Frost Climatology in Illinois

frost2It is that time of year to discuss the nemesis of every gardener out there – the late spring frost or freeze. We have a full suite of maps showing the earliest, 1 in 10 years, median, 9 in 10 years, and latest frost (32 degrees) and freeze (28 degrees).

One of the things you notice about this map, and all the others in the series, is that even though the date for the last spring frost/freeze gets later the farther north you go, you can get big differences between nearby locations. The reason for that is that most of these last frosts occur during calm, clear, cold nights. As a result, subtle local features can make a big difference. For example, cold air can settle in low-lying areas, causing frost pockets. Or locations out in the countryside may be just a few degrees cooler than in town, due to warm buildings, that may make them vulnerable to a later frost. Also, keep in mind that we measure the air temperature at 5 feet above the ground while the ground and some objects may cool to lower temperatures on calm, clear nights. Therefore, you may see frost even if the temperature is reported as 34 or 35 degrees.

Personally, I use these maps as a general guide. However, I watch the latest weather forecasts closely for a few weeks after I have new plants in the ground for any surprises. The NWS does put out hard freeze warnings (temperatures at or below 28 degrees with or without frost), freeze warning (temperatures at or below 32 degrees with or without frost), and frost advisories for widespread frost.

Click to enlarge.

Trouble Ahead from the Warm Weather?

Illinois has just come off its third warmest winter on record. And the warm weather continues in March. The first 7 days have been 3.2 degrees above normal and the NWS forecast for the next 14 days shows high odds of above-normal temperatures.
As a result, plants have begun to think spring is here. On the left are some daffodils in my yard in Champaign today. The blooms are just about to open. It’s the same story for the Magnolia tree at work (picture right). And this is central Illinois. Southern Illinois will be even farther along than here.Photo by Ben Woloszyn.
So what’s the problem? Well, it’s March 8 and we could still have temperatures drop into the 20s in March and April. In fact the median date for the last 28 degree reading in spring in Illinois somewhere near early to mid April in most locations. Our GIS expert Zoe produced this map (left) of the median dates in spring. Click the thumbnail to enlarge.

April Freeze of 2007

An April freeze is exactly what happened in 2007. A warm March was followed by a cold-air outbreak on Easter weekend that caused temperatures to drop into the low 20s and produced high winds.
Below is the plot of the daily average temperature from March 1 to April 30, 2007, for Illinois. There was a small stretch of warm temperatures in early March and then a long stretch in late March and early April with temperatures 10 to 20 degrees above normal. Trees and shrubs rapidly sprouted tender leaves and flowers during this warm spell. Things took a turn for the worst in early April when temperatures plummeted to 10 to 20 degrees below normal for about 2 weeks.
temperature plot for March - April 2007
That particular freeze cause over a billion dollars in damages across the Midwest and Southern U.S. to fruit crops, especially apples and peaches, as well as alfalfa and winter wheat. Here are some reports that outlined the conditions and damages.