Severe or not to be severe: the thunderstorm question 

Posted: 7:38 am Tuesday, April 17th, 2018

By Kirk Mellish

Many things go into if there will be thunderstorms or how widespread they will be and how strong they will be.

There are different types of thunderstorms. I won’t go over all that now but have covered that in past blogs and will no doubt do so at some point in the future.

This is a simplified explanation of what goes into SEVERE THUNDERSTORM and particularly TORNADO potential. A severe storm is defined as one that can cause damage through wind or hail. (lightning can and does do damage and kill, but all thunderstorms by definition have lightning even weak ones not just severe ones).

Thunder is caused by lightning, an old time description of a thunderstorm was “an electrical storm”. This was both because of the electricity of the lightning but also because the electric power often went out during storms when lightning struck power poles or tree branches hit wires. This is less common in modern times thanks to the improved power grid and proactive pruning.

We analyze the weather set-up using current observed and model forecast atmospheric soundings or Skew-T thermodynamic diagrams and Hodographs obtained from the twice a day release of weather balloons that sample the atmosphere.

Here are a couple examples of what they look like:


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Severe weather forecasting is ingredients based. Here is the basic 4 part recipe:


We look for surface dew points of at least 55 with 65 or higher better, we look at the precipitable water values that show the total moisture available in a vertical column of air measured in inches, the more the heavier. There are many other measures of moisture we look at I wont go into here. We also look at CAPE. Convectively Available Potential Energy, a measure of buoyancy and instability, or ability for air parcels to rise up, measured in Joules per kg. The more above 500 the more unstable the air mass. There are other measures of instability I am not covering here such as lapse rates.

WIND SHEAR/Helicity (Kinematics):

This is very complicated so I won’t cover every nuance. Wind shear is the change of speed and/or direction of the wind as you go vertical. (Vertical speed shear and vertical directional shear). There are hundreds of different variables we look to analyze. The more the wind changes direction with increasing height from surface to the stratosphere the more likely storms are to rotate. So for example a Southeast wind at the surface and a Southwest flow at jet stream level is favorable for thunderstorm rotation. For reasons too complex to go into this helps storms generate larger hail and grow stronger in all facets. The SAME for increasing wind SPEED as you go up from the surface to the stratosphere. Speed shear of at least 25 knots is needed. The greater the bulk wind shear the greater the threat of rotating and thus severe storms. PVA or positive vorticity advection at jet stream level can aid this process. But we also want to know if there will be a tendency for wind updrafts and downdrafts from cloud base to earth to rotate. This is helicity and we look for values of 100 m2/s2 or higher in the 0-1km layer. There are dozens to hundreds of other parameter formulas for rotation and spin and updraft velocity that forecasters look at that I wont cover here.


There are low-level jets and the more familiar main jet stream aloft at various levels and locations. The presence and configuration of jet streams are related to wind shear and lift in the atmosphere, the patterns of diffluence and divergence that aid in air wanting to rise must be assessed. Again, too complicated to go into all the details since this isn’t a college course. Suffice to say, that crossing jet streams that are strong at least 45 knots, and the higher the jet winds the more favorable for severe weather.


A surface boundary such as a trough of low pressure, a warm front or a cold front and or low pressure center, or a buoyancy gradient can all help focus lift and all the other ingredients. Other types of boundaries such as small scale difference in temp or moisture or mesoscale outflow boundaries can all serve the same purpose of focusing the potential for thunderstorms into reality.

Note how research finds a correlation between Helicity (SRH) and CAPE in tornado potential:


There is often a trade-off between key ingredients for a severe thunderstorm so that the maximum positive ingredients are not in sync, they are out of phase with one another. So you might have plenty of one but not enough of the other making it a more complicated forecast, because severe storms are still possible, but predicting how likely or unlikely is a more difficult call.

That happens a lot here in the Southeast if we have a strong system approaching but we are in one of our “wedge” (CAD) patterns. It also happens when a strong system to our West moves into our area but at a cooler less favorable time of day: late night or morning. Here’s an example from March 2018. See how two main ingredients (buoyancy and vertical shear/rotation) are not at a maximum at the same time at the same location:

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Sometimes there’s enough of one to overcome the lack of the other, that’s what we mean by the trade-off. It make the severe weather threat far more uncertain in such cases.

In that case, a wedge helped kill storms as they tried to move into it (blue shade) while most of the tornadic storms formed on the edge of the greatest CAPE (buoyancy) instead of where it was at a maximum. The blue shade CINH or Convective Inhibition, is where conditions want to resist upward air motions. High CINH tends to inhibit the release of buoyant energy.

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The greatest threat of tornadoes and other forms of severe weather occurs where moist buoyant unstable air (high CAPE) and maximum wind shear and rotation (Helicity) overlap.

We saw this happen again Sunday April 15th, much like the March 19th event, although this time there was no wedge in place. However, the low pressure and front arrived in Atlanta in the morning when the air was cooler and more stable, hence the same effect.

Where the storm arrived at the warmest part of the day to our West the storms were worse, when it arrived here weaker with the cool morning, then the storms fired up again to our East as the front arrived there later in the warmest part of the day:

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The Atlanta area was again fortunate to be where key parameters were out of sync and not overlapping, that Sunday morning we had a lot of spin but not enough instability/buoyancy. But to our South and East where the front arrived later the ingredients again became more juxtaposed:

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Like many things in life, timing matters and the atmosphere is complex with many interactions. There are hundreds of charts a meteorologist must look at to make a forecast, not just some “Future radar” animation. That won’t cut it and neither will automated web pages or phone APPS, except on days the weather is simple which isn’t often.

This is why in both cases I down-played the threat as minimal instead of hyping the risk.

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