Category: Cold

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Mild October Had a Chilly End

The preliminary statewide average October temperature was 56.7 degrees, 1.9 degrees above the 1991–2020 average and tied for the 31st warmest on record going back to 1895. The preliminary statewide total October precipitation was 3.46 inches, 0.20 inches above the 1991–2020 average and the 41st wettest on record.

Data are provisional and may change slightly over time.

Mild October Temperatures with Some Extremes Thrown In

For my money, October is the best weather month of the year. The summer heat mellows out and mixes with the first real shots of chilly air. We had both summer- and winter-like weather last month, but a little more of the former than the latter. As the daily temperature departures from normal in Mt. Vernon show, most October days had above average temperatures, including a few days in the final week of the month that were 15 to 20 degrees above normal (Figure 1).

Figure 1. Daily October average temperature departures in Mt. Vernon.

October average temperatures ranged from the low 50s in northern Illinois to the low 60s in southern Illinois, between 1 and 2 degrees above normal in most places (Figure 2). Most of the state saw high temperatures in the upper 80s or low 90s in early October, including 94 in Quincy and 90 in Hoopeston. The brief taste of winter in the last few days of the month brought widespread low temperatures in the low 20s, including 21 degrees in Mt. Vernon and 23 in Bloomington.

The warm periods last month broke 31 daily high maximum temperature records and an incredible 66 daily high minimum temperature records. Aledo in Mercer County broke its all-time October high temperature records last month with a high temperature of 93 degrees on October 3. The cold end to the month broke 18 daily low maximum temperature records and 37 daily low minimum temperature records. 

Figure 2. Maps of (left) October average temperature and (right) October average temperature departures from normal.

Overall, the preliminary statewide average October temperature was 56.7 degrees, 1.9 degrees above the 1991–2020 average and tied for the 31st warmest on record going back to 1895.

Dry Start and Wet End to October

The month-end precipitation totals across the state do not tell the entire story of October precipitation. The first two-thirds of the month were somewhat to very dry across the state, as most places were 1 to 2 inches drier than normal through October 24. More active weather brought multiple rounds of rain–and some snow–to Illinois, raising month-end totals near or above the 30-year normals. As Figure 3 shows, Peoria picked up more rain in the last 7 days of October than in the first 24 days, and the month ended just slightly wetter than normal in Peoria. The dry start to the month was ideal for fall harvest, while the wetter end of the month helped improve dry conditions that prevailed in August and September.

Figure 3. Plot of October daily precipitation accumulation in Peoria (shaded area) versus the normal daily accumulation (black line).

October precipitation ranged from nearly 6 inches in northeast Illinois to less than 2.5 inches in the St. Louis Metro East. Most of the state north of Interstate 70 was 1 to 2 inches wetter than normal, while areas farther south were just slightly drier, up to 2 inches drier than normal (Figure 4).

Figure 4. Maps show (left) October total precipitation and (right) October precipitation departure from average.

Last month wasn’t an extremely wet or extremely dry month anywhere in the state; however, the wetter conditions in the northern half of the state helped reduce drought extent from 23 percent of the state on October 3 to 7 percent on October 31. Small areas of western and southern Illinois remain in moderate drought due to longer-term precipitation deficits. For example, the first 10 months of the year have had the third lowest precipitation total in Quincy with 20.17 inches, around 10 inches below average. While topsoil moisture in western Illinois has improved from the wetter end of October, deeper layer moisture and water table levels remain less than ideal because of the long-term dryness.

Overall, the preliminary statewide total October precipitation was 3.46 inches, 0.20 inches above the 1991–2020 average and the 41st wettest on record.

A Spooky, Winter-y Halloween

Like all the horror film characters who inevitably run back into the haunted house, mild October temperatures lulled all of us into a false sense of security. The intense cold, wind, and for some folks, snow on Halloween was more shocking than the sound of a revving chainsaw. Halloween nighttime temperatures dipped into the low 20s and high teens across much of the state, including 18-degree lows in Monmouth and Rochelle (Figure 5). The daytime high temperatures on Halloween were the coldest on record in several spots across the state, including at Chicago’s Midway airport (37 degrees) and Mt. Vernon (43 degrees). The low temperatures on Halloween night also set records in many places, including in Kankakee (24 degrees) and Olney (22 degrees). In fact, this year was the coldest Halloween in Olney since observations began there in 1896.

Halloween snow in northern and central Illinois is not necessarily a rarity but happens once every 4 to 6 years. Measurable snow, with totals exceeding 0.1 inches, was recorded in much of the state north of Interstate 74, with totals as high as 1.5 inches in Mundelein and 0.9 inches at O’Hare (Figure 5). For reference, the average first measurable snow comes in the third or fourth week of November for most of northern and central Illinois, so this year’s event came about two to three weeks early. It’s important to note that an early snowfall does not mean we will necessarily have a very snowy winter… but this State Climatologist can hope.

Figure 5. Maps of (left) nighttime low temperatures and (right) total snowfall on Halloween. Maps are from the Lincoln National Weather Service Office: https://www.weather.gov/ilx/illinois-daily.

Outlooks

November doesn’t get the love it deserves, because it is so often associated with cloudy, blustery weather. But November typically gives us a great mix of pleasant and not-so-pleasant weather and portends the beautiful winter season in Illinois. The latest Climate Prediction Center outlooks lean into El Niño with higher chances of below normal November precipitation. November temperature outlooks are more mixed, with equal chances of warmer and colder weather this month (Figure 6).

Figure 6. Maps of (left) temperature and (right) precipitation outlooks for November.

Meanwhile, NOAA leans even more heavily into El Niño for climatological winter (December–February) outlooks (Figure 7). The outlooks show highest chances of above normal temperatures in winter, with mostly equal chances of above or below normal precipitation.

Figure 7. Maps of (left) temperature and (right) precipitation outlooks for winter (December–February).
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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.