2019 Midwestern United States and southern Canada derechos

the **2019 Midwestern United States and southern Canada derechos** were 5 separate very intense derechos, as well as several other derechos, albeit weaker, associated with a major heat wave from July 19 through July 23, 2019. Several areas reported all time record highs, with Blackford County, Indiana breaking it's previous all-time record high of 106°F with a new record high of 109°F being achieved around 3:15 pm on July 20. Many areas were placed under Excessive Heat Warnings in advance, with vividly worded ones being issued by the National Weather Service offices in Indianapolis and Northern Indiana on July 18.

## Meteorological history

After a previous heat wave effected portions of the southern United States in early July, which saw average temperatures exceed 100°F in much of the southeast, a heat wave began on July 19 in the Midwest, with various locations in Indiana breaking record highs for the month of July.

The Storm Prediction Center had first noted the possibility of a progressive squall line on July 16 across the upper Midwest, where temperatures could briefly exceed 100°F on July 19, issuing a slight risk of severe thunderstorms for the Twin Cities-Southern Wisconsin area for the early afternoon hours of July 19, and another slight risk for later in the evening for much of northern Indiana, concerning the possibility of several progressive squall lines.

By July 18, a unofficial derecho probability system placed a 93% chance of a derecho on July 19, and a 97% on July 20, the Storm Prediction Center had merged the two slight risks into one on July 17, and had introduced a rare D2 moderate risk for the Twin Cities area on July 18.

### Twin Cities derecho (July 19)

Ahead of the system bringing the excessive heat, scattered thunderstorms developed around 6:30-7:45 am CDT on July 19 in eastern South Dakota, the Storm Prediction Center had by this point introduced a very rare high risk of damaging winds for the Twin Cities area, marking only a enhanced risk for tornadoes.

Several supercells merged into a squall line around 7:30 am CDT and became severe around 7:50 am CDT, the SPC issued a Particularly Dangerous Situation (PDS) Severe Thunderstorm Watch for southeastern North Dakota, northeastern South Dakota, far northwestern Iowa and southwestern Minnesota at 7:47 am CDT, noting the possibility for "Widespread damaging wind events of up to 70 mph and scattered very damaging wind events of up to 95 mph", and noting the squall line in the watch discussion.

As the squall line crossed the Minnesota state line from South Dakota, several severe thunderstorm warnings were in effect, a few noting the possibility for tornado-like wind speeds. As the storm continued eastward, a 113 mph wind gust was recorded at 10:27 am CDT near Redwood Falls, Minnesota, where trees were uprooted and houses were heavily damaged.

The storm continued violently towards the Twin Cities area, which was placed under a PDS Severe Thunderstorm Watch at 11:03 am CDT, noting the likelihood for "Widespread destructive wind gusts of up to 100 mph, with occasional instances exceeding 120 mph". At this point, a off-schedule SPC outlook was issued, marking a impromptu 75% "extreme" risk of damaging winds for the Twin Cities area.

The squall line reported a all-time record high wind gust of 132 mph near Waconia, Minnesota, or the equivalent to a low-end Category 4 hurricane, or high-end EF2 tornado. Various houses were damaged beyond repair in Waconia, with several trees being torn completely from their stump and into houses.

The squall line thankfully began to weaken before it struck the Twin Cities area, with Minneapolis/St. Paul International Airport reporting a maximum wind gust of 103 mph, with gusts reaching 120 mph at 12:27 pm CDT. Over half of Minnesota's population was left without electricity as the major line moved into Wisconsin at around 12:50 pm CDT, La Crosse reported a wind gust of 109 mph with actual winds of 87 mph as the squall line impacted the region, and the line continued to weaken.

The Chicago area, which was where the derecho seemed to be aiming, was placed under a PDS Severe Thunderstorm Watch at 1:32 pm CDT, noting a lower, 80% chance of damaging winds, and 60% chance of >65kt winds. The derecho began to weaken rapidly, with Kenosha reporting winds of 59 mph, and gusts of 81 mph at 2:41 pm, or just barely severe.

As the squall line entered the Chicago area, it weakened below severe limits and broke up, with O'Hare International Airport reporting the last severe winds of 57.1 mph and gusts of 77.5 mph at 3:31 pm. The strong thunderstorms associated with the remnants of derecho would attribute to another derecho later in the evening.

### Fort Wayne derecho (July 19-20)

The Storm Prediction Center had placed northern Indiana under a moderate risk of severe thunderstorms during the late morning hours of July 19, concerning strong winds. Temperatures across the region had reached 93°F by 5:30 pm, and scattered thunderstorms were in progress across northern Illinois, associated with the remnants of the Twin Cities derecho.

Several of the thunderstorms became severe between 5:30 and 6:30 pm, and merged into a squall line around 7:00 pm, the Storm Prediction Center issued a severe thunderstorm watch ahead of the squall line at 7:19 pm, noting the possibility for "Widespread damaging wind gusts up to 65 mph, with isolated wind gusts of 75 to 85 mph.", also noting the possibility for a few tornadoes, which would become a secondary hazard.

The squall line began to gain a bowing shape as it entered northwestern Indiana, with a wind gust of 83 mph being recorded at Gary. Widespread wind gusts of 70 to 75 mph occurred across northwestern into northern Indiana between 7:45 and 8:45 pm, with a new severe thunderstorm watch being issued, this one PDS, for much of central, east central and northeastern Indiana at 9:03 pm, concerning widespread winds of 75 to 85 mph, and isolated wind gusts of over 90 mph.

The squall line knocked out power in Lafayette, with Indianapolis International Airport reporting a gust of 89 mph ahead of the line, knocking a tree onto one of the runways. The squall line caused moderate damage in Kokomo between 9:45 and 10:00 pm, completely knocking out the city's power and damaging several buildings, as well as uprooting various trees. A personal weather station in Kokomo recorded a wind gust of 93.6 mph before being knocked offline.

The squall line marched into east central and northeastern Indiana, now a derecho, causing widespread power outages in Warsaw and Marion between 10:20 and 10:45 pm, and uprooting various trees in the region. Muncie Municipal Airport's last weather observation reported a thunderstorm, a temperature of 83.6°F and wind gusting upwards of 84.5 mph before it was knocked offline.

Significant damage occurred in Hartford City with the derecho, with a PWS in southwestern Blackford County recording a wind gust of 76.8 mph at 10:58 pm, although it survived and remained in use. Various houses were heavily damaged in Hartford City, and most of the city's trees were knocked down or uprooted, with the top of the Historic Blackford County Courthouse being knocked down into buildings below.

Fort Wayne International Airport sustained heavy damage as the dangerous squall line continued eastward, with a wind gust of 101.2 mph being recorded at 11:03 pm, a tree fell into the main structure, and a plane was flipped, killing 3 people. Several houses in and around Fort Wayne were heavily damaged, and all of Columbia City lost power.

The squall line started to weaken as it entered Ohio, with Toledo recording a wind gust of 83.5 mph. At 12:22 am, a downstream severe thunderstorm watch into northern Ohio was issued, although this one was not PDS. Various other locations across northern Ohio reported severe wind gusts, with the highest being recorded in Defiance, with a wind gust of 91.3 mph at 12:27 am.

The squall line was much weaker as it approached the Pennsylvania state line, and there was no downstream severe thunderstorm watch issued into Pennsylvania and New York, some of Pennsylvania recorded severe weather before the squall line diminished early in the morning of July 20.

### Rockford derecho (July 20)

Behind the Fort Wayne derecho, which had left more stable conditions in northern Illinois early on July 20, a few supercells merged into a squall line in northeastern Iowa. The squall line continued southeast, with a normal severe thunderstorm watch being issued at 1:13 am for northern Illinois and Indiana ahead of the squall line, which was showing a bowing pattern.

The squall line caused moderate wind damage across northwestern Illinois from 1:15 through 1:50 am, with some PWS' recording wind gusts of 70 to 75 mph, the stable environment greatly reduced the severity of the derecho, and through most of it's history, it maintained sustained wind gusts of 65 to 75 mph, unlike the Fort Wayne derecho, which had sustained wind gusts of 90 to 100 mph along the line.

The squall line continued into Rockford around 2:21 am, with a peak wind gust of 83.2 mph being recorded there. Some trees were knocked down and several houses were heavily damaged. A plane was heavily damaged. The squall line gradually weakened as it entered northeastern Illinois.

The squall line continued to weaken, and the severe thunderstorm watch was cancelled at 3:04 am. The line broke up and diminished through 4 am.

**THIS IS A WORK-IN PROGRESS!**