Category: Cold

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October: Heat to Snow

We saw highly variable temperatures across the state this month, with record-breaking heat in the early part of October, and record-breaking cold in the latter part. The preliminary statewide October average temperature was 53.7 degrees, less than 1 degree below our 30-year normal. Temperatures were near normal in eastern Illinois, and between 2 and 6 degrees below normal across western Illinois. Preliminary data suggest October was considerably wetter than normal for the entire state. The statewide average October precipitation total was 5.20 inches, approximately 2 inches above the 30-year normal. The wet deviations were particularly large in the northern and southern reaches of the state.

Data are provisional and may change slightly over time

Temperature Variability

Record-breaking high temperatures persisted from September into early October. Average temperatures during the first four days of October were 10 to 14 degrees above normal in the southeast part of the state, and 3 to 8 degrees above normal for the northwest part (see map below). Maximum temperatures broke 90 degrees and minimum temperatures remained in the 70s for several days in southern Illinois. Stations in Saint Clair and White Counties reached 96 degrees on October 2.  Between October 1 and October 4, 46 daily high maximum temperature records and 73 daily high minimum temperature records were broken across Illinois, according to the NOAA National Centers for Environmental Information (NCEI). Additionally, 9 stations broke their all-time October high maximum temperature records, and 13 stations broke their all-time October high minimum temperature records. In one particularly extreme event, the October 1 nighttime minimum temperature at Kaskaskia Lock and Dam in Randolph County was 72 degrees, 10 degrees above the previous daily record and 2 degrees above the all-time October minimum temperature record at that station.

Seasonable temperatures ensued after the heat was broken toward the end of the first week of October. Temperatures from October 5 to October 27 were near normal in eastern Illinois and between 3 and 6 degrees below normal for western Illinois (see map below). Nearly all the state experienced the first fall frost event in the second week of October. Nighttime minimum temperatures dipped below 32 degrees as far south as Pope County and below 28 degrees in Warren and Jo Daviess Counties.

The heat wave that started the month was matched by a strong burst of cold air to close out the month. Temperatures between October 28 and October 31 were 8 to 16 degrees below normal. Similarly, 48 daily low maximum temperature records and 12 daily low minimum temperature records were broken across Illinois over the last four days of the month. Nighttime minimum temperatures dropped below 30 degrees as far south as Pulaski County. The lowest minimum temperature observed in October was 14 degrees in both Carroll and Lee Counties on Halloween night. Halloween was also the coldest on record for 51 stations across Illinois.

The temperature contrast between the start and end of this month may be best summarized in the graph below, which shows daily maximum and minimum temperatures at Springfield Abraham Lincoln Capital Airport this last month. There was a 57° difference between daily maximum temperatures on October 1st and October 31st in Springfield, both of which broke daily records. In fact, 10 Illinois COOP stations broke their daily high maximum temperature record on October 1st and their daily low maximum temperature record on October 31st.

Precipitation

October precipitation was above normal for virtually all of Illinois. The statewide total precipitation in October was 5.20 inches, approximately 2 inches more than the 30-year normal. Areas of far northern and southern Illinois received over 7 inches of rainfall in October. CoCoRaHS observers in New Lenox in Will County and Riverwoods in Lake County recorded over 12 inches of precipitation in October. Expressed as a percent of the long-term mean, areas of northeastern Illinois received more than 200 percent of mean October precipitation, and a broad swath of southern Illinois received over 150 percent of mean October precipitation (see maps below). Significant rainfall helped improve drought conditions in southern Illinois. The latest U.S. Drought Monitor map – current as of October 29 – shows no drought in Illinois for the first time since early August.

The cool down that came at the end of this month brought a variety of precipitation, including snowfall and some snow accumulation in northern and western Illinois. Total snowfall accumulation over the last week of October ranged from over 8 inches in northwestern Illinois to just over a tenth of an inch as far south as Nokomis in Montgomery County. The highest October snowfall total, 8.5 inches, was in Orangeville in Stephenson County. Although late October is early for the first snowfall in Illinois, it is certainly precedented. The map below shows the date of the earliest recorded snowfall (> 0.1 inch) at COOP stations across the state.

Short- and Long-Term Outlooks

Short-term 8- to 14-day outlooks from the Climate Prediction Center show strongly elevated odds of below normal temperatures persisting into the first couple of weeks of November. Concurrently, probabilities are elevated for below normal precipitation out to 14 days, as drier weather is likely to prevail following the first winter storm of the season in Illinois.

Longer-term outlooks for November also show increased odds of below normal temperatures and increased odds of above below normal precipitation. Winter (December–February) outlooks show greater odds of a wetter than normal winter.

 

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Illinois First Fall Freeze Climatology

Due to significant planting delays across most of the Midwest this year, I have heard many concerns about an early fall freeze and its potential effects on immature crops. Most plants experience damage from a hard freeze or “killing freeze”, which is typically designated by a daily minimum air temperature at or below 28°F. Even in normal growing seasons, an early fall freeze can cause considerable impacts and yield losses for crops. Delayed planting, as was the case this season, increases the risk of freeze damage because crops are less mature going into our normal fall freeze time.

The maps and summary below show first fall freeze dates across Illinois using temperature observations over the period 1979 to 2018. The maps show the earliest and latest fall freeze dates over this 40-year period, as well as the median date, which represents the middle value in the range of dates. The median is preferred over the mean or average, as it is less sensitive to very early or very late freeze dates. Also shown are the 10th (1 in 10 years) and 90th (9 in 10 years) fall freeze dates. All station temperature data were provided by the Midwest Regional Climate Center (https://mrcc.illinois.edu); the shaded areas between stations on the map were interpolated and do not represent actual observations.

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. An early freeze anomaly can be seen at the Mt. Carroll station (Carroll County), which experienced a minimum temperature of 27°F on September 7, 1988. Interestingly, the observed all-season Illinois minimum temperature record was broken earlier this year at the Mt. Carroll station (-38°F).

Tenth percentile first fall freeze dates (i.e., 1 in 10 years) range from early October in northwest and central Illinois to mid- to late October in southern and eastern Illinois.

Median first fall freeze dates range from mid- to late October in northwest and central Illinois to late October/early November in southern and northeastern Illinois. Approximately half the years between 1979 and 2018 experienced the first fall freeze before the median dates. Also, the median dates map clearly shows the effects of the developed Chicagoland area on nighttime minimum temperatures. The median first fall freeze date at Chicago Midway is 10 to 15 days later than in some of the collar counties.

Ninetieth percentile first fall freeze dates (i.e., 9 in 10 years) range from early November in northwest and central Illinois to mid- to late November in southern Illinois. Based on the 40-year climatology, one could say that there is a 90% chance that the first fall freeze on any given year will occur on or before the dates in the 90th percentile map.

Finally, the latest first fall freeze dates across the state range from mid- to late November in northwest Illinois to early to mid-December in southern Illinois.

Note that air temperatures can vary considerably on smaller or micro-scales. For example, plants near heated buildings or other development can be spared when minimum temperatures dip below the 28°F threshold in the countryside. More information and useful freeze products are provided by the Midwestern Regional Climate Center as part of their Vegetation Impact Program (https://mrcc.illinois.edu/VIP/indexFFG.html).  Higher quality, full-page maps can be accessed by clicking the following links:

10th_Percentile 90th_Percentile Earliest Latest Median

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Late April 2019 Snow Event

4/29/2019 – A strong and considerably cold low pressure system took aim at Illinois on Saturday April 27th, bringing a large area of widespread precipitation and unseasonably cool weather to a majority of the state.  The greatest impacts were felt in Northern Illinois, where temperatures were cold enough to support a variety of wintry precipitation types. This included a late season and uncommon accumulating snow for many from Chicago and points north and west.

According to the National Weather Service, the final snowfall total at Chicago O’Hare of 2.5 inches was the latest accumulating snowfall for the city since 1989, and the latest 2+ inch single calendar day snowfall in station history. The two day period of May 1-2, 1940 saw 2.2 inches.

Typically, Chicago O’Hare can expect 1.2 inches of snow in April, with the record for the month being 11.1 inches which was set in April 1975.  To date, April 2019 has seen 7.9 inches.

The official snowfall total in Rockford of 3.7 inches was the latest accumulating snow since 1994, and ranks as the latest 2+ inch snow event on record.  Beating out April 23, 1986 with 3.8 inches, and April 23-24, 1910 with 2.5 inches.

A station in Stockton (Jo Daviess County) reported 6.0 inches of snow with this event.  See the interpolated map below of preliminary snowfall accumulation across the state. (Note: Locally higher snowfall reports were common)

Behind this system, and with the aid of fresh spring snow cover, temperatures dropped across Northern Illinois, with Rockford tying the record daily low temperature on the morning of April 28th with a reading of 28°F.

In total 26 weather stations reported temperatures below freezing on the morning of April 28th, with Stockton (Jo Daviess County) reporting the lowest weekend reading in the state with just 21°F.

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Previous records slashed with monumental cold conditions in Illinois

CHAMPAIGN, Ill., 1/31/19: Illinois has been experiencing some of the coldest weather that has been seen in decades and, in some locations, ever.

The National Oceanic and Atmospheric Administration cooperative weather observer at Mt. Carroll in northeastern Illinois reported a temperature of -38 degrees on the morning of Jan. 31.

“The temperature in Mt. Carroll may be a new state record, if officially confirmed,” said Brian Kerschner, spokesperson for the Illinois State Climatologist’s Office at the Illinois State Water Survey.

When it appears that a state record temperature may have been broken, a state climate extremes committee reviews the observations to assess its validity. This team typically includes the State Climatologist’s Office, the Midwestern Regional Climate Center, and federal climate experts, such as from the National Weather Service and the National Center for Environmental Information.

Most of Illinois has been in the deep freeze for the past two days. With a recording of -31 degrees, Rockford broke their all-time low temperature, which was previously -27 degrees on Jan. 10, 1982, the National Weather Service Forecast Office in Chicago reports.

During the mornings of Jan. 30 and 31, numerous locations in northern Illinois reported temperatures in the -20s and some locations going below -30 degrees. Minimum temperatures were below 0 degrees throughout most of the state, except for the southern regions. Daily mean temperatures were generally 15 to 20 degrees lower than the 30-year average temperature.

Some other notable temperatures included -35 degrees at Elizabeth, -32 at Galena, -30 at Rochelle, -33 at Aledo, -26 at DeKalb, -22 at Joliet, -21 at Galesburg, -17 in Champaign, and -16 in Decatur.

In the southern counties, Cairo reached 12 degrees and Carbondale and Rosiclare saw 4 degrees.

Numerous schools, businesses, and government offices were closed throughout the state because of the dangerously cold wind chills.

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Note: Data for this press release were obtained from the Office of the Illinois State Climatologist, Midwestern Regional Climate Center, and National Weather Service Offices responsible for the state.

Media Contacts: Brian Kerschner, (217) 333-0729, statecli@isws.illinois.edu

David Kristovich, head of the Climate and Atmospheric Science Section, Illinois State Water Survey, (217) 333-7399, dkristo@illinois.edu