A Preliminary Analysis of Convective Reports Received by the NWS in Chicago (2000-2002)

During the summer of 2002, the National Weather Service in Chicago developed a database for all warm season (convective) weather reports. This database was then filled with logged reports back to January 2000. Now it is possible to sort and run statistics on weather reports called in by the media, law enforcement, and the public. The following is a study of the trends that are seen in the local severe weather report data from 2000-2002. Since there are only three years of data, this study is viewed as a preliminary look at the local severe weather report trends.

Severe Weather Report Distribution

Convective Warnings issued 2000-2002

There is certainly a downward trend over the past three years with severe weather reports. The total amount of warnings issued also decreases at about the same rate. Therefore, the drop off in severe weather reports is believed to be a result of the drop off of severe weather, and not a result of decreased spotter activity.

Convective Reports by Month

During the past three years, the severe weather reports were rather low until April, showing the slow start to the severe weather season. May and June were the most active months, followed by a steep drop off in July. The July drop off makes sense since climatologically. Warm temperatures aloft tend to decrease the hail reports, while weak winds aloft would tend to decrease wind reports. The August through October period was somewhat active, although not to the extent of the late spring months. The convective report in January is a result of a lightning strike injury.

Hail Report Trends

Severe Hail and all Hail Reports by Month

Local hail reports between 2000 and 2002 started slowly in March. An obvious peak of hail reports is seen during the months of April and May. During the months of June and July, there is a rapid decrease in reported hail, and indication of the warming temperatures higher in the atmosphere. The hail reports picked up from August to October, with October representing a hail maximum during the fall months. Rather unexpected though, is the ratio of severe hail reports to all hail reports is about equal each month. This shows that when severe hail reports increase, reports of smaller hail also increase at about the same rate.

Wind Report Trends

>49 knot winds and wind damage reports by month

Wind damage reports clearly dominate the number of actual recorded wind speeds that exceed 49 knots or 57 mph as expected. Storms produced the most amount of wind damage in May and June, with a secondary peak in August, as upper level winds strengthen again. A slight increase of wind reports in October from September may reflect the cold front induced squall lines typical for that time of the year.

Area Distribution of Reports

Severe Weather Reports by county

With only three years of data, the area distribution of severe weather reports may be slightly skewed towards locations that saw more active weather between 2000 and 2002. As expected, population density dominates where the reports come from. However, this is not always the case. LaSalle County is one example of a sparsely populated county which relays severe weather reports consistently. Another example is rural Benton county Indiana. Active spotter networks may be a reason for the increased weather reports from these locations. This affirms the need to expand local spotter networks.

Steve Rogowski
Warning Statistics supplied by Jim Allsopp
January 2003

For further information on local severe weather reports, our severe weather database output from 2000, 2001, 2002, and for the entire 2000-2002 period are supplied. Please note that this data is unofficial.

Last Updated 01-16-2003

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