Here are the median dates of the first time we hit 32 degrees in the fall. Since we don’t have reports of when frost occurs, we use 32 degrees as a proxy. It is no surprise that the frost date occurs earlier in northern Illinois than southern Illinois.
If you look closely, you may see differences of several days between neighboring sites. Nighttime temperatures, especially on calm, clear nights in the fall, can be very sensitive to the local conditions. Sites in town tend to be a little warmer than those in the countryside, and result in delaying the arrival of colder temperatures.
Chilly temperatures have already visited the northern part of the state this weekend. Mt. Carroll reported a low of 29 on September 23 and Elizabeth reported a low of 28 on September 24.
NOAA’s Climate Prediction Center released their latest monthly and seasonal outlooks today (Thursday). In the figure below, the outlook for October in Illinois is for an increased risk of above-normal temperatures and below-normal precipitation. The 3-month outlook for October-December in Illinois is for an increased risk of above-normal temperatures. Their precipitation outlook is neutral at this time.
That’s not the best news for drought recovery but it might make it easier on farmers for fall harvest.
One factor that could come into play this winter is El Niño. In fact, the CPC says an El Niño event is likely to arrive some time in September, according to their latest advisory. However, in the last two winters the Arctic Oscillation has played a major role in our winter weather. Two winters ago it was in the negative phase and dumped lots of cold air into Illinois. Last winter it was in the positive phase and prevented a lot of cold air from reaching us. Unfortunately, we can only forecast the Arctic Oscillation out to 14 days.
The statewide average precipitation for the first half of September was 3.77 inches, which is already above the monthly normal precipitation of 3.24 inches.
However, that precipitation was not distributed equally around the state. The figure below shows that large areas across eastern and southern Illinois received 3 to 6 inches of rain (shades of dark blue and green). Meanwhile, areas north of Interstate 80 and in western Illinois were much drier with amounts of less than 2 inches. Most of the heavy precipitation fell from the remnants of Hurricane Isaac that passed through Illinois on Labor Day weekend.
Reports from individual stations ranged from 8.08 inches for the CoCoRaHS station Bush (IL-WM-4) in southern Illinois to only 0.57 inches for the CoCoRaHS station Geneva (IL-KN-1) in northeast Illinois.
Large areas of Illinois were upgraded by one or two categories of the US Drought Monitor. Most notable is the reduction in two most severe drought categories: D3 and D4. The D4 category went from 7.8 percent to zero percent. The D3 category went from 69.6 to 6.96 percent – a ten-fold reduction. D4 was present in parts of 10 counties in northern Illinois. Much of the state still remains in D1 or D2 drought. We have an area in northeast Illinois categorized as just abnormally dry.
While the impact from TS Isaac has been remarkable, concerns remain for some lakes and rivers as well as shallow groundwater sources. The statewide average precipitation after Isaac is at 20.8 inches, still about 6.3 inches below normal.
In an earlier post, I discussed the sizable rainfall totals across central and southern Illinois from Tropical Storm Isaac. Its impact on soil moisture has been dramatic. We measure soil moisture at 19 sites across Illinois as part of the Illinois State Water Survey WARM program. Here are the 4, 8, and 20 inch soil moisture maps based on readings at midnight on September 3.
We measure the soil moisture as the fraction of water in the soil by volume. So a value of 0.4 means that 4/10 of the soil is water by volume. Or you could view it as 40 percent of the soil is water by volume.
Values in the 0.4 to 0.5 range means the soil is close to or at saturation;
Values in the range of 0.3 to 0.4 are typical of a moist soil;
Values less than 0.3 are getting dry;
values between 0.1 and 0.15 are pretty much at the lower limit of what plants can use.
Below are the maps for 4, 8, and 20 inches. For most of Illinois, the fraction of water by volume is in the range of 0.3 to 0.4 – that’s good, especially for this time of year. There are a few places over 0.4, which means they are getting close to saturation at that level of the soil. A month ago a lot of stations were in the 0.10 to 0.15 range, which is pretty much the lower limit of what plants can use.
These numbers suggest that soil moisture conditions in much of Illinois have improved dramatically after Tropical Storm Isaac. However, they still have room for improvement in northern Illinois. And the rains came too late for the corn crop and much of the soybean crop. Even so, it is reassuring that we have gone a long ways towards recharging the soil moisture as we move into fall.
By the way, you’ll notice the 0.06 values in west-central IL. Those are in an extremely sandy soil near Havana in the Illinois River floodplain. They drop quickly after a rain event and stay very low most of the year.