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.

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.

A typical February ends an otherwise atypical winter

February was slightly warmer and wetter than average across Illinois, bringing an end to a very warm climatological winter season. The preliminary statewide average February temperature was 31.2 degrees, 0.30 degrees above the 30-year normal and tied for 42nd warmest on record. Preliminary statewide average total precipitation for February was 2.13 inches, 0.07 inches wetter than the 30-year normal and tied for 46th wettest on record.

Data are provisional and may change slightly over time

Mostly Warm, Briefly Cold in February

The first two months of the climatological winter season were much warmer than average, with very few cold air incursions. As the temperature plot from Bloomington shows (below), February started consistently warmer than average, but multiple cold air incursions in the latter half of the month led to an overall near-normal temperature month.

Average temperatures in February ranged from the high teens in northwestern Illinois to the high 30s in southern Illinois. Average temperatures ranged within a degree of the long-term average for all but a southeast sliver of the state. The statewide average February temperature was 31.2 degrees, which is 0.30 degrees above the 30-year normal and tied for 42nd warmest on record.

The warm weather that started the month resulted in 115 daily high maximum temperature records and 12 daily high minimum records being broken across the state. One truly incredible departure was a 70 degree high temperature in Charleston in Coles County on Feb. 3. This was nearly twice the long-term average daily high temperature of 38 degrees for that calendar day in Charleston and 5 degrees above the previous record. Eleven days later the Charleston station broke its daily low maximum temperature, reaching only 15 degrees on Valentine’s Day. In total, cold air incursions in February resulted in 23 daily low maximum temperature records and 34 daily low minimum temperature records being broken, most of which were recorded on Valentine’s Day.

The highest temperature recorded in the state in February was 72 degrees in Effingham County, while the lowest temperature was -18 degrees in Jo Daviess County.

An End to a Very Warm Winter Season

February brought an end to climatological winter, which will be best known for persistently warmer than average weather. The preliminary December to February statewide average temperature was 32.6 degrees, which would result in this being the 12th warmest climatological winter season on record in Illinois.

This winter season’s mildness is characterized well by the Accumulated Winter Season Severity Index (AWSSI), which uses daily accumulation of points, based largely on temperature, to characterize winter season severity. The map below shows accumulated winter season severity as of March 3 for individual stations across the U.S. All stations in Illinois have so far exhibited a mild or moderate winter season (i.e., below average severity). AWSSI data and maps are created by the Midwestern Regional Climate Center (https://mrcc.illinois.edu/research/awssi/indexAwssi.jsp).

Wet South, Dry North

Winter precipitation depends strongly on the location of the subpolar jet stream, which often characterizes the track for winter storms. Last month storms tended to track south of the state, bringing the most rain to southern and south-central portions of Illinois. Total February precipitation ranged from over 10 inches in far southern Illinois to just over a tenth of an inch in northwestern Illinois. These totals range from over 150 percent of average February precipitation in southern Illinois to less than 25 percent of average in northern Illinois.

This was the driest February on record at Gladstone Dam in Henderson County, with only 0.04 inches of precipitation recorded, despite the extra February day this leap year. The wettest place in the state last month was Cairo in Alexander County, with 7.34 inches total precipitation.

Overall the preliminary statewide average total February precipitation was 2.13 inches, 0.07 inches wetter than the 30-year normal and tied for 46th wettest on record. As a point of reference, the statewide total precipitation for February 2019 was 3.21 inches and for February 2018 it was 4.88 inches.

While southern Illinois was experiencing persistent rain, most of northern Illinois was treated to abundant snowfall in February. Snowfall totals from last month ranged from just over 15 inches in Kane County to less than an inch along Interstate 70. Thanks to a mid-month storm, a small area of Mason, Cass, and Menard counties received 6 to 8 inches more than average snowfall in February, whereas most of the rest of the state north of Interstate 70 received near average to just a couple of inches above average snowfall last month.

Snowfall totals over the climatological winter season (December–February) are within 5 inches of the long-term average for most of the central and western portions of the state, while a snowfall deficit exists along the Indiana border and for most of southern Illinois. The largest departures from average are in the Chicagoland area, where winter season snowfall has been 15 to 20 inches less than average.

Outlooks

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

Longer-term 30-day outlooks show probabilities remain elevated for warmer than normal conditions across the state for the remainder of March, while elevated probabilities of wetter a wetter than normal March persist in the southern half of the state.

Spring flooding outlooks from the National Weather Service continue to show above normal to much above normal risks of flooding in the Mississippi River Basin, including 473 gauges with a 50 percent or greater chance of flooding between March and May (see below). Most gauges along the Mississippi River in Illinois have a 50 percent or greater chance of major flooding this spring, with gauges along the Illinois, Kaskaskia, Wabash, and lower Ohio rivers having a similar chance of minor to moderate flooding before May.

The elevated flood risk is due to: (1) very wet soils across the entire region; (2) large snowpack in the Upper Midwest; and (3) outlooks for a wetter than normal climatological spring season. For those wanting more information related to current hydrologic conditions and/or spring flood outlooks, I have included links to relevant partners and information below.

National Weather Service River Forecast Center: https://www.weather.gov/ncrfc/

NOAA Climate Prediction Center Soil Moisture: https://www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/US/Soilmst/Soilmst.shtml

NOAA Climate Prediction Center Seasonal Outlooks: https://www.cpc.ncep.noaa.gov/products/predictions/90day/

A Warm, Wet Start to 2020

Our first month of 2020 was quite a bit warmer and wetter than average across the state. The preliminary statewide average January temperature was 31.4 degrees, 5 degrees above the 30-year normal and the 17th warmest on record going back to 1895. The preliminary statewide average total January precipitation was 4.41 inches, 2.34 inches above the 30-year normal and the 9th wettest on record.

Data are provisional and may change slightly over time

Warm Conditions Persist from 2019

Following a warmer than average December, temperatures remained persistently above average for the first half of January, with only a short cold air incursion between Jan. 15 and 23. January average temperatures ranged from the mid-20s in northern Illinois to the high 30s in southern Illinois, and were 2 to 7 degrees above average across the state.

The majority of January days were warmer than average across the state. For example, the daily mean temperature in Dwight in Livingston County was below the long-term average in only 5 of the 31 January days. Dwight has only experienced 14 days with below average daily mean temperatures since the beginning of meteorological winter on Dec. 1.

Last month, 21 local daily high maximum temperature records and 36 daily high minimum temperature records were broken in Illinois. The highest temperature recorded across the state in January was 65 degrees in Cairo and Carbondale on Jan. 11. Overall, the preliminary January 2020 statewide average was 31.4 degrees, which was the 17th warmest January on record, following the 15th warmest December (2019) on record.

Temperatures since the start of meteorological winter on Dec. 1 are 2 to 7 degrees above the long-term average across the state. As an example, this winter through Feb. 2 is the 4th warmest on record in Moline with an average temperature of 31.1 degrees.  The three winters that were warmer through Feb. 2 in Moline are 2002, 2012, and 2016.

Warmer conditions have helped make for a mild winter so far across the southern half of the Midwest, as indicated by the Accumulated Winter Season Severity Index (AWSSI). AWSSI uses a daily accumulation of points, based largely on temperature, to characterize winter season severity. Despite an early snowfall this season, all AWSSI-monitoring stations in Illinois indicate a mild or moderate winter season so far (see map below). AWSSI data and maps are created by the Midwestern Regional Climate Center (https://mrcc.illinois.edu/research/awssi/indexAwssi.jsp).

A Few Days Make for a Wet January

January total precipitation was above normal across the state, with monthly precipitation totals ranging from 2 inches in northwestern Illinois to nearly 8 inches in south-central Illinois. The highest accumulation was along the Interstate 70 corridor between the St. Louis Metro East and Effingham. This region experienced nearly 300 percent of its average January total precipitation. Vandalia in Fayette County experienced its third wettest January on record with 7.99 inches, over 5 inches above the long-term average. As with most of the wettest parts of the state, Vandalia received 85 percent of its total January precipitation in just three days. Much of this was the result of a single winter storm that moved through the Midwest between Jan. 9 and 12.

Total precipitation between Jan. 10 and 12 broke three-day total January precipitation records at Charleston (5.85 inches), Taylorville (4.35 inches), Champaign (3.63 inches), and Marseilles (3.41 inches). This event resulted in flooding along many of Illinois’ larger rivers, including the Kaskaskia River breaking through the Vandalia levee and causing a temporary closure of highway 51. Rivers crested between 24 and 72 hours after the precipitation ended. As of the end of the month, minor flooding persisted in only the lower Illinois and Kaskaskia Rivers in the state (see map below).

Source: https://www.weather.gov/ncrfc/

Snowfall totals this last month ranged from less than a tenth of an inch in southern Illinois to over 12 inches in northwestern Illinois. There was a strong southeast-to-northwest January snowfall gradient across the state, resulting from a similar gradient in air temperature. The highest January snowfall total was 15 inches in both Stephenson and Bureau Counties. Rockford experienced the highest 1-day snowfall maximum across the state with 6 inches on Jan. 25.

January continued winter season snowfall patterns, with the western third of the state experiencing slightly above normal snowfall since Oct. 1, and the eastern two-thirds of the state experiencing near normal to well below normal snowfall over the same period. A broad area along the Indiana border from Cook County to Lawrence County has experienced between 50 and 75 percent of normal winter season snowfall, whereas most of the state south of Interstate 64 has experienced less than 50 percent of normal snowfall since Oct. 1. In contrast, snowfall totals across a broad area between Jo Daviess County and Madison County in western Illinois has experienced between 100 and 125 percent of normal season snowfall.

As is often the case, Illinois spans the continuum of conditions in the Midwest. Most areas northwest of us have experienced well above normal snowfall where areas to the southeast are in a snowfall deficit. In particular a region spanning the Dakotas to the Upper Peninsula of Michigan have experienced well above normal snowfall this winter season. Two CoCoRaHS stations in the Upper Peninsula have recorded over 210 inches of snowfall since Oct. 1. Heavy snowpack in the Upper Midwest (see map below) can often exacerbate unusually wet conditions in early to mid-spring here in Illinois, especially if a brief warm spell and/or heavy rain causes rapid snowmelt. Therefore Upper Midwest snowpack is an important variable to consider when planning for potential flooding-related impacts in the spring.

Source: https://www.nohrsc.noaa.gov/normals/

Outlooks

Short-term 8–14-day outlooks from the NOAA Climate Prediction Center show strongly elevated odds of above normal precipitation, with the highest probabilities across the southern two-thirds of the state. Short-term temperature outlooks show slightly elevated odds for a continuation of above normal temperatures across the eastern part of the state.

Longer-term 30-day outlooks show elevated odds of both above normal temperature and above normal precipitation in southern Illinois. Seasonal outlooks for spring (March to May) continue to show elevated odds of above normal precipitation across the state, with an equal chance of above normal, normal, and below normal temperatures.

Source: https://www.cpc.ncep.noaa.gov/products/forecasts/