Fall Frost in Illinois

 

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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.

The actual date varies from year to year. For tender plants in the fall, subtract two weeks from the average date to be on guard against an early frost.

Open areas are usually the first to experience frost, while areas under trees are more protected. Homeowners can protect plants by covering them when a frost is expected. Plants near heated buildings sometimes are spared too. Because of the abundance of trees and warm buildings, we see frost arrive in town a little later than in the countryside.

Here is the median date of the first occurrence of 32 degrees in fall. Additional maps for spring and fall, 32 and 28-degree thresholds, etc. can be found on the frost webpage.

fall-freeze-32-median

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.

Precipitation

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.

Temperatures

Everything is pumpkin spice these days, including the shading on the temperature departure map below. Illinois and the Midwest were much warmer than normal for the month. The two lightest shades of pumpkin spice are 1 to 3 degrees above normal, while the darker shades common across Illinois were 3 to 5 degrees above normal. The unseasonably warm temperatures are likely to end by Monday after the next major system passes through Illinois on Sunday.

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When Is Fall? Astronomical, Meteorological, Climatological?

Here are some thoughts on Fall that I wrote in 2014 …

Illinois State Climatologist

00quad_1280x1024 The Quad – University of Illinois in fall color.

No doubt today (September 22) will be announced as the “first day of fall” because of the fall or autumnal equinox. However, that concept refers to the date when we get equal amounts of daylight and dark.  I don’t think it was ever intended that this astronomical event would be the start of fall. In fact, this equinox would be the start of spring in the southern hemisphere. So to be fair to everyone we should call it the September equinox and leave fall out of it.  ;-)

Climatologists and meteorologists prefer to use calendar months to define the four seasons in the US. For example, fall would start September 1 and end on November 30. Not only is this more convenient, because you can use monthly data, but it lines up better with the typical or average temperature pattern for Illinois…

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Lake Michigan Warmer than Average in 2016

Lake Michigan has been warmer than average throughout 2016, according to the NOAA Coastwatch site.

Here are what the current surface water temperatures look like across the Great Lakes. Temperatures on the southern end of Lake Michigan are in the 70s and in the low 70s or upper 60s in much of the rest of the lake. There are some spots along the Wisconsin shore that are in the low 60s and 50s.

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New Seasonal Outlooks for Fall and Winter

The NWS Climate Prediction Center released their new monthly and seasonal outlooks for October, October-December, and January-March. Illinois has an increased chance of having above-normal temperatures for the rest of 2016. That is no surprise since every month in 2016, except May, has been above-normal for Illinois.

According to the NWS, the current ocean/atmosphere conditions in the Pacific reflect so-called ENSO-neutral conditions. This means that we are not in El Niño or La Niña conditions. There is only a 55-60 percent chance of La Niña showing up this fall or winter. And if it does show up, it is likely to be a weak event.

In the NWS forecasts, the term equal chances (EC) is used to identify areas where there is no clear signal of how temperature or precipitation might behave. The other way to look at it is that those are areas without an increased risk of being much above or below normal.

October

For October, Illinois has an increased chance of above-normal temperatures. Western Illinois has an increased chance of above-normal precipitation. The rest of the state has equal chances (EC) of above, below, and near-normal precipitation. Click to enlarge.

October-December

For October-December, Illinois has an increased chance of above-normal temperatures. Illinois has equal chances (EC) of above, below, and near-normal precipitation.

January – March

For January-March, Illinois has equal chances of above, below, or near-normal temperatures and precipitation. There is an area with an increased chance of above-normal precipitation centered on Michigan that just touches northeast Illinois. However, I don’t give much credence to areas right on the edge of a particular zone.

September – Wetter, Warmer than Normal

September so far in Illinois has been warmer than normal with areas of widespread rain. The statewide average temperature is 72.4 degrees, 1.9 degrees above normal. The statewide average precipitation is 2.03 inches, 0.99 inches above normal.

Rainfall

The rains in September have been widespread across the state with heavier amounts of 3 to 5 inches east of St. Louis and across central Illinois (left panel, blue shading). That is about 1 to 3 inches above normal for the first 12 days of September (right panel, darker green shading). Click to enlarge.

Temperature

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Wettest August, Wettest July-August in Illinois

Summary: Illinois has experienced its wettest August (6.89 inches) and wettest July-August (13.74 inches) on record.

August

The statewide average rainfall for August was 6.89 inches, 3.30 inches above normal and the wettest August on record. It just beat the old record of 6.86 inches set back in 1977. The rainfall for this August is based on preliminary data and may change in the next several days as late reports trickle in. The highest rainfall total with reports on all 31 days was Sterling (IL-LE-5) with 14.01 inches from the CoCoRaHS network.

The statewide average temperature for August was 75.9 degrees, 2.3 degrees above normal. That is tied with 1943 as the 15th warmest August on record. The average high temperatures were close to normal. In fact, most of Illinois never got hotter than the low to mid-90s in August. However, the high humidity levels did not allow temperatures to cool off at night. As a result, nighttime temperatures were 3 to 5 degrees above normal.

July-August

Combine a very wet July with a very wet August, and you have the wettest July-August on record. The rainfall total of July-August was 13.74 inches, which is 6.07 inches above normal. It beat out the old record for July-August of 12.83 inches, set back in 1915. The highest 2-month rainfall total was Downers Grove (IL-DP-135) with 22.93 inches of rain.

Maps

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