 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. To learn more about other important education stories in atmospheric and oceanic science, be sure to click here! © 2018 Meteorologist Ash Bray
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 DISCUSSION: In the wake of what is now a departing Tropical Storm Chris after formally being Hurricane Chris over the past 24 hours or so, there still do remain one or two concerns along north-south oriented beaches along the Mid-Atlantic and the Northeast United States. Although it would now seem to appear that the primary threat this latest offshore tropical cyclone is now well offshore, there is still a very legitimate coastal concern or two which is on the mind of many. This is the threat of both rip currents as well as mild-to-moderate coastal beach erosion. Even when a hurricane or tropical storm is tens to hundreds of miles offshore from a given coastline, there is always a legitimate threat for there to be a prominent and persistent ocean swell and wave action along the chance even far away from the storm.
This is a result of the fact that closer to the storm’s immediate proximity there are much larger waves which are generated by the stronger wind’s much closer to the storm’s center of circulation. Hence, as these larger waves continue to move away from the center of what is now (as of this evening) Tropical Storm Chris, they will gradually begin to lose some energy and magnitude with increasing distance from Chris, but not nearly enough to eliminate all the potential and kinetic energy from these incoming waves. To clarify, potential energy with wave action is referring to the net amount of energy which may end up having the ability to reach a given coastline. On the flip side, kinetic energy with wave action refers to the net amount of energy which does ultimately reach a given coastline during some given period. Further, such wave action reaching a given coastline can also induce substantial amounts of regional coastal beach erosion. This a result of the unrelenting wave action acting to quite literally “tear up” coastlines and remove a lot of sand from both parts of the inner coastal shelf, beaches, and critical protective sand dunes which help to more effectively protect coastal communities. Lastly, tropical storms and/or hurricanes can also induce what are most commonly referred to as rip currents along coastal beaches normal to the axis of the incoming wave action. Rip currents occur as a result of strong incoming wave action racing back out to sea and creating locally strong undertows in the vicinity of the outgoing ocean water from the aforementioned incoming wave action. This is visually reflected the graphic attached above courtesy of the NOAA National Weather Service network. To learn more about other important education stories in atmospheric and oceanic science, be sure to click here! © 2018 Meteorologist Jordan Rabinowitz  DISCUSSION: As the Northern Hemisphere heads deeper in the Summer of 2018, it is no surprise that thunderstorm occurrence frequency is increasing along with corresponding rising average day-time high temperatures across many parts of the world. More specifically, across the South-Central, Central, and North-Central Plains states of the United States in North America, there is no question that thunderstorm activity frequency experiences a substantial increase. Having said that, one of nature’s greatest natural dangers is the well-known lightning strike.
With severe weather, many people across the United States and many other parts of the world know first-hand about how dangerous lightning can be. First off, it is important to note that an average lightning strike can have a maximum instantaneous temperature of around 53,000 degrees Fahrenheit as opposed to the surface of Earth’s Sun which has a temperature of around 10,000 degrees Fahrenheit. Second, lightning which is most commonly found directly in association with strong to severe thunderstorms in places all over the world can also sometimes strike many miles away from a given thunderstorm cell. Sometimes the reasons for such a displaced electrical discharge away from a given thunderstorm are not perfectly clear, but, the bottom line here is that it can and does happen at times. Thus, when you may have the curiosity during this Summer to go outside and watch an incoming thunderstorm, remember that lightning can strike both unexpectedly and unpredictably in many cases. Hence, always be sure to have the utmost respect for the natural power of thunderstorms during any season regardless of when they may occur. Remember the old phrase from the NOAA National Weather Service network: “When thunder roars, go indoors.” It may initially seem comical at the face of that phrase, but at some point in your life, this may just end up being a phrase which separates you from encountering a potentially life-threatening experience due to a run-in with one of Mother Nature’s most intense weather phenomena on the planet. Remember that you can always replace a memory card in a camera for the next thunderstorm event, but you can never replace a life. To learn more about other important educational topics in global atmospheric science topics, be sure to click here! © 2018 Meteorologist Jordan Rabinowitz |
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