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April 2019: An Active Weather Pattern and Late Season Snow!

April 2019 will be a month remembered for a continuation of an active and stormy weather pattern across Illinois, with two short lived, yet notable and uncommon late season snow events which impacted many in northern portions of the state.

The month finished noticeably wetter than average, with temperatures marginally below the long-term average. The preliminary average statewide April temperature was 52.0°F, which is 0.6°F below the long term average. The preliminary average statewide precipitation was 4.58 inches, which is 0.80 inches above the long term average.

Note: Data are provisional and may change slightly over time.

Precipitation

Preliminary statewide average precipitation for April was 4.58 inches, which is 0.80 inches above the long term average.

Precipitation in Illinois has been above average since December 2018. Preliminary data would rank this as the 7th wettest December – April period in state history. The end of April 2019 marked six months since any portion of the state has been classified as in drought, or abnormally dry by the U.S Drought Monitor..

An active spring weather pattern resulted in a majority of regions in northern, eastern, and southern portions of the state receiving over 100% of average monthly precipitation, with localized amounts approaching 200%.  Smaller regions of the state in west-central Illinois – roughly between the Mississippi and Illinois rivers are the only locations to receive near or below average precipitation for the month (see map below).

The heaviest precipitation fell in extreme southern Illinois, especially in counties bordering the Wabash and Mississippi Rivers, where 6 to 8+ inches were common (see map below).  The highest total in the state was reported at a station near Cobden (Union County) with 8.31 inches.

Heading into May, flooding remains an ongoing concern for the state, as soil moisture percentiles remain in the 90th to 95th percent range.

As of April 30th, 134 river gauges affecting Illinois were reported with crests in minor, moderate, or major flood stages. The greatest threat continues to remain along the Mississippi River. River flooding concerns were intensified by several days or heavy rains during the last week of April.

Snowfall

A majority of April snowfall came from two short lived, yet notable late season events that impacted many in northern portions of the state.

The first event on the weekend of April 13th and 14th brought widespread reports of 1 to 5 inches along a corridor extending from near Quincy, and running northeast toward Chicago and the Illinois/Wisconsin border. Several localities west and north of Chicago reported 6 to 8+ inches. Notable accumulations were measured as far south as Peoria. An accumulation of 5.4 inches at Chicago O’Hare Airport tied the record for the snowiest calendar day this late in the season.  April 16, 1961 also recorded 5.4 inches of snow.

The second snow event on April 27th brought a variety of wintry precipitation types to locations in northern Illinois, including another round of accumulating snow for many from Chicago and points north and west.  With 3.7 inches, this was the latest 2+ inch snow event on record for Rockford (Winnebago County).

The maximum monthly snowfall in Illinois was reported at a station near St. Charles (Kane County) with an impressive 13.5 inches.

Temperature

Preliminary results show that April 2019 finished with a statewide average temperature of 52.0°F which is 0.6°F below the long term average.

Despite several significant temperature swings throughout the month, which are common during springtime, statewide April temperatures finished right around the long term average. A few pockets in extreme southern Illinois reported slightly above average, and sporadic regions in northern Illinois reported slightly below the long term average.  This is a welcome change from last year,  April 2018, which ranked as the second coldest April on record.

The highest temperature in the state was recorded at a station near Jerseyville (Jersey County) with a reading of 85°F on April 23.  In contrast, the lowest temperature of 18°F was recorded at a station in Altona (Knox County) on April 1.

May 2019

Looking ahead at the rest of May 2019, the Climate Prediction Center (CPC) is favoring probabilities of above average temperatures for the southeast half of the state, with equal chances of above, near, or below average temperatures for the northwest half of the state. Unfortunately, an active weather pattern looks to persist, as the May outlook favors probabilities of above average precipitation statewide.

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Late April 2019 Snow Event

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

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

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

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

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

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

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

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April Already 4th Wettest on Record

Based on preliminary data, this April in Illinois is already the 4th wettest on record with 6.58 inches of precipitation. Average statewide precipitation for April is 3.77 inches. So we are already almost two inches above average.
According to the NWS forecast, more rain is expected across southern Illinois on Friday and Saturday. Therefore, it is possible that we will move up the list by the end of the month. In addition, some sites may have already reached their record for April, including the city of Chicago.

Top 5 Wettest Aprils

  1. 2011 with 7.40 inches
  2. 1957 with 7.13 inches
  3. 1927 with 6.95 inches
  4. 2013 with 6.58 inches
  5. 1944 with 6.50 inches

Precipitation Pattern Across the State

Here is how the precipitation has been distributed around the state, based on our multi-sensor precipitation product, with the actual amounts and the departures from average. Many parts of central and northern Illinois have more than double their average April precipitation (shades of purple in the second map).

April precipitation in inches for the first 24 days of the month. Click to enlarge.
April precipitation in inches for the first 24 days of the month. Click to enlarge.

April precipitation as departures from average. Click to enlarge.
April precipitation as departures from average. Click to enlarge.

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Frost in Illinois

frost2For much of Illinois April is typically the last month that we see freezing temperatures until Fall (at least we hope). Below are the maps showing the median dates when we see 28 and 32 degrees for the last time in Spring. The median represents the middle value in the range of dates and is less sensitive than the average to unusually early or late dates.
We have more discussion and maps, including the earliest and latest dates of freezes during the 1981-2010 period on our frost webpage.
In addition you can track the status of this spring (2013) in terms of how things stand on hitting 28 and 32 degrees from the Midwestern Regional Climate Center.

Click to enlarge.
Click to enlarge.

Click to enlarge.
Click to enlarge.

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Cold March

I dropped my car off at the shop for some brake work this morning here in Champaign and decided to walk to work. The air temperature was 21 degrees and the wind chill was 4 degrees. That was an uncomfortable stroll even with a winter coat, hat, and gloves. A year ago this morning (March 20, 2012), the air temperature was 63 degrees and well on its way up to a high of 82 degrees, a record high for this date.
So far this year, the statewide March temperature is 33.3 degrees. A year ago through this date, the statewide temperature was 51.1 degrees. That’s a whopping 17.8 degree difference. March went on to become the warmest March on record at 55.3 degrees, 14.2 degrees above the long-term average monthly March temperature of 41.1 degrees.
The rest of March does not look promising. The 1-5 day forecast shows temperature 10 to 14 degrees below average for Illinois. The 6-10 and 8-14 day forecast show widespread cold weather across Illinois and the central US. Figures below.
The long-term average temperatures for March were calculated from the 1981-2010 period.
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Spring Frost in Illinois

Frost in spring is a concern to farmers, landscapers, and gardeners. Frost in Illinois is usually not measured directly at weather stations. Instead, it is inferred from the air temperature – when the air temperature crosses the threshold of 32°F.
The average date of the last spring occurrence of 32°F ranges from April 7 in far southwestern Illinois to April 28 in northern Illinois (see map below). The actual date can vary from year to year. The spring dates are getting earlier by about 5-10 days over the last few decades.
The actual date varies from year to year. For tender plants, add two weeks to the average date in the spring to protect against late season frost.
Although 32°F is the temperature traditionally used to show frost, visible frost can be seen on the ground and objects when temperatures are slightly above 32°F on calm, clear nights that allow cold, dense air to collect near the ground. Under these conditions, the temperature near the ground actually can be a few degrees cooler than at the 5-foot height of the official National Weather Service thermometer.
Open, grassy areas are usually the first to experience frost, while areas under trees are more protected because the trees help prevent the heat from escaping. Homeowners can protect tender plants by providing this same type of protection if they cover their plants when a frost is expected. Plants near heated buildings sometimes are spared too. Because of the abundance of warm buildings and trees in towns, they tend to experience frost less often than those living in the country.

Average dates of last spring frost in Illinois.

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Fourth Warmest Spring in Illinois

Based on preliminary data, the statewide average temperature for spring in Illinois was 55.3 degrees, 3.3 degrees above normal and the fourth warmest spring on record. Warmer than normal conditions prevailed in the spring months of March (+2.5 degrees), April (+6.2 degrees), and May (+1.3 degrees).
The statewide average precipitation for spring was 11.97 inches, just 0.7 inches above normal. This is notably less than the 15.83 inches that fell in the spring of 2009 and the 14.21 inches that fell in the spring of 2008.
Climatologists and meteorologists tend to use the calendar months of March-May to define spring and not the astronomical definition from the vernal equinox (usually March 20) to the summer solstice (usually June 21). The calendar months better match the climate of Illinois. Our winter weather usually diminishes by early to mid March across much of Illinois and summer weather usually arrives long before June 21.

Spring temperatures in Midwest.
Spring temperature departures for the Midwest for 2010.

Spring precipitation in the Midwest
Spring precipitation in the Midwest for 2010.

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Warmest April on Record for Illinois

Based on preliminary data, the statewide average temperature for Illinois in April was 58.4 degrees, 6.2 degrees above normal and the warmest April on record. This beats the old record of 58.2 degrees set in 1955. Official statewide average temperature records go back to 1895. The warm temperatures in April were not unique to Illinois – the entire Midwest was much above normal (see map below).
April rainfall was 3.5 inches, just 0.3 inches below normal. Areas in western Illinois around received the most precipitation, over 5 inches in some locations. A CoCoRaHS observer in Matherville (Mercer County) reported 6.50 inches for the month.

April temperature departures
Temperature departures for April across the Midwest (click to enlarge).

Midwest precipitation for April
Precipitation totals for April across the Midwest (click to enlarge).

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First Documented Radar Hook Echo

The Illinois State Water Survey is home to the first ever documented radar hook associated with an actual tornado. Water Survey staff captured the historic event on film on April 9, 1953. This was a major turning point in monitoring severe weather, demonstrating that tornadoes could be identified by radar. This discovery helped lead to the first national weather radar network in the United States.
The radar was located at Willard Airport, south of Champaign IL, and was being used along with a rain gauge network to relate radar signals with rain rates. Don Staggs, the radar technician, had stayed late to complete repairs on the radar. While testing the repairs, he noticed an interesting radar return and began recording the radar scope using the mounted 35 mm camera. As a result, he captured a well-defined hook echo (see photo) on film. Afterwords, researchers related this information to damage and photos along the tornado’s path. More images and information of this event are in the links provided under the photograph.

Radar hook echo
First recorded radar hook echo of a tornado, April 9, 1953, near Champaign, IL (photo courtesy of the Illinois State Water Survey, INRS, University of Illinois).

Additional information on this event:

I talked about this event when Tom Skilling, WGN-TV, hosted his annual Tornado and Severe Storm Seminar at Fermilab in Batavia, IL on April 10, 2010. The video of the presentation is on the WGN-TV web site.