Sunday, June 26, 2011

The Tornado

          Hi it's Rebecca again, I'm back with another installment of my blog series. Whenever I'm around people, once they find out that I like to chase storms, the conversation inevitability changes to questions about storms, and especially what is it like to see a tornado.  This post will go in quite a bit of detail on what a tornado is, how it forms, and what  it looks like. I will also touch on a few cousins of the tornado.

What is a tornado:

            Do a web search and you will find most explanations start with: a tornado is a violent, dangerous, rotating column of air that is in contact with both the ground and a cumulonimbus cloud.

            In a paper by Charles A. Doswell III a severe convective storms researcher; he said he did not like the phrase tornado touchdown. I must confess I feel the same way, in my option, the word leads to a big and dangerous misconception.  When most people hear that a tornado is forming they expect to see a funnel coming down out of the clouds. In my opinion, this idea can put lives in danger. I want to make one thing very clear, there is nothing coming down, in the sense that something is falling out of the cloud.   Doswell said:  "What actually goes on when a vortex is present in the atmosphere is that the vortex either (a) is already present at the surface, or (b) wraps around itself, like a smoke ring". So while the vortex can build downward. it's not a tornado descending.  A vortex can also intensify the exact opposite way, by that I mean from the ground up; this is how many landspouts (weak tornadoes) form. The funnel cloud  is also called a condensation funnel, because what your seeing is really the condensation of water vapor into cloud material. I will go into this in more detail later.

            So let's look back at the web definition of a tornado, a  tornado is a violent rotating column of air extending from a thunderstorm to the ground. Perhaps a better description would be, a tornado is a vortex of air extending upward from the surface  into a cloud base that has deep moist convection, that is intense enough at the surface to cause damage. They can appear suddenly without warning and can be invisible until dust and debris are picked up.

 How does a tornado form:

            In a regular garden variety thunderstorm warm moist air shoots upward meeting colder, dryer air. Because the warm moist air is lighter than the cold dry air it will form a strong updraft within the thunderstorm.  During the storm, the cold air and warm air combine in a set pattern: the cold air drops as the warm air rises. As the warm moist air rises, it may meet varying wind directions at different altitudes (wind shear). The wind shear creates an invisible horizontal spinning effect in the lower atmosphere. Now because we have warm moist air in the updraft, it hits this horizontal tube of spinning air and tilts it into a vertical position. As the updraft tightens the spin and it speeds up (much like a when an ice skater pulls in their arms and spins faster.  The warm air eventually twists into a spiral and forms the funnel cloud that we all associate with a tornado.

            There are two types of tornadoes: those that develop out of a supercell thunderstorm and those that form out of a regular thunderstorm.

Supercell tornadoes:

            Tornadoes that form from a supercell thunderstorm are the most common, and often the most dangerous. In the blog post "Types of Thunderstorms" I talked about Supercells and a little about the tornadoes they form. This kind of tornado  has a life cycle. First, the mesocyclone , along with the rear flank downdraft( RFD), starts moving towards the ground. At this time a small funnel appears to build up at the base of the wall cloud. Once the RFD reaches the ground, the surrounding dirt rises up, causing damage to objects on the ground. The funnel touches the ground immediately after the RFD, forming a tornado.

            The next stage starts when the RFD, begins to cool. The distance the tornado covers, depends on the rate at which the RFD cools. The long lived tornadoes during the Super Dixie Outbreak were a good example of what happens when there is plenty of warm moist air for the tornado to feed on.  Once the RFD cannot provide any more warm air to the tornado, it begins to die. The lack of a warm air supply causes the  vortex to weaken and contract . As the tornado weakens, the mesocyclone also starts to dissipate. There is one important thing to keep in mind,  a new mesocyclone can start very close to the dying one.  So you don't want to let your guard down too quickly.

                                                                    Diagram courtesy of Weatherzone

Non-supercell tornadoes:
            These are circulations that form without the aid of the rotating updraft found in supercells. Non supercell tornadoes develop in normal thunderstorms in the process I described above.  One type of non-supercell tornado is the gustnado, typically they look like a swirl of dust or debris along the leading edge of the thunderstorm outflow (the gust front).  There is usually no condensation funnel or other visible connection to the cloud base above.  Gustnadoes, like all tornadoes, are potentially dangerous to both life and property. While most are very weak, a few have been known to reach EF1 strength. Gustnadoes are most commonly seen in lines of thunderstorms, especially bow-echoes. Another non-supercell tornado is the landspout.   Landspouts are most commonly seen in lines of towering cumulus clouds or on the backside of weak thunderstorms. Unlike a gustnado, landspouts are normally visible; most of them have a narrow, rope like condensation funnel extending from the base of the cloud to the ground. These tornadoes are typically short-lived and weak. However, it is not unheard of for them to reach EF2 status.

              If you recall my discussion on weather radar, you will remember, I said doppler radar can't see wind; it can only see objects like rain, hail, or even birds. Also, doppler radar in general cannot see tornadic scale rotation, it is much too small. What we see on radar is the much larger scale rotation of the entire thunderstorm rotating. Gustnadoes and landspouts pose a very significant challenge to forecasters. not only because they can form in rather benign environments. But also, most of the time they form before precipitation is detected on radar. Another thing is most of the rotation occurs close to the ground, which is below where the radar can see.Rarely does radar give us a good view of non-supercell tornadoes. Because of this, non-supercell tornadoes are next to impossible to predict.


I saw this landspout a little over a week ago, on the 17th, around 6:17 PM on the Tughill. It just spun up out of nowhere and with no warning; there was some thunder going on but that was about it. I saw the  condensation funnel start to form on the backside of the storm.  As it got a little lower, I saw the circulation come up from the ground to meet it..It hung out for about two minutes and then dissipated. The next day I went down and checked the area: some of the hardwood trees had branches up to two inches in diameter snapped. Also there was a few small trees uprooted. I estimate the 3 sec max winds were 65 mph or so. That would rate this an EF0.

What does a tornado sound like: 
                       I'm sure you've heard people mention the sound of a freight train, when they describe a tornado. While this loud rumbling is true of some tornadoes, it's not true of all. In fact tornadoes can have many different sounds; it depends on many factors: closeness, intensity, and what it's eating at the time. Besides the continuous rumble, I've heard them shriek like some crazed banshee. Sometimes, a tornado produces a loud whooshing sound, like what you hear when the car windows are open, only much louder. In fact tornadoes can make noises that range from whistles to humongous roars. no matter what it sounds like, if the tornado is close by the air rushing into the storm is impossibly loud. For some the sound is awe-inspiring, for others it inspires terror. But, I can assure you it's a sound you will never forget. 
 What does a tornado look like:
            Most people' mental image of a tornado is like the one in the "Wizard of Oz", I guess this is because for most of us this was our first glimpse of a tornado. However tornadoes come in a variety of shapes and sizes. . A tornado often goes through a life cycle starting as a classic funnel shape, then broadening and widening in its mature stage. Then it enters the dissipating stage where it becomes thinner, long and often very distorted. This is called the rope stage. I should add, that the size and shape of a tornado is no indication of it's strength. Below I will briefly discuss the major shapes.
The wedge tornado:
            A wedge tornado doesn't have your typical classic funnel shape.  They have especially large funnels, which can be over two miles wide. the distance between the ground and the cloud base can be very short.
                                                                            A Wedge Tornado
Elephant trunk:
            These look just like the name suggest. The funnel starts out wide and gradually gets narrower as it gets closer to the ground; it has a slight curve to the shape as well.
                                                                 Picture of an Elephant trunk tornado

Rope Tornado:

            Sometimes they appear as roiling billows of smoke, other times a twisting rope, or a barely visible swirl of dust.
                                                                        A image of a  Rope tornado

            A stovepipe tornado typically has straight sides. the top of the tornado has about the same width as the base of the tornado.
                                                                 A picture of a Stovepipe tornado
Multiple Vortex Tornado:

              There is a lot we don't completely understand about tornadoes especially near the base of the tornado. A multiple vortex tornado (sub-vortices or suction vortices) is one that has mini vortices inside the bigger main vortex. I think most toradoes have these suction vortices. Most of the time no one can see them because they are rain wrapped or hidden by debris in the funnel. I've seen a few multi vortex tornadoes. In the ones I've seen these sub-vortices formed at the base of the tornado. Inside the main vortex there are several forces at play: inflow and outflow angle, rotational motion, centrifugal forces, pressure gradient forces, and even the winds in and around the tornado vortex. I think these complex forces form relatively calm areas inside a tornado, therefore areas inside the parent tornado will be spinning faster that others. Sub-vortices can cause narrow areas of extreme damage inside the main tornado damage path. Even though a tornado can range from less than one hundred yards to over two miles in width, these smaller vortexes may only be 60 or 70 feet in diameter and follow one another, this is often referred to as training. The winds in these sub-vortexes can easily spin in excess of 150 mph and are most likely responsible for a majority of a tornadoes destruction. They are one of the reasons people think tornadoes can skip over one house and hit another house across the street.

                                                                           Multiple vortex tornado
Satellite tornado:
            Something that's similar in nature to multiple vortexes is the satellite tornado. It is different from a multiple-vortex tornado in that it's a separate but weaker tornado which forms close to the main tornado within the same mesocyclone. As its name implies it orbits the main tornado like a satellite. Putting my chaser hat back on, satellites can be extremely dangerous. If you're  not paying attention to the main inflow band  of the tornado; you might have an uninvited guest sneak in from behind.  From my experience, satellite tornadoes like to form within striated rain bands. This is because within the bands are small shear/convergence zones that can easily spin-up a tornado. So it's always a good idea to stay out of them, if you can.
            Not too long ago, a close and dear friend asked me, what is an outbreak and how many tornadoes does it take to make one. Because of that, I thought I would mention it here. A tornado outbreak occurs when you have at least six within a 24-36 hour time frame from the same general weather system.   There are two major kinds of outbreaks; cluster outbreaks and corridor outbreaks.  A cluster outbreak is when you have four or more tornadoes which occur within a roughly circular area of between 5,000 and 6,000 square miles. Whereas, corridor outbreak is when there are three or more tornadoes that generally move from west to east within a narrow corridor of land. Over half of the corridor outbreaks occur between March 1 - May 15 which peaks during the last half of April. On the other hand, Over 70% of the cluster outbreaks happen from May 16 - June 30 with a well defined peak in early June. Tornado outbreaks are often sub-divided into three groups.

Local outbreak: normally this is at a county or state level.

Line outbreak: in this case the tornadoes form around the same time along a line. A line outbreak can be at the state level. However, normally it encompass several states.

Progressive outbreaks: are when several tornadoes form over a 12 to 72 hour time frame. this kind of outbreak progresses toward the NE, E, or SE. A progressive outbreak is like the one we just saw in the end of April.
Cousins of the tornado:
            These are similar to landspouts, except they occur over water. So called "fair weather waterspouts" grow from the bottom up. The first sign of a waterspout is a dark spot on the water's surface;  it's a good sign that a invisible vortex is present from the surface to the cloud base. As the waterspout grows stronger, it begins kicking up a ring of sea spray around the dark spot. As the spout grows it begins to carry the spray upward in a circular pattern known as a spray vortex.  As low air pressure inside the vortex falls the funnel begins to become more and more visible. once it reaches the cloud base the spout is at its peak, and is moving across the surface of the water. Once it's warm water supply is cut off it begins to dissipate. The rain behind the spot will cool the air thereby killing the waterspout. Most waterspouts are weak. However, tornadic waterspouts, are tornadoes which moved from the land to the water, or form over water in the first place. They are very dangerous, Tornadic waterspouts form under a rotating storm or supercell.
                                                                       Image of a waterspout
Dust Devils:                                                            
            Dust devils are created when air near the surface becomes a lot warmer than the air above it. This causes a lot of instability which allows the warm to rise quickly. They form on hot days, generally over areas of fairly bare ground, including parking lots. They are not associated with thunderstorms. Dust devils rarely cause anything more than minor damage.
                                                                           Picture of a dust devil

Fire whirls:

          Fire whirls sometimes called fire tornadoes or fire devils, are seen in intense fires. Very strong updrafts over the firefront result in rapid upward air movement. Because the air was displaced strong horizontal winds form as air rushes in to replace that in the updraft. Under these conditions small intense whirlwinds will often form,  outlined by bright flames, along the fire front.
            Well that's about it, I hope you found this both educational and enjoyable to read. It's not meant to replace SKYWARN training, instead it's a supplement to it. Now while I feel the SKYWARN program must be revamped, in my option it has a few serious gaps. I feel the classroom time should be increased, with more emphasis on such things as video. However, it is still an excellent program that I encourage each of you to take.  I have a strong believe that reading books, watching video, reading stuff like this blog series, along with SKYWARN training will keep you safe when severe weather strikes. I was going to end the series with this installment. However, I've decided to add one more, the next one will be on such things as watches, warnings, and what to do in different severe weather events.

Rebecca Ladd.

Friday, June 17, 2011

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.
            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.
            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
            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.
            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
            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.
            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.

Wednesday, June 8, 2011

Clouds associated with severe weather.

            Clouds can be fascinating to watch. I don't know about you, but I've spent time laying on the ground just watching clouds. OK I will admit it,  I didn't have a lot going on that day,  still watching the clouds was enjoyable. There are 10 Main cloud Types, at sometime in the near future it is my hope to do a couple of post on the subject of clouds.  In this segment, I will give you a little description of a few of the different clouds associated with severe weather. This will help you identify what you're seeing when severe weather is approaching.

Shelf Cloud:

            This type of cloud is common and is found along a storm's gust front. a shelf cloud is attached to the parent cloud and looks a lot  like it's name suggest. A shelf cloud is created by the rain-cooled air from the storm. As most of you know, the air is warmer at the surface and colder aloft.  There is often a atmospheric cap several thousand feet off the ground where the temperature raises briefly. Most of you have heard the WTEN weather team say " The atmosphere is capped" what this means is there's a warm pocket of air aloft that's keeping a lid on things. A cap works like a closed bottle of soda, if you shake the bottle-up all is fine as long as the bottle cap is in place. However, if you remove the cap... the soda inside will explode. An atmospheric cap works the same way, as long as the cap is in place nothing happens. But, if  the cap weakens or breaks the developing thunderstorms will explode. The warm air being lifted above the cap is what you are seeing when you see a shelf cloud.  The process develops like this:  as the rain cooled air rushes outward it becomes a wedge that forces the warm air up. this forms more updrafts in turn the updrafts will form more precipitation which leads to more cold air flowing outward and new updrafts created and so on.

In this image you can see the downdraft behind and the warm air in front moving up and over the shelf cloud

Roll cloud:
            This type of cloud is fairly rare,  unlike shelf clouds, roll clouds are completely unattached from a parent storm cloud. This type of cloud is long and tubular in shape. they can be startling to see because they appear to be rolling across the sky. Although you may think it looks like a tornado turned  sideways, it is not associated with tornadoes at all.

                                                              Image of a roll cloud.
            These are  rounded sack-like protrusions hanging from the underside of an anvil cloud. This type of cloud doesn't produce severe weather. But, it's normally seen in severe thunderstorms. They like to form upwind of the updraft. As a chaser, you always look for this type of cloud, while it doesn't guarantee a tornado will form,  it does indicate the storm has a good chance to produce one.
                                                  Image of mammatus clouds

Beaver tail and Tail clouds:

            These types of clouds are inflow bands into a storm. Some confuse tail clouds and beaver tails. The major difference is a beaver tail will be attached to storm base area, whereas the tail cloud attached to the wall cloud lowering.

                                                  Above are images of beaver tail and tail clouds
Scud clouds:
            Scud's are low detached ragged looking clouds. They tend to rise and may exhibit lateral movement. Scuds are not dangerous. However, scud clouds can often be mistaken for a developing tornado.  You tell the difference  by watching the cloud to see  if there's any rotation with it;  if you see any rotation then a tornado has a high chance of forming.

                                                     Pictures of scud clouds

Funnel Cloud:
            A funnel cloud is a funnel-shaped cloud spinning at high velocity.  Normally it extends from the base of a cumulonimbus or towering cumulus cloud. A funnel cloud is usually seen as a cone-shaped or needle like protrusion from the cloud base. If a funnel cloud touches the ground it becomes a tornado, or a waterspout it was over water. If you spot a funnel cloud that's nearby take shelter immediately, as it may suddenly become a tornado.  However, if you feel there is time to safely report it you should do so.
                                                    Below are three pictures of funnel clouds

Cold air funnels:
            Unlike the normal funnels associated with severe thunderstorms, cold-air funnels are generally associated with partly cloudy skies after the passage of a cold front. it is very rare for them to touchdown. But if they do they become a weak tornado. My next blog post will be on Non-tornadic severe weather
                                                         Picture of a cold air funnel

Wall Cloud:
            A wall cloud marks the lower portion of a very strong updraft, usually associated with a supercell or severe multicell storm. It typically develops near the precipitation region of the cumulonimbus. Wall clouds can range from a under half a mile wide up to around  five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. Wall clouds that exhibit significant rotation and vertical motions often precede tornado formation by a few minutes to an hour.
                OK so you think you see  a wall cloud. Here are a few things to ask yourself,  First is it in the right region of the storm? Are you seeing a low hanging cloud on the forward flank of the storm or the rain free base? Another question to ask is  this feature pointing toward or away from the rain? Shelf clouds generally point out away from the precipitation while wall clouds generally point toward the rain. Is this feature rising? Slowly rotating? Intense rotation? Or bowing out? If you determine you're seeing a wall cloud report it to the NWS or local law enforcement.
            One other question I've been asked  Is it possible to have a tornado that doesn’t have wall cloud ? Absolutely!  Also if you're dealing with a  HP supercell the wall cloud could be in the NE quadrant of the supercell. So unless you're close, you would never see it from a distance away because you will be blocked by wrapping rain curtains.

                                                   picture of an developing wall cloud

           Well that's it for this installment. I hope you found this informative. The next installment will be on non-tornadic severe weather.  I will go into more detail on such things as Bow echo's, MCS, Microburst, and Hail.

Rebecca Ladd.

Thursday, June 2, 2011

Severe Thunderstorm Structure

          Hi  it's Rebecca again, with all the thunderstorms this afternoon; I thought it would be a good time for this segment on thunderstorm structure. From a storm chaser viewpoint,  It is important to know basic storm structure before attempting to chase. The reason is, people immediately draw a parallel to dark skies as being the worst part of the storm. They would not know the importance of a rain free base on the back side of a supercell. This is the location of the updraft region and the area that you want to look for the development of a wall cloud. However, reading storm structure is not just important to chasers. Everyone can benefit, there are times you won't be around a TV or radio, you could be hiking, boating, or just at a ballgame. By watching the storm, you would be able to tell what type of storm it was, its relative strength, and its general direction of movement. This information would help you know what to expect when the storm arrived at your location. 

Severe thunderstorm structure:

            Visual clues in and around the storm will tell you it's direction of movement. If you can see the anvil of the cloud(Image below)  it usually is stretched out in the direction of the upper level winds which is typically the direction the storm is moving. One other clue to look for is rain, if you can see the rain coming from the base of the cloud and it is slanted, you can determine the movement of the storm. For example, if the slant from the base to the ground is to the left, the storm is moving to the right. (If it is moving right at you or directly away from you, no slant would be noticed.) Sometimes you can notice other, closer cumulus clouds moving rapidly across the sky. The storm would tend to move in the same direction.

                 Color can sometimes give you an indication of how powerful a thunderstorm is: A very dark (black) thunderstorm or one taking an peculiar look ( green, yellow, or even brownish cloud colors) may be an indication of a severe thunderstorm. The colors and darkness of the cloud are caused by the storm's massive size and the blockage of sunlight.

                                               Image credit Skywarn

The diagram above shows a severe thunderstorm, in fact, it's a supercell. You may remember, in my last blog installment I said, the major difference between supercell and multicell storms is the element of rotation in supercells.


            The Anvil is the top flat portion on a storm, it is the most impressive part of a severe thunderstorm. how the  anvil looks  can give you a clue is to updraft strength.  If the updraft is weak, the anvil will have a fuzzy appearance. However, this doesn't mean the storm won't later strengthen.  On the other hand, if the edges of the anvil are very sharp you know there is a very strong updraft in the storm.
Overshooting Top:
             An overshooting top shows where the updraft is strongest. Generally the larger and the higher it is the more intense the updraft producing it is.
                                                Image of overshooting top

Back sheared Anvil:
             This is when the anvil cloud spreads upwind against the stronger flow aloft. It can be an  indicator of a intense updraft.  Mammatus clouds are found in this region of the storm. I will go into more detail on these cloud features in the next blog post.

Forward-flank Downdraft(FFD):
        The FFD is downwind of the updraft.  This is the outflow from the rain-cooled air of the storm's downdraft.  The FFD is the main downdraft of the storm.  Most of the precipitation falls in this region.
 Rear-flank Downdraft(RFD):
        The air in rear flank downdraft tends to be warmer than the forward flank downdraft.  The FFD and RFD interact with each other.  It works like this, because of the lack of evaporational cooling in the RFD area of the storm,  Shear is enhanced along these flanking downdraft boundaries and the shear can be magnified along where the two flanks merge. If you have a the right setup of shear and instability it can lead to a tornado. Because of this, the RFD can give you several clues that a tornado is about to happen. I will go into this in the last blog entry, which will be about the tornado.

Flanking Line:
            This can be seen as a line of developing cumulus clouds extending from the storm. The cumulus closer to the storm tend to be more mature and eventually merge into the parent storm. Like I said in the last blog post, it often looks likes steps leading up into the storm. The flanking line often feeds into the updraft of the storm. Remember that RFD behind the storm? That sinking air acts as lift that  gets new updrafts going. It is called the Gust Front. The flanking line is the new storms forming due to the new updrafts. It was this same basic process that caused the second tornado in Springfield, MA earlier tonight.
                                               Image of a flanking line
The diagram also shows areas of heavy rain and hail; these areas are called the rain/ hail core. the heaviest precipitation is found in the outer edge of the updraft and downdraft. There is often extreme turbulence in this spot; which can lead to large hail growth. As the hail drops a lot of it melts and hits the ground as rain. if the hail size started out extreme in size all of it won't melt so it will reach the ground as hail.

The wall cloud and rain free base are unique to supercells.

Wall Cloud:
             The wall cloud is located in the updraft area of a supercell. The base of the wall cloud is close to the ground.  The wall cloud will often be seen as rotating since directional wind shear (change in wind direction with height)  acts on the updraft as it rises. Tornadoes can occur under the wall cloud.
                               Image of an almost fully  developed wall cloud in Kansas.

Rain-Free Base:
             The updraft region in supercells will often lack precipitation. This is much more true of developing supercells and for classic/LP supercells, than it is for HP supercells.  The reason for this is:  most of the time rain will wrap around the updraft and eventually infiltrate into the updraft region. This  will allow the updraft to tilt with height. This will move most of the rain away from the updraft.  
 Well that's it for this installment of my series. I hope you found it informative. the next installment will be on different kinds of clouds and cloud features associated with severe weather.
Rebecca Ladd