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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Hot, Humid Weather and Crops

Temperatures in Illinois this week have ranged from the upper 90s to the low 100s. At times the night-time lows have been in the upper 70s and low 80s as a result of the high humidity.

Emerson Nafziger, University of Illinois Extension, has a thoughtful discussion about the effects of these conditions on crops in Illinois. In general, high day-time temperatures are not a major concern for corn until they get above 100 degrees. Other potential problems include:  high night-time temperatures leading to higher losses of sugars available for crop growth; high humidity levels increasing the risk of foliar disease; and the lack of rain in parts of Illinois since the beginning of July leading to reduced photosynthesis.

You can read the full story on the University of Illinois web site “The Bulletin: Pest Management and Crop Development Information for Illinois” High Temperatures and Crops.

100 Degree Mark in Illinois

Based on preliminary reports, Geneseo and Bentley Illinois reached 101°F yesterday. Another nine places reached 100°F. They include Illinois City, Moline, Mt. Carroll, Rockford, Prairie City, Normal, Rantoul, Streator, and Urbana. The Chicago Botanical Garden was close with 99°F.

Here is the list with the last time they saw 100°F:

  • Geneseo: July 25, 2005
  • Bentley: August 11, 2010
  • Illinois City: July 24, 2005
  • Moline: July 17, 2006
  • Mt. Carroll: August 18, 1988
  • Rockford: July 10, 1989
  • Prairie City: July 26, 2005
  • Normal: July 26, 2005
  • Rantoul: July 22, 2002
  • Streator: June 26, 2009
  • Urbana: July 13, 2005 1995 (thanks Chris G.)

By the way, the last time Chicago at O’Hare reported 100°F was on July 24, 2005.

You notice all these sites were in central and northern Illinois instead of southern Illinois. Much of central and northern Illinois have been dry. As a result, more of the sun’s energy is devoted to warming up the surface and lower atmosphere and less for evaporation and transpiration in plants.

Heat Index Extremes for Illinois

Since heat and humidity is on everyone’s mind these days, I pulled out the record high heat index values that we calculated for the Illinois Climate Atlas from a few years ago.

For that analysis we looked at the few sites with long-term temperature and humidity records. Here is what we found for the highest heat index value at each site:

  • Chicago’s record is a heat index of 118 degrees on July 13, 1995 (temperature 100°F, relative humidity 50%)
  • Rockford’s record is a heat index of 119 degrees on July 13, 1995 (temperature 98°F, relative humidity 57%)
  • Peoria’s record is a heat index of 121 degrees on July 13, 1995 (temperature 99°F, relative humidity 53%)
  • Springfield’s record is a heat index of 118 on July 15, 1980 (temperature 98°F, relative humidity 56%)
  • St. Louis’s record is a heat index of 119 on July 13, 1995 (temperature of 100°F, relative humidity 51%)

As you may have noticed, most of the cities set their record during the deadly July 1995 heat wave. I included St. Louis because it is just across the river.

I have seen unofficial heat index values even higher from locations at smaller airports.  Some of those sites have reported heat index values in the mid to upper 120s. However, we don’t normally use them for record keeping. For one thing the humidity sensor has a reputation of becoming unreliable at times. For another thing the archive of those observations extends back to 15 years or less at most sites.

Dangerous Heat and Humidity in Illinois

The National Weather Service (NWS) has issued a variety of heat warnings and advisories for Illinois this week. The combination of temperatures in the upper 90s and high levels of humidity mean a greater risk of heat-related illnesses and death.

One measure that combines both the effects of temperature and humidity is the heat index. Explanations of the heat index can be found on Wikipedia and NWS. Below is a chart showing the heat index for a given temperature and relative humidity. If you like to do your own calculations, here is the NWS heat index calculator.

While the heat index incorporates relative humidity to give a better idea of what the temperature feels like, there are some important underlying assumptions. It assumes a person who is 5′ 7″, 147 lbs, walking at 3 mph, wearing long pants and a short-sleeve shirt, in the shade with a light breeze. It is estimated that working out in the sun would increase the heat index by 15°F.

Here are some resources to consider for monitoring the heat and what to do during the current heat wave:

HEAT INDEX
RELATIVE HUMIDITY (%)
Temp. 40 45 50 55 60 65 70 75 80 85 90 95 100
110 136
108 130 137
106 124 130 137
104 119 124 131 137
102 114 119 124 130 137
100 109 114 118 124 129 136
98 105 109 113 117 123 128 134
96 101 104 108 112 116 121 126 132
94 97 100 103 106 110 114 119 124 129 135
92 94 96 99 101 105 108 112 116 121 126 131
90 91 93 95 97 100 103 106 109 113 117 122 127 132
88 88 89 91 93 95 98 100 103 106 110 113 117 121
86 85 87 88 89 91 93 95 97 100 102 105 108 112
84 83 84 85 86 88 89 90 92 94 96 98 100 103
82 81 82 83 84 84 85 86 88 89 90 91 93 95
80 80 80 81 81 82 82 83 84 84 85 86 86 87
Category Heat Index Possible heat disorders for people in high risk groups
Extreme Danger 130° or higher Heat stroke or sunstroke likely.
Danger 105 – 129° Sunstroke, muscle cramps, and/or heat exhaustion likely. Heatstroke possible with prolonged exposure and/or physical activity.
Extreme Caution 90 – 105° Sunstroke, muscle cramps, and/or heat exhaustion possible with prolonged exposure and/or physical activity.
Caution 80 – 90° Fatigue possible with prolonged exposure and/or physical activity.

30-Day Dry Spot in Corn Belt

As of July 13, 2011, the map of 30-day precipitation departures (figure below) showed a large area of below-average precipitation extending from eastern Iowa, through Illinois and Wisconsin and into parts of Michigan, Indiana, and Ohio. At first, this was not a concern because of the wet conditions that occurred through much of the spring. However, the lack of rainfall and hot conditions in the first part of July have begun to dry out the topsoil in Illinois. The rainfall departures range from 0.5 to 3 inches below average across northern and eastern Illinois.

Comparing the NASS report for  July 5 and July 11:

  • the Northwest crop reporting district (CRP) went from 11% dry to very dry topsoil to 30% dry to very dry in one week
  • the Northeast CRP went from 4% dry to very dry to 29% dry to very dry in one week
  • the East CRP went from 12% dry to very dry to 37% dry to very dry in one week.

The potential loss of water from soils from evaporation and transpiration from crops is on the order of 0.2 inches per day in Illinois now (link to potential evapo-transpiration maps). That means that we need about 1.4 inches of rain per week in July just to keep up with the demand from the crops. If the crops don’t get the water from rainfall then they have to rely more on soil moisture. So a week with no rain and temperatures in the 90s can dry out the topsoil quickly.

According to the Crop Watchers in the Illinois Farm Bureau’s FarmWeek, corn and soybeans in the sandy soils and in the thin soils on the ridges are showing signs of stress. Today we have seen slightly cooler temperatures across Illinois, along with some rain. However, the NWS is forecasting a return of hot weather over the weekend. In addition the NWS 6-10 day and 8-14 day forecast show a greater chance of above-average temperatures across all of Illinois. Those forecasts show a greater chance of below-average rainfall across the southern two-thirds of the state and near-to-above average rainfall in the northern third of the state.

While northern Illinois has been dry, much of southern and western Illinois has received much-above average rainfall, by as much as 2 to 5 inches or more in many locations. The wet conditions have led to flooded fields and widespread planting/replanting delays.

30-day precipitation departure
The 30-day precipitation departure as of July 13, 2011. Area circled in red shows less-than-average amounts. Source NOAA. Click to enlarge.