Annual Averages

Averages by Report

The table below shows the average number of severe weather reports in our local area broken into several groups:

• Any Severe Weather (Tornado, Damaging Winds, and/or Damaging Hail)
• Tornadoes
• Damaging Winds
• Damaging Hail 

10-year average range: 2011-2020   |   20-year average range: 2001-2020   |    30-year average range: 1991-2020   |   Database average range: 1950-2020

Averages by Days with at least One Report

When considering days with a report of severe weather, individual hazards are treated independently to provide a representative climatology of days on which a specific hazard may occur. However, since a day on which damaging winds was reported may overlap with a report of hail on the same day (or both non-significant or significant reports of the same hazard), each respective group cannot be added together to get the "Any Severe Weather Days" metric, as this would inflate the statistics. Instead, the "Any Severe Weather Days" statistics were created by identifying days on which any report of severe weather was received, thereby removing duplicates. Such a treatment of data is different when considering bulk report statistics (as was done in the previous section). 

Since all days with any severe weather reported are treated equally regardless of the number of reports (e.g. a day with 100 reports is treated the same as a day with 2 reports), the table below provides the best "snapshot" of the frequency of severe weather as well as individual hazards in our local area. More specifically, the 20- and 10-year averages are the most representative. 

10-year average range: 2011-2020   |   20-year average range: 2001-2020   |    30-year average range: 1991-2020   |   Database average range: 1950-2020

Time of Year

In this section, the climatology of severe weather by time of year in the NWS Chicago forecast area is discussed. Note the following: 

• To avoid any skew introduced by severe weather outbreaks, a framework of using days with at least one severe weather report (also known as "Severe Weather Days") will be analyzed instead of bulk report statistics. In such a framework, any day with at least one report of severe weather is treated equally, and a specific month with an abnormally large number of severe weather reports will not bias the statistics. 

• In addition, the majority of plots represent data collected between 2001 and 2020 to avoid a low bias in severe weather reports that would be introduced if including data prior to the year 2001. The exception is at the bottom of the page looking at the percent of days with at least one report to provide context of the 1950 to 2020 period of record. While minor differences may exist based on the period of record shown, using data only from 2001 to 2010 provides a representative look at local severe weather climatology. For more information, please see the "Averages" and "Trends" tabs above. 

20-Year Averages in Days with Severe Weather Reports (Tornado, Hail, and/or Wind)

​Plots use the database of reports from 2001 to 2020. 

• The "Severe Weather Season" runs from April through September with an average of at least one day per month with at least one report of severe weather (~90% of all days in the database). 

• The most active months for severe weather are June and July with an average of at least 6 severe weather days. 

• The most common type of severe weather is damaging winds, with an average of 23 days per year with at least one report. 

• The least common type of severe weather is significant tornadoes with an average of one day per year with at least one report. (No complaining here!)

• There is an average of 10 days per year with at least one flash flooding report. 

20-Year Average Hazards Breakdown

​Plots use the database of reports from 2001 to 2020. 

Tornadoes

(Any day with at least one report of a tornado)

Tornadoes are most frequent in May and June with an average of one day per month with at least one report. A  secondary spike in days with at least one tornado report is in August. 

Damaging Hail

(Any day with at least one report of hail >1" in diameter)

Damaging hail is most frequent in May and June with an average of two to three days per month with at least one hail report. 

Damaging Winds

(Any day with at least one report of damaging winds >58 mph)

Damaging winds are most frequent in June and July with an average of five to six days per month with at least one damaging wind report. 

Flash Flooding

(Any day with at least one report of flash flooding)

Flash flooding is most common from May through August with at least one to two reports of flash flooding per month.

Generally speaking, hazards during the severe weather season transition from tornadoes, hail, damaging winds as the year progresses. However, all hazards can occur at any point throughout year. 

 Breakdown of Significant and Non-Significant Days

​Plots use the database of reports from 2001 to 2020. 

When treating days with significant or non-significant reports separately, June stands out as the month with the most frequent significant severe weather with around 2 days with at least one report every year. Altogether, significant severe weather is most frequent from May through July. 

(Note that each distribution is treated separately but plotted on the same axes. Accordingly, the two distributions cannot be compared to each other in individual months to make an assessment as to which "type" of, say, damaging wind, is more likely in a certain month.  In other words, number of days with at least on report used to generate each  dataset is different.)

1950 to 2020 Severe Weather Days Breakdown 

​Plots use the database of reports from 1950 to 2020. 

When looking at the entire database of days with at least one severe weather report, June sticks out as the month with the most frequent severe weather for most severe weather types (nearly 25% of all days for significant and non-significant severe weather!)

Time of Day

In this section, the climatology of severe weather by time of day in the NWS Chicago forecast area is discussed. Note the following: 

• The period of record used is from 1950 to 2020, except for flash flood data which starts in 1996. 

• The plots will show the percent of total reports rather than bulk report statistics. 

• Daylight Savings Time was not taken into account. So, all time shown is in local time. 

• For plots that discriminate Significant from Non-Significant reports, each distribution is treated separately but plotted on the same axes. Accordingly, the two distributions cannot be compared to each other at individual hours. For example, it would be incorrect to say "Significant Severe Weather is more likely at 4 PM than non-significant severe weather since the hatched bar is higher than the solid bar" since each bar is from a different dataset (e.g. the total number of reports used to generate each percentage is different). 

Any Severe Weather (e.g. Tornado, Damaging Winds, or Damaging Hail)

In general, severe weather is most frequent between 1 and 9 PM local time, regardless of the intensity or magnitude of the severe weather. A secondary "spike" in reports occurs near sunrise, which appears to be driven by damaging winds (see below). Accordingly, the "sunrise spike" may be related to decaying thunderstorm complexes originating in the Plains moving into Illinois toward daybreak, which is a common regime during the summer months. 

Tornadoes

Tornadoes are most common between 12 and 10 PM local time, and especially between 4 and 8 PM, regardless of intensity. 

Damaging Winds

Damaging winds are most common between 2 and 8 PM local time, though the tail of the distribution extends outward into nighttime hours likely owing to a common summertime regime of nocturnal mesoscale convective systems. 

Damaging Hail

Damaging hail is most frequent from 1 to 9 PM local time, which coincides strongly with daylight hours. Interestingly, significant hail tends to occur slightly later than non-significant hail, which may relate to a tendency for a supercell storm mode most often in the late afternoon hours coinciding with peak heating, and "marginal" severe storms earlier in the afternoon. Interestingly, damaging hail is relatively unlikely during nighttime hours. 

Flash Flooding

Finally, the distribution of Flash Flood reports is vast with only a modest peak during the evening hours. In other words, flash flooding is more or less equally likely at any time of day! 

Trends in Severe Weather Reports

In this section, trends in the climatology of severe weather in the NWS Chicago forecast area is discussed. Note the following: 

• To avoid any skew introduced by severe weather outbreaks, a framework of using days with at least one severe weather report (also known as "Severe Weather Days") will be analyzed instead of bulk report statistics. In such a framework, any day with at least one report of severe weather is treated equally, and a specific month with an abnormally large number of severe weather reports will not bias the statistics. 

• Since the frequency of severe weather can vary significantly from year to year, 10-year averages of days with at least one severe weather report will be shown. Such a framework allows for a representative signal for severe weather trends without rapid fluctuations. 

Any Severe Weather Type (Tornado, Damaging Wind, and/or Damaging Hail)

On average, our area sees severe weather 25 to 30 days out of the year. In addition, our area experiences about 10 days per year with Significant Severe Weather with an enhanced threat to life and property. We believe there are four factors to the apparent rise in the frequency in reported severe weather in our local area from the 1990s to 2000s:

• The rise of the internet, social media, and mobile phone usage to provide the ability to report severe weather to the NWS through the early 2000s.

• A gradual increase in population in the Rockford and Chicago areas translated to more people reporting severe weather.

• An increase in "Storm Spotter" training and other education by the NWS

• The number of days with at least one tornado as well as significantly damaging winds or hail have remained relatively constant, suggesting the apparent rise in the reports of damaging winds and hail are largely driven by Non-Significant severe weather (See plots below).

While it is possible the frequency of reported severe weather has indeed increased from the 1990s to 2000s, it is more likely factors listed above are actually most responsible. 

Trends in Tornado Days

In general, tornado days has remained relatively constant in our area, with between four and six days per year with at least one tornado.

Interestingly, the number of days with a Significant Tornado, or tornado with a damage rating of F/EF2 or greater on the Enhanced Fujita Scale, have decreased from the 1970s and 1980s to 2010s from about two days per year to around one. We believe improvements in building codes and construction practices may help explain the apparent downward trend in Significant Tornadoes especially considering the frequency of days with Non-Significant Tornadoes or Any Tornado have remained constant (if not slightly increased). 

In the entire severe weather database (1950-2020), the number of days with at least one tornado have ranged from zero (1952, 1953, 1964, 2005) to ten (1965, 1975, 2015). 

Trends in Damaging Wind Days

Our local area typically experiences 20 to 25 days with at least one report of damaging winds, and about 6 or 7 days per year with at least one report of Significant Damaging Winds with wind speeds of 75 mph or greater.

In the entire severe weather database (1950-2020), the number of days with at least one report of damaging winds has ranged from zero (1950-1954,1993) to 32 (2006).

Trends in Hail Days

Our local area typically experiences around 15 days with at least one report of damaging hail, and about two to three days per year with at least one report of Significant Hail with a diameter of 2" or larger. We suspect the number of days with damaging hail may be higher than shown since hail that falls in rural areas is often underreported. 

In the entire severe weather database (1950-2020), the number of days with at least one report of damaging hail has ranged from zero (1950-1954,1959,) to 22 (2006). 

Trends in Flash Flood days

Our local area typically experiences around 10 days with at least one report of flash flooding and has remained relatively steady. Since the period of record for flash flooding starts in 1996, data are only shown from 2005 onward coinciding when a 10-year average is available. 

In the Flash Flooding database (1996-2020), the number of days with at least one report of flash flooding has ranged from one (1997, 2005) to 16 (2007).

Specific Hazard Information

In this section, bulk report statistics are provided allowing for an idea of the relative rarity of specific report types (e.g. 2.00" hail or an EF-4 tornado). The period of record used in this section runs from 1950 to 2020, and may be updated after subsequent years. 

Tornado Reports

• Four out of five tornadoes (~80%) in our area have a rating of F/EF-0 or F/EF-1, which are classified as non-significant. Hence, the chance that any given tornado report is an EF-2 or greater is only about 20%. 

• The chance that any given tornado report is rated an F/EF-3 or greater is only about 5% (about 1 in 20).  

• There has been only one F/EF-5 rating in our area -- the August 30, 1990 Plainfield Tornado. We're glad there haven't been more. 

Damaging Hail Reports

​

• Roughly one in two reports of hail in our area are for hailstones that are 1.00" in diameter, or roughly the size of a quarter. The 2nd most frequent hail size reported is about the size of a golf ball, or roughly one in four reports. We      believe the actual frequency of golf ball size hail is likely lower than actually reported, since "golf ball" is an easy-to-remember size even if the hail is    actually smaller. 

• Roughly 1 in 20 hail reports are for hailstones that are larger than 2.00" in diameter. Accordingly, there is about a 95% chance that report of hail is for a hailstone less than 2.00" in diameter. 

• The largest hailstone reported in our area was about 4.75" in diameter, or the size of a softball, and fell on June 10, 2015 near Minooka. The hailstone is considered the record largest hailstone to ever be reported in the state of  Illinois (More information)!

Damaging Wind Reports

• Four in five (~80%) damaging wind reports are associated with wind speeds less than 70 mph, while only about 4% are associated with wind speeds greater than 80 mph. 

• Reports with wind speeds greater than 100 mph are relatively rare (0.4%, or about 1 in 250 reports).

• The highest wind speed report in our area is listed as 130kt, or ~150 mph, on June 29, 1990, when a microburst impacted Streamwood in northwestern Cook County and killed one person. We believe the wind speed assigned at the time was based on the original Fujita Scale, which overestimated wind speeds. (StormData Entry, local news clip)

Severe Weather Rankings

Have you ever wondered in which year our local area experienced the most severe weather? Or, the largest tornado event in our area? Check out the links below to be brought to individual webpages. 

Annual Rankings by Days with at least One Severe Weather Report 

Annual Rankings by Number of Reports 

Annual Top-5 Events Lists