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.
Here are the ten coldest December-February periods in Illinois since 1895. The 1981-2010 statewide average is 29.0 degrees.
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.
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.
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.
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.
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).
Late last week, the NOAA Climate Prediction Center released their latest monthly and season forecast. They expect all but far southern Illinois to have an increased chance of below-average temperatures in March. They expect the northern third of Illinois to have an increased chance of below-average temperatures in March/April/May. On the precipitation side, there was nothing to say about March but they do expect an increased chance of wetter conditions in the southern third of Illinois.
On the more immediate time-frame, the latest 6-10 and 8-14 day forecasts show that another blast of Arctic air will reach into the Midwest.
Thursday was an interesting weather day in Illinois. Many areas experienced a rapid melt of the snowpack as warm, humid air moved into Illinois. The by-products of this included heavy fog, rain, severe weather, and localized flooding. I was driving back from a meeting in Pittsfield and was chased by severe thunderstorms and tornado warnings all the way back to Champaign. As I approached Champaign, the visibility on Interstate 72 dropped to 20-40 feet as the warm, moist air blowing over the cold, snowy fields produced heavy fog.
Below is a map of the preliminary storm reports for Thursday, including several possible tornadoes in Illinois. Click on the map to link to the reports from the NOAA Storm Prediction Center.
February tornadoes are relatively rare but they do happen. The second graph is from a study of 2,239 tornado reports in Illinois (link to more plots). Of those, just 1.3 percent occurred in February. However, one event that occurred after that study was the deadly Harrisburg tornado that killed 8 people on February 29, 2012.