September 2011 – Page 3 – Illinois State Climatologist

September 13, 2011

Possible Frost Wednesday Night in Northwest Illinois

The National Weather Service office in Davenport has issued this statement of a possible frost on Wednesday night:

Canadian high pressure will bring the threat of an early season frost by late Wednesday night. Areas along and north of Interstate 80 may dip down into the middle to lower 30s after midnight. These kind of temperatures will produce patchy to areas of frost by early Thursday morning. People in these areas should plan ahead and be ready to take the necessary precautions to prevent damage to cold sensitive vegetation. A Frost Advisory may eventually be issued for portions of the area. North winds maintaining 5 to 10 mph will help keep temperatures in the upper 30s south of I-80 and thus limit the frost potential in those areas.Normal 1st 32 degree temperatures range occur around early October in Northeast Iowa and Northwest Illinois, to mid October in West Central Illinois and Northeast Missouri.

You can check freezing temperatures from around the Midwest at the Midwestern Regional Climate Center…

Frost maps — Frost and Freeze map for the Midwest, using temperature thresholds of 32 and 28 degrees. Courtesy of Midwestern Regional Climate Center.

September 12, 2011

La Niña Is Back

According to the NOAA Climate Prediction Center press release, La Niña conditions returned in August and are expected to gradually strengthen and continue into this winter. This La Niña event first appeared last summer and continued in fall, winter, and early spring. By May, it had faded away. In fact, from May to July La Niña was not present in the Pacific according to the Climate Prediction Center. Only in August did it seem to return.
While La Niña events can continue for more than one year, it is very rare for one to disappear in spring and then reappeared in late summer. In fact, there are no other case like this in the historical records that go back to 1950, according to one of the primary indices used to measure La Niña and El Niño.
If La Niña continues to redevelop, the impacts on Illinois could include a fall that is warmer and drier than average and a wet winter all along the Ohio River Valley. The next set of seasonal outlooks from the Climate Prediction Center will consider the redeveloped La Niña. The new outlooks will be released on Thursday of this week.
La Niña is the name given to abnormally cold waters along the equator in the eastern Pacific Ocean. The changing weather patterns associated with La Niña effect not only the Pacific Basin but the U.S. as well. See the Climate Prediction Center link for more information current conditions and possible impacts.

September 6, 2011

August – Drought in Central Illinois

The statewide average precipitation was 2.06 inches, 1.6 inches below average and 56 percent of average. This was the 13th driest August on record since 1895, based on preliminary data.
The driest area was south of and parallel to Interstate 74 (Figure 1). Rainfall amounts in that region were generally an inch or less. One of the driest spots in Illinois was Lovington (Moultrie County in east-central Illinois) with only 0.03 inches for the entire month. Other familiar sites with extremely low readings included: Decatur with 0.15 inches, Springfield with 0.25 inches, and Quincy with 0.25 inches.
Only northern Illinois received average to above-average rainfall in August (Figure 2). One of the wettest spots in Illinois was Morris (Grundy County) with 7.02 inches, followed closely by Earlville (LaSalle County) with 7.01 inches.
Moderate to severe agricultural drought arrived in Illinois in August (Figure 3), according to the US Drought Monitor. This was the result of much less than average rainfall in July and August, as well as warmer than average temperatures. Damage to corn and soybean crops appears to be significant in places. However, the full extent of the damage will not be known until harvest.
Temperatures were above average for much of Illinois (Figure 4). Highs in the 90s and even low 100s were common across the state. The hottest spots in the state were Cairo with 103 degrees on August 4 and Bentley with 103 degrees on August 25. Both Quincy and Grand Chain Dam were close with 102 degrees on August 2 and August 4 respectively.
The statewide mean temperature (an average of both the high and low temperatures) was 74.9 degrees, 1.2 degrees above average.

Figure 1. August 2011 precipitation for Illinois. Click to enlarge.

Figure 2. August 2011 precipitation departure for Illinois. Click to enlarge.

drought monitor — Figure 3. US Drought Monitor for August 30 2011 for Illinois. Click to enlarge.