National Weather Service Weather Forecast Office

A Climatology of Severe Weather Occurrences

in Northeast Illinois 1980-1999

Severe weather in this study was 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

     ** hail greater than 3/4 inch diameter

The intent of the work is to obtain some determination about when these severe weather phenomena occurs in northeast Illinois. Using the above criteria a tabulation was made of the hours during which these phenomena were reported. Time was assumed to be when the event was first observed. If multiple occurrences of the same activity were noted in the same hour, it was counted only once for that hour. Thus an attempt was made to isolated just the time element for spacial distribution. Information was obtained from "Storm Data" entries.

The geographical region was over northeast Illinois roughly bounded by a line from Rockford to Pontiac (Livingston County) to Watseka (Iroquios County) during the 1980-1999 period.

We have broken the data down into various time or seasonal elements but maintained a separation of weather type. One of the displays graphs shows both wind and tornadoes combined. Each is also displayed separately. All times are CST using a 24 hour clock. .ie. 1300 is 1PM, 1400 is 2PM etc.

Summation graphs of all data types over the period of record clearly shows the diurnal nature of these events. The peak for all elements is around 1700 CST (5PM). Tornadoes are highly concentrated in the daylight hours from around noon to 8PM CST (2000). Other phenomena show occurrences during all times of the day and night but obviously skewed toward the afternoon and evening hours. By further refining these data into other spacial realms we can get a better idea of the true climatology and perhaps related it to meteorological patterns.

The spring months (defined as March through May) show a highly diurnal flavor to most events perhaps in part because of the dependence upon the heating from the increasingly strong sun. A few instances of hail and wind occur during the early morning hours especially a little after midnight. This may be partially due to the longevity of synoptically induced convective storms.

Summer (June through August) has a peak of activity by number of hours. The maxima of activity, while still during the late afternoon, is much broader..beginning an upward trend just after sunrise especially for wind and hail events. During this time the solar heating is maximized and the rapid onset of activity during the morning reflects this.

Activity winds down during the autumn (September through November) while the late afternoon diurnal bias dominates once again. As expected the winter season (December through February) shows a minima of destructive storms although it is of note that anything that does occur is not necessarily driven by daytime heating. Some of the wind events during the autumn, winter and spring may not be related to convective storms.

Monthly Data

HAIL: The seasonal evolution from winter into early summer is accompanied by a corresponding jump in hail frequency during the daylight hours. The distribution during April, which jumps sharply at 1600 CST (4PM) broadens out into a much flatter time curve by June. This seems to reflect the dependence of hail producing storms on maximum surface heating. Later in the summer into autumn while the late afternoon cycle remains in tact the number of hours counted drops off considerably to almost nil in September.

WIND: Winds of 50 knots or greater are reported even during the coldest months. During January and February some if not most of these may be wind from large scale storms rather than thunderstorms. As one approaches and enters the spring months wind concerns increase and are again concentrated during the afternoon and evening hours. Closer examination of the data for April through June suggests a broader peak in time and a bit later in the day (on average) than hail. During may and particularly June there is a noticeable peak in strong wind events during hours shortly after midnight CST. One explanation may be that these reflect the remains of convective complexes which either formed over the Plains in response to forcing by the low level jet or were associated with still active synoptic scale systems.

By July this nocturnal maxima is over and the wind threat tails off rapidly by September. Still, a smattering of strong winds are reported into December possibly more related now to big synoptic storms. The diurnal cycle appears to be reversed in November suggesting any wind threat during this time of year is not from convection.

TORNADOES: At these latitudes storms that could produce a meaningful tornado (F1 or higher) are highly dependent upon both surface heating and/or active synoptic situations. This is reflected in the distribution both in month and time of day for this phenomena. By any measure the data shows a peak in activity for northeast Illinois during May with building activity in April and as a measurable threat continues into September. The highly diurnal nature of tornadoes is seen quite easily.

HOURLY OCCURRENCES

Looking at the data from another viewpoint we have broken it down to an hour by hour level for the types of threat. In this context the hour is delineated by month. Thus for each hour of the day what is the activity during each month.

HAIL.. During the predawn hours there is minimal activity although a slight bias toward the month of June is seen. June also seems to jump the gun as noon approaches. Later in the day the peaks shift toward April and May. The early evening hours (1800-1900) are particularly active in April and May.

WIND: Wind has a similar history to hail early in the day with a post-midnight peak hovering for several hours during the month of June. This month also starts the daily cycle off before noon (CST) relative to other months. During the late afternoon and evening hours wind problems are most common during the heat of the summer, June and July.

TORNADOES: Tornadoes are the most diurnally driven of these events. In any given hour between midnight and noon there is a very slim possibility of having to deal with this threat. Once past noontime (1200 CST) however things change. Activity begins to appear during the traditional months beginning in April. The data shows that the afternoon hours between 5PM and 7PM (1600-1800) during May to be the most active for tornadoes in the area. The threat extends into the evening hours especially during April.

While this brief climatology of 'severe weather' time of occurrence in northeast Illinois covers less than 20 years it does highlight the diurnal nature of these events. In attempting to explain some of the variations such as the nocturnal June maxima one can speculate on synoptic or climatological forcing mechanisms which come into play. Studies such as this can lead to a more complete understanding of the meteorological regimes which are reflected in the weather we see each day.

Acknowledgement

The majority of the work done in this study was completed by Mr. Steve Rogowski, a student at Penn State University, during a student voulunteer assignement at WFO Chicago. His efforts are much appreciated by the staff of this station.