Forecasting deep moist convection, i.e., thunderstorms, is a major challenge for forecasters. One practical approach is the ingredient-based method of moisture, instability and lift. This approach originated in SELS (Severe Local Storms Unit of the National Severe Storms Forecast Center) in the early to middle 1970s, perhaps earlier. Over the subsequent twenty years numerous researchers explored the various components of this approach, refining and improving our understanding of thunderstorm forecasting.
In recent years mesoscale numerical models have dominated thunderstorm forecasting. Nevertheless, a knowledge of the moist, instability and lift approach to the development of deep moist convection can help improve our interpretation of what the numerical model is telling us.
This web page presents an integration of the work of many researchers combined with the web-author's observation of thunderstorm development and movement over thirty years of Central Plains weather. Although most of the ideas were based on Central Plains thunderstorms, the concept presented here is sufficiently broad to have application over a wider area.
The ideas presented here are a conceptional approach to forecasting deep moist convection, i.e., thunderstorms. The concept is a variation on the original SELS moisture, instability and lift idea. This variation states that moist, instability and lift are needed to produce non-severe thunderstorms while three additional factors need to be considered to allow these storms to become severe. Similarly lift is divided into a synoptic scale component and mesoscale component. The latter component helps to better define thunderstorm development in time and space. The details of this conceptual approach are discussed below.
It has been known since the 1940s that unstable air is needed for the development of thunderstorms. In addition to the well-recognized convective instability, conditional symmetric instability (CSI) may also contribute to thunderstorm development.
Convective instability refers to a vertical column of air in which the wet-bulb potential temperature decreases with height. A vertical column with this structure allows a rising parcel of air to accelerate upward once that parcel passes the level of free convection (LFC).
Over the years numerous stability indices have been developed to evaluate this upward acceleration. These indices range from the Showalter index, to the Lifted Index, to the more inclusive index called CAPE (convective available potential energy). These indices are a "snapshot" of the convective instability at a specific time and place. Numerical forecast models provide maps of these indices as well as images of vertical temperature profiles, i.e., soundings, into the future.
However, indices alone are only one part of the information needed to assess the instability component of forecasting deep moist convection. If the convection originates in the surface layer, the effect of the near-surface convective inhibition energy (CIN) must be examined. CIN measures the energy in the negatively buoyant area below the LFC. As the negatively buoyant area increases, the magnitude of the near-surface forcing must increase if the rising air parcels are to move from the boundary layer to the LFC and subsequently accelerate into the thunderstorm updraft. If CIN is large enough, updrafts may be suppressed due to the inability of rising air to reach the LFC. If CIN is low, it is easier for boundary level air to reach the LFC.
The discussion above applies to convection originating in the boundary layer. There are situations where the initial updraft may be forced aloft. This is usually referred to as "elevated convection."
The importance of both the horizontal and vertical distributions of moisture has been recognized for over 50 years. Forecasters typically analyze isodrosotherms, lines of constant dewpoint, on surface and upper level charts. These analyses identify areas of moisture advection and/or moisture gradients. Dew point analyses can anticipate short-term changes in the moisture available for thunderstorm development. These changes can substantially affect convective instability or reinforce or modify existing moisture gradients. Advective changes can be very rapid in some situations. Moisture advection by the nocturnal low-level jet rapidly moves low-level moisture northward to produce the "moist tongue" recognized by forecasters as a favorable pattern for thunderstorm initiation. Horizontal moisture gradients such as the dryline are also favorable areas for thunderstorm development. Parameters such as mixing ratio and equivalent potential temperature can also be used (instead of dew point) to access moisture patterns.
Variations in the vertical distribution of moisture have been associated with different types of thunderstorm events. A relatively high moisture content through a deep tropospheric layer is favorable for the occurrence of heavy rainfall and possible flash flooding. Typical soundings associated with wet and dry microbursts have been identified. Forecasters use these characteristic soundings as a pattern recognition process to better anticipate the type of thunderstorm event that is likely on a given day.
The lift needed to produce thunderstorms can be discussed in terms of a synoptic-scale component and a mesoscale component. Studies of upper-tropospheric divergence and lower-tropospheric warm-air advection indicate that synoptic-scale upward motion is present during most thunderstorm events. This synoptic-scale lift is usually associated with mid- and upper-tropospheric troughs, jet maxima, and warm-air advection. By itself synoptic-scale left does not generate or trigger convection but produces an environment conducive to the development of deep moist convection. Synoptic-scale left alone can destabilize an atmospheric column if the column is convectively unstable and if enough time is available. Another source of lift is needed to focus the ascent on the mesoscale.
Thunderstorms are mesoscale phenomena, and the source of lift needed to initiate them must be sought on mesoscale space and time scales. In the discussion of instability above it was noted that rising parcels of air frequently must move upward through a negatively buoyant layer below the LFC before positively buoyant upward acceleration is achieved. Upward movement through the negatively buoyant layer is provided by mesoscale mechanical lift produced by low-level convergence. Near-surface mesoscale convergence starts and focuses the forcing in the near-surface layer. This low-level convergence and upward lift must be strong enough to carry the near-surface air to the LFC and higher. In situations where the negative buoyancy is very weak or absent, the role of the mesoscale lifting mechanism is more to initiate and maintain the supply of near-surface air to the developing storm.
Near-surface lift is provided by three main features: boundaries, differential heating, and wind interactions with terrain. Boundaries are curvilinear discontinuities characterized by cyclonic shear and convergence (in the horizontal plain). Boundaries include fronts and convective outflows (gust fronts). The best example of differential heating that produces a near-surface convergence zone is the sea breeze. The convergence produced by near-surface wind flow interactions with terrain is a less obvious and often subtle lifting mechanism.
The identification of near-surface lifting mechanisms is frequently the key to anticipating the occurrence of thunderstorms in a specific location at a specific time. A skilled surface analysis combined with satellite imagery and derived fields such as moisture flux convergence or surface wind divergence assists forecasters in timely identification of near-surface convergence areas.
In certain situations, thunderstorms occur in the presence of weak synoptic-scale subsidence. In this case thunderstorms develop due to strong near-surface convergence and high convective instability that compensates for any synoptic-scale downward motion that might impede convection in cases of weak forcing or weak convective instability.
The discussion above relating moisture, instability and lift to the formation of deep moist convection applies to the development of non-severe thunderstorms and does not imply that those thunderstorms will reach severe intensity. Three factors differentiate a severe thunderstorm environment from a non-severe thunderstorm environment. These factors are the presence of extreme instability, strong low-level vertical wind shear, and midlevel dry air or the intrusion of dry air at mid-levels.
Extreme convective instability, when released, provides the large positive buoyancy needed to develop strong thunderstorm updrafts that support large hail. Although "extreme instability" has not been explicitly defined, experience shows that CAPE values above 3000 m2s-2 can be used as approximate lower bounds.
Strong low-level vertical wind shears contribute to the development of rotation within a thunderstorm, that is, the development of mesocyclones and/or tornadoes. Mid-level dry air has been cited as factor important to the development of strong convective wind gusts. The entrainment of dry air into rain-saturated downdraft results in evaporational cooling that enhances the negative buoyancy of the downdraft.
When anticipating the movement of deep moist convection a forecaster must differentiate between cell movement and mesoscale convective system (MCS) propagation. Individual thunderstorm cells move with the mean wind in the cloud bearing layer. This value is usually available from computer forecast models. A good approximation to cell movement can be obtained from an average of the 700 mb and 500 mb winds.
A mesoscale convective system is a cloud and precipitation system with a spacial scale from 20 to 500 km and a temporal scale from 3 to 12 hours that includes deep convection during some part of its lifetime. This multicellar storm or group of interacting storms implies some type of mesoscale organization in its forcing. The MCS typically lasts longer than any individual thunderstorm cell that is part of it.
An MCS tends to move or propagate in a direction different from the individual thunderstorm cells within it. Propagation is the movement of the mesoscale system as a result of new cell development on one flank of the system. When new cells develop on the leading edge of the system, forward propagation has an accelerating effect and the system moves faster than expected from the wind field alone. When new cells develop on the rear edge of the system, backward propagation has a decelerating effect that results in three possible motions: slower than expected system movement, quasi-stationary motion, or backward propagation. Essentially, an MCS will propagate in the direction that provides the best inflow of moist unstable air that will sustain the system's existence. Expressed in a less scientific manner, you have to feed the mesoscale beast if you do not want it to die.
This web page outlines a variation on an ingredients-based approach to forecasting deep moist convection that has been around for over 50 years. This variation states that moist, instability and lift are needed to produce non-severe thunderstorms while three additional factors (extreme instability, vertical wind shear, and mid-level dry air) need to be considered to anticipate storms reaching severe limits. Similarly lift is divided into a synoptic scale component and mesoscale component. The latter allows forecasters to better anticipate thunderstorm development in time and space.
The ideas outlined on this web page have been used in operational forecasting. It is the web-author's opinion that they provide a useful approach to forecasting deep moist convection.
An example would be useful to illustrate this concept, but that example is not currently available.
If the reader is interested in specific peer-reviewed papers on the topics discussed above, please refer to the article cited below.
McNulty, Richard P., 1995: Severe and Convective Weather: A Central Region Forecasting Challenge. Weather and Forecasting, 10, June, 187-202.