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.
There have been some mention in the ag media about the linkage between above-average November-December precipitation and lowerbelow-trend corn yields for the next summer (usually understood to be related to some form of drought). It’s an intriguing case because the drought of 2012 was preceded by a wet November-December. However, when you dig into the statistics, the relationship is more complicated.
For this comparison I looked at the 11 wettest November-December events for the Corn Belt (map at bottom)and the US corn yield from the USDA. I picked the top 11 wettest because it gives us 10 previous cases to examine.
The 11 Wettest Corn-Belt November-December
2015 with 7.17 inches
1982 with 6.59 inches
1983 with 5.95 inches
1909 with 5.93 inches
1985 with 5.78 inches
1931 with 5.76 inches
1992 with 5.53 inches
1972 with 5.25 inches
1990 with 4.98 inches
2011 with 4.94 inches
1973 with 4.83 inches
Here are how the corn yields for the growing season following a wet November-December looks like (orange dots), compared to all corn yields (blue line), and the yield trend (green line). We did not consider yields in 1910 or 1932 since those predated modern hybrids. Of the remaining 8, 1974, 1983, 1991, and 2012 experienced below-trend yields from drought and 1993 experienced below-trend yields from flooding.
After writing about the 7th warmest and 11th wettest winter on record for Illinois yesterday, today we look more closely at the winter snowfall season. While the core winter months are December, January, and February, I have extended the snowfall analysis all the way back to September 1 since areas in northern Illinois can get snow in the fall.
So far, northern Illinois has received 15 to 25 inches of snow (areas in blue, first map) for the current snowfall season, while central and southern Illinois have received 5 to 10 inches. However, most of Illinois is running below-average on snowfall (second map). In some places, it’s up to 50 percent less snowfall. The one area with above-average snowfall is far southeastern Illinois.