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Thunderstorms often form larger clusters known as mesoscale convective systems (MCS) that spread flooding rain, high winds and prolific lightning over multiple states in spring and summer.

An MCS is smaller than low-pressure systems attached to cold and warm fronts, but is far larger than any single thunderstorm.

T​hey were first identified in satellite imagery. In the mid-1970s, they were first named mesoscale convective complexes – basically large versions of an MCS – delineated by the expansive and persistent nature of its satellite signature.

E​xcept for an intense hurricane or low-pressure system, there are few more impressive blowups on satellite loops than an MCS, often resembling a large fried egg.

Satellite images pick up the expansive cirrus clouds, as well as the tops of thunderstorms that make up the MCS. In radar imagery, an MCS will usually appear as a line of thunderstorms either moving or seemingly stuck in place.

It's a common summer feature in the central U.S. According to a recent study of 22 years of radar data in the U.S., MCSs are most numerous in summer, peaking in June and July.

T​hey most often track through the Plains and Midwest from May through August, but also happen in the Deep South from fall through spring.

They're typical in many parts of the world, including sub-Saharan Africa, equatorial and mid-latitude South America, southeast Asia, Indonesia and northern Australia.

T​hey're usually most active at night. This may sound weird, since one ingredient for thunderstorms is a source of warm, humid air near the ground. Once the sun sets, don't you lose that energy source?

It turns out the cooler air near the ground at night helps the intensification of a jet stream much lower than what your typical commercial airliner uses.

This low-level jet – typically 1,500 to 3,000 feet above the ground, riding atop the cooler air near the ground – is what feeds warm, humid air to the developing MCS.

New thunderstorms form where the low-level jet collides with the rain-cooled outflows from thunderstorms that had formed earlier.

An MCS will also be stronger overnight because of the increased instability from net cooling at the cloud-top level – as cloud tops radiate (lose) energy into outer space with no incoming solar radiation to absorb – coupled with the heat given off from condensation of water vapor into clouds.

An MCS is a prolific lightning generator. A late-April 2014 MCS along the Gulf Coast produced 6,076 cloud-to-ground lightning strikes in just 15 minutes, according to the University of Wisconsin-Madison's CIMSS Satellite blog.

Given all that thunder and lightning, an MCS can certainly lead to a sleepless night.

Particularly insidious about an MCS is its ability to generate lightning well after the area of heaviest rain has passed, sometimes lingering for an hour or more.

Some of these lighter-rain MCS lightning strikes contain more current than an average bolt, with a greater potential to start a house fire or kill someone.

So when these thunderstorm clusters are nearby, it's best to remain indoors and avoid using electrical equipment and plumbing until 30 minutes after the last lightning flash.

It can also produce damaging winds. If an MCS is pushed forward by stronger upper-level winds, damaging straight-line winds can down trees and power lines, cutting power to thousands, sometimes in the heart of a sweltering summer.

Since this may happen at night, victims may claim their home was hit by a tornado, which sometimes do occur at the leading edge of the MCS.

The more extreme version of this is called a derecho. These MCSs bring a swath of wind damage hundreds of miles long, often with peak gusts over 75 mph.

One such derecho in August 2020 traveled 770 miles from southeastern South Dakota into western Ohio, with winds over over 100 mph over central and eastern Iowa. It inflicted over $12 billion in damage to homes, vehicles, businesses and crops, according to NOAA, making it one of the most damaging severe thunderstorm events since 1980.

Flash f​looding is often another danger. If an MCS stalls or moves slowly, flash flooding usually occurs.

This typically happens when winds at the upper jet stream level are relatively weak, and/or the aforementioned low-level jet is oriented toward the west or southwest edge of the MCS.

Instead of thunderstorms clearing an area, new storms form and track over the same area, bringing repeated rounds of heavy rain over several hours.

T​hat's what happened in the St. Louis flash flood of July 2022.

A​nother scenario for flash flooding is when two MCSs either merge or follow each other in succession over the same area.

Sometimes one area can see repeated MCSs for weeks at a time. When that happens, major river flooding can become widespread.

Two notable examples of this were the succession of MCSs in the Plains in May 2015, as well as the Great Mississippi River Flood of 1993.

B​ut their rain is a necessity. Despite the flood danger, MCSs provide the widespread late-spring and summer rain needed to sustain crops in the nation's heartland.

A 1986 study found 30 to 70% of the April through September rainfall over much of the area between the Rockies and Mississippi River comes from MCSs.

Without this widespread, soaking rain, the corn and wheat belts would shrivel up as the hot summer sun saps moisture out of the soil.

T​hey can generate their own weather systems. One fascinating thing about MCSs is their ability to manufacture their own area of low pressure that can trigger yet another MCS later on.

Heat released from the condensing of copious water vapor into clouds generates an area of spin in the MCS known as a mesoscale convective vortex, or MCV.

T​his circulation might persist long after the thunderstorms in the MCS have died off the following morning. Meteorologists can often detect this remnant spin in satellite imagery.

L​ater in the afternoon, that area of spin plus the warm, humid air mass can reignite thunderstorms that once again congeal to form an MCS.

T​hey can even spawn hurricanes. Yes, you read that right.

Occasionally, the areas of spin generated by an MCS have pushed into the Gulf of Mexico or off the Southeast coast, helping to initiate sufficiently persistent thunderstorms to form a tropical cyclone.

T​he best recent example was in July 2019, when a July Fourth MCS over the central Plains spawned an area of spin – an MCV. That spin then pivoted southward and eventually gave rise to Tropical Storm (later a hurricane) Barry a week later over the northern Gulf of Mexico.

A​ similar scenario eventually allowed Tropical Storm (later hurricane) Arthur to form off the Southeast coast in as July 2014 began. A​nd one of Houston's most damaging hurricanes, Alicia in August 1983, had its origin from an MCS near the northern Gulf Coast.

So the next time you notice an impressive blob on satellite imagery, it's probably an MCS and is having, or will have, significant impacts.

M​ORE ON WEATHER.COM:

'​Bolts From The Blue' A Lightning Danger Even When It's Not Raining

W​hy Flash Flooding Peaks In Late Spring And Summer

W​hy You Shouldn't Ignore Severe Thunderstorm Warnings

T​here's More Than One Way A Hurricane Forms

Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Twitter and Facebook.

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