Author: Trent Ford

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Warm, Wet, and Active Spring

March was warmer and wetter than average across the state, continuing the pattern from winter. The preliminary statewide average March temperature was 43.5 degrees, 2.20 degrees above the 30-year normal and the 28th warmest on record going back to 1895. Preliminary statewide average total March precipitation was 3.96 inches, 1 inch wetter than the 30-year normal and tied for the 34th wettest on record.

Data are provisional and may change slightly over time

Persistent Warmth in March

Much like the first two months of 2020, March temperatures were consistently above the long-term average.

The first two months of the climatological winter season were much warmer than average, with very few cold air incursions. The plot below shows the March daily average temperature as a departure from average in Rockford. Since the start of 2020, over 70 percent of days in Rockford have been warmer than the long-term average. This has caused 2-inch and 4-inch soil temperatures to generally remain above freezing over this time, according to observations from the Illinois Climate Network (https://www.isws.illinois.edu/warm/soil/).

Average temperatures in March ranged from the high 30s in northern Illinois to the low 50s in southern Illinois. Temperatures ranged between 1 and 5 degrees above the long-term average. The statewide average March temperature was 43.5 degrees, which is 2.20 degrees above the 30-year normal and tied for the 28th warmest on record. March marked the fourth consecutive month with statewide average temperatures above the 30-year normal. By comparison, the first three months of 2019 were all 1 to 3 degrees below the 30-year normal, considerably cooler than 2020 so far.

The warm weather in March resulted in three daily high maximum temperature records and seven daily high minimum temperature records being broken across the state. The few cold, cloudy days we had in March also resulted in six daily low maximum temperature records and one daily low minimum temperature record being broken across the state. The long-running station in Carbondale was only 1 degree away from its all-time March high minimum temperature record on March 28, thanks to a strong mid-latitude warm sector bringing warm air from the south. Multiple stations in southern Illinois recorded daily maximum temperatures at or over 80 degrees during the last week in March. At one of these stations, Fairfield in Wayne County, this was two weeks before the average first 80-degree day based on the long-term record.

The highest temperature recorded in the state in March was 81 degrees in Alexander, Pope, and Hardin Counties, while the lowest temperature was 8 degrees in Jo Daviess, Knox, and Whiteside Counties.

Storms Bring Heavy Rain to Northern and Southern Illinois

Frequent precipitation persisted from February into March for the southern part of the state. Most areas south of Interstate 64 received over 6 inches of total precipitation in March, and some areas received over 8 inches. This represents between 150 percent and 200 percent of normal March precipitation in southern Illinois (see map below). Most stations in southern Illinois received 50 percent or more of their total March precipitation in one 24-hour period between March 20 and 21, thanks to a series of storms that tracked across the region.

One-day precipitation totals reached 4.50 inches in Clay County, which set the all-time March one-day precipitation record at the station in Clay City and broke the previous record by over three-quarters of an inch. Other large one-day totals from the March 20 storms included 3.85 inches in Mt. Vernon in Jefferson County and 3.55 inches in Olney in Richland County. With these one-day totals subtracted, March 2020 was very close to March 2019 total precipitation in southern Illinois; however, because of this event, most areas in southeast Illinois received between 1.5 and 2.5 times the amount of March 2019 precipitation this month.

Although March precipitation totals in northern Illinois were not as generous as those in the south, northern Illinois was not averse to very large one-day precipitation totals. A series of storms that moved through on March 28 generated between 3 and 4 inches of precipitation in a less than 24-hour period for a stretch of Illinois between the Quad Cities and the western suburbs. One CoCoRaHS observer in Prophetstown in Whiteside County recorded 5.34 inches on this day. Unfortunately, the heaviest precipitation missed the longer-term COOP stations in the region, but the storm did manage to break the all-time March one-day precipitation total in DeKalb.

Total March precipitation ranged from over 8 inches in far southern and southeast Illinois to just over 2 inches in central Illinois. These totals ranged from over 200 percent of average March precipitation in southeastern and northern Illinois to just over 75 percent of average March precipitation in western Illinois. Most of central Illinois received between 75 percent and 125 percent of average March precipitation. This combined with above average temperatures allowed soils to dry a bit across central and western Illinois.

Last month was the wettest March on record at Rock Island Lock & Dam 15, with 6.17 inches recorded. The wettest place in the state last month was Clay City with 8.31 inches.

Overall, the preliminary statewide average total March precipitation was 3.96 inches, exactly 1 inch more than the 30-year normal and the 34th wettest on record. Although the March average does not reflect the 5- to 6-inch differences in precipitation between central and northern/southern Illinois.

Most of the northern half of the state experienced measurable snowfall last month. March totals ranged from around 6 inches in northeast Illinois to just over one-tenth of an inch along the Interstate 70 corridor. A winter storm on March 22 and 23 accounted for the vast majority of snowfall in the northern part of the state, with one-day totals exceeding 6 inches in Grundy County. Morris in Grundy County was the snowiest point in the state in March, with just over 7 inches of total snowfall.

March total snowfall departures mimic the spatial patterns for the entire winter season. Most areas of western, northwest, and west-central Illinois had totals within 5 inches of the long-term average, whereas most counties south of Interstate 70 as well as counties in the Chicagoland metro area have experienced 5 to 10 inches below average snowfall since the first snow of the season.

Severe Weather

Illinois experiences severe weather and storms in all calendar months, but March often begins the unofficial severe weather season. This last month we had numerous severe weather and storm reports, ranging from snowstorms to large hail and a few tornadoes. Trained spotters reported 2-inch hail in both Williamson and Vermilion Counties last month, with many more reports of 1.5- to 1.75-inch hail across southern and central Illinois. The AWOS station in Hyde Park in Cook County recorded a 61 mph non-thunderstorm wind gust on March 29. Finally, multiple tornadoes were reported in Illinois last month in southern and west-central Illinois, including three tornadoes between Peoria and the Quad Cities on March 28. One of these, an EF-1 tornado, developed just a quarter mile east of the Peoria International Airport, according to the Lincoln National Weather Service https://www.weather.gov/ilx/032820Tornadoes).

 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 temperatures across the state.

Longer-term 30-day outlooks are similar to the 8-14 day outlooks, with continued elevated chances of warmer and wetter conditions across the state for April.

Heavy precipitation in northwest and southern Illinois, combined with continual snowmelt in the Upper Midwest has continued the threat of flooding along most major rivers in Illinois. Currently, gauges along the Mississippi, Ohio, Illinois, and Wabash Rivers are at or above minor flood stage, with nine gauges in moderate flooding, according to the National Weather Service River Forecast Center.

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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/

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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/

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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.