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).

Cool Fall Temperatures Continued in October

October was much cooler and slightly drier than average across Illinois. The preliminary statewide average October temperature was 51.8 degrees, 2.6 degrees below the 30-year normal, and tied for the 19th coolest on record. Preliminary statewide average total precipitation for August was 3.03 inches, 0.21 inches less than the 30-year normal, and the 54th wettest on record.

Data are provisional and may change slightly over time

October Temperatures

Cool weather persisted from the end of September to the first week of October across the state. As the plot below of daily temperature departures from normal in Moline shows, the second week of October was 5 to 10 degrees warmer than average across the state. However, this was followed by a predominantly cooler than average second half of the month.

Eleven daily high maximum temperature records and eight daily high minimum temperature records were broken last month. Concurrently, 66 daily low maximum temperature records and 19 daily low minimum temperature records fell in October. The cool, cloudy day of October 27 broke the month’s low maximum temperature records at seven locations in Illinois. This included the 34-degree high in Kewanee in Henry County, which broke the previous October low maximum temperature record of 36 degrees, set just last year on Halloween.

The maps below show October average temperatures and their departure from the long-term average. October average temperatures ranged from the high 40s in northwest Illinois to the high 50s in southern Illinois, between 2 and 5 degrees below average. The preliminary statewide average October temperature was 51.8 degrees, 2.6 degrees below the 30-year normal and tied for the 19th coolest on record. This follows the cooler than normal months of August and September statewide.

Harvest Fires

The combination of high temperatures, very low humidity, and strong winds created a significant fire risk during the second week of October. The plot below shows a daily mean surface vapor pressure deficit, a measure of atmospheric humidity, at Springfield between January 1 and October 18 of this year (red line) compared to all previous years (gray shaded area). A vapor pressure deficit has been linked to wildfire risk, with higher vapor pressure deficit values indicating a higher risk of wildfire outbreak.

The very high values of vapor pressure deficit in Springfield, near record values on October 14, indicated a warm, dry atmosphere. Existing drought and crops drying down in fields provided ample, dry fuel for wildfire potential. Indeed, several small- to medium-sized field fires broke out across central and south-central Illinois during the second week of October.

First Snow of the Season

The map below shows the climatological earliest first measurable snowfall on record at stations across the state. The dates range from early to mid-October in northern Illinois to mid-November in southern Illinois.

The map below shows the climatological earliest first measurable snowfall on record at stations across the state. The dates range from early to mid-October in northern Illinois to mid-November in southern Illinois.

A Wet End to the Month in Southern Illinois

The first half of October was very dry for the southern half of the state, continuing a very dry September. However, around the middle of the month the atmosphere moved into a pattern more conducive to bringing precipitation into southern Illinois. Areas that received less than a tenth of an inch of rainfall in previous weeks saw widespread 4- to 9-inch precipitation totals during the final two weeks of October.

Belleville, for example, recorded its third wettest October with a total of 7.38 inches, following its wettest August and its second wettest July on record. At the end of October, Belleville was only 4 inches away from its wettest year on record, which was 2008 with 56.83 inches of rain.

Drought Continues in Central Illinois

In contrast to the wet end of October in southern Illinois, much of central Illinois ended yet another month with below average precipitation. The maps below show 30- and 180-day precipitation deficits across the state. Most of central and western Illinois received between 1 and 3 inches below average precipitation in October, adding to existing precipitation deficits from dry months in August and September.

Total six-month precipitation deficits in central Illinois exceed 7 inches in Fulton and Mason Counties and 8 inches in parts of Logan and Macon Counties. At Mount Pulaski in Logan County, for example, the June 1 to November 1 precipitation deficit of 8.47 inches is the third largest on record, smaller only than 1988 and 1893. However, it should be noted that Mount Pulaski and the surrounding areas of Logan and Macon Counties received more precipitation in the first four months of this year than in all of 1988. Therefore, drought conditions, although severe, are not nearly to the extent of those in 1988.

The most recent, October 27th version of the U.S. Drought Monitor showed continued widespread moderate drought across central Illinois in response to the rainfall deficits and below normal soil moisture and streamflow.

Outlooks

Pleasant weather is in store for the state during the first week of November. However, the Climate Prediction Center’s 8- to 14-day outlooks indicate an imminent pattern change in the second week of November, with odds moving back to below average temperatures, especially to the northwest, and above average precipitation statewide.

The most recent one-month outlooks for the entire month of November continue to show a strong La Niña influence, with elevated odds of warmer than normal conditions throughout the central U.S. and drier than normal weather in the south.

Finally, the three-month outlooks for climatological winter, December–February, show equal chances of warmer or colder than normal conditions in Illinois. However, the odds increase for wetter than normal conditions later in winter.

Warm, Dry December Concludes a Cold, Wet Year

December temperatures were well above the long-term average across the state, breaking dozens of local daily maximum and minimum temperature records. The preliminary statewide December average temperature was 35.2 degrees, about 5 degrees above the 1981-2010 normal and the 18th warmest on record. Preliminary data show December was drier than average for most of the state. The statewide average December precipitation total was 2.03 inches, 0.66 inches below the 30-year normal.

Data are provisional and may change slightly over time

Warm Weather

Temperatures during the first half of December were very close to average. This was followed by a brief period of well below average temperatures caused by cold air incursion from the north on the back of a strong upper atmosphere trough to our west. On December 20 the predominant wind direction changed to southwesterly, bringing warm, dry air into the region. Temperatures between December 20 and 29 ranged from 5 to 25 degrees above normal across the state. In total, 104 daily high maximum temperature records and 27 daily high minimum temperature records were broken over this time period, including a few dozen records on December 25. In fact, it was the warmest Christmas day at 68 stations across the state. As shown in the figure below, the daily average temperature in Decatur in Macon County on Christmas was nearly 20 degrees above the 30-year normal.

The station in Elgin (Kane County) broke its previous Christmas day high maximum record by 10 degrees. The highest temperature recorded in the state was 70 degrees on December 26 in Wayne County and again on December 29 in Pope County. The lowest temperature was -4 degrees on December 15 in Rock Island County.

A shift in the upper atmosphere and the passage of a cold front late in the month allowed temperatures to moderate. December average temperatures ranged from the low 30s in northern Illinois to the mid-40s in southern Illinois. Monthly average temperature departures ranged from 7 degrees above the long-term mean in northwestern Illinois to just over 1 degree above average in south-central Illinois.

The preliminary 2019 statewide average December temperature was 35.2 degrees, which was the 18th warmest December on record. December’s warm weather was an aberration in an otherwise colder than average 2019 in Illinois. Only three months this year–July, September, and December–exhibited a statewide average temperature above the 30-year normal.

Precipitation

December precipitation was below the long-term average for the entire state. Areas in far southern Illinois received 2 to 3 inches less than average in December, approximately 50 percent of normal December precipitation. The statewide average total December precipitation was 2.03 inches, approximately 0.66 inches below normal. This last month was the 50th driest December on record in Illinois and marked the second straight month of below average statewide precipitation. Preexisting wetness and reduced evaporative demand, typical for this time of the year, have prevented impacts from the prolonged dry conditions. Despite two straight months of well below average precipitation, streamflow and soil moisture were both near normal across the state.

Snowfall totals this last month ranged from less than a tenth of an inch in far southern Illinois to over 10 inches in south-central Illinois. A strong system came through in mid-December and brought several inches of snow to an area spanning the St. Louis Metro East to the Champaign-Urbana area. The highest 24-hour snowfall total was 5.6 inches in Lovington (Moultrie County) on December 17, although CoCoRaHS observers in Mascoutah in St. Clair County and Columbia in Monroe County both recorded 7.5 inches on December 17.

The December snowfall glut in south-central Illinois turned into snowfall deficits of 8 to 10 inches in northern Illinois. This last month was only the 10th December with 1 inch or less of snowfall in Stockton (Jo Daviess County). Despite the small snowfall totals this last month, the seasonal total snowfall was above average for most of the state between interstates 80 and 64. A broad area between Peoria and the St. Louis metro east received over 4 inches of above average snowfall, whereas the Chicagoland region has so far this season experienced a snowfall deficit of 4 to 6 inches.

Outlooks

Short-term 8-14-day outlooks from the NOAA Climate Prediction Center show strongly elevated odds of both above normal precipitation and above normal temperature.

Thirty-day outlooks show elevated odds of wetter and warmer than normal conditions to persist throughout January in southern Illinois. Outlooks for January through March and March through May continue to show elevated odds of above normal precipitation for the entire state.

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.