Many areas across northern Illinois have yet to experience their first freezing temperatures of the 2011 autumn season. However, the growing season is quickly coming to an end across the region. So, in spite of the fact that a widespread freeze has not yet occurred across northeastern Illinois and northwest Indiana, the Chicago National Weather Service will stop issuing headlines for frost advisories and freeze warnings for all of northern Illinois and northwest Indiana on October 31st. Frost and freeze headlines will resume during the spring of 2012.
Below is a table displaying the probabilities of exceedance for the date of the first autumn season freeze. To interpret this chart, consider the 30th and the 10th percentiles. For Chicago O'Hare these correspond to dates of October 16th and October 30th, respectively. This means that 70% of the previous years in the period of record experienced the first fall season freeze on or before the 16th of October. Likewise, 90% of previous years experienced freezing temperatures on or prior to October 30th. So, we are running late for freezing conditions across much of the area.
|Percentage of Exceedance||Chicago O'Hare||Chicago Midway||Rockford|
|50%||October 13th||October 26th||October 10th|
|30%||October 16th||November 1st||October 14th|
|10%||October 30th||November 8th||October 27th|
|Latest||November 6th||November 19th||November 4th|