With the severe weather of this past weekend in our rear view mirror, we can now look back and see what caused it. Meteorologists often do this after large severe weather events in order to learn what caused the set-up in the first place. This allows us to catch onto patterns that led up to the event. With this information, we can look for similar patterns in the models which can hint at the possibility of severe weather.
Upper Level Divergence
One of the patterns that led to such a strong storm system is upper level divergence. Divergence occurs where the air is being removed or pushed out of an area. Since you can’t have a void of air in the atmosphere, air rushes in to replace the air being removed.
There is actually two types of divergence that occur in the atmosphere. The first is speed divergence (stretching). This divergence occurs when the wind speed increases downstream of the region. This pushes air out of the area. The second type is directional divergence also called diffluence. Upward motion is created as air spreads out downstream of the area in different directions.
A good way to illustrate this concept is to push water out of the way in a bathtub or pool.
Materials Needed to Conduct Experiment:
- Pot or bowl to push water downstream
- A source of water such as a bathtub or pool
- Camera to record video in slow motion
What happens when you do this? More water rushes in from underneath and the sides to replace the water you pushed away. The exact same thing happens in the atmosphere.
When divergence occurs in the upper levels of the atmosphere, air rushes in from the lower levels to replace it. This rushing/filling in of air is called convergence and is the complete opposite of divergence. These two processes are important in keeping the atmosphere balanced. Remember the atmosphere does not like imbalance.
The converging air collides at the surface and rises to replace the void left by the diverging winds aloft. This rising motion can help create or enhance low pressure systems. The stronger the divergence is aloft, the more forceful the upward motion is to replace it.
This stronger lift in the atmosphere can sometimes lead to explosive storm development and stronger surface winds.
We had a large amount of both directional and speed divergence over us for Saturday night’s event (11/14/20). This is indicated by the yellow concentric lines. Notice where the divergence is occurring as well. It is located downstream or to the right of the trough axis. In this particular example the trough axis cuts through the central United States, which places NW Arkansas and the River Valley to the right or downstream of the trough.
Watch the upper level charts (300-200 mb) patterns. The next time that we end up on the downstream (rightside) of a trough you can expect the lift to be increasing across the region. Depending on how much moisture we see return you could either see cloud cover or even precipitation.
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