Cold Front Dropping South Across the Western U.S.; Watching Threat for Tornadoes and Flooding in the South

A cold front will push south across the Western U.S. into Tuesday with mountain snow and areas of gusty to high winds. An area of low pressure will form along this front on Tuesday and bring a potential for severe thunderstorms with tornadoes and excessive rainfall in the lower to mid Mississippi River Valley. To the north, heavy snow is possible in parts of the upper Midwest. Read More >

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Tornadoes In Northern Illinois Saturday Evening, June 5, 2010
Updated 6/10/2010

National Weather Service survey teams have confirmed twelve tornadoes moved across portions of northern Illinois Saturday evening, causing significant damage across portions of La Salle, Livingston and Kankakee Counties. Six of these tornadoes appear to have been associated with particular cyclic supercell storm.

Extreme Southwest LaSalle County
Streator
Second Streator
St. Anne
Second St. Anne
Livingston County (between Streator and Dwight)
Dwight (northern tornado)
Dwight (southern tornado)
Chatsworth
East of Dwight
Ottawa
Limestone-Kankakee

Here is the breakdown of the intensities of the June 5th, 2010 tornadoes.

EF-0:	6
EF-1:	2
EF-2:	2
EF-3:	2

The last tornado event to produce so many tornadoes in the NWS Chicago's area of responsibility was back during the Utica, IL tornado outbreak on April 20, 2004 when 13 tornadoes occurred.

Meteorological Overview:

Showers and thunderstorms had moved east across portions of central and northern Illinois during the overnight and early morning hours of Saturday morning. This left much of northern Illinois cloudy and somewhat rain-cooled after sunrise, which inhibited development of storms for a good portion of the day. However, clouds began to thin by mid-day across parts of north central Illinois, which allowed temperatures to climb into the lower 80’s during the afternoon hours. The following image is a visible satellite image at 202 pm CDT. Note the thinning cloud cover over La Salle and Livingston County.

Also note in the preceding satellite image the band of clouds extending from central Illinois west into northern Missouri. This is likely enhanced low level cloudiness along the outflow boundary from the morning showers and storms.

As the afternoon wore on, temperatures in the lower 80s combined with dew points in the lower 70s to make for very warm, humid and unstable conditions. Meteorologists measure instability in the form of Convective Available Potential Energy, or CAPE. The following image depicts the unstable character of the air mass using surface based CAPE at 8 pm CDT, with values of 2000-3000 j/kg over north central Illinois at that time.

At the same time, a mid level disturbance was approaching from the west. This disturbance was helping to increase mid level winds to 60-70 knots across the Midwest, resulting in strong deep layer wind shear, which would result in rotating storms to develop, which are known as supercells. The following image depicts 500 mb (around 18,000 feet MSL) conditions at 00Z (7 pm CDT). The orange colors denote the mid level wind speeds, indicating the jet maxima across Nebraska and Iowa at this level.

While increased deep layer shear supported the development of rotation within thunderstorms, low level shear was also increasing, which supported the potential for these storms to produce tornadoes. The outflow boundary which was lifting north from central Illinois likely aided in increasing low level shear and helicity, which led to an environment supportive of tornadic supercells across the area by the evening hours.

The following image shows surface to 1 km Storm Relative Helicity (SRH), which is a measure of the low level wind shear. Values of SRH approaching 400 m2/s2 are more than sufficient for tornado development, given the instability and deep layer shear present in the storm environment.

The Storm Prediction Center (SPC) produces a “Significant Tornado Parameter” which combines instability, deep and low level shear, and low level moisture in addition to a couple of other parameters favorable for tornado development. The next image is the SPC Significant Tornado Parameter for 01Z (8 pm CDT). Note the peak values near 7, just upstream of La Salle county.

Within this environment supportive of producing tornadic supercells, thunderstorms indeed developed across western Illinois around 7 pm CDT. These storms developed ahead of a cold front approaching from the west, as seen in this surface map from 00Z (7 pm CDT).