I work with and counsel quite a few people that are afraid of Severe Thunderstorms or in general Severe Weather. I think that this fear basically lies in the fact that they do not know what to look for or they do not understand severe weather. If you do not know what to look for then you really do not know what to expect which then plays on the underlying fear of severe thunderstorms which is the fear of the unknown. So we are going to try to put together something that will help these individuals and hopefully allow them to over come that fear.
Severe Weather Ingredients…
Lets begin with the 4 primary ingredients.
1 Instability. You will often find this discussed as CAPE or Convective Potential Energy..
Instability is important because it causes air to accelerate into the vertical. Think of it this way..
As an example, imagine a basketball at the bottom of a swimming pool. Once the basketball is released it accelerates upward to the top of the pool. The basketball rises because it is less dense than the water surrounding it. A similar process occurs when instability is released in the atmosphere. Air in the lower troposphere is lifted until it becomes less dense than surrounding air. Once it is less dense it rises on its own.
An example of CAPE is above which shows the greatest CAPE as being 1000-2000.
2, Wind Shear
Strong vertical wind shear is important to severe thunderstorm development. Wind shear influences a storm in potentially several ways:
VERTICAL SPEED SHEAR- Significant increase of wind speed with height
VERTICAL DIRECTIONAL SHEAR- Significant change of wind direction with height
1. A significant increase of wind speed with height will tilt a storm's updraft. This allows the updraft and downdraft to occur in separate regions of the storm the reduces water loading in the updraft. The downdraft will not cut-off the updraft and actually it will even enforce it.
2. Strong upper tropospheric winds evacuates mass from the top of the updraft. This reduces precipitation loading and allows the updraft to sustain itself.
3. Directional shear in the lower troposphere helps ignite the development of a rotating updraft. This is one component that is important to the development of a mesocyclone and the development of tornado genesis. Strong lower tropospheric winds and directional shear together will generate high values of Helicity and thus this increases the tornado threat when severe storms develop.
On the above example you can see shear as strong as 60 knots across the NE region..
Next up is the third ingredient..
Lifting or what is referred to as the Lift Index..
Now while instability can be described as a basketball rushing to the top of the pool as described above..Lifting is caused by air that is forced to rise. Forced lifting is like picking up a bowling ball or bench pressing! This air will not rise until an object forces it to do so.
Some lifting mechanisms or what we call triggers:
Frontal boundaries
Lower Level Warm Air Advection or WAA
Upslope Flow
A low pressure system
Low Level Moisture Advection
Number 4 is moisture. Moisture is important ingredient for severe weather. If you have no return moisture …even though you have CAPE…and other severe parameters…then the likely hood of severe weather is low..
Other things that are important to look at ..
Helicity & EHI
These are two important Severe ingredients or parameters to look for if you are looking for tornado potential. First one is Helicity and the second one is the EHI..
Essentially anything over a 1 on EHI is tornadic potential..
The higher the Helicity ..the better the Tornado chances will be..
What is Helicity?
A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e. vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2.
EHI is Energy Helicity Index…
For super cells to develop, wind factors (such as helicity) and thermodynamic factors (such as CAPE, or convective available potential energy) must combine to produce an environment that is favorable for the formation of rotating thunderstorms. Therefore, one important aspect of forecasting the likelihood of super cells (and by association, tornadoes) is estimating and assessing the combination of helicity and instability.
Lift Index…
(abbrev. LI)- A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms. However there are no "magic numbers" or threshold LI values below which severe weather becomes imminent.
Example map of LI
These are the major severe weather ingredients one has to look at and decipher on the guidance. If all these ingredients come together with the proper lifting mechanisms mentioned above you have the ingredients for severe weather to occur,
However… the severity of the severe will depend on the combination of the ingredients as well as other factors such as Dry Air..
Above is a water vapor image and you can see how there is mass amounts of dry air in place. This shows up as a brownish orange color.
Also another thing to watch out for is dry air in the upper levels of the atmosphere . Such as the 700 mb level..
The green colors represent moisture while the gray colors represent dry air.
As we have just seen this past weekend places like TX escaped majority of the severe weather because of the dry air in place. This was also the reason that the storms fell apart as they moved east because of all the dry air that is across places like PA for example.
So as you can see there are many ingredients that go into Severe Weather. However..the more you understand about these ingredients and how to look for them and find them the more you will understand whether or not you even have to worry about Severe Weather.
If you are really interested in learning more about severe weather and weather in general a good starting place would be the following:
Met Ed (click here its Free Training with videos )
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