Pecatonica, IL F1 Tornado of August 28, 1990

An F1 tornado developed southeast of Pecatonica in Illinois on the early evening of August 28, 1990. This tornado was part of the same cold front, which sliced through northern Illinois and eventually dropped the Plainfield, IL F5 tornado later in the afternoon.

The Forecast

According to the NWS Chicago, IL (2015), the atmospheric conditions that led to this tornado were typical of those leading to most severe weather cases. An upper level shortwave trough (Figure 1) was moving through the Great Lakes, while a cold front was pushing through the area (Figure 2).

Figure 1. 500mb geopotential height contours at 12Z and pressure centers on August 28, 1990 (Plymouth State University, 2019).

According to the NWS Chicago, IL (2015), the National Severe Storms Forecast Center (NSSFC) in Kansas City, MO upgraded their thunderstorm outlook from SLIGHT to MODERATE risk of damaging storms. At 1:28 pm CDT, the NSSFC issued a severe thunderstorm watch for portions of northern Illinois.

Figure 2. Surface analysis at 21Z for August 28, 1990 with mean sea-level pressure (MSLP) contours and surface plots showing temperature (C) and dew points (C). Illinois is highlighted in orange and the storm responsible for an F5 tornado is circled in red (WPC, 2017, modified by Francis Lavigne-Theriault).

Thunderstorms began initiating around noon near the central Illinois and Wisconsin border. Around the same time, it became obvious that conditions across northern Illinois were favoring supercell storms. By 3:00 pm CDT, instability values had risen to an incredible 7000 J/kg (Figure 3). The cold front could be observed crossing the state line at the same time.

Convective Available Potential Energy (CAPE) for August 28
Figure 3. Convective Available Potential Energy (CAPE) at 18Z for August 28, 1990 (NWS Chicago, IL, 2015).

Temperatures ahead of the cold front were in the mid to upper 90s (F) with dew points in the upper 70s (F). According to the NWS Chicago, IL (2015), even with the lack of low-level shear, the impressive parameters for supercell growth along the cold front allowed a storm to produce tornadoes west of Rockford at 1:42 pm CDT. Eventually, a dominant storm emerged and ventured into the extreme instability axis (Figure 3) and strong shear, where it exploded to a height of 65 000 feet.
Figure 4. Sounding taken at Peoria, IL at 00Z (7:00 pm CDT) on August 28, 1990 (NWS Chicago, IL, 2015).

Figure 4 depicts the vertical profile of the atmosphere in Peoria, IL, which is roughly 100 miles from Plainfield, IL. The sounding shows large instability values and 50-60 kt winds at the mid-levels of the atmosphere, which contributed to favorable conditions for supercells (NWS Chicago, IL, 2015).

Series of radar images from 2:27 pm through 3:49 pm. Picture from the Natural Disaster Survey Report.
Figure 5. Series of radar images from 2:27 pm through 3:49 pm CDT showing pronounced hook echo as storm moved into northwestern Will County. Times shown in UTC from (a) through (h) (NWS Chicago, IL, 2015).

The tornado reached its maximum strength (F5) from 3:15 pm to 3:45 pm CDT ([e] through [h] in Figure 5) as it moved through Plainfield and Crest Hill. The tornado dissipated in Joliet after the parent supercell produced damage and fatalities across northern Illinois for four and a half hours.

Visible GOES satellite images showing the development of the tornadic thunderstorms.
Figure 6. Visible GEOS images showing development of tornadic supercell across northern Illinois at (a) 12:46 pm; (b) 1:46 pm; (c) 2:46 pm; and (d) 3:46 pm CDT on August 28, 1990 (NWS Chicago, IL, 2015).


According to the NOAA’s Storm Event Database (2019), an F1 tornado developed just southeast of Pecatonica, IL in Winnebago County at 1:42 pm CDT. The tornado travelled due south for three miles and had a maximum width of 103 yards to end north of Seward, IL. The tornado caused no fatalities or injuries but caused $25 thousand dollars in property damage. The storm responsible for this tornado would later re-intensify and develop an F5 tornado in Plainfield, IL.


NWS Chicago, IL. (2015). 25 Years Later: The August 28th, 1990 Plainfield Tornado. Retrieved from:

Plymouth State University. (2019). Plymouth State Weather Center. Retrieved from:

NWS Weather Prediction Center. (2017). Surface analysis 21Z Tue Aug 28 1990. Retrieved from:

NOAA’s National Centers for Environmental Information. (2019). Storm Events Database. Retrieved from: