Category: Heavy Rains

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July Rainfall in Illinois Varied from Too Dry to Too Wet

The statewide average rainfall for July in Illinois was 4.12 inches, only 0.29 inches above average. However, rainfall across the state varied widely from too dry to too wet.
Rainfall amounts in the northern third of the state were impressive with widespread areas in excess of 6 to 8 inches. The heaviest rains fell around Galena. A CocoRaHS observer (IL-JD-2) reported a monthly rainfall total of 19.21 inches while the nearby Galena NWS Coop observer reported 17.78 inches. In nearby towns:

  • Elizabeth reported 16.74 inches (NWS),
  • Freeport reported 12.08 inches (CoCoRaHS), and
  • Mt. Carroll reported 11.33 inches (NWS).
Chicago O’Hare airport had a July rainfall total of 11.15 inches, their wettest July on record, thanks largely to the 6.86 inches that fell in three hours on July 23.

The middle third of Illinois was exceptionally dry. Some of the smaller rainfall totals for July were just south and west of the heavy rainfall in northwest Illinois. One of the drier sites was Aledo with only 0.55 inches. Amounts of only 1 to 2 inches were common in central Illinois. This equates to rainfall departures that were generally 50-75 percent of average (or worse) across the region. Champaign-Urbana experienced its 20th driest July on record with only 1.58 inches, 3.12 inches below average. Springfield reported 1.09 inches and Peoria reported 1.66 inches.
The southern third of Illinois was wet, where amounts of 4 to 8 inches or more were common. The largest July rainfall total was at the NWS Du Quoin site with 8.88 inches.

July 2011 rainfall in Illinois
July 2011 rainfall in Illinois. Click to enlarge.

July 2011 rainfall departure for Illinois
July 2011 rainfall departure from average for Illinois. Click to enlarge.

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Heavy Rain in Chicago on July 23

Chicago has a new all-time daily record rainfall when 6.86 inches fell during the early morning hours of Saturday, July 23, 2011, at O’Hare airport. The previous daily record was 6.64 inches set on September 13, 2008 (a little less than three years ago).
Even more amazing, the 6.86 inches of rain fell in slightly over three hours from 1 to 3 a.m. According to the ISWS Bulletin 70, the so-called 100-year, 3-hour storm for the Chicago area is 4.85 inches. Obviously, this storm far exceeded that. On a side note, the phrase “100-year storm” is a misnomer because it implies that you have a space of 100 years between storms. The slightly better phrase is “a once in 100 year return period on average“.  It’s best to think of it as the size of storm with a 1% chance of occurrence in any given year.
The folks at the Iowa Environmental Mesonet of the Iowa State University Department of Agronomy produced an amazing plot of the rainfall at O’Hare using 1-minute data. The black line is the hourly rate over 1 minute (rainfall rate in inches/hour). The green line is the hourly rate over 15 minutes. The red line is the hourly rate over 60 minutes. The blue line is the accumulated rainfall over time. The point of the 1-minute and 15-minute lines is to show that it was not a steady rain but contained several short periods with tremendous rain rates. One of those minutes exceeded 8 inches per hour (o.14 inches in one minute X 60 minutes = 8.4 inches/hour).
You can read more about the July 23 event at the Chicago NWS office link. Several CoCoRaHS observers had amounts that were even higher than O’Hare including two Arlington Heights observers with 7.25 inches (IL-CK-87) and 7.14 inches (IL-CK-81), one in Des Plaines (IL-CK46) with 7.24 inches, and one in Elks Grove Village (IL-CK-63) with 7.17 inches.

July 23 2011 Chicago rainfall
Plot of the 1-minute, 15-minute, and 1-hour rainfall rates and accumulated rainfall for the record July 23, 2011, rainfall in Chicago at O'Hare Airport. Figure courtesy of Daryl Herzmann, Iowa Environmental Mesonet. Click to enlarge.

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Wet Year So Far in Illinois

The statewide precipitation for June is 6.6 inches, 2.5 inches above the 1971-2000 average (normal). That makes it the ninth wettest June on record (based on preliminary numbers as of June 28, 2011).
The statewide precipitation for the first six months of 2011 is 27.2 inches, 7.7 inches above the 1971-2000 average. That makes it the fourth wettest January-June on record. Statewide records go back to 1895.
The figure below shows the June departure from normal precipitation across the US. Cool coolers (green, blue, purple) show abnormally wet conditions. Warm colors show abnormally dry conditions. Wet conditions have prevailed from Montana, into the Dakotas, Nebraska, Iowa, northeastern Missouri, across Illinois, and much of the lower Ohio River valley. However, dry conditions are not far from Illinois borders, especially to our southwest.

June rainfall across the US (courtesy NOAA).
June departure from normal precipitation across the US (source NOAA). Click to enlarge.

The figure below shows the year-to-date departure from normal precipitation across the US. Like June, the band of above-normal precipitation extends from Montana, down the Missouri River, and up the Ohio River valleys. This area has seen abundant atmospheric moisture and a strong, persistent jet stream for much of winter and spring. Meanwhile, the Southwest and South have struggled with dry conditions. BTW, it is not unusual to have one part of the US experiencing drought while another part experiences heavy rains and flooding. It is amazing that you can go from southern Illinois with its 8 to 16 inches of above normal precipitation to Arkansas and Mississippi and find areas that are 8 to 16 inches below normal on precipitation.
Year to date precipitation departure for the US.
Year to date departure from normal precipitation across the US (source NOAA). Click to enlarge.

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Record-Setting April Rainfall in Illinois

Record-Setting Rainfall

[Updated May 2]
Based on preliminary data, the statewide average rainfall for April (as of May 2) is 7.59 inches, beating the old record of 7.13 inches set in 1957. Statewide records go back to 1895.

Rain, April 2011 (click to enlarge).

Rain, percent of average, for April 2011 (click to enlarge).

Selected Rainfall Totals

April rainfall has been heaviest in southern Illinois with several stations reporting over 10 inches of rain. Here are the rainfall totals through April 30 across the state, ranked from high to low. These are provisional and may change as more data comes in and as the data are processed through quality control at the national archive.

16.90 DU QUOIN 4 SE
15.29 BROOKPORT DAM 52
15.13 CAIRO 3 N
15.10 SMITHLAND L&D
15.04 MURPHYSBORO 2 SW
14.84 CARBONDALE SEWAGE PLT
13.94 CHESTER
13.92 GRAND CHAIN DAM 53
13.74 HUTSONVILLE PWR PL
13.68 CENTRALIA
13.31 OLNEY 2S
13.18 ROBINSON
12.97 SHAWNEETOWN OLD TOWN
12.56 LAWRENCEVILLE
12.53 MT VERNON 3 NE
12.40 PALESTINE 2W
12.15 MT CARMEL
11.92 KASKASKIA RIV NAV LO
11.91 PINCKNEYVILLE 2 N
11.87 CLAY CITY 6 SSE
11.87 CLAY CITY 6 SSE
11.78 CASEY
11.71 KINMUNDY
11.67 HIDALGO 3SW
11.66 MARTINSVILLE 10S
11.37 BEECHER CITY
11.12 EFFINGHAM SE
11.06 NEW ATHENS
11.04 FAIRFIELD RADIO WFIW
10.98 NEOGA
10.67 IUKA
10.54 EFFINGHAM
10.49 FLORA
10.47 SALEM
10.32 GRAYVILLE
10.29 PARIS WTR WKS
10.23 MATTOON
10.22 SULLIVAN
10.22 VANDALIA
10.19 RAMSEY
10.08 NASHVILLE 1 E
9.81 CHARLESTON
9.60 NEOGA 4NW
9.25 ST DAVID
9.07 FISHER
9.01 RANTOUL
9.01 GREENVILLE
9.00 SHELBYVILLE DAM
8.97 CARLYLE RSVR
8.73 WINDSOR
8.70 LEBANON
8.55 PRAIRIE DU ROCHER
8.49 HIGHLAND
8.40 TRIMBLE 1E
8.32 PANA
8.17 DANVILLE SEWAGE PLAN
8.02 TUSCOLA
7.94 EDWARDSVILLE 2 W
7.91 MT OLIVE 1 E
7.91 BELLEVILLE SIU RSRCH
7.87 TAYLORVILLE
7.81 ALTON MELVIN PRICE
7.72 DANVILLE
7.72 HOOPESTON 1 NE
7.66 PEORIA 5NW
7.59 ARTHUR 1W
7.50 GERMANTOWN HILLS
7.44 NEWMAN 3W
7.42 URBANA
7.38 PAXTON
7.35 BOURBONNAIS 3 NW
7.33 PEORIA GTR PEORIA AP
7.33 SIDELL 5 NW
7.32 ST ANNE
7.27 HAVANA
7.21 MOWEAQUA 2S
7.11 PHILO
7.10 DECATUR
7.08 LOVINGTON
7.02 OGDEN
6.95 MORRISONVILLE
6.94 MILFORD 5 NW
6.85 WATSEKA 2 NW
6.70 CLINTON 1 SSW
6.65 DAHINDA
6.52 SAYBROOK 2N
6.42 STREATOR
6.38 PARK FOREST
6.32 KINCAID
6.30 MARSEILLES LOCK
6.29 JERSEYVILLE 2 SW
6.27 CAMP GROVE
6.27 SHERMAN 1 ESE
6.24 CHANNAHON DRESDEN ISLAND
6.22 OTTAWA
6.19 DWIGHT
6.18 SPRINGFIELD #2
6.16 CHENOA
6.15 STANFORD 2S
6.08 PRINCEVILLE 2W
6.08 KEWANEE 1 E
6.04 BLOOMINGTON 5W
6.02 ROMEOVILLE LEWIS UNIV AP
6.02 PONTIAC
6.00 SPARLAND 6 SW
5.96 LANSING
5.95 LITTLE RED SCHOOL HSE
5.91 NEWARK 2 SSE
5.86 ELBURN
5.86 LA GRANGE
5.84 KNOXVILLE
5.83 FARMER CITY 3W
5.81 MASON CITY
5.73 CHICAGO MIDWAY AP 3 SW
5.73 ROANOKE
5.70 YORKVILLE 3 SW
5.65 ST CHARLES 7 NW
5.62 ELGIN
5.62 BRADFORD 3SSE
5.60 ELIZABETH
5.57 MUNDELEIN 4 WSW
5.55 MORTON
5.50 JOLIET BRANDON RD DM
5.49 NORMAL 4NE
5.49 NORMAL 4NE
5.46 SPRINGFIELD CAPITAL AP
5.45 PERU
5.45 PERU
5.45 MINONK
5.42 ALTONA
5.36 CARLINVILLE
5.35 EARLVILLE 3 S
5.35 MORRIS
5.33 CHICAGO BOTANICAL GARDEN
5.32 MENDOTA 2 SE
5.32 OAK BROOK 2W
5.32 JACKSONVILLE 2
5.31 SPRING GROVE
5.30 CHATSWORTH
5.30 WINCHESTER
5.28 EUREKA
5.25 PLAINFIELD 3 NE
5.22 LINCOLN
5.21 ALBERS 1 W
5.16 WOODSTOCK 5 NW
5.13 STREAMWOOD
5.08 BARRINGTON 3 SW
5.08 GLEN ELLYN 4 S
5.06 JACKSONVILLE 2 E
5.04 MOLINE QUAD CITY INTL AP
5.01 PRAIRIE CITY 2S
5.01 CONGERVILLE 2 NW
4.99 LAKE SPRINGFIELD
4.90 CHICAGO OHARE INTL AP
4.88 MCHENRY STRATTON L&D
4.87 VIRGINIA
4.80 MACKINAW
4.78 PITTSFIELD #2
4.78 CHICAGO MIDWAY AP
4.73 MORRISON
4.73 BLOOMINGTON WATRWRKS
4.68 BUFFALO
4.65 QUINCY DAM 21
4.63 WHITE HALL 1 E
4.57 RIPLEY
4.54 FAIRBURY WWTP
4.54 FREEPORT WWP
4.51 TOULON
4.44 AUGUSTA
4.41 DE KALB
4.35 MT PULASKI
4.35 GREENFIELD
4.34 HARVARD
4.33 ATHENS 2 N
4.30 STEWARD 3S
4.25 GENOA 2 SW
4.24 KANKAKEE METRO WWTP
4.21 CISCO
4.20 ROCHELLE
4.20 ROCHELLE
4.18 RUSHVILLE
4.16 ALEDO
4.14 GALESBURG
4.11 SHABBONA 3S
4.11 QUINCY RGNL AP
4.04 PERRY 6 NW
3.98 BELVIDERE
3.97 MONEE RSVR
3.94 BENTLEY
3.91 ROSCOE 2 SE
3.71 STOCKTON 3 NNE
3.66 BEARDSTOWN
3.65 AMBOY
3.62 PAW PAW
3.61 MT CARROLL
3.47 WHEATON 3 SE
3.45 MARENGO
3.40 ROCKFORD GTR ROCKFORD AP
3.20 GRIGGSVILLE
2.61 MONMOUTH 4 NW