What Patterns Cause Severe Weather?

There are four main ingredients for severe weather – shear, lift, instability, and moisture (or SLIM for short). Almost every severe weather outbreak has some combination of these four ingredients.

Shear

Shear means winds changing speed and/or direction with height. Sometimes this is referred to as vertical wind shear. Shear is important because it separates the updrafts and downdrafts within a thunderstorm so that they are not competing with each other. This allows for stronger updrafts, which are more likely to produce things like tornadoes and large hail. Shear values supportive of severe weather would generally be at least 20 knots between 0 and 6 kilometers above the surface.

Lift

Lift refers to a sort of mechanism to get the ball rolling when it comes to creating the updraft of a thunderstorm. The most common types of lift are warm and cold fronts. Lift can also be found from stationary fronts and dry lines. Sometimes during summertime thunderstorms, lift can simply come from a very hot surface due to daytime heating.

Instability

Instability means how likely the atmosphere is to support thunderstorm development once there is initial lift. If lift is pushing a ball off a hill, instability is the height of the hill. In other words, instability is a measure of how much updrafts will continue once given a little push by a lifting mechanism. Instability is commonly measured with a variable called CAPE (Convective Available Potential Energy). CAPE can be measured with weather balloons that are launched across the United States by the National Weather Service twice a day. There are many other variables that can be used to measure instability, but CAPE is the most widely used and understood. Typical severe thunderstorms have CAPE values of at least 500 joules per kilogram (J/kg). The most impactful severe weather events can have CAPE values exceeding 5,000 J/kg.

Moisture

Moisture is pretty self-explanatory – it is how much water vapor is in the air. This moisture is needed to condense the water vapor into clouds and precipitation. When an updraft occurs, the water vapor wants to condense out of the air at higher altitudes. This leads to the formation of clouds and thunderstorms. The most well known way to approximate moisture is relative humidity, which is a percentage of how much water vapor is currently in the air, compared to how much total water vapor the air can physically support. However, relative humidity does not actually measure how much water vapor is in the air. Instead, meteorologists often use the “dew point” temperature to measure the liquid water vapor content of the air. Dew point temperature refers to the temperature at which water vapor would be forced out of the air in order for temperatures to cool further, which is how dew forms in the early morning hours. It really depends on the environment, but severe thunderstorms typically have dew point temperatures over 50 degrees Fahrenheit.

Other Considerations

There are many other factors to consider when forecasting a severe weather event. The four topics discussed here are just a broad overview of the ingredients which are usually necessary. One other important factor is the jet stream. Typically, a strong jet stream, which leads to greater shear, is necessary to dynamically support strong thunderstorms. Some orientations of the jet stream are more favorable than others for thunderstorm development. Topics like this will be covered in more advanced lessons.