The meteorology behind Monday’s Derecho, tornadoes confirmed to Stateline


ROCKFORD, Ill. (WIFR) – A well-organized, long-lasting storm complex produced significant and widespread wind damage across Iowa, northern Illinois and northern Indiana during the day Monday, August 10 . Much of that high wind was heavy (75 mph or more) causing many trees to fall, several overturned on semi-trucks, and many communities suffered at least some minor structural damage.

The derecho moved across several states. In total, the National Weather Service says there have been more than 600 reports of high winds (58 mph or more) from the Nebraska-Iowa border, passing through Iowa and northern Illinois before ending in northern Indiana. The wind was most extreme over Iowa and northern Illinois before the storms spread outward and brought slightly weaker winds to a much wider area.

A derecho produces a band of particularly damaging thunderstorm winds (in particular, gusts of at least 58 mph for most of its length with several well-separated gusts of 75 mph or more) over an area of ​​at least 250 miles long. These are primarily classified as straight line winds rather than tornadoes. The wind speed in a derecho can exceed 100 mph, which is equivalent to that of an EF1 tornado, but over an area much larger than a tornado would impact.

The Monday system is known as the “progressive” derecho with a main bow echo or curved squall line at the leading edge of the storm. This squall line contained the strongest winds.

This complex of severe and long-lasting thunderstorms has produced hundreds of damage reports and possibly a few tornadoes.(NWS Chicago)

Derechos thrive in an environment with very hot and humid air at the surface, colder air at higher altitudes, and moderate to strong winds at higher levels of the atmosphere. The “back surge” jet, as it is called, pushes the center of the squall line forward and causes storms to “tilt”.

The Stateline was quickly upgraded to a Category 4 moderate risk for severe weather with a 45% probability of high winds. A few hours earlier, the region was only subject to marginal category 1 risk. This shows how quickly the system has evolved.

SPC Day 1 Severe Thunderstorm Outlook released at 10:57 a.m. August 10.
SPC Day 1 Severe Thunderstorm Outlook released at 10:57 a.m. August 10.(Storm Forecast Center)

The National Weather Service in Chicago, which covers Rockford, is currently investigating storm damage. In their first investigation, officials confirmed a tornado that occurred in Rogers Park on the north side of Chicago. It was the first tornado within Chicago city limits since September 3, 2018.

In the Rockford area, the National Weather Service investigated a potential tornado that occurred on the northeast side of Rockford and on the southwest side of town. An EF-0 tornado touched southwest of Rockford and was 1.5 miles long with maximum winds of 75 MPH. On the northeast side of town, it advanced 9.2 miles into Caledonia and was rated EF-1 with maximum winds of 100 MPH.

An EF-0 and EF-1 tornado hit Rockford on Monday.
An EF-0 and EF-1 tornado hit Rockford on Monday.(WIFR)

Widespread storms swept across the Stateline and left hundreds of thousands of people across Illinois without power. Many tree branches were knocked over, decks and other outdoor furniture were blown up, power lines were blown, and several trees were uprooted. A tree was knocked over and fell on a couple’s home just outside Rock Valley College on Monday afternoon.

Officials say there were no deaths from the derecho on Monday. However, the National Weather Service says five injuries were reported during the storms in Forreston, Illinois. Village officials issued a declaration of disaster after 48 blocks of trees and cables occurred in the village as well as damage to the structure of 13 blocks.

In addition, a few storms, primarily along the Highway 20 corridor west of Rockford, produced significant hail, with several reports of golf ball ping pong hail near the Freeport area.

More than 100 of the reports of wind damage come from northern Illinois. Here is the full list of reports of high winds, rain and hail from the Stateline.

  • Strong wind reports
    • 92 MPH, Dixon: A private weather station observed wind gusts of 92 MPH just south of Dixon at 2:39 PM. The same station also reported 20 to 30 minutes of winds blowing above 50 MPH
    • 75 MPH, Lee: Winds estimated at 70 to 80 mph and penny-sized hail were reported near Lee in DeKalb County at 2:53 PM.
    • 62 MPH, Monroe: At 2:25 p.m. in Monroe, Wisconsin, a thunderstorm wind gust of 62 MPH was reported
    • 58 MPH, Waddams Grove: The home weather station in northwest Stephenson County reported gusts of 58 MPH and pea-sized hail
    • 54 MPH, Rockford Airport: At 3:23 p.m., Chicago Rockford International Airport reported its maximum gust of 54 MPH
  • Hail reports
    • 2 inches of hail, Freeport: Monday at 1:35 p.m., a public report of 2 inch hail (the size of a chicken egg or a lime) came from Freeport
    • 1 inch hail, Cherry Valley: At 2:53 p.m. Monday, a trained storm watcher reported 1 inch hail (quarter hail) in Cherry Valley
    • 1 inch of hail, Belvidere: At 3:00 p.m. on Monday, a trained storm watcher reported 1 inch hail (quarter hail) at Belvidere
  • Rain reports
    • 2 inches of rain, Freeport
    • 1.37 inches of rain, Stockton

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