Mesoscale Convective Systems: Why Thunderstorm Clusters Are Both Important and Dangerous

Jon Erdman
Published: May 24,2016
Thunderstorms can be hazardous enough. Sometimes they combine into a large cluster spread over multiple states.
Known as mesoscale convective systems (MCS), these thunderstorm clusters are smaller than low-pressure systems with cold and warm fronts, but larger in scale than any single thunderstorm.
They may even grow large enough to affect the weather for the next day in the surrounding area.
First identified in the mid 1970s from infrared satellite imagery, they were named mesoscale convective complexes, simply larger versions of MCSs delineated by areal coverage and persistence of their satellite signature.
Other than intense hurricanes or low-pressure systems, there are few more distinct satellite signatures than an MCC.

Infrared satellite image of a mesoscale convective complex over parts of Nebraska, western Iowa, and southeast South Dakota on August 8, 2010 at 6:58 p.m. CDT.
(UW-CIMSS)

They're quite common in many parts of the world, including the central and southern U.S., sub-Saharan Africa, equatorial and mid-latitude South America, southeast Asia, Indonesia and northern Australia.
Let's delve into the mystery of how these clusters form and the typical dangers associated with them.

Why They Cluster

A low-level jet (green arrows) above the surface helps feed the MCS during the overnight/early morning hours.

Visible satellite image of a mesoscale convective vortex centered over Missouri at 11:15 a.m. CDT on July 8, 1997.
(UW/SSEC/CIMSS)

The typical MCS in the nation's heartland in late spring and summer becomes most active at night, continuing into the early morning hours.
One of these may have kept you awake with an hour or more of vivid lightning and crashes of thunder piercing an otherwise tranquil night.
This may sound puzzling, since one ingredient for thunderstorms is a source of warm, humid air near the ground. Once the sun goes down, don't you lose the energy source?
It turns out the cooler air near the ground at night helps the intensification of a jet stream much lower than that which 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.
MCS will 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.
This so-called latent heat may be enough to generate an area of low-pressure aloft near, but behind the main line of thunderstorms in the MCS.
Known as a mesoscale convective vortex (MCV), this circulation may persist well after the thunderstorms in the cluster have died off the following morning. It may appear in satellite imagery, sometimes resembling an inland tropical storm.
Interestingly, this MCV can help instigate additional thunderstorms later in the day, perhaps forming another MCS.
Occasionally, these MCVs have pushed into the Gulf of Mexico, helping to initiate sufficiently persistent thunderstorms to form a tropical cyclone.
An MCS can track over hundreds, if not over 1,000 miles and can persist for 12 hours or more.

Most Common Impacts

A significant part of the impact of an MCS is determined by how fast it moves.
"MCS motion can be quite a forecast challenge," says Dr. Michael Coniglio, research meteorologist at NOAA's National Severe Storms Laboratory. "In some cases it's clear that the MCS will move in a certain direction but the forecasters may struggle with how long it will last."
Coniglio participated in the PECAN (Plains Elevated Convection At Night) field program in 2015, helping to gather data on nocturnal MCSs in the Plains.
Coniglio says forward propagation of an MCS versus backbuilding - essentially, standing in place - can make a huge difference in impact.
While an MCS can produce hail and tornadoes, there are three main impacts that dominate when an MCS is in progress in the U.S.
(BLOG: Stu Ostro on MCSs)

Damage to the Fischer Store Road bridge seen in Wimberley, Texas, on Sunday, May 24, 2015.
(Twitter/Bridget Spencer)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Flooding Rain

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 thunderstorms form and track over the same area, bringing repeated rounds of heavy rain over several hours.
Rain rates of several inches an hour quickly trigger flash flooding, particularly dangerous when occurring overnight.
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 are the succession of MCSs plaguing the Plains in May 2015, as well as the Great Mississippi River Flood of 1993.
Despite the flood danger, MCSs provide the widespread late-spring and summer rainfall needed to sustain crops in the nation's heartland. Without them, the corn and wheat belts would shrivel up as the hot summer sun saps moisture out of the soil.
A 1986 study from Penn State University found 30-70 percent of the April - September rainfall over much of the area between the Rockies and Mississippi River comes from MCSs. The study lead by Dr. J. Michael Fritsch postulated MCSs are "very likely the most prolific precipitation producer in the United States, rivaling and even exceeding that of hurricanes."

Damaging Winds

In other cases, the MCS is propelled forward by a more vigorous upper-level jet stream.
When this happens, damaging straight-line winds can down trees and powerlines, 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 feature a swath of wind damage at least 250 miles long, often with peak gusts over 75 mph.
One such derecho traveled roughly 800 miles from near Chicago to the Mid-Atlantic seaboard on June 29, 2012, with an average forward speed of 65 mph.

Satellite history of the June 29, 2012 Midwest-East derecho (Credit: NASA). Radar history compiled by digital meteorologist Ari Sarsalari.Over 4 million customers were without power and over 600 reports of high winds or wind damage were attributed to this derecho.

Infrared satellite image and 15-minute lightning strike data for the Pensacola flash flood of April 30, 2014 at 6:15 a.m. CDT. There were 5379 negative lightning strikes, denoted by teal blue minus signs, and 697 positive lightning strikes, denoted by red plus signs, in a 15-minute period ending at 6:15 a.m. CDT.
(UW-CIMSS)

Prodigious Lightning Rates

Lightning flash rates in an MCS can truly boggle the mind.
For instance, the MCS responsible for the Gulf Coast flash flood of late April 2014 produced 6,076 cloud-to-ground lightning strikes in just 15 minutes, according to the University of Wisconsin-Madison's CIMSS Satellite Blog.
A Texas MCS on June 11, 2009, produced roughly 31,000 lightning strikes in six hours, including about 7,500 in Dallas and Tarrant Counties alone over that time.
Such is the power of a large cluster of thunderstorms with millions of collisions between ice crystals and graupel separating charge.
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 your average bolt, with a greater potential to start a house fire or kill someone.
So, it's best when these thunderstorm clusters are near to remain indoors and avoid use of electrical equipment and plumbing until 30 minutes after the last lightning flash.
Given the nocturnal tendency of an MCS, we asked how late the PECAN team work into the overnight hours during their 2015 field program?
"The latest observations will generally be made at 4-5 a.m. local time,"  Dr. Coniglio said. "Our assets usually need to leave for their deployment in the late afternoon to early evening, making for long nights."
"The mornings are for sleeping on this project."
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.

 

MORE: Southern Plains Flooding - May 2015