Frost is the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. If a frost period is severe enough to end the growing season or delay its beginning, it is referred to as a “killing frost”.
Frost in both spring and fall can be a concern to farmers, landscapers, and gardeners. However, we usually do not directly measure frost at weather stations in Illinois. Sometimes observers may note the presence of frost in their comments on the forms. To get around the lack of direct observations, we use a temperature threshold of 32° for frost and 28° for a hard freeze. You can see the full suite of maps for both thresholds here.
Median date of last spring occurrence of 32 degrees
You can keep track of soil temperatures at 2, 4, and 8 inches and soil moisture at 2, 4, 8, and 20 inches at the Illinois State Water Survey’s WARM soils page. WARM stands for our Water and Atmospheric Research Monitoring program, used for statewide monitoring of water resources.
Here is this morning’s soil temperatures at 2 inches. They have cooled off a little after the chilly weekend but will warm back up later this week.
The NWS Climate Prediction Center released their latest outlook for April and the growing season. In general, the concerns of a higher risk of a warm, dry spring and early summer shown in earlier outlooks seem to have dissipated in Illinois.
Our very strong El Niño is expected to linger until late spring or early summer, which is quite normal for these events. This summer is expected to be “ENSO-neutral” with a strong likelihood of transitioning to La Niña in the fall. An excellent discussion of El Niño and La Niña can be found at https://www.climate.gov/enso.
Illinois has equal chances (EC) of above, below, and near-average temperatures in April. The northern half of Illinois has EC on precipitation while the southern half has an increased chance of above-average precipitation. You might consider the phrase “equal chances” to mean they have no strong indication of conditions moving too far from the average. Sometimes I like to call it a neutral forecast.
After a mild winter, temperatures continue to be well above average for March in Illinois and the other Midwestern states. Illinois is running about 6 to 8 degrees above average (map below), as are Indiana, Michigan, Ohio, Kentucky, and eastern Missouri and Iowa through March 13. The warmer than average conditions are even stronger in the western portion of the Midwest. Temperature departures of 9 to 12 degrees are common in Wisconsin, Minnesota, and western Iowa and Missouri.
For Illinois in particular, the statewide average temperature for March currently stands at 44.3 degrees, 6.6 degrees above average. If we maintain that departure to the end of the month, we could be the 7th warmest March on record. March 2012 remains the warmest March on record with temperature departures of 14.2 degrees above average.
According to the latest numbers, Illinois just experienced its 5th warmest winter (4.8 degrees above average), and its 3rd warmest fall on record (3.3 degrees above average). Statewide records go back to 1895.