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What's an MCS? This is...

Updated: May 1, 2019

If you read any of the NWS discussions (such as the Area Forecast Discussion), there's no doubt that the acronym MCS is tossed around quite a bit during the warm season.  MCS stands for mesoscale convective system.  These are not just a complex of thunderstorms as they are a "system" of thunderstorms.  If you fly during the summer months (most of us do), they will play a major role in your preflight analysis if you fly anywhere east of the Rocky Mountains.

Their typical signature can be found on the color-enhanced IR satellite as a large oval-shaped shield of very cold cloud tops like the one shown below.  It's not unusual to see them in pairs.  In fact, the clouds at the bottom of this image are convective debris from another MCS that had previously dissipated.

Tucked under these systems is a line of strong to severe thunderstorms that have a bowing structure.  As with any bow-shaped line you can expect some strong straight line winds.  In most cases either a very strong rear-inflow jet or a strong downdraft deforms the leading active line of convection into a bow-shaped arrangement.

In fact, when these persist for several days, they can lay down a path of wind-related destruction over a huge region.  These long lasting systems are called a derecho (Spanish word that is literally translated as "straight ahead").   Below is the path (red arrow) of storm reports for this system that developed in southern Iowa and curved to the southeast into western Tennessee.  Notice most of these are wind reports (blue triangles) with about a half dozen reports of sustained winds exceeding 65 knots (black squares).  The other reports mostly in Alabama are from the previous MCS.

The other detail you will notice in the NEXRAD loop above is the gust front ahead of the main line of convection.  This is where you'd expect to see the strongest straight line winds.  The outflow or "gust front" shows up on radar as lower reflectivity returns shown by the yellow arrow below.  This gust front is a low level event with the radar detecting dust, insects and other debris as well as the density discontinuity of the leading edge of the cold, dense outflow of air or "front."

If you currently get datalink weather in the cockpit from FIS-B or through SiriusXM, you will notice these gust fronts and other non-precipitation returns are typically filtered out.  The mosaic you see in the cockpit is scrubbed (QC'ed) to only show reflectivity from actual areas of precipitation.  Normally that's what you want to see. Here's what you would have likely seen on the radar depiction downloaded from the datalink broadcast.  Notice the lack of any gust front. Personally, I'd also want to see the unfiltered mosaic given that the gust front can often lead the primary precipitation by 10 to 20 miles in some cases.  Just when you think you are beating the precipitation to the airport, you are, in fact, timing it perfect to hit the gust front on short final.

What you will also see from these systems is a stratiform rain area behind the main line of convection.  While most of the lightning occurs along the primary line of storms, you can still get some strikes in that stratiform rain area.   Below is an example from an MCS that skirted just south of Charlotte a few days ago.  The primary region of heavy precipitation is at the bottom of the image with the stratiform area to the north.  Notice that most of the lightning strikes are in the region of higher reflectivity, but there are still several strikes in the stratiform area to the north.

Similar to hurricanes, MCSs are very seasonal. Occurring mostly east of the Rocky Mountains, they start out in the Southern Plains and Deep South during the month of May. As the jet stream moves north through the summer months of June and July, they tend to occur in the Central Plains, Middle Mississippi Valley as well as the Tennessee and Ohio Valleys. Finally, into July and August, they are seen more in the Northern Plains, Upper Mississippi Valley and Upper Great Lakes regions.

You were probably taught that the early morning hours are the best time to fly to avoid thunderstorms. That’s usually sound advice (especially in the western one-third of the U.S.) unless you are dealing with an MCS that will often develop and mature in the overnight hours and persist into the next day. So they are often nocturnal beasts that almost seem to create their own environment to feed on.  They also tend to occur in benign-looking upper level flow under a 500 mb ridge (shown below) as was the case for this MCS that moved southeast through the middle Mississippi Valley. When looking at the 500 mb pattern, they will form generally along the most southern line of constant height (588 dm line in this example).

Give the ridging pattern, they tend to occur in regions with high Convective Available Potential Energy (CAPE).  In some cases this CAPE can be very high.  For this system CAPE values approached 9000 J/kg!  In this case, the MCS had plenty of CAPE available along it's path (green dashed arrow) as shown below to maintain it's strength for an extended period of time.

You can also see this high CAPE in a forecast sounding from a point in extreme northeast Missouri.  The surface-based and most unstable CAPE value is more than 8700 J/kg!  That's some instability.

Most pilots are weatherwise, but some are otherwise™

Scott Dennstaedt

Weather Systems Engineer

CFI & former NWS research meteorologist

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