Tornadic Winds versus Straight Line Winds.

Hi it's Rebecca here again, With the Tornado in Elmira, NY last week and the microburst in Sidney this week; There has been quite a bit of severe weather lately. When the NYS damage survey team determined it was  straight line wind damage in Sidney, NY on Monday; it  caused a bit of controversy, many still feel it was a tornado. This is based on several people saying they saw what looked like a tornado, the rotating wall cloud, and people heard a roaring sound. Because of this,  I thought I would talk about the difference between tornadic and straight line winds.


On Sunday August 5 2012,  thunderstorms ahead of a strong cold front produced quite a bit of damage reports; most of these were in Western NYS. However, one storm near Sidney, NY in Delaware County caused the most widespread damage.

What is a storm survey?

when storm conditions and damage occur as happened in Sidney on Sunday; the local NWS field office assigned to that area sends out a damage survey team; whose job is among many things to determine if the damage was caused by a tornado or straight line winds. No two surveys are ever the same..Some surveys are easier than others...but there is no such thing as an easy storm damage assessment survey. There are two types of damage surveys..ground and aerial... In the case of a ground survey, the highly trained team will go to the damaged area. If the damage path is known the team will most likely start at the beginning and walk through to the end notating damage as they go. If the damage path is not known they will often start at the most damaged area and fan out normally into two person teams....They are always looking for and at the big picture.  

A damage survey consist of many things: interviews, taking pictures and video of damaged areas, documenting the type and quality of construction of the damaged building, the type of construction materials use in the building, the type of damage that occurred to the trees including the type of tree it was. These are just a few of the hundreds of things and parameters they are looking at.  

The ground survey can also take a considerable amount of time so often the amount of detail collected is determined by the scope of the event and available time/resources. A small, isolated damage spot might favor detailed examination while widespread or extensive damage might need to be summarized by a few, quick stops.

What's the difference between a tornado and straight line winds?

A tornado is a vortex of air extending upward from the ground  into the base of a thunderstorm, that is intense enough at the surface to cause damage. The condensation funnel may not always be visible from cloud to ground.

Straight line winds are very strong winds that produce damage that shows a lack of a rotational damage pattern. Straight line winds are common with the gust front of a thunderstorm or originate with a downburst from a thunderstorm.

But the one thing they have in common is both contain winds capable of causing extreme damage on the ground. Straight line winds can be just as damaging or even more damaging than a tornado.

Types of straight line wind:

The setup for straight line winds is: when you have strong updrafts and downdrafts overhead, the middle layers of the atmosphere have dry air, and the line of storms are moving very fast.  We had all of that last Sunday.

As most of you know, meteorologist use lots of terms that seem to talk about the same thing. Well this applies to straight line winds as well, straight line winds can be called convective wind gust, downburst, outflow, and as was the case in Sidney.... Rear Flank Downdraft (RFD).

A downburst is a strong downdraft which causes damaging winds on  or near the ground.

Downburst can be classified as a microburst or a macroburst. The only real difference between the two is the size of the damaged area. But both can cause widespread tornado like damage.

Microburst:

Damaging winds occur over an area of 2 1/2 miles or less. A microburst can last as long as15 minutes.

Macroburst:

Damaging winds occur over an area larger than 2 1/2 miles. A macroburst can last as long as 30 minutes.

The other type of straight line wind I want to talk about is RFD.

RFD is a type of downdraft that develops at the back edge of a rotating thunderstorm (mesocyclone). This warm and dry air is forced down out of the mid levels of the thunderstorm. I think this is caused by barometric pressure that is rapidly lowering very close to the ground. The presence of RFD is a very good indication that a tornado is in the process of forming. RFD gives birth to and causes the death of tornadoes.  The RFD descends to the ground along with tornadic circulation. RFD often causes damage when it hits the ground and is wrapping around the mesocyclone. RFD causes the death of a tornado by wrapping around the mesocyclone and cutting off the inflow to the tornado. In the case of Sidney last Sunday, I feel the tornado was within a minute of forming. When the RFD hit the ground it interfered with the inflow just enough that the tornadic circulation was interfered with. Because of this the circulation was unable to complete the process of establishing a connection between the base of the storm and the ground.   

The best way to determine if it was straight line winds or a tornado is offen from the air. Here are two pictures of storm damage. At first look they both look very much the same. However, when you take a closer look you will see differences between the two.

A damage picture taken by Gary Klindt, airport manager at the airport near Sidney. The picture was taken  near the Sidney Airport...In the picture most of the trees are more or less in the same direction..........some of the trees are in a slightly different orientation on the left and right hand sides but that is about it.....I grabbed this from the NWS Binghamton Facebook page.

This is a picture of tornado caused tree damage taken after a tornado moved across Wolfeboro NH Through Effingham NH back in 2008. If you look you can clearly see how the trees are laying in all different directions across the picture.

Why are downburst mistaken for tornadoes?

They both have damaging winds.

Wind in a tornado can range from around 60 mph to over 200 mph. Downburst have had documented winds of over 165 mph.

Both can cause trees to have that twisted look often associated with a tornado.

If you've ever looked at a tree you should have seen the tree is different all the way around it. The limbs and leaves are in different places, even the bark can have differences from one part of the tree to another. All of these things cause the wind to hit some parts of the tree harder than others. Because of this the tree will start to twist. So even if the winds are straight line if they're strong enough they will force the tree to tear in a twisting motion.

As I said above, some people heard a roaring sound in the storm. Winds that are very strong are also very loud.  This can cause people to think they heard a tornado when in fact it was straight line winds.

I hope this helps clear up some of the confusion and sometimes mistaken ideas some of us had. As always I will be more than happy to answer any questions you might have.

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.