Ingredients-Based Forecasting

It has been discussed in this course and in the Meteorology 101 course that forecasting severe weather can be broken down into four main ingredients: shear, lift, instability, and moisture. Often times when looking for areas which may experience severe weather on a certain day, forecasters will look for the combination of these four ingredients. These four ingredients must work together in order to generate severe weather. If there is a lot of instability present but no lifting mechanism to initiate thunderstorms exists, then thunderstorms may never happen. Similarly, if there is a strong lifting mechanism but there is very little instability, then thunderstorms do not have the energy they need to survive. Here, we’ll investigate each of the four ingredients, including how to forecast them for severe weather.

Shear

Shear refers to vertical wind shear, or winds changing speed and/or direction with height. Shear is a necessary component of severe thunderstorms because the changing winds separate competing updrafts and downdrafts. Without enough wind shear, thunderstorms would collapse in on themselves before becoming strong enough to generate large hail or tornadoes. Generally, the environmental wind shear needs to be at least 30 knots between the surface and 500 millibars to support severe weather.

Large shear values are typically found wherever there are strong wind speeds in the middle and upper troposphere. These strong wind speeds are often associated with the jet stream. From this perspective, the presence of a strong jet stream is typically a necessary component for developing severe weather. This doesn’t always have to be the case as some severe weather events can occur in the Florida peninsula, where there is typically no strong jet stream aloft. In those cases, either there are localized effects creating enough shear or the severe weather which occurs is not very impactful.

Lift

Lift refers to the initial lifting mechanism, which lights the match for thunderstorms to begin. This often occurs in the form of a cold front or warm front, but can also occur from a dry line or stationary front. As discussed in the sounding lesson, air parcels need to be lifted to their LFC (or Level of Free Convection) before they will begin rising on their own due to instability in the atmosphere.

Lifting mechanisms can be forecast by identifying surface-based fronts, which may initiate thunderstorms. Besides temperature differences, fronts are usually identified by a change in wind speed along a certain boundary. This change in wind speed often causes air to converge, which forces it upwards and can thus begin the life cycle of a thunderstorm.

Instability

Instability is the ingredient for thunderstorms which provides the energy needed to generate strong updrafts and precipitation. Instability can be measured as how buoyant air parcels are compared to its environment if they are lifted by a lifting mechanism. One parameter to measure instability is CAPE, or Convective Available Potential Energy. CAPE values can be calculated based on Skew-T diagrams within soundings, and are measured in J/kg (Joules per kilogram). Typically, severe weather environments have at least 500 J/kg of CAPE, but the strongest severe weather events often have 4,000+ J/kg. The measure of CAPE is also affected by how moist the air is, such that if there is more moisture at the surface and less moisture aloft, there will generally be more CAPE.

Regions of greater instability often occur where there is warm, moist air near the surface, and cool, dry air aloft. These conditions are often met in the Great Plains region of the central United States as warm, moist air from the Gulf of Mexico is present at the surface and cool, dry air exists aloft due to the jet stream going over the Rocky Mountains to the west. This makes the Great Plains region a global hot spot for severe weather, including tornadoes, to form, which is why it gets its nickname Tornado Alley. Cloudiness can affect instability by limiting how warm the surface temperatures become during the day. Generally, CAPE values and instability are greater when there is at least some clearing of the skies to heat the surface.

Moisture

The final ingredient necessary for severe weather is moisture. This refers to having a continuous source of moist air parcels to feed into thunderstorms, which will produce condensation and precipitation. Moisture near the surface is also a key ingredient for instability, which is one example of how these four ingredients work in tandem to generate environments favorable for severe weather.

Regions of moisture are often static as the moisture is supplied by nearby bodies of water such as the oceans, large lakes, and rivers. For example, the source of moisture for Tornado Alley is the Gulf of Mexico, as southerly winds bring moisture from the Deep South up into the Central Plains.

Putting it All Together

Searching for environments which have all four of these ingredients is an important part of severe weather forecasting. Typically, favorable severe weather environments are near a strong jet stream (which adds shear into the equation), are near a strong frontal boundary (which adds lift into the equation), have some clearing leading to warm surface temperatures (which adds instability into the equation), and are fed moist air along the surface by a body of water (which adds moisture into the equation).