Thunderstorms are often a "one and done" sort of deal during the summer months, impacting an area for a relatively short period of time before clearing out in the wake of the storm.

However, in some instances, numerous thunderstorms or showers will re-develop and track over the same area over and over with only short breaks in between. This is referred to as the training of thunderstorms.

Why and How Does Thunderstorm Training Occur?

Thunderstorm training occurs when...

1) Enough moisture and instability are present for storms to develop.

2) A stationary or slow-moving lifting mechanism in the atmosphere or near the surface allows multiple thunderstorms to develop over the same areas.

Let's go over some of these features that can lead to thunderstorm training...

Stalled Cold Front or Stationary Front

A front is a separation between two air masses and is a common trigger for thunderstorms. Converging winds and moisture along a boundary forces warm, moist air parcels near the surface to rise, which can develop into thunderstorms.

If a front remains stalled over an area for an extended period of time, then multiple storms can form along the boundary and track over the same areas.

Slow-Moving Trough or Shortwave

A trough of low pressure is a larger-scale feature that can trigger widespread showers and thunderstorms over an area. Slow-moving troughs that linger over an area for an extended period of time without any significant changes in temperature, moisture, or wind can result in multiple thunderstorms developing over the same area. 

Shortwaves are smaller scale and more subtle features in the mid to upper levels of the atmosphere that cause moist/unstable air to rise. These are common thunderstorm "triggers" during the summer months. If a shortwave is moving slowly enough, then it can cause numerous thunderstorms to redevelop over the same areas.

Jet Stream and "Jet Streaks"

When the jet stream is located nearby, it also acts as a trigger for thunderstorms to develop – and often re-develop over the same areas. This is due to diverging air streams caused by strong winds aloft, which in turn leads to rapid acceleration of rising air currents to "fill the void" left by the diverging air in the upper atmosphere. 

The position of the jet stream and localized wind maxima within the jet stream relative to a location play in role in potential thunderstorm development. The strongest wind currents in the jet stream are known as “jet streaks”, and areas located near the right entrance regions and left exit regions of jet streaks are most favored for the lifting of moist/unstable air and thunderstorm development. 

Orographic Lift

Orographic lift from a warm season thunderstorm perspective refers to when winds transport warm, moist air toward a mountain barrier. This air is then forced to rise vertically in conjunction with the rise in terrain and can lead to the development of thunderstorms.

When any of the previously mentioned lifting "features" interact with mountainous terrain in a way that enhances vertically rising air, then the threat of thunderstorm training will increase with impacts possible over relevant mountain ranges and surrounding areas.

Thunderstorm Training and Lightning Danger

Thunderstorm training can make for a dicey situation if you are outdoors and particularly if you are hiking or climbing in exposed terrain.

Breaks in between thunderstorms are usually limited during thunderstorm training situations, giving you little time to cross over any high passes or other exposed areas.

It's always important to keep an eye on clouds for new development once a storm is exiting your area and to evaluate your terrain and shelter options should another storm pass over you.

Thunderstorm Training and Flash Flooding Danger

Significant levels of moisture in the atmosphere along with at least modest instability increase the threat of heavy rainfall and flash flooding with thunderstorms.

In addition to moisture and instability, there are two other key factors that influence flash flood potential: 1) the speed of thunderstorms (will storms be slow moving?) and/or 2) the potential for training of thunderstorms.

Anytime there is a situation with significant moisture and weak/erratic winds aloft, this results in higher flash flooding potential due to slow-moving or even stationary thunderstorms that can set up over an area for an extended period of time.

Thunderstorm training is a little bit different. Storm motions can be faster during training scenarios, limiting rainfall potential somewhat under individual thunderstorms.

However, if numerous thunderstorms re-develop and track over the same areas, producing consistent heavy rainfall with only short breaks, then flash flooding becomes a significant threat in the same way that a slow-moving single thunderstorm does.

Learn More → Flash Floods and Outdoor Recreation

Alan Smith, Meteorologist