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.


The outlook for November through January shows the southern third of Illinois with an increased chance of above-normal temperatures. The rest of Illinois is labeled EC for equal chances of above, below, or near-normal temperatures. All of Illinois is labeled EC on precipitation. Since they consider November in Illinois to have a higher chance of being warmer than normal, most of the uncertainty in this three-month period lies in the outlook for December and January.

December-February (Winter)

Here is the outlook for the core winter months of December through February. These maps most resemble the typical La Nina winter pattern (shown earlier) with an increased chance of above-normal temperatures south of Illinois and below-normal temperatures north of Illinois. Meanwhile, an increased chance of above-normal precipitation is expected in much of eastern and northern Illinois.

This will be a great forecast if La Nina manages to remain intact through this winter. However, with the odds at only 55%, as noted earlier, what happens if La Niña fades out? The Climate Prediction Center also maintains a set of climate models that make projections out to six months. For the past several months, this ensemble of models (below) has indicated that Illinois and almost all of North America have a chance for a warmer than normal winter. The precipitation forecast closely resembles the official precipitation forecast above with wetter conditions in Great Lakes region.

Other thoughts on this winter:

  1. There are other actors in the winter forecast besides La Nina. These include the North Atlantic Oscillation (NAO), other patterns in the Pacific including one called the Madden-Julian Oscillation (MJO), as well as above-normal ocean temperatures in the Atlantic and much of the Pacific Oceans.
  2. The historical trends in Illinois indicate a move towards warmer, wetter winters in recent decades. However, much variability still exists from one winter to the next.
  3. It is important to remember that most winters in Illinois are not 3 months of unrelenting bitter cold and snow. Most often they are mixtures of periods with mild winter weather and more severe conditions. Last winter included a very warm December with little snow, near-normal temperatures and below-normal snow in January, followed by above-normal temperatures and near-normal snow in February.

March-May (Spring)

For the core spring months of March through May, most of Illinois has an increased chance of above-normal temperatures. It’s EC for precipitation in Illinois; however, the northern states in the Midwest have an increased chance of above-normal precipitation.

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).


Snapshot of the NWS forecast, late morning October 12, 2016. 


Most of Illinois has not seen 32 degrees yet, according to this map from the Midwestern Regional Climate Center’s VIP program. On average, most of Illinois will see their first fall freeze sometime in October.


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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Fall Frost in Illinois


Fall frost near my office a few years ago, Jim Angel 2013.


Summary: the median dates for fall frost in Illinois range from early October in northern Illinois, to mid-October for central Illinois, and late October for southern Illinois.

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.

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September So Far – Warm and Rainy

So far September has been much warmer and somewhat wetter than normal for Illinois. The statewide average temperature for the month is 72.2 degrees, 4.3 degrees above normal. The statewide rainfall is 3.10 inches, 31% above normal for the month to date. Warm and rainy also applies to much of the rest of the Midwest.


Below are maps of the rainfall across the Midwest for September. The left panel is the actual rainfall, the right panel is the departure from normal. Rains have been widespread across the Midwest in September and generally near to above normal across much of MN, IA, MO, WI, IL, and IN. Meanwhile, MI, OH, and KY were mostly below normal. On closer examination, central and southern Illinois were near to above normal while northern Illinois has been a little drier.

The heaviest rains of 5 or more inches have occurred in two blue blobs (left panel): one blog from Kansas through southwestern Illinois; and another blob from NE and SD, eastward through IA, MN, and WI. The heavy rains in IA, MN, and WI may cause minor to moderate flooding in the next several days along the Mississippi River above St. Louis.


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