Mesoscale Convective Systems (credit: Meteorologist Ash Bray)

​As if thunderstorms aren’t hazardous enough, sometimes they can combine into a large cluster known as a mesoscale convective systems (MCS). These thunderstorm clusters are smaller than our typical low-pressure systems with warm and cold fronts, but are larger in scale than a singular thunderstorm.

First discovered in the mid-1970s by infrared satellite imagery, they were originally named Mesoscale Convective Complexes--larger versions of MCS’s characterized by areal coverage and persistence of their satellite signature. Like major hurricanes and low-pressure systems, MCC’s have very distinct satellite signature. These storm clusters frequently occur in many parts of the world, including the southern and central United States, South America (the equatorial and mid-latitude areas), southeast Asia, Indonesia, and northern Australia.

So, why do these thunderstorms “cluster” and why are they so dangerous? The typical MCS that we see in the Midwest during the spring storm season becomes most active in the evening, lasting clear into the early morning hours. If you’ve experienced an MCS, you probably noticed an abundance of vivid lightning and multiple crashes of thunder on an otherwise peaceful evening. This may be a bit puzzling seeing as a major ingredient for thunderstorms is warm, humid air near the earth’s surface. Once the sun sets, wouldn’t you lose your main energy source? As it turns out, the cooler air near the surface at night helps with the intensification of a jet stream much lower than what a typical airliner uses. This low-level jet--roughly 1,500 to 3,000 feet above the surface, gliding atop the cooler air below--feeds the warm, humid air into the developing MCS. Thus, new thunderstorms form where the low-level jet collides with rain-cooled outflow from previous thunderstorms.

MCS’s become stronger overnight due to the increased instability from cooling at the cloud-top level--as cloud tops radiate energy back into space with no incoming solar radiation to absorb--teamed up with the latent heat, heat given off from condensation of water vapor into surrounding clouds. Latent heat alone may be enough to generate an area of low-pressure aloft, but behind the main line of thunderstorms in the MCS. This circulation, known as a mesoscale convective vortex (MCV), may last well after the thunderstorms in the cluster have died off the following morning. It may also appear in the satellite imagery, occasionally resembling an inland tropical storm. Interestingly enough, this MCV can help regenerate additional thunderstorms later in the day, even forming another MCS. An MCS can track over 1,000 miles and last in excess of 12 hours.
A major part of the impact of an MCS is determined by how quickly it moves. These systems can be quite difficult to forecast. In many cases, it’s pretty clear that an MCS will move in a particular direction, but forecasters may struggle to determine how long it will cast. In 2015, the PECAN (Plains Elevated Convection At Night) Field Program ventured out to collect data on nocturnal MCS’s in the Plains. Some of the data collected showed that forward propagation of an MCS versus backbuilding--standing in place--can make a huge difference in impact. While it is possible for an MCS to produce hail and tornadoes, there are three main impacts that occur when an MCS is in progress: Flooding rains, damaging winds, and immense lightning rates.

If an MCS stalls or moves rather slowly, flash flooding becomes a possibility. This typically occurs when the winds in the upper-level jet stream are relatively weak, and/or the aforementioned low-level jet is oriented towards the west or southwest edge of the MCS. Rather than thunderstorms clearing an area, new thunderstorms are produced and train over the same areas, bringing multiple rounds of heavy rain over an extended period of time. It’s not uncommon to have rain rates of several inches fall within an hour, quickly triggering flash flooding that’s particularly dangerous when occurring at night. Sometimes one area can see repeated MCS’s for weeks at a time! When this occurs, major riverbed flooding can become widespread. Two notable examples of this are the succession of MCS’s plaguing the Plains in May of 2015 in addition to the Great Mississippi River Flood of 1993.

Despite the flood dangers that accompany MCS’s  they also provide widespread spring and summer rainfall that’s needed to sustain crops in the Midwest. Without these storms, corn and wheat would shrivel up as the hot summer sun depletes moisture from the soil. Penn State University’s 1986 study found that 30%-70% of the rainfall between April and September over much of the area between the Rockies and Mississippi River comes from MCS’s. This study, lead by Dr. Michael Fritsch, also stated MCS’s are “very likely the most prolific precipitation producer in the United States, rivaling and even exceeding that of hurricanes.”

In other cases, MCS’s can be pushed forward by a more vigorous upper-level jet stream. When this occurs, damaging straight-line winds can cause tornado-like damages to powerlines, small buildings, and crops. Since these instances may occur at night, victims believe their property was indeed hit by a tornado (which can occasionally occur on the leading edge of an MCS). The more extreme version of this is called a derecho. These MCS’s feature massive wind damage at least 250 miles long, often producing gusts over 75mph. One of the more recent destructive derechos occurred on June 29th, 2012, traveling nearly 800 miles from Chicago to the Mid-Atlantic coast with an average pace of 65mph!

The lightning rates within an MCS can truly blow one’s mind. For example, the MCS responsible for a flash flooding event along the Gulf Coast in 2014, produced 6,076 cloud-to-ground lightning strikes in just 15 minutes--according to the University of Wisconsin-Madison’s CIMSS Satellite Blog. Another MCS occurred in Texas on June 11th, 2009, producing roughly 31,000 lightning strikes within a six-hour timeframe, including 7,500 in Dallas and Tarrant Counties alone! This is just a glimpse into the power of a large cluster of thunderstorms with thousands of collisions between graupel and ice crystals separating charge.

Another dangerous characteristic of an MCS is its ability to produce lightning well after the strongest portion of the storm has passed, occasionally lingering for an hour or more. Some MCS that produce lighter rain can still produce lightning strikes that contain a more powerful current than the average lightning bolt. When your area is in the path of an MCS, it’s best to practice lightning safety: Remain indoors, stay back from windows, and avoid using electrical or plumbing equipment until 30 minutes after the last lightning flash.

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© 2018 Meteorologist Ash Bray