The Incredible Super Derecho Of June 29th 2012

Hi it's Rebecca again, After the destructive derecho last Friday; I thought I would do a write up about it. This post will explain what a derecho is as well as talk about the timeline of the super derecho on the 29th. 

What is a derecho? 

A derecho is a widespread and long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms that assume a curved or bowed shape. The bow-shaped storms are called bow echoes. Bow echoes typically arise when a storm's rain-cooled outflow winds are strong, and move preferentially in one direction.  Although tornadoes can be produced by a derecho most of the damage is from straight line winds. Normally if the wind damage swath extends more than 240 miles and includes wind gusts of at least 58 mph (93 km/h) or greater along most of its length, then the event is called a derecho. The most severe derechos are given the adjective “super.” Derechos fall under the heading of  Mesoscale Convective System ( MCS).  A MCS is a  complex of thunderstorms which becomes organized on a scale larger than storm scale and smaller than synoptic scale.

In meteorology there are four basic weather scales.

 The largest scale is synoptic-scale (also known as large scale or cyclonic scale) is a horizontal length scale of the order of about 600 miles or more. This is the  high and low pressure systems you hear mentioned on TV weather reports (e.g. extratropical cyclones).

Meso-scale meteorology is the study of weather systems smaller than synoptic scale systems but larger than  storm-scale systems. Horizontal dimensions generally range from around 5 miles  to few hundred  miles or so. Examples of meso-scale weather systems are sea breezes, squal lines, and mesoscale convective complexes.

Storm-scale is a scale of sizes of weather systems on the order of individual thunderstorms.

Miso-scale is the scale of meteorological phenomena that ranges in size from a few hundred feet  to about 3 miles. It includes rotation within a thunderstorm.

Well  I don't know about you but to me that's a lot of gobblygook.  Therefore I will give this definition:  A derecho is basically a large cold pool of air that gets dragged down from the upper atmosphere. When the cold air hits the warm moist air mass, storms explode and they self propagate.  They will keep going as long as the air mass being pushed into has enough warm moist air for it to feed on. Once on the move they produce incredible straight line wind damage over hundreds of miles.

The Set Up:

The set-up was classic for a derecho. We had extreme instability and extreme heat over the entire  region. Also there were other conditions present at are necessary for the development of a derecho.  The first is a strong jet streak overhead (See image 1).   Having a midlevel SE flow ( see fig 2). It is also an aid to development.  Third there was a boundary zone that was separating a dry and cool air mass to the north from the humid and hot air mass to the south.

                                                                              fig 1

fig 1 shows where the 250 mb jet streak was located.

                                                                                  fig 2

fig 2 shows the 500 mb level (18,000) feet.  the lines (isotachs) show supporting winds (the yellow arrows show the direction). when you see isotachs lined up like this it means there is a very strong high pressure system over the SE U.S. A  setup like this aids in the establishment of a derecho. On the 29th there was a moderate to strong vertical wind shear above ground in the lowest 2.5 km. The upper flow was also northwesterly. This type of setup allows small impulses to ride along the northwest flow at the edge of the upper-level heat dome to the south. . The winds at the 500 mb level also help stear weather systems like the super derecho.

 
All of these conditions were more than met across the  Midwest and Mid-Atlantic on the 29th  as there was extreme instability in place 6000 MLCAPE across Kentucky  and Ohio with 4500s into West Virginia. The heat dome over the region had been responsible for hundreds of record highs being broken during the days preceding the derecho. On the 29th the surface air and dewpoint temps were at record high for June; many place were in the mid 90's into low 100's. with dewpoint in the mid 60 to low 70's. there was a strong upper level jet streak just to the north of this region and extreme heat enveloped the entire region along with a boundary zone set up just to the north.

  
A blog post I did that covers a bit more on derechos can be found here.

A blog post that talks about the "Ring Of Fire" by my good friend Andy Gregorio can be found here.

The June 29th Super Derecho.

The derecho on the 29th began as a cluster of storms that developed in Eastern Iowa. As it marched east into Northern Illinois and Indiana. The cluster  erupted during Friday afternoon near Chicago, IL and then rapidly grew in intensity and coverage as they raced southeastward.  The MCS developed into a bow echo in Indiana as it continued to intensify. Over Ohio it matured into a derecho . The derecho  would continue expand and envelop  larger and larger areas as it headed into the Mid-Atlantic. The derecho reached the coast at around midnight.  The path of extreme damage was about 650 miles long in just 11 hours.  (see fig 3 and 4) Winds of 70 to 80 mph were common along the damage path,  with some seeing wind gusts that ranged between 90 to 100 mph. This event was very widespread.Of the over 1200 damage reports   Over 800 were wind damage reports that will cost  several million dollars (see fig 5). A thing that surprised me was that a derecho of this intensity only produced two confirmed tornadoes.  There were millions of people who lost power. Unfortunately the storm caused 23 fatalities. As power crews race to restore power to around a million people, this number could climb farther if people circum to the heat of the ongoing heat wave.

                                           
                                                                                     fig 3

                                                         Showing the progression of the derecho

fig 4

          Another view of the damage path

                                                                                

                                                                                         fig 5

SPC damage reports 

Timeline of June 29th 2012 super derecho:


  At 8:00 a.m. EDT

A few thunderstorms start to breakout in Eastern Iowa.

At 8:30 a.m. EDT.

The storms are intensifying as they cross into western Illinois.

At 8:51 a.m. EDT.

 The NWS starts to issue severe thunderstorm warnings for Northwestern Illinois.

At 11:35 a.m. EDT

The cluster of thunderstorms starts to form a bow echo west of Chicago.

11:50 a.m. EDT

The storms have cleared Illinois and are rapidly intensifying as they enter Northwest Indiana.  At about the same time, the National Weather Service in Sterling, Va.,  introduces enhanced wording that there is a increased chances for thunderstorms for the Washington DC area, feeling that the thunderstorms  most likely will keep rolling east.

12:14 p.m. EDT

 The Storm Prediction Center (SPC) begins tracking the evolution of the developing MCS in Illinois and Indiana. However, they fail to truly comprehend the  true nature and danger that is developing. At one point an operational forecaster states that the "extent of the severe threat should be limited to areas west of the Appalachian Mountains."

12:50 p.m. EDT.

The SPC  issues a severe thunderstorm watch for northeastern Illinois and the northern half of Indiana.

At 1:08 p.m. EDT

The MCS has grown in both size and intensity.

At 1:17 PM EDT

The first tornado warning of the day goes up in northwest Ohio.

1:54 p.m. EDT.

 Fort Wayne International Airport records a wind gust to 91 m.p.h. Emergency officials in parts of Indiana begin reporting "massive damage" following passage of storms.

2:15 a.m. EDT

The MCS has developed into a wicked bow echo as it continues to gain strength and momentum.

3:30 p.m. EDT.

The SPC  upgrades parts of Indiana, Ohio, and West Virginia to a moderate risk of severe weather and warns of "significant winds."

4:03 p.m EDT.

 The SPC classifies the "widespread/locally significant wind damage"   as a derecho.

Between 5:00 - 5:20 p.m. EDT.

  TV meteorologist go on air  taking about the possibility of severe thunderstorms in the Ohio Valley and Mid Atlantic states.

5:37 p.m. EDT.

 The NWS, introduces increased chances for thunderstorms - some severe weather  - for the region. They are becoming increasingly concerned about the ongoing derecho over the Ohio Valley.

6:30 p.m. EDT

The derecho has pushed across the Ohio River and has moved into West Virginia, falling trees have trapped numerous people in their homes.

7:20 pm EDT.

 Yeager Airport,  in Charleston, recorded a 77-mph wind gust.

7:50 p.m. EDT

The SPC realizes the derecho moving over  West Virginia and warns it will  continue to roll beyond the east slopes of the Appalachians and move into  Virginia.

8:00 p.m. EDT.

The SPC upgraded parts of Virginia, Maryland, and Washington D.C., to a moderate risk of severe weather and issues a severe thunderstorm watch with extremely high winds likely.

A little after 9:00 p.m. EDT. (this is an interesting side note) Amazon's Cloud Service fails, this in turn took down Netflix, Pinterest, Instagram, and other services across North Virginia.  

9:08 p.m. EDT

The National Weather Service enhances the severe thunderstorms warnings in Virginia using the  wording warning of "destructive winds."

10 p.m. EDT.

The derecho is  approaching the Washington, D.C. metro area. It is still  producing powerful damaging wind gusts. Between 10:00 pm and 11:00 pm EDT. Airports in Virginia and Maryland are reporting wind gust of between 70 and 80 mph .

10:10 p.m. EDT

 The NWS issues the warning for the people in Washington DC and the surrounding counties "This is a dangerous line of storms... These storms are capable of producing destructive winds in excess of 80 miles per hour. This is a serious situation. You need to take cover now."

 12:50 a.m. EDT

The last two people  lose their lives when powerful winds toppled a pine tree onto a tent.

 1 a.m. EDT 

The derecho was crossing over southern New Jersey, and the final reports of wind damage came in around 1:40 a.m. in Tuckerton, N.J., where winds were reported to have gusted to 81 mph.

A timelapse of  NEXRAD base reflectivity of the 29 June 2012 derecho.  The timelapse starts in  Davenport, Iowa and ends in  Richmond, Virginia. It can be found here.

Here we go again, lets all blame the NWS:

Another meteorologist friend of mine Matt Lanza brought an article out of the Baltmore Sun to my attention.

The article  talks about the derecho and how forecasters didn't anticipate the extent of how things would unfold on the 29th. Now that much is true. As I pointed out in this blog post. forecasters didn't expect the type of storm that developed last Friday. However the way the article is written it implies that power crews, government offices and the general public were caught off guard with no warnings at all, until after 10 p.m EDT. This is far from true. As I've shown in the timeline of the storm. The SPC, the NWS local field offices, and local weather TV meteorologist  were giving warnings well before that time. It always seems that when a rare event like this happens, the first thing just about everyone does is point a finger at the meteorologist and say...  I didn't know this was going to happen, because you never told me... This blame the weatherman for everything is starting to wear on me. Everyone needs to listen for warnings and respond to them. This hide your head in the sand mentality is also starting to wear on me. I've wrote several post where I berated individuals for  falling to heed warnings from the NWS, be it come outside to see what going on, drive to the store, or just flip the channel on the TV and sit there, during a tornado warning  The general public has to except their share of the responsibility for failing to act on any warnings given.  Now I agree the SPC and the NWS fumbled the ball on the June 29th super derecho; but, I'm sure the local NWS field offices issued warnings with enough lead time for people to take shelter. The SPC did underestimate the strength of the derecho. This event shows,  while we understand many things about  the science of  meteorology, there are still many things we don't completely understand. But we're getting better every day. Matt Lanza said "The whole warning system needs an overhaul in my opinion. You need tiered warnings for events. I don't know how this gets accomplished in a way that makes sense...I just know that this is what needs to be done. Unfortunately you can't always delineate between EF-0 and EF-3 TOR on radar...which makes this idea difficult to accomplish in reality. But something needs to get done somewhere". I completely agree with this statement. I've said in the past that the severe warning net can and should be improved. But people are the weak link in the chain. For people to sit there and blame the NWS for failing to issue warning everytime something like this happens is ridiculous.     

Here is one of the post I've writen abut this subject. It can be found here.

A link to the article can be found here.

                                                                                 fig 6

Fig 6 was taken by Aviation Dave as the super derecho rolled into the Cincinnati/Northern Kentucky International airport


                                                                                      fig 7

Fig 7 shows the derecho climatology for the United States. As you can see the Northeast normally see's a derecho once every four years.

Well that's it. I hope you enjoyed reading this post and maybe learned a thing or two. As always questions and remarks are always appreciated.   

Rebecca   

Non-tornadic severe weather

          Hello, it's Rebecca Ladd again, This blog post might be a little more complicated than the others have been. We associate many storm elements with severe thunderstorms. Lighting and thunder, gusty winds, hail, flash floods, and tornados are the most well-known features, but we cannot forget their cousins , the microburst, mesoscale-convective systems (MCS),  heat bursts,  and derechos. This post will try and shed some light on these things.

Microburst and Macroburst:

            A downburst is an area of rapidly descending air beneath a thunderstorm. When this downdraft  hits the ground, it quickly spreads out in all directions, causing very strong, straight-line winds. These winds are commonly as strong as 40-60 mph but can exceed 125 mph at times. These downburst are broken down into two groups. The first is called a microburst; In order to be called a microburst the ground area impacted by the downburst is less than 2.5 miles in diameter. The other group is called a macroburst; a macroburst is physically the same thing as a microburst, but over a much larger space scale - Sometimes the area affected is greater than 5 miles in diameter. A downburst can last as long as 15 minutes.

            If you remember, in the thunderstorm life cycle. I said, rain aids in the creation of a downdraft. The process is the same here. Inside a thunderstorm, water vapor condenses into raindrops.  On their way to the ground,  these raindrops will fall through drier air which will make the drops start to evaporate. The evaporation process cools the air, causing it to become denser than the air around it. This rain-cooled air, along with the falling raindrops, accelerates downwards; it is this down-rushing air that eventually hits the ground and is forced to spread out in all directions causing the damaging straight-line winds. Microbursts are sub-divided as dry or wet, depending on how much rain accompanies the microburst when it reaches the ground.

                                                    Photo of a downburst.

Heat Burst:

                A heat burst is an extremely rare event. A heat burst is a downdraft of hot and dry air that typically occurs in the evening or overnight hours after thunderstorms are ending.  It is caused when rain falls into very dry air, high up in the atmosphere. The rain quickly evaporates as it falls through the dry parcel of air and that parcel cools rapidly. This dense mass falls rapidly toward the ground, heating up as it compresses. When this hot ball of air hits the ground it spreads out in every direction creating very strong, warm and dry winds. Wichita, KS was actually hit by one last week on Jun 9. National Weather Service meteorologist Stephanie Dunten says the heat burst hiked temperatures from 85 to 102 degrees in 20 minutes, beginning at 12:22 a.m. Thursday. She said a pocket of air in the upper atmosphere collapsed, and when it hit the ground it sent winds of more than 50 mph through parts of the city.

Velocity radar image shows a very small area of strong winds, approximately 50 kts or 58 mph. These winds as highlighted in the circle resulted in the heat burst across the area.

Lightning:

            Another factor of thunderstorms that is sometimes taken for granted is lightning. If you can hear thunder, you are at risk for being hit by lightning. Seek shelter indoors. A hardtop vehicle offers excellent protection from lightning.

Hail:

            Hailstones generally begin forming on small frozen raindrops or soft ice particles known as graupel. However, hail has been known to form around pebbles leaves or anything that has been drawn into the cloud by the updraft.  In strong thunderstorms you have the potential to get really big hail. The updraft that sweeps the rain high in the clouds continues to sweep up any falling frozen rain. Each time the frozen rain gets swept back up in to the high clouds, it gathers more moisture which freezes and gets larger. This cycle continues until the hail eventually breaks free from the cycle and falls to the earth. You can find baseball size hail if you get a thunderstorm with an updraft of 100 miles per hour. Therefore, large hail greater than two inches forms mostly in supercells.

            How dangerous is hail?   I'd probably say 3/4" diameter hail and larger would start causing damage.  I've been hit by quarter size hail before....Let me tell you it hurt. So you can image what golf ball or softball size hail will do.  Large hail can demolish houses and mobile homes.  So you can see, hail is very dangerous. Therefore, when hail is expected, your best defense  is to take shelter in a substantial building away from windows.

                                                              Large hailstones.

          In the blog post on types of thunderstorms, I briefly mentioned squall lines, bow echoes, and MCS's.  In this post, I will go a little more in depth on Bow echoes, MCS's, and especially the Derecho.

Bow Echo:

            While lines of strong thunderstorms often become severe, their less-common cousins known as 'bow echoes' can grow even more intense. When they occur, their usually within  a grouping of multicell storms that are arranged into a squall line. A thunderstorms speed and direction is greatly influenced by upper level winds.  Along a squall line these upper level winds will not always be constant. Therefore, in areas where these winds are stronger that portion of the squall line will push outward.  Because of evaporative cooling these winds are drier than other areas. This will help accelerate the downdraft even more; therefore the faster the downdraft the faster that portion of the line moves forward.

                                                     Image of a bow echo

MCS:

            Mesoscale-Convective Systems (MCS), I dare you to say that three times real fast.  You may have experienced an MCS without ever knowing its name. Let's break it down ...

"Mesoscale" on the whole means medium-sized relative to the big picture, When you're dealing with events on the mesoscale they're a lot smaller than lets say a low pressure system which can encompass a large portion of the country (known as "synoptic scale"), however it's much larger than an "microscale" event such as  a tornado.

"Convective" this just means thunderstorms and their upward and downward air motions.

"system" according to Webster's,  it's defined as a group of interacting elements comprising a unified whole.

            In other words,  an MCS is simply a decent-sized and well-organized area of multiple thunderstorms.  The thunderstorms in an MCS form from the same things that  trigger normal thunderstorms: fronts, upper-level disturbances, daytime heating, etc. The difference is how close the thunderstorm cells are to one another. When the cells are very close together, they begin sharing and combining their various downdrafts and updrafts, intensifying one another.

            Once the MCS forms, it becomes its own creature and is capable of producing its own weather independent of the larger scale weather pattern. An MCS can even move in ways that would seem to defy the  upper-level wind pattern. An MCS can last for hours, some MCS's have lasted over 20 hours. As long as it can inject enough moisture, heating and  instability it will keep going. . An MCS can be hundreds of miles wide, though more frequently, it's about 50-75 miles in diameter. The major concern with an MCS is high winds. However, if the MCS is moving slowly flooding can be a problem. They can produce large hail and the occasional tornado. If a tornado develops it's normally found at the edges or ends of the cluster or line.  A long-lived bow-echo MCS that produces damaging straight-line winds over hundreds of miles of terrain is sometimes referred to as a derecho. I will go into that next.

Derecho:

            A Derecho is a very rare storm that is known for its strong straight line winds of 60 to over 130 mph; that cause extreme damage for hundreds of square miles.  It may last for several hours. Therefore, the dangers associated with derechos arise from both the strength and duration of the wind. The storms width is normally 50-100 miles wide. But, some have had widths close to 300 miles. Derechos like to form along nearly stationary fronts. Normally the front will separate very warm, moist, and unstable air from  relatively cool, dry air. The derecho typically moves eastward along the front, veering toward the warm air mass. There are three types of derechos, The first two the progressive and serial, have slightly different formation processes and the time of year for their peak occurrence.

            The first type of derecho is called a serial derecho. They can occur anytime of the year. However, their most often encountered during the spring and fall. A serial derecho usually forms out of a strong low-pressure system. This type of derecho is formed when there are several bow echoes in a strong squall line.  Normally it is hundreds of miles long. Serial derechos do not need the strong unstable conditions required of its brother the progressive derecho. But it does need an environment that will support convection.  The second type of derecho is called a progressive derecho. They generally form in the spring and summer spawned by the plentiful solar energy that heats the surface and the lower atmosphere. Normally they look like a relatively short line of thunderstorms (40 miles to 250 miles in length)  it can take the shape of a single bow echo, especially early in its lifecycle. Like any derecho it can travel for hundreds of miles. The third type of derecho is known as a hybrid derecho; these have characteristics of both the progressive and serial types.

            Over the last 20-30 years there have been several derechos  which impacted NYS.  I will briefly discuss three of them.

            The Adirondack  derecho occurred on July 15, 1995; this derecho closely resembled the progressive type.  The storm moved out of Ontario and into Jefferson and St Lawrence counties in northern NYS around 4:30 AM;  where winds of at least 100 mph caused severe wind damage. It then moved through the  Adirondack Mountain region, In the Adirondacks the storm leveled mile after mile of trees and unfortunately killed several people and injured dozens. The derecho entered western New England about 7 AM causing extreme damage to an apartment building in Holyoke. It also killed one person when a tree fell on them..

If anyone is interested, you can find more information here

http://cstar.cestm.albany.edu:7773/research/derecho.html

            September 7, 1998 is unique and will always standout. The reason is two severe derechos struck NYS on that labor day.  The northernmost derecho nicknamed "The Syracuse Labor Day Derecho and referred to by many in the North Country as "The Labor Day Storm". This derecho caused wide spread damage. Some of the worst damage occurred at Rochester, Syracuse, and Utica; where wind speeds were measured 70-115 mph. To make matters worse the derecho had an embedded supercell that produced several tornadoes.  The derecho killed three people and injured several. Damage was estimated at $130 million (1998 dollars). Many in the region were without electricity for over a week. The 2nd derecho formed as the first one moved into New England. This one followed a path just south of the first. This derecho was more powerful than the first; when it slammed into New Jersey and New York City it caused tremendous damage. The storm killed a total of 4 people and produced at least 6 tornadoes.

Here is a site that has more information on the Syracuse storm.

http://www.erh.noaa.gov/bgm/laborday/

            Clearly a derecho is a dangerous storm. So if you hear that one is approaching you must act quickly to protect the lives of your family and yourself.  And even if the severe thunderstorms are not a derecho, they are still deadly. There may actually be more deaths in regular severe thunderstorm, non-derecho, events.

Well that's it for this post, the next one will be on the tornado itself.

Rebecca Ladd.

Types of thunderstorms

     In this segment, I will go into a little more detail on a subject that is on everyone's minds lately... thunderstorms.  There are four main types of thunderstorms, single cell, multicell clusters, squall lines, and the infamous supercell. The difference between the types of thunderstorms has nothing to do with their lifecycle. instead it has to do with the amount of cells in the thunderstorm and how they are positioned.  Now, I'm sure someone is saying ...So what, there are four kinds,  why should I care? I feel having a better understanding of the various types of thunderstorms can help you distinguish between severe and non-severe thunderstorms, this will help you keep yourself and your family safe.

The single-cell

    A single cell thunderstorm also can go by two other names: A pulse thunderstorm or an airmass thunderstorm. This type of storm only has one main updraft.  It's a thunderstorm that goes through  its life cycle and dissipates without creating any other cells, the term "cell" refers to the number of principal updraft points in the storm. Single-cell thunderstorms usually last between 20-30 minutes. They are usually poorly organized and seem to occur at random times and locations, making them difficult to forecast. Single-cells are rarely severe, They may contain heavy rain and can also produce occasional downbursts, small hail, and (rarely) weak tornadoes, storm chasers call these kind of tornadoes landspouts, but these are very rare in single cell storms. However, there is a special class of single cell that is always severe, I will discuss this special class later.

                                                                         Single cell thunderstorm

 

Multicell Cluster Thunderstorms:

     Multicell thunderstorms are groups of cells adjacent to one another that move together, which are all in different stages of the lifecycle.  Because they are in different stages of development they have a much longer life span that a single-cell. In my last blog post I talked about a thunderstorms lifecycle; in a multi-cell it works the same way, with a slight twist. Here's how it works.  As cumulus develop, one of the cumulus begins to grow faster than the other cumulus; eventually it will  produce some light precipitation. As this precipitation and corresponding downdraft descends it cools the air around it (evaporative cooling). The evaporative cooling accelerates the downdraft, as the downdraft hits the ground it spreads outward. Sometimes this outward movement of air can act as a wedge as the colder out flowing air undercuts the warm moist air in the regions surrounding the main cell. This can have the effect of intensifying updrafts in the surrounding cells nearby. In-turn, These cells move into their mature stage as the new cell  sends down precipitation it becomes the dominant cell. Simultaneously, the newer cell produces downdrafts that stops the updraft of the original cell. This cycle will keep going as long as atmospheric conditions allow it. If you've ever watched on radar when there is a lot of thunderstorm; you might have noticed a group of cells will be moving one way, then all of a sudden move in another. This is because of the unusual structure of multicells. This happens, because the developing and dissipating process causes the storm to have a motion veering slightly at an angle to each cells line of motion. On average, multicell cluster storms last for about 20-30 minutes, however the whole line may persist for several hours. Multicell thunderstorms can become severe. All types of severe weather can be experienced from severe multicells including giant hail, severe winds and tornadoes.

                                                                 Image of  multicell cluster thunderstorms


Squall lines:

     Squall line thunderstorms can also be called multicell line storms. These systems of thunderstorms arranged in a line. Sometimes this line can extend laterally for hundreds of miles. At first glance, a squall line looks like a long system of multicell thunderstorms, with cells developing on one end and dissipating on the other. However, the storm looks like on large thunderstorm with a large anvil extending well ahead of the main body. The approach of a squall line is a astounding sight. As it approaches, the observer will normally see a very dark shelf cloud with an extensive precipitation cascade. A shelf cloud is a low, horizontal wedge-shaped cloud. that  is attached to the base of the parent cloud.  If you're facing the squall line a strong warm wind will form at your back, this is the inflow updraft feeding the storm. As the squall line gets close, there will be a  brief lull in the wind soon to be replaced with a sudden blast of wind from the storm in the opposite direction. this is the outflow downdraft. Sometimes Bow echoes can form within squall lines,  bringing with them even higher winds. Bow echoes get their name because of what they look like on weather radar. A bow echo brings with it very high and often damaging winds. An unusually powerful type of squall line is called a derecho, this an very  intense squall line that travels for several hundred miles. There is one more thing I should mention. Some of you  may have heard the term mesoscale convective system (MCS);  an MCS is just a fancy name for is a complex of thunderstorms that becomes very organized on a scale that can impact several states at the same time. squall line systems often form within a  MCS. Now on to the last type of thunderstorm.

                                                                                                                A shelf cloud

                                                                                       Radar image of bow echoes

The Supercell:

                This is the special class of single cell thunderstorm I mentioned above. Supercell thunderstorms are the largest and the most severe of all types of thunderstorms. Most of the large tornadoes and giant hail events you've heard about over the last month were spawned by supercells.  The reason why supercells are the most severe is because of their rotating structure.  when a thunderstorm spins it is called a mesocyclone. A mesocyclone is basically an area of extremely strong updrafts which spin as the air moves upwards.  The supercells are one of nature's most destructive but beautiful constructs.  I think I will stop for today. There are two main types  of supercells that I will explain in the next installment.

                                                                                              A supercell in Oklahoma

 

     I hope you found this post both enjoyable and informative. I feel, the more you understand about the weather; the more you will be able to appreciate the wonder and beauty of nature. Even though thunderstorms can be destructive they are also very beautiful at the same time.

Rebecca Ladd.