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