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

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.

Previous records slashed with monumental cold conditions in Illinois

CHAMPAIGN, Ill., 1/31/19: Illinois has been experiencing some of the coldest weather that has been seen in decades and, in some locations, ever.

The National Oceanic and Atmospheric Administration cooperative weather observer at Mt. Carroll in northeastern Illinois reported a temperature of -38 degrees on the morning of Jan. 31.

“The temperature in Mt. Carroll may be a new state record, if officially confirmed,” said Brian Kerschner, spokesperson for the Illinois State Climatologist’s Office at the Illinois State Water Survey.

When it appears that a state record temperature may have been broken, a state climate extremes committee reviews the observations to assess its validity. This team typically includes the State Climatologist’s Office, the Midwestern Regional Climate Center, and federal climate experts, such as from the National Weather Service and the National Center for Environmental Information.

Most of Illinois has been in the deep freeze for the past two days. With a recording of -31 degrees, Rockford broke their all-time low temperature, which was previously -27 degrees on Jan. 10, 1982, the National Weather Service Forecast Office in Chicago reports.

During the mornings of Jan. 30 and 31, numerous locations in northern Illinois reported temperatures in the -20s and some locations going below -30 degrees. Minimum temperatures were below 0 degrees throughout most of the state, except for the southern regions. Daily mean temperatures were generally 15 to 20 degrees lower than the 30-year average temperature.

Some other notable temperatures included -35 degrees at Elizabeth, -32 at Galena, -30 at Rochelle, -33 at Aledo, -26 at DeKalb, -22 at Joliet, -21 at Galesburg, -17 in Champaign, and -16 in Decatur.

In the southern counties, Cairo reached 12 degrees and Carbondale and Rosiclare saw 4 degrees.

Numerous schools, businesses, and government offices were closed throughout the state because of the dangerously cold wind chills.

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Note: Data for this press release were obtained from the Office of the Illinois State Climatologist, Midwestern Regional Climate Center, and National Weather Service Offices responsible for the state.

Media Contacts: Brian Kerschner, (217) 333-0729, statecli@isws.illinois.edu

David Kristovich, head of the Climate and Atmospheric Science Section, Illinois State Water Survey, (217) 333-7399, dkristo@illinois.edu

Dryness Across Northern and Central Illinois

After a wet start to the 2014 growing season, we have seen a significant drop in rainfall across parts of northern and central Illinois in the last few weeks. Here is the 30-day rainfall as a percent of average. Areas in the orange are 25 to 75 percent of average while the areas in red are less than 25 percent of average. There are reports of soil moisture running low in some areas. On the other hand, southern Illinois has received above-average rainfall in the last 30 days.

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Besides the switch from too wet to too dry in northern and central Illinois, and too much rain in southern Illinois, the other issue is that temperatures have been running about 4 degrees below average for the past 30 days. We are getting some heat this week. However, the longer-term forecasts indicate a return to cooler temperatures and more rain after this week through September 1.

If you look at the last 90 days the heavier rains in June and early July masks the recent dryness (map below). In fact, at the 90 day time scale rainfall in Illinois is generally at or above long-term average (1981-2010), as denoted by the grays and greens. This is one of the challenges of drought monitoring – sorting out short-term dryness versus long-term wetness or vice versa.

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Cool July – Cool August?

As we approach the end of July the statewide average temperature in Illinois is 70.6 degrees, which currently puts it in second place for the coldest July on record. I will post more on this at the end of the week.

Here is how the previous top 10 coldest July temperatures for Illinois looked and what happened in the following August (table below). In 8 out of the 10 cases, the following August was colder than average. However, two of those “colder” August’s were marginally so (1924 and 1996). The one spectacular reversal was in 1947, where August was 7.2 degrees above average after the 3rd coldest July. Therefore there is a historical tendency for cooler weather to prevail into August.

 

Temperature (degrees F)
Year July August August Departure
2009 70.3 70.9 -2.7
1924 71.1 73.2 -0.4
1967 71.7 68.2 -5.4
1971 71.9 71.6 -2.0
1950 72.0 69.2 -4.4
1915 72.2 66.9 -6.7
1947 72.3 80.8  7.2
1904 72.4 69.8 -3.8
1905 72.5 74.1  0.5
1996 72.6 73.5 -0.1

We are using the 1981-2010 average for August (73.6 degrees) as the benchmark for this comparison. Statewide records go back to 1895.

The NWS forecasts are pointing towards a colder than average start to August, based on the latest 6-10 and 8-14 day forecasts.

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March Cold and Dry in Illinois

Based on preliminary data, March 2014 in Illinois was cold and dry.

Temperature

The statewide average temperature was 33.8 degrees, which was 7 degrees below average and the 8th coldest March on record. Combined with the colder-than-average January and February made this the 4th coldest start (23.6 degrees) for Illinois for the year to date.

This was the fifth month in a row with temperatures much below average in Illinois. At this point, it was the second coldest November-March on record for Illinois at 29.1 degrees. See the bar graph below showing monthly temperature departures since January 2013.

If this cold March felt familiar, it was because last March was cold as well. The statewide average temperature for March 2013 was only 34.1 degrees.

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Precipitation

The statewide average precipitation was 1.49 inches, which strangely enough was 1.49 inches below average and the 11th driest on record. The statewide average precipitation last March was much higher at 2.74 inches. Eight out of the last nine months have had below-average precipitation. As a result, the statewide precipitation departure since July 1 was 7.2 inches.

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The first map below is the accumulated precipitation for March (rain plus the water content of any snow event). Most of the state received between 1 to 2 inches of precipitation. It was wettest in the far south and driest in the northwest.

The second map shows the precipitation departures from average for March, showing all areas of the state with below-average precipitation. This would be of more concern if March had been warm. However, with the colder conditions very little drying took place.

Snowfall

The third map shows the snowfall for March. Amounts were in the 1 to 5 inch range in the southern half of the state and 5 to 15 inches or more in the northern half. Mendota reported the highest monthly total of 17.9 inches.

The fourth map shows the snowfall departure from average for March. The entire state was above-average on snowfall for the month. While it seems like a contradiction to report above-average snowfall and below-average precipitation for March, it really is not. The problem is that we have had few rainfall events in March, which was unusual. So we ended up with a lot of snow but the water content of all that snow did not make up for the lack of rain. map_btd

 

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First Half of March – Cold and Dry

Winter’s grip on Illinois is slowly releasing. However, we remain cold and dry for March. The statewide average temperature for March 1-16, 2014, was 30.3 degrees, which is almost 8 degrees below average. The good news is that the average, or normal, temperatures are climbing rapidly through the month. As a result, being 8 degrees below average in mid-March is still warmer than this winter. The NWS Climate Prediction Center 6-10 and 8-14 day forecasts show the below-average temperatures to continue through the end of March.

Besides the cooler temperatures, another concern at this point is the below-average precipitation (in shades of yellow and orange) for March so far. This is true not only across Illinois but much of the Midwest.

MonthPDeptMRCC
March Precipitation Departure from Average (inches). Click to enlarge.

Below are the 90-day precipitation departures from average. Above-average precipitation (shaded in green) can be found in IL, IN, OH, and MI, due for the most part to our generous snows and a few rain events. However, parts of southeastern IL are 1.5 to 3 inches below average. This is part of a larger area of dryness covering Missouri and parts of KS, OK, and AR. Of course this pattern could change quickly as we get out of winter and into spring. A few good spring rains could erase most of this deficit. In the meantime, we will be watching this area closely.

90dPDeptMRCC

February Seventh Coldest on Record

feb2014-floodedfield
Flooded farm field in east-central Illinois after a period of snowmelt and rainfall in the third week of February, 2014.

Based on preliminary data, the Illinois statewide temperature for February was 18.7 degrees. That was 12.1 degrees below the long-term average and the seventh coldest February on record. No surprise there.

The statewide average temperature for the three core winter months of December, January, and February was 20.8 degrees. That was 8.2 degrees below average and the fourth coldest December-February period on record. Incidentally, it is in a 3-way tie with 1917-18 and 1976-77. The coldest winter was 1977-78 at 19.6 degrees. The winter of 1978-79 was in second place at 19.9 degrees. Overall, this winter was comparable to those in the late 1970s.

Below is the table of the ten coldest February’s on record for Illinois. It’s really hard to beat those cold February’s in the late 1970s. The 1981-2010 statewide average is 30.8 degrees.

Rank Year Avg.
1 1978 16.9
2 1979 17.5
3 1936 17.7
4 1905 17.8
5 1895 18.1
6 1899 18.5
7 2014 18.7
8 1902 19.3
9 1914 20.9
10 1958 21.4

Here are the ten coldest December-February periods in Illinois since 1895. The 1981-2010 statewide average is 29.0 degrees.

Rank Year Avg.
1 1977-1978 19.6
2 1978-1979 19.9
3 1935-1936 20.6
4 1917-1918 20.8
4 1976-1977 20.8
4 2013-2014 20.8
5 1903-1904 21.6
6 1962-1963 21.9
6 1904-1905 21.9
7 1981-1982 22.8

The snowfall for February was above average across the state. The total snowfall ranged from 4 inches in far southern Illinois to 15-20 inches in north-central Illinois. The snowfall departures from average ranged from 1-5 inches south of Interstate 70 and between 10 and 18 inches between Interstates 70 and 80.

feb-snowfeb-snow-dep

The statewide precipitation for February was 2.28 inches. That was just 0.17 inches above average. Precipitation includes both rain events along with the water content of any snowfall. The result in February was that the above-average snowfall did not translate to above-average precipitation because several of those snowfall events occurred in colder conditions when snow density is lower (i.e, fluffier).

By the way, here are the snowfall totals for the entire snowfall season. You may recall that we saw snow flurries back in October and some measurable snow in November. Some of largest snowfall totals this winter are in the Chicago area and include Lincolnwood with 79.8 inches and Oak Park with 78.6 inches.

win-snowwin-snow-depart

Frost Depth in Illinois

One of the effects of this exceptionally cold winter has been that our soils have remained frozen at considerable depths. We have hourly soil temperatures under grass at 19 sites across the state at 4 and 8 inches, available through the WARM website, that give us glimpses of soil conditions.

Here are snapshots of the daily low soil temperature at 4 inches yesterday and a week ago when temperatures were much warmer. The 4-inch temperature responded to the warmer weather and showed signs of thawing before re-freezing this week. In many parts of the state, the 8-inch soil temperatures remained frozen during this period. Click on each map to enlarge.

feb21          feb26

Soil temperatures depend on soil types, soil moisture, vegetation, snowcover, and exposure. In general, drier soils warm up and cool faster than wet soils. Both vegetation and snow can insulate the soil for air temperature extremes. I recall the morning of January 5, 1999, when we had a foot of snow on the ground and an air temperature of 25 degrees below zero. Because the winter had been mild up to that week, the soil temperature at 4 inches was 32 degrees, a difference of 57 degrees between 4 inches below ground and 5 feet above ground!

While the above site tracks temperatures at specific depths, the NOAA North Central River Forecast Center maintains a web site with observed frost-depths in Illinois and points to the north. For most of this winter, the frost depth has run in the neighborhood of 10 to 20 inches across Illinois with a few sites going deeper. Here is a screenshot of this morning’s map. While it doesn’t work on the screen shot, you can mouse-over the points on the map on the website and see the individual reports.

North Central River Forecast SoilT

Finally, Wayne Wendland, the former State Climatologist for Illinois, did a frost-depth study in Illinois using data collected from grave diggers from 1980 to 1996. He developed a network of sites across Illinois through the Illinois Cemetery Association and provided post cards that the grave diggers filled out every two weeks in winter. They noted frost depth, soil moisture, soil texture, ground cover, and exposure. The deepest observed frost depths during this period ranged from 5 inches in far southern Illinois to 30 inches in far northern Illinois. The results were published in the Transactions of the Illinois State Academy of Science (pdf).

Illinois Soil Temperatures – Then and Now

You can find 4 and 8-inch soil temperatures for Illinois on the Water Survey’s web site at www.isws.illinois.edu/warm/soiltemp.asp. These data are from a network of 19 sites around Illinois that is maintained by the Survey.
Below are the maps of what the 4-inch soil temperatures looked like on Wednesday and a year ago on the same date. This year the soil temperatures are barely above freezing in northern Illinois and barely above 40 degrees in southern Illinois. On the same date in 2012, the 4-inch soil temperatures were in the upper 50s and lower 60s across the state.
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