For the second winter in a row, La Niña will influence weather patterns across the country, but as usual, it’s not the only climate factor at play. The ‘wild card’ is the lesser-known and less predictable Arctic Oscillation that could produce dramatic short-term swings in temperatures this winter.
Overall, northern Illinois is expected to have increased odds of being colder and snowier than normal – similar to last year. Here are the details. Below the forecast is a refresher on the winter of 2010-2011 that we all knew and loved.
For Illinois, the northern third of the state is expected to have an increased chance of below-normal temperatures. The southern two-thirds of the state has equal chances of above, below, or near-normal temperatures.
Almost all the state, except for far western Illinois, is expected to have an increased chance of above-normal precipitation. Far western Illinois has equal chances of above, below, or near-normal temperatures. Although NOAA does not offer a winter snowfall forecast, increased precipitation in the winter months usually means increased snowfall.
Winter of 2010-2011
Here are the temperature map and snowfall map for the Midwest for the winter of December 2010 to February 2011. Both maps are expressed as departures from the 1971-2000 averages. The winter was both colder and snowier than average across the Midwest. Meanwhile, precipitation (rainfall and the water content of snow) was right at average for the winter.
According to statistics compiled by NOAA’s Storm Prediction Center, so far in 2011 Illinois has had 58 reports of tornadoes, 298 reports of significant hail (1 inch or more), and 674 reports of wind damage. That’s about average for us. Fortunately, we escaped the worst of the 2011 season that centered on the southeastern U.S. and Joplin, MO.
See the first figure below for the distribution of events. In some cases, we may have multiple tornado reports from the same tornado as it travels along.
The second figure shows the distribution of severe weather reports by month for Illinois. After a quiet start to 2011, severe weather activity picked up in April and continued through July before tapering off in August and September. You can see the monthly raw data and chart in Google Docs.
The first 10 days of October have been warm, dry, and sunny in Illinois. The statewide average temperature is 61.3 degrees, 3.8 degrees above average.
The statewide average precipitation is zero. Only a handful of sites in far western Illinois have reported anything. The highest total so far in the state was 0.44 inches from a CoCoRaHS site near Carthage, Illinois.
The driest October on record in Illinois was 1964 with only 0.20 inches for the entire month. Last year (2010) was the 19th driest October with only 1.35 inches. However, October 2009 was second wettest with 8.40 inches.
Here is a look at how October precipitation has behaved since 1895 in Illinois. The green dots are the individual years. The alternating brown and green shaded area show below and above average precipitation, respectively. The average is based on the period of record (1895-2010). You can see a lot of year to year variability in the numbers and no clear long-term trends.
Based on preliminary numbers, the statewide average temperature for September in Illinois was 63.2 degrees, 3 degrees below average. That makes it the 13th coolest September on record for Illinois. The coolest September was 1918 with 59.3 degrees. Statewide records go back to 1895.
Despite the overall cool readings for the month, Labor Day weekend was very hot. Highs in the upper 90s and low 100s were common during that time. One of the hottest was Quincy with 104 on September 1. In all, at least 65 stations in Illinois either tied or broke daily records for high temperatures in early September (text file with details).
On the other extreme, the coldest spot for the month was Paw Paw with 32 degrees on September 15. Twenty-two stations either set or broke daily records for low temperatures (see text file for details).
The statewide average precipitation for September was 3.53 inches, 0.34 inches above average. Rainfall was stubbornly sparse in the areas hardest hit by drought where only 1-2 inches were common. Amounts in far southern and northern Illinois were much wetter at 4-8 inches.
The highest rainfall total for the month was by a CoCoRaHS observer in Bush, Illinois, with 8.42 inches. In second place was a CoCoRaHS observer in Harrisburg, Illinois, with 8.04 inches. The reported driest spot in the state was a CoCoRaHS observer in Warsaw with only 0.69 inches of rain for the month. CoCoRaHS observers are volunteers that use standard equipment and training. Therefore, their results are comparable to other networks.