Overrunning--A condition existing when an air mass is in motion aloft above another air mass of greater density at the surface. This term is usually applied in the case of warm air ascending the surface of a warm front or a quasi-stationary front. (Definition from the Glossary of Meteorology).

This term is perfectly acceptable if used merely to describe (called a kinematic statement) the fact that in certain situations, cross-sections show that potentially warm air is moving above colder denser air (as in the classical warm frontal model).

It is an improper use of the term to imply that overrunning is a mechanism to lift air and generate clouds and precipitation.

Synoptic-scale lift is associated with synoptic-scale upper tropospheric divergence which is in turn connected to mid-tropospheric upward vertical motion via the equation of continuity. The general descriptive relationship that comes out of the synoptic-scaling of the Equation of Continuity is called "Dine's Compensation." The synoptic-scale lift can be assessed or diagnosed using the quasigeostrophic omega equation.

Suppose we are interested in qualitatively characterizing the vertical motion at 500 mb. There are two so-called "forcing terms" which appear in the quasigeostrophic omega equation that can be used to estimate such motion:

• vorticity advection becoming more positive with height in the layer centered at the level at which the vertical motion needs to be estimated (in this case, the 500 mb level, often erroneously approximated by pva at 500 mb)
• warm advection at the given level at which the vertical motion needs to be estimated (in this case, the 500 mb level)

Since both are associated with vertical motion (say, in mid troposphere), then both are related to upper tropospheric divergence (which is what many operational meteorologists mean by "dynamics"). Many forecasters use "dynamics" to mean positive vorticity advection and forget that warm advection is an equal or greater partner in synoptic scale lift, but that is a subject for another rant and rave.

Illustration of Misuse of Rule of Thumb that Warm Advection = Overrunning (Warm Frontal Lifting)

The trouble is that in the minds of many operational meteorologists, warm advection means "warm frontal" lifting. Also, since many forecasters then use the term "overrunning" as a synonym for the shallow lifting assumed to occur in the classic model of the warm front, then, incorrectly, the term overrunning is used often as a synonym for warm advection. (See Chuck Doswell's discussion on the misuse of this term.) If forecasters really mean the rising motion associated with warm fronts when using the term "overrrunning," they need to keep in mind that frontal lifting (either cold or warm) is really a mesoscale process and is not the same thing as the synoptic scale lift related to warm advection.

Regions of warm advection are prominent features associated with the "low latitude" storm type (often referred to as "pineapple connection") along the West Coast. When such "warm advection regions" begin appearing in the forecasts, it is also common to see the term "overrunning" applied to the mechansim that will bring cloud and precipitation to California, for example. For the sake of argument, if "overrunning" is a synonym for the lifting that occurs along a shallow warm frontal boundary (which is indeed characterized by warm advection), then soundings should show that clouds and precipitation are associated with an elevated warm moist layer (with a cool and relatively dry layer below). This is not always the case.

The maps and charts at right illustrate this point.