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.


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