Friday, April 25, 2014

The Basics of a severe weather sounding.

Severe weather season is quickly approaching. So, here is another installment on severe weather  parameters and indices. This installment will deal with SKEW-T soundings.

I did a similar one quite a while back. But I think that one was a bit too complex. So I will do this one that I hope is easier to understand.

If you want to read that one, you can find it here.

                                                A 18Z sounding taken the other day.

When you look at a sounding you're looking at a vertical profile, so the bottom of the chart is the surface, and the top of the chart is up near the top of the atmosphere.

The black numbers on the far left side of the Skew- T diagram are all pressure values measured in millibars. They go from 1000mb to 100mb, 1000mb is very close to the average sea-level pressure, 500mb would be in middle of the atmosphere, and 100mb is very close to the top of the atmosphere.

The red numbers on the left side of the diagram are altitude values in kilometers. SFC means the surface altitude where the balloon was launched.

The black numbers on the bottom of the diagram are temperature values in degrees Celsius. Note the temperature lines skewed (tilted up) to the upper right .

The red line represents the temperature of the atmosphere, while the green line represents the dewpoint. (the dewpoint line is always on the left and the temperature line is always on the right.)

On the 18z skew T you can see the surface dewpoint is 51 and the surface temperature is 80. You can also see the wind barbs on the right side of the diagram. Each wind symbol represents the wind speed and direction at that level of the atmosphere. This allows you to see the shear (twist) in the air.

Don't know if you know how to read a wind barb. But, the end of the staff points in the direction the wind is blowing and the barbs show wind speed.

A full blacked pennant or triangle is 50 knots of speed.

A full line on the wind barb is 10 knots of speed.

 A half line is 5 knots of speed.

 Now look at the winds. The surface wind is out of the south southwest at 20 knots, but the wind direction changes to the southwest and speed increases with height. A clockwise wind direction change is called veering, when it the other way it's called backing. Twisting in the atmosphere is a major component in promoting severe thunderstorm development.

In the upper right hand side is a diagram called an hodograph. A hodograph shows the change of wind speed and direction with height (vertical wind shear), though the layers of the atmosphere. The wind barb data next to the SKEW-T is plotted on the hodograph and a line is drawn connecting the different layers. It makes it easier to see the twisting action going on.  
Note the temperature (red line) decreases with height from the surface up to about 925 mb. The temperature then briefly increases with height (This is called a temperature inversion), before going on to decrease with height. The dewpoint follows a pattern that looks similar.  All thunderstorms require low level moisture, instability and a trigger. Severe thunderstorms also require decent wind shear - speed and/or directional change of the winds with height.

The sounding immediately shows the moisture profile. The better thunderstorm setups will have plenty of moisture in the lowest few km of the atmosphere with dry air aloft. But not too much low level moisture - you want to see the temperature and dew point lines with a few degrees of separation . If the temp/DP lines are very close together or the same values , it will be very cloudy and raining.

The brown dotted line near the temperature line is called the Theoretical Air Parcel Plot (TAPP) is a representation of how a parcel of air may rise from near the surface. Where the brown dotted line is to the right of the temperature trace it is unstable. the further to the right of the Temp trace the  line is the more instability. And if brown dotted line is to the left of the temp trace then things are relatively stable.

 If the surface layer moisture increases or decreases more than expected. Increasing or decreasing the dew point makes a much bigger difference to overall instability than a changing surface temperature. You can also see the complete wind shear profile at a glance. Wind speeds will typically increase the higher up you go. The better thunderstorm setups will have winds backing (anticlockwise) with height in the lower few km. For example N-NE at the surface, N at 925hPa, NW at 850hpa and W by 700hPa is a typical scenario along the east coast. This turning helps establish good updraft/downdraft separation and hopefully a supercell. Of course it is not that simple.

On the SKEW-T you can see where they have labeled the LCL. The LCL will be where cloud bases will initially form - ie. the first cumulus clouds. As more and more convection occurs the air will mix - and cloud bases will develop lower down. Numerous thunderstorm indices can be determined on the Sounding. The for now I will only deal with Lifted Index (LI) and Convective Available Potential Energy (CAPE). The higher the CAPE and lower the LI means there is generally a better shot at severe weather.

On the sounding you provided you can see that CAPE is around 1700 and LI is at -7....showing a moderate severe set up...but nothing that great. The CAP (the temperature inversion) is a relatively warm layer of air that may delay or completely suppress the formation of thunderstorms. On the sounding it can be seen where the temperature stays the same or often increases with height. You can see there is a CAP starting just below 800mb. This is a moderate CAP. You can see another CAP at 550hPa. These higher CAPS do not necessarily stop thunderstorms but may slow updraft strength.

If you want to go into more detail on CAPE. use this link 

Regarding storm height, this is easily read form a sounding. The level at which the Theoretical Air Parcel Plot line (TAPP) crosses the temperature trace is known as the Equilibrium level (EL). This is the level where storms anvil out. Note though, a strong thunderstorm can overshoot this level resulting is storm tops above the EL. For this reason, on any given day, the taller storms are most likely to be the strongest but storm height is largely dependent on the height of the EL.

A few examples of severe soundings:

When looking for severe potential certain shapes can get your attention.  

One of these is called the Inverted V.

Here is an example of an Inverted V.

When you see an inverted V it means there is a good chance for severe weather capable of strong downdrafts and gusty winds.

Another is called a loaded Gun sounding, or sometimes called a goal post sounding.
Here's an example

When you see this type of sounding, it means there is tons of instability, but there is a strong cap in place that is keeping the lid on. It's like a pressure cooker, it keeps the pressure on until the lib blows. When this happens you can see very explosive thunderstorm development.

Here are two soundings that show the  environment that was in place for the severe weather outbreaks over Oklahoma in May of 2013. One is the sounding for the Newcastle/Moore and the  El Reno Tornadoes.

                        Taken on May 20th, 2013, just 3 hours before the Newcastle/Moore EF5  

                                      Sounding of the May 31,2013 El Reno Tornado
That's the basics

As always I will answer any question you may have........

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