NWS Lincoln, IL -- SWOP event summary

July 15-16, 2024

A powerful line of thunderstorms known as a derecho swept across eastern Iowa and the northern halves of Illinois and Indiana during the evening of July 15th into the pre-dawn hours of July 16th.  The National Weather Service defines a derecho as a swath of thunderstorm wind damage greater than 240 miles in length with wind gusts in excess of 58mph along the entire path.  

Surface analysis from the afternoon of July 15th revealed a 1004mb low over western Iowa with a weak cold front extending southwestward into Kansas...and a warm front arcing southeastward along the I-74 corridor.  The airmass within the warm sector of the storm system was hot and humid with temperatures in the lower to middle 90s and dewpoints in the middle 70s across central Illinois.

SURFACE MAP 4PM CDT JULY 15

Due to the increasing risk for severe weather, both NWS Quad Cities and NWS Lincoln conducted special upper air balloon flights to better assess the pre-storm environment.  The 19z/2pm NWS Quad Cities sounding showed extreme instability with a Surface Based Convective Available Potential Energy (SBCAPE) value of 6683J/kg.  The yellow-shaded area in the image below depicts the SBCAPE, which represents the energy source for any potential thunderstorms.  In addition, deep-layer wind shear was on the increase with 0-6km bulk shear of 46kt.  Note the very light easterly surface winds at 5kt just north of the warm front...veering to west-northwest and increasing to 40kt at 6km aloft.  This degree of wind shear suggests a high confidence for robust storm organization.

QUAD CITIES UPPER AIR SOUNDING 2PM CDT JULY 15

The upper air pattern was dominated by a prominent ridge of high pressure over the Desert Southwest and a deep low anchored near Hudson Bay, Canada.  In between these two features, fast west-to-east zonal flow was noted across the northern U.S.  The 500mb chart from the evening of July 15th showed the jet stream extending from the northern Rockies to the western Great Lakes, with an embedded short-wave (black dashed line below) across Wisconsin/Iowa.  This wave was enhancing deep-layer wind shear in the region, further aiding storm organization.

500MB MAP 7PM CDT JULY 15

As the short-wave trough interacted with the surface low over western Iowa, thunderstorms initiated north and northwest of Des Moines by mid-afternoon.

RADAR IMAGERY 330PM CDT JULY 15

Fed by the extreme instability and further invigorated by the approaching jet stream and embedded short-wave trough, thunderstorm clusters rapidly grew upscale into a fast-moving linear feature by early evening.

RADAR IMAGERY 530PM CDT JULY 15

RADAR IMAGERY 730PM CDT JULY 15

As a strong cold pool became established behind the squall line, the rear-inflow jet caused the line to bow forward as it accelerated east-southeastward into Illinois.  The radar image below clearly shows the bowing segments, where enhanced winds were most likely.  In addition, forecasters were keeping a close eye on the numerous kinks and notches forming along the leading edge of the storms as these were the most favorable spots for the development of fast-moving tornadoes.

RADAR IMAGERY 830PM CDT JULY 15

By late evening, the main push of the derecho had raced into north-central Indiana while the less intense trailing end had sagged southward toward the I-70 corridor.  Meanwhile further west, additional thunderstorms were firing along the outflow boundary left behind by the feature, aided by a 40-50kt 850mb jet streak oriented from the southern Plains northeastward into Illinois.  These storms remained semi-stationary, dropping copious amounts of rain from Macomb southeastward to just west of Lincoln.  Storm total rainfall across this area ranged from 6 to 8 inches, resulting in flash flooding across parts of Fulton County.

RADAR IMAGERY 1030PM CDT JULY 15

The July 15th derecho left behind a wide swath of wind damage primarily along and north of a Des Moines, Iowa...to Lincoln, Illinois...to Indianapolis, Indiana line.  The most significant and widespread damage in central Illinois occurred across Stark County and northern Peoria County eastward into Marshall and northern Woodford counties where numerous trees and power lines were blown down.  This was the location within one of the bowing segments of the derecho where wind gusts of over 80mph were common.  In fact, there were two reports of home weather stations logging winds in excess of 100mph near Speer in Stark County and in Dunlap in Peoria County.  Further southeast, many reports of trees being blown down were received...including some that blew onto vehicles and homes.  Significant power outages occurred as well, with many residents of north-central Illinois still without power as of the morning of July 17th.  In addition, the NWS is currently assessing storm damage to determine how many tornadoes spun-up on the leading edge of the storms.  This number will be updated in the coming days once all surveys are complete.   

JULY 15 STORM REPORTS