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.
Late Season Snowfall in Chicago and Rockford
Gino Izzi of the NWS office in Romeoville rounded up these statistics for late season snowfall in Chicago and Rockford…
PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE CHICAGO IL
1128 AM CDT FRI MAR 25 2011 /1228 PM EDT FRI MAR 25 2011/
...LATE SEASON SNOWFALLS...
NOTHING SAYS SPRING TIME IN ILLINOIS MORE THAN TEMPERATURES IN THE
20S AND GRAY CLOUDY SKIES...BUT HISTORICALLY SPEAKING IT MAY BE A
BIT TOO SOON TO STOW AWAY THOSE SHOVELS. IN FACT...PORTIONS OF
MISSOURI AND DOWNSTATE ILLINOIS ARE SEEING SNOW THIS MORNING. SO
HOW COMMON IS IT TO SEE SHOVEL-ABLE SNOW IN OUR AREA THIS TIME OF
YEAR?
IN CHICAGO...54 OF THE PAST 139 YEARS (OR ABOUT 39%) HAVE HAD AT
LEAST ONE DAY WITH AN INCH OR MORE OF SNOWFALL ON OR AFTER MARCH
25TH. IN FACT...17 OF THOSE YEARS HAD MULTIPLE DAYS WITH OVER AN
INCH OF SNOWFALL INCLUDING 6 DAYS WITH OVER AN INCH OF SNOW BACK IN
1926! HERE ARE SOME OF THE RECENT LATE SEASON (AFTER MARCH 25TH)
CHICAGO SNOWFALLS...
SNOWFALL DATE
3.0 4/11/2007
3.0 4/ 7/2003
2.1 4/ 5/2009
1.9 3/27/2008
1.6 4/ 7/2000
1.2 3/29/2009
HERE ARE THE TOP 10 BIGGEST LATE SEASON SINGLE DAY SNOWFALL TOTALS
FOR CHICAGO...
RANK SNOW DATE
1 13.6 3/25/1930
2 9.4 4/ 5/1982
9.4 4/ 2/1975
4 9.0 4/ 6/1938
5 8.9 3/26/1970
6 8.2 4/ 1/1970
7 7.8 3/30/1926
8 7.7 3/29/1954
9 7.1 3/29/1964
10 6.6 3/26/1934
IN ROCKFORD...47 OF THE PAST 118 YEARS (OR ABOUT 40%) HAVE HAD DAYS
WITH AN INCH OR MORE OF SNOWFALL ON OR AFTER MARCH 25TH. OF THOSE
YEARS...10 HAVE HAD MULTIPLE DAYS WITH OVER AN INCH OF SNOW
INCLUDING 4 DAYS IN 1970 AND 1926. HERE ARE SOME RECENT LATE SEASON
ROCKFORD SNOWFALLS...
SNOWFALL DATE
1.9 4/11/2007
1.8 3/29/2009
1.2 4/ 7/2000
1.1 4/ 1/2002
1.0 4/12/2007
HERE ARE THE TOP 10 BIGGEST LATE SEASON SINGLE DAY SNOWFALL TOTALS
FOR ROCKFORD...
RANK SNOW DATE
1 13.5 3/31/1926
2 10.4 3/29/1972
3 7.0 4/18/1912
4 6.3 4/ 5/1982
5 6.0 4/ 6/1938, 6.0 3/25/1933
7 5.0 3/29/1954
8 4.8 3/29/1964
9 4.6 4/ 2/1975
10 4.5 4/ 2/1936, 4.5 3/28/1894
WHILE NO BIG SNOWS ARE CURRENTLY IN THE FORECAST FOR THE REGION...IT
IS WORTH NOTING THAT LATE SEASON SNOWS ARE NOTORIOUSLY DIFFICULT TO
PREDICT MUCH IN ADVANCE. CURRENT INDICATIONS ARE THAT THE COLD AIR
OVER THE AREA NOW WILL REMAIN IN PLACE THROUGH THE WEEKEND INTO
EARLY NEXT WEEK.
$$
IZZI
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.
