Halloween Weather in Illinois

Here are some Halloween statistics for locations across Illinois. Typical for Illinois, Halloween can be either near freezing or downright balmy. This year’s Halloween will likely be warmer than normal with highs ranging from the low 60s in northeastern IL to the upper 70s in southern IL (map below), but not record breaking. Last year was much cooler with highs in the 50s and low 60s, and a band of rain from St. Louis to parts of Chicago.
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November Trending Warmer, Wetter

Based on historical data for Illinois, the weather in November is trending towards warmer and wetter conditions over time. Based on the latest NWS forecasts, this November is likely to continue that pattern.

Historical Trends


The statewide average temperature for November shows a wide variation from year to year – typical of all months in Illinois. However, there is an underlying warming trend of about 2 degrees over the last century.



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A Very Warm October in Illinois

[Edited on October 26 to reflect update] Based on data through October 25, the statewide average temperature for Illinois in October is 60.7 degrees. That is 6.3 degrees above normal and the fourth warmest October on record. Temperatures for the rest of October are expected to be 3 to 5 degrees above normal. Therefore, this October could slip in the polls. Here are the top ten warmest Octobers

  1. 1963 with 63.6°F
  2. 1947 with 62.2°F
  3. 1971 with 61.3°F
  4. 2016 with 60.7°F
  5. 1900 with 60.6°F
  6. 1956 with 60.2°F
  7. 2007 with 59.9°F
  8. 1897 with 59.7°F
  9. 1950 with 59.3°F
  10. 1920 with 59.2°F

Here are the U.S. temperature departures for October so far . Illinois and the eastern two-thirds of the U.S. are experiencing above-normal temperatures (shown  in various shades of pumpkin spice, appropriately enough). Speaking of pumpkins, the harvest for this year looks much better than last year.
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Warm November, Normal Winter in Store for Illinois?

The NWS Climate Prediction Center released their outlook for November and beyond. The primary consideration for the outlook was the likely return of La Niña in the Pacific Ocean.

La Niña

In a recent blog post on climate.gov, Emily Becker writes

… now forecasters think there’s a 70% chance that La Niña conditions will develop this fall. However, any La Niña that develops is likely to be weak, and forecasters aren’t quite as confident that La Niña conditions will persist long enough to be considered a full-blown episode, giving it a 55% chance through the winter.

Here is the typical winter pattern for La Nina, as shown on climate.gov, with warmer and drier conditions south of Illinois, colder conditions north of Illinois, and wetter conditions with an active storm track right over Illinois. This active weather pattern tends to produce more snow in the Great Lakes region as well.


The outlook for November (below, click to enlarge) shows Illinois with an increased chance of warmer than normal conditions across Illinois and much of the US. Areas in Illinois along the Mississippi River are part of a larger area with an increased chance of being drier than average. My confidence in the temperature forecast remains high. I am less confident in the precipitation forecast. Some of their own precipitation forecasts that extend into early November suggest a persistent wet pattern over the next 4 weeks in Illinois.


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Jack Frost Getting Closer to Illinois

The latest NWS forecast has a frost advisory for northwest Illinois and an extensive area with freeze warnings that cover KS, NE, SD, MN, IA, and WI.
A frost advisory is issued when the minimum temperature is forecasted to be 33 to 36 degrees on clear, calm nights during the growing season. They are issued in the fall until the end of the growing season (marked by the occurrence of that first widespread freeze). Tender plants can be damaged by frost and should be covered or moved indoors.
Air temperature is measured at a height of 5 feet. Colder, denser air near the ground can drop below freezing even when the measured air temperature at 5 feet is 33 to 36 degrees.
A freeze warning is issued when significant, widespread freezing temperatures are expected. They are issued in the fall until the end of the growing season (marked by the occurrence of that first widespread freeze). Continue reading “Jack Frost Getting Closer to Illinois”

Warm Start to October in Illinois, Midwest

Illinois and the Midwest are off to a warm start in October, as this temperature departure map shows. Temperatures across southern Illinois, as well as southern and western portions of the Midwest are running 4 to 6 degrees above normal. Meanwhile, central and northern Illinois, as well as the rest of the Midwest, are running 6 to 10 degrees above normal.
So far, October precipitation has been light across the southern half of Illinois and for much of the southern Corn Belt. A band of 1 to 3  inches of rain extends from KS/OK through MO, IL, IN, and up into Michigan and another band extends up into NE, IA, MN, and the eastern Dakotas. Meanwhile, eastern IA and much of WI saw little precipitation.

Trends in September, October in Illinois

Changes in the climate of Illinois can sometimes be seen in the monthly temperature and precipitation and sometimes not. Unlike the other three seasons, fall in Illinois tends to be non-trendy (like my wardrobe). Here are the time series of temperatures and precipitation for September and October for Illinois.
All four plots show a considerable amount of year to year variability. Some decades were more volatile than others. Some of the biggest swings in temperatures in September occurred in the 1920s and 1930s. On the other hand, if you take out the three really wet Octobers of 1919, 1941, and 2009, the October precipitation is fairly consistent from one year to the next and usually within 2 inches of the 1981-2010 normal.
There are no significant trends in temperature over the last century. September precipitation has a small downward trend of 0.5 inches over the last century while October precipitation shows a slight upward trend of 0.6 inches over the last century. As a result, they largely cancel each other out.
We will cover November trends in a separate post, but it’s fair to say that November is much more interesting. The horizontal line in each graph is the 1981-2010 average (aka normal).

September Temperature

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Sixth Warmest September in Illinois


[Update 10/6/2016] New numbers released by NCEI indicate that September was the 5th warmest on record for Illinois, instead of the 6th warmest. The statewide average temperature was 70.9 degrees, 4.7 degrees above normal. That was 0.2 degrees warmer than the preliminary number I provided at the end of the month. It is tied in 5th place with 1998. The warmest September on record was a tie between 1933 and 1925 with a statewide average temperature of 72.2 degrees.
The average statewide temperature for January through September of this year was 57.8 degrees, 2.1 degrees above normal and the 5th warmest January through September on record.  Here is the month by month breakdown of statewide temperature departures for 2016. Every month, except May, has been warmer than the 1981-2010 normal.image010


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