September

October 3, 2012

September in Illinois: Cooler and Wetter Than Normal

The statewide average temperature for September in Illinois was 64.6 degrees, 1.6 cooler-than-normal. This was the first cooler-than-normal month of 2012, and the first cooler-than-normal month since September of 2011. See graph below.
The statewide average precipitation for September was 4.9 inches, 1.7 inches wetter-than-normal. This is the first wetter-than-normal month in 2012. See second graph below.
Much of the September rainfall came from the remains of Hurricane Isaac that passed over Illinois on Labor Day weekend. Additional rains fell later, especially in south-central Illinois. In general, areas south of Interstate 80 had monthly totals in the 3 to 12 inch range. A few sites in that region reported over a foot of rain with the largest total at Centralia with 15.89 inches. See first map below.
Precipitation totals north of Interstate 80 were around 1-2 inches. One of the driest spots in the state was Elburn (Kane County IL-KN-30) with only 1.28 inches for the month. Chicago and Rockford were not far behind with O’Hare Airport reporting 1.76 inches while the Rockford airport reported only 1.74 inches for September.
By the end of September, drought conditions had eased somewhat according to the US Drought Monitor. Only 6.7 percent of the state was in the worst two categories of drought (D3 and D4). This compares to 70 percent of the state in the two worst categories at the end of August. Even so, 82 percent of the state still remained in some stage of drought at the end of September.

January-September

Even with a wet September, the January-September statewide precipitation total of 22.38 inches was 8.34 inches below normal and the fifth driest on record. Here are the top five:

1988 with 19.49″
1901 with 19.84″
1936 with 21.76″
1940 with 22.17″
2012 with 22.38″

Remarkably, the precipitation over the last two months has erased the precipitation deficit since January 1 in much of Fayette, Washington, Clinton, Bond, and Montgomery counties. Sizable deficits remain across much of Illinois, especially western and northern Illinois as well as far southern Illinois. See second map below.
The January-September statewide average temperature of 59.6 degrees was 4.1 degrees above normal. It was the second warmest January-September on record and just slightly cooler than the record of 59.7 degrees set in 1921.
Notes: “normal” refers to the 1981-2010 averages. Statewide temperature and precipitation records began in 1895.

temp-depart — Statewide temperature departures from normal for September in Illinois. Click to enlarge.

prcp-depart — Statewide precipitation departure from normal for September in Illinois. Click to enlarge.

IL_prcp_mpe_030_tot_20120929 — 30-day precipitation total through the morning of September 29 for Illinois. The 30-day map for September 30 failed to run. Click to enlarge.

IL_prcp_mpe_y2d_dev_20120929 — Year to date precipitation departures from normal through September 29 for Illinois. Click to enlarge.

September 17, 2012

First Half of September – Wet in Eastern and Southern Illinois

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.

IL_prcp_mpe_m2d_tot_20120916 — Precipitation totals for September, 2012, in Illinois through the morning of September 16. Click to enlarge.

September 4, 2012

Impact of Tropical Storm Isaac on Illinois Soil Moisture

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.

4inch-sep3 — Four-inch soil moisture, expressed as the fraction of water in the soil by volume. Click to enlarge.

8inch-sep3 — Eight-inch soil moisture, expressed as the fraction of water in the soil by volume. Click to enlarge.

20inch-sep3 — Twenty-inch soil moisture, expressed as the fraction of water in the soil by volume. Click to enlarge.

September 4, 2012

Rainfall Totals from Tropical Storm Isaac

Tropical Storm Isaac dropped a lot of rain on Illinois over the Labor Day weekend. As noted in an earlier post, tropical storms are rare in Illinois but they do happen. However, this one was especially interesting because the heavy rains fell over areas in drought across the state. In a normal year, such heavy rains would have produced widespread flooding in low areas, streams, and small rivers. But not this year. I have not heard any reports of significant flooding. This suggests that the dry soils absorbed most of the rainfall. And what little runoff was produced, ended up in streams that were much below-normal. More on this in another post.
The first map shows the 7-day rainfall totals from Tropical Storm (TS) Isaac. The areas in red had over 4 inches of rain. Areas north of Interstate 80 saw little of the rainfall.
The second map shows the 30-day rainfall totals. Parts of Illinois already had substantial rainfall before TS Isaac. With the addition of the rainfall from Isaac, much of central and southern Illinois had accumulated over 4 inches of rain. In fact, some areas had over 6 inches of rain (areas shaded in yellow or red).
The third map shows the 30-day rainfall departures from normal. Areas in green and blue are above normal. The areas in the two shades of blue are 2 to 4 inches above normal. The areas in purple and violet (?) are 4 to 8 inches above normal. Clearly, areas with much above normal rainfall will have a significant impact on the drought.