, compounded by social and economic vulnerability, resulted in more than 700 deaths over a five day period.

The chart below shows the daily progression of maximum and minimum temperatures between July 1st and 20th in 1995. Daytime temperatures between July 10th and 16th reached 90 degrees, and over triple digits on the 13th and 14th. Minimum temperatures during this time period were in the 70s and, on two days, the low 80s. The combination of very hot days and very warm nights enhanced heat stress and reduced nighttime recovery. The multiple day duration of the heat wave was another important contributor to excess mortality.

Social and Environmental Factors Contributed to Heat Impacts

The heat wave was made worse by the urban heat island effect, which is caused by the high concentration of buildings and roads in urban areas. These surfaces tend to absorb more heat in the day and result in more heat in the atmosphere at night. Therefore, urban areas usually experience less cooling at night than nearby rural sites.

Deaths attributed to the 1995 heat wave were primarily among elderly, poorer Chicagoans, due to a number of social and economic factors such as lack of access to air conditioning and/or air conditioned cooling centers. In addition, death rates among non-hispanic black Chicagoans were 50% higher than those among non-hispanic white Chicagoans (Whitman et al. 1997), highlighting racial disparities in both risk and vulnerability to extreme heat. Other factors that contributed to the high number of deaths were an inadequate local heat wave warning system, power failures, and inadequate emergency services and hospital facilities.

Current Conditions

This summer so far has also been very warm, although without a heat wave as extreme as 1995. Daily average temperatures in Chicago have been consistently 5 to 10 degrees above normal since the beginning of June, and July 12th was the first day this month with a below normal daily average temperature. Maximum temperatures have mostly been in the mid-80s to low 90s for several weeks. Chicago O’Hare, for example, has experienced 16 days with a daily high temperature of 90 degrees or more since June 1st. This is about twice the to-date average frequency of 90 degree days, and fourth most on record at O’Hare.

The NOAA Climate Prediction Center outlooks indicate strongly elevated chances of above normal temperatures across Illinois persisting for the next four weeks. Although temperatures are not expected to reach the peak levels of the 1995 heat wave, the persistence of our current heat, in combination with high humidity levels, can result in negative human health outcomes without proper precaution.

A Look Forward

According to the NOAA National Centers for Environmental Information Illinois Climate Summary (Frankson et al., 2017, https://statesummaries.ncics.org/chapter/il/), summer temperatures are projected to increase and likely to exceed historical record levels by the middle of the 21st century. The report states that future heat waves in Chicago are likely to be more intense with temperature increases due to climate change.

Figure shows observed and projected changes in near-surface air temperature for Illinois. Observed data re for 1900-2014. Projected changes for 2006-2100, under lower and higher greenhouse gas emissions scenarios. Taken from Frankson et al. (2017).

Given these projected changes, continued and further work is critical to decrease social vulnerability to extreme heat in Chicago, ensuring Chicagoans are resilient to extreme heat now and into the future.

Additional Reading

Changnon, S. A., K. E. Kunkel, and B. C. Reinke, 1996: Impacts and Responses to the 1995 Heat Wave: A Call to Action. Bulletin of the American Meteorological Society, Vol. 77, No. 7, 1497-1506.

Kunkel, K. E., S. A. Changnon, B. C. Reinke, and R. W. Arritt, 1996: The July 1995 Heat Wave in the Midwest: A Climatic Perspective and Critical Weather Factors. Bulletin of the American Meteorological Society, Vol. 77, No. 7, 1507-1518.

References

Frankson, R., K. Kunkel, S. Champion, B. Stewart, D. Easterling, B. Hall, and J.R. Angel, 2017: Illinois State Climate Summary. NOAA Technical Report NESDIS 149-IL, 4 pp.

Whitman, S., G. Good, E.R. Donoghue, N. Benbow, W. Shou, and S. Mou, 1997: Mortality in Chicago attributed to the July 1995 heat wave. American Journal of Public Health, 87, 1515-1518, https://doi.org/10.2105AJPH.87.9.1515.

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A Mostly Warm, Dry Start to Summer

June was warmer and drier than average across the state. The preliminary statewide average June temperature was 73.7 degrees, 1.8 degrees above the 30-year normal and tied for the 26th warmest on record going back to 1895. The preliminary statewide average total June precipitation was 3.70 inches, 0.51 inches below the 30-year normal and the 48th driest on record.

Data are provisional and may change slightly over time

Persistent Above Average Temperatures Last Month

June was warmer than normal for most of the northern two-thirds of the state following a warm end to May. June average temperatures ranged from the high 60s in northern Illinois to the high 70s in southern Illinois. In the northern part of the state, June was 1 to 6 degrees warmer than normal, with the highest departures in northern and northeast Illinois.

The highest temperature recorded in Illinois last month was 99 degrees in Cook County on June 20. In contrast, a few cloudless nights following a strong cold front in the middle of the month resulted in several stations recording minimum temperatures in the 40s, including 43 degrees in McHenry County on June 15.

Overall, 20 daily high maximum temperature records were broken last month, including a 148-year-old daily maximum temperature record on June 2 at Rock Island Lock and Dam. High humidity throughout the state also resulted in 31 daily high minimum temperature records broken last month, including a daily record 76-degree minimum temperature in Olney on June 10.

The station at Chicago’s O’Hare airport only recorded six days in June with an average temperature below the long-term daily mean (see plot below). The average June temperature at O’Hare of 74 degrees tied 2012 for the second warmest on record, following only the 74.2-degree average of June 2005. The June average daily maximum temperature at O’Hare was 84 degrees, which was the sixth highest on record. It should be noted that the O’Hare temperature record only goes back to 1958, and therefore does not include the 1930s during which most summer high temperature records were set in Illinois.

Because of the heightened influence of the land surface on temperature during the summertime in Illinois, monthly precipitation and air temperature (particularly daily maximum temperature) are often negatively correlated. Drier summer months tend to result in reduced soil moisture, which can also decrease evaporation and increase air temperature. This is why the driest summers (think 1988, 2005, 2012) are also frequently the warmest summers. The graph below shows the total June precipitation on the x-axis and average daily maximum June temperature on the y-axis, both from the station at O’Hare. Each scatter point represents a calendar year from 1959 to 2020. In general, wetter Junes correspond with lower maximum temperatures in Chicago. However, despite last month being slightly wetter than average in Chicago, it was much warmer than average.

The added heat last month helped crops progress after well below normal temperatures in May. However, base 50 growing degree days since April 1 are still below normal across the state and well below normal in southern Illinois.

June: Dry for Most, Very Wet for Some

Most summer precipitation in Illinois comes from local- to meso-scale systems, such as convective thunderstorms. These storms can produce large precipitation totals over short time periods but typically only affect a small geographic area. This can result in a “have and have not” summer precipitation pattern, which is what occurred last month.

The June total precipitation ranged from over 8 inches in parts of western and east-central Illinois to less than an inch in central and south-central Illinois. The totals represented between 200 percent and less than 25 percent of normal June precipitation. For example, only 60 miles separates COOP stations in Fisher in Champaign County, which recorded over 8 inches of precipitation in June and Morton in Tazewell County, which recorded just 0.52 inches in June.

High humidity and active large-scale atmospheric patterns resulted in several instances of thunderstorm-driven heavy precipitation across the state last month. Six stations recorded single-day precipitation totals of over 4 inches. This included 4.48 inches of rainfall recorded in West Frankfort in Franklin County, which was the seventh highest single-day total since precipitation observations began at that station in 1972. Heavy precipitation in Adams and Pike counties resulted in two different flash flood warnings, one on June 21 and a second on June 30.

In contrast to the few areas of wet extremes last month, most of the state was drier than normal, including a few areas with less than 25 percent normal June rainfall. The station in Highland in Madison County experienced their driest June on record going back to 1977. Incredibly, the same station in Highland recorded nearly 3.5 inches more precipitation than normal in January and May of this year and, despite the very dry June, is still well above 100 percent of normal precipitation for the first six months of the year. Thankfully, this station also recorded over 2 inches of rainfall on July 1, helping reduce some of the dryness from the previous month.

Below normal precipitation and declining soil moisture resulted in the U.S. Drought Monitor identifying abnormally dry (D0) conditions at a number of areas in the state, most notably in south-central and central Illinois. It is important to note that at this time the conditions in these marked areas are considered only abnormally dry and not officially in drought.

Outlooks

Short-term 8- to 14-day outlooks from the Climate Prediction Center show strongly elevated odds of above normal temperatures across the state through the middle of July.

The early to middle part of July is, climatologically, the hottest time of the year in Illinois, and this outlook suggests temperatures will be above the climatological normal. Accordingly, the Climate Prediction Center is indicating a moderate risk of excessive heat over the same period between July 8 and 14 for most of the state.

Similar to what we experienced in June, the high temperatures will likely be coupled with very high humidity, resulting in hazardous conditions that increase the risk of adverse human and animal health outcomes.

The precipitation outlook over the same time period indicates slightly elevated odds of drier than normal conditions in Illinois.

Given that the 8- to 14-day outlooks tilt toward persistence of warm, dry weather to start July, dry conditions in central and south-central Illinois likely will worsen before they are alleviated. The July U.S. Monthly Drought Outlook indicates likely drought development in south-central Illinois in response to expected dry conditions, exacerbated by elevated evaporation due to high temperatures.

Longer-term 30-day outlooks from the Climate Prediction Center show continued elevated odds of warmer than normal conditions through the entirety of July. Precipitation outlooks indicate slightly elevated odds of wetter than normal conditions for July as a whole, hopefully reducing the potential for drought development this month.

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Very Dry June (so far)

Four out of the first five months of 2020 have been wetter than normal. As the map below shows, most of the state, entered June with a calendar year precipitation surplus of 2 to 10 inches.

However, more recently rain has been scarce in most of the state. Ironically, the driest area in the state through May has received abundant June precipitation, while most other areas have seen less than 1.5 inches of total June precipitation (through the 21st). In areas of southwest and south-central Illinois, totals so far represent less than 25% of average precipitation by this time of June, including the driest first 21 days of June on record in Pinckneyville in Perry County.

Three full weeks with little to no precipitation and increased evaporative demand has resulted in depletion of surplus soil moisture in many places around the state. The plot below shows 8-inch soil moisture observations at the Illinois Climate Network (ICN) station in DeKalb. The thick, red line shows the daily evolution of soil moisture in DeKalb between June 1st and 22nd of this year, with respect to all other record years back to 2003. I denoted soil moisture conditions during other, noteworthy dry years in northern Illinois. In general, the plot shows the rapid decline in soil moisture in response to dry, warm conditions this month.

For a broader view of soil conditions the maps below show soil moisture on June 21st from all ICN stations at 4, 8, and 20 inches (from left to right). Generally speaking, observations exceeding 0.35 to 0.40 indicate saturated or nearly saturated conditions, while observations in or below the teens indicate conditions at or approaching the wilting point. Not surprising, 4 inch and 8 inch soils have dried quite a bit more than the 20 inch soil; however, nearly all stations are still drier than average for this time of the year at 20 inches.

Early summer is a challenging time to experience moisture deficit, both because crops and other vegetation are actively growing and because evaporative demand and evaporation tend to increase as temperatures continue to rise into July. However, the 7-day forecast indicates potential for rainfall to alleviate some dryness in northeast and east-central Illinois. There is less of a chance of reprieve from dryness for southern Illinois over the next week.

At the same time, the 6- to 10-day and outlooks from the Climate Prediction Center indicate elevated odds of wetter than normal conditions through the end of June and into July.

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Cool, Wet May ends Wet Spring

May was much colder and moderately wetter than average across Illinois, bringing an end to a wet climatological spring season. The preliminary statewide average May temperature was 60.5 degrees, 2.2 degrees below the 30-year normal and the 39th coldest on record. Preliminary statewide average total precipitation for May was 5.41 inches, 0.81 inches wetter than the 30-year normal and the 25th wettest on record.

Data are provisional and may change slightly over time

Cool Start to May, Very Late Freeze

Following a cool April, temperatures for the first half of May were mostly below average. As the temperature plot from Quincy shows (below), May temperatures were 10 to 15 degrees below average between May 8 and May 13.

Most stations in the northern two-thirds of the state observed daily minimum temperatures below freezing during the six-day stretch. The map below shows observed minimum temperatures on the morning of May 10, and below freezing temperatures as far south as Hardin County. The 24-degree minimum temperature observed in Normal was the second lowest May temperature observed at that station since observations began there in 1893. Although May freezes are not uncommon in Illinois, many tender perennials and flowering trees were at a more vulnerable state because of the warm start to this calendar year.

Minimum temperatures observed on the morning of May 9, 2020.

Between May 6 and May 13, 37 daily low minimum temperature records and 40 low maximum temperature records were broken across the state. In addition, stations in Paxton, Joliet, Normal, and Princeton broke their all-time May low minimum temperature records on May 9. All four stations have data records going back at least 30 years.

A shift in the predominant winds brought warmer and more humid weather for the last two weeks of May. The dew point temperature is a good indicator of humidity because the dew point is the temperature to which the air would need to be cooled to reach saturation (relative humidity of 100%). Therefore, higher dew point temperatures indicate higher humidity. The air will begin to feel humid when dew point temperatures are between 55 and 60 degrees, and dew points exceeding 70 degrees indicate conditions that are oppressive, or very humid. Dew point temperatures were well above average throughout the state during the last 10 days in May. At 6 p.m. on May 26, the station at the Effingham County Memorial Airport observed a 77-degree dew point temperature. This was only the third time since observations began at the Effingham airport in 1948 that a May dew point temperature exceeding 75 degrees was observed.

May average temperatures ranged from the high 50s in northern Illinois to the low 60s in southern Illinois. May was colder than average in all but the very northeastern corner of the state. Overall, the May statewide average temperature was 60.5 degrees, which was 2.2 degrees below the 30-year normal and 39th coldest on record.

Both April and May this year were colder than normal statewide, following three consecutive warmer than normal months to start the year. Although official numbers will not be released until later this month, it is likely the 2020 climatological spring was very close to the 30-year normal.

Another Record-Breaking Wet Spring in Chicago

May started dry, with most areas of the state receiving less than an inch of rain in the first two weeks. However, a series of storms moved across the Interstate 55 corridor around the middle of the month, leaving widespread totals of 2 to 4 inches between the St. Louis Metro East and Chicagoland.

The months of May through September tend to have a larger percentage of precipitation that comes from small-scale convective storms than cold season months. Despite their size, these storms can generate large precipitation totals in a matter of hours; however, because of their size and relatively short lifespan, they can create large disparities in precipitation over short distances. The storms that moved through the state in the middle of last month generated very heavy rainfall over the Chicagoland area and resulted in over 7 inches of rain in four days over a large area of the city and suburbs. Over 3.5 inches was recorded at Chicago’s O’Hare airport on May 14, the largest single-day May precipitation total ever recorded at that station.

The rainfall resulted in the wettest May on record at both Chicago O’Hare (9.51 inches) and Midway (7.65 inches) airport stations. At O’Hare, this last month broke the previous May record set last year in 2019, which at that time broke the previous record set in 2018. This makes three consecutive years that the O’Hare station has broken May total precipitation records, and all five of the wettest Mays on record at O’Hare have occurred since 2004.

Heavy rainfall across northern and central Illinois led to flash flooding in developed areas as well as inundated fields and standing water in rural parts of the state. Gauges in both the Illinois and Des Plaines Rivers topped flood stage, including a likely new record crest on the Illinois River at Morris.

Overall, May total precipitation ranged from over 9 inches in northeast Illinois to just under four inches in southeast and far western Illinois. Most of the state received between 100 percent and 150 percent of normal May precipitation, while the Chicagoland area received over 200 percent of normal. Only southeast Illinois, from Washington to Massac Counties, was drier than normal but still received around 90 percent of normal May precipitation.

The statewide average total May precipitation was 5.41 inches, which is 0.81 inches more than the 30-year normal and 25th wettest on record.

May ended a wetter than average climatological spring, with most areas of the state receiving between 100 percent and 125 percent of normal precipitation. Interestingly, the statewide average spring total precipitation has been above the 30-year normal only 5 times in the past 10 years: 2020, 2019, 2017, 2013, and 2011. However, the average departure from normal in those five years was 4.75 inches, whereas the average departure from normal on the five drier than normal springs was only -1.38 inches. This means that although we’ve had an even split between drier and wetter than normal springs over the past decade, the wet springs have been very wet.

Although only trace snowfall occurred this last month, most areas in northern Illinois experienced more snowfall than average in the climatological spring. Spring snowfall totals ranged from less than an inch in most of central Illinois to over six inches along the Interstate 80 corridor.

Outlooks

Short-term 8- to 14-day outlooks from the NOAA Climate Prediction Center show elevated odds of both above normal precipitation and above normal temperatures across the state.

Longer-term outlooks for climatological summer show continued elevated odds of wetter than normal conditions, but equal chances of above normal, normal, and below normal temperature.