National Weather Service Weather Forecast Office

Updated Report on the Climatology of Severe Weather

in the Chicago County Warning Area

1980 - 2003

When focusing on climatology we measure many variables similar to a doctor doing a diagnostic measurement. Instead of the blood pressure, we look at measurements of atmospheric pressure. Instead of body temperature, we measure air temperature. We identify naturally recurring long and short term atmospheric and oceanic features.�Climatology, studies of the interrelationships among climate variables, is one tool we use to help put in context observations of the atmosphere and oceans.

The intent of the work is to update a severe weather climatology work done in 1999 (Rogowski) to cover the period 1980-2003 for severe weather phenomena occurring in northeast Illinois and northwest Indiana.

The work now covers a 23 year period. Data used is defined as tornadoes of F1 (Fujita Scale) or higher, reported winds in excess of 50 knots (55mph) or higher, winds causing $5000 or more of damage, and hail greater than 3/4 inch diameter.

Tabulations were made of the hours during which these events were reported. Time was assumed to be when the event was first observed. Thus an attempt was made to isolate just the time element for spatial distribution. Information was obtained from "Storm Data".

Selected data has been broken down into various time or seasonal elements but maintained a separation of weather type.�All times are Central Standard using a 24 hour clock.

Yearly and Seasonal Data:

Graphs of all data types over the period of record clearly show the diurnal nature of these events. The peak for all elements is between 1700 and 1900 CST (5PM-7PM CST). Tornadoes (>=F1) are highly concentrated during the daylight hours from around noon to 8PM CST (2000 CST).�Annual hail distribution is strongly biased toward afternoon, decreasing rapidly after 2100 CST.�Because wind events >=49kts include some synoptic systems during the cool months as well as effects from decaying convective complexes especially in June, the diurnal focus is spread across most nighttime hours.

The spring months (March, April, and May) show us that most of the severe activity is held to the afternoon hours (1600PM-1800PM CST). Tornadoes, for example, focus between 1600 CST and 1800CST with only one instance in 23 years between midnight and noon.�It implies dependence of this phenomena on daytime heating.�Hail and the tornado/wind combo also have the diurnal bias but not as exclusively (nocturnal winds from synoptic storms in early spring add to the nighttime distribution).

The summer months (June, July, and August) shows the most numerical activity in severe weather.�Tornadoes continue to have a afternoon maxima but earlier, at 1400 CST. A few events even occurred at night. Activity is spread out diurnally. In contrast with spring severe weather events in the summer months show a more diverse time horizon but are still heavily biased toward afternoon and evening . While hail was not reported between 0300 and 0600 CST during this period of record during JJA, wind events were found during every hour of the day. The increase in wind between 0200 and 0400CST was due almost entirely to the month of June.��

The onset of autumn (September, October, and November) brings a decrease in severe weather as the convective regimes give way to synoptic scale systems.�Total events decrease as expected but distribution displays some unexpected traits. Tornado occurrence is low with a peak at 1700 CST.�However two events occurred shortly after sunrise (both in September).�Wind has a defined diurnal bias and it is suspected that some of the nocturnal events were synoptically related.�While the number of instances is low, hail showed a bimodal tendency with an early afternoon (1400CST) and near midnight maximum.�The majority of the 2300CST peak occurred in October.�It is difficult to associate this latter time with any daytime heating so they must be related to synoptic storms where deeper convective elements are forced in a cooler environment.

Winter (December, January, and February) is very minimal. Most reports are of high wind likely associated with synoptic features that are not convective.

BD, Aug 2004