R.A.C.E.S. Weather Spotter NE-408

A typical approaching thunder storm over Fort Worth, TX A slight bit of rotation was observed just behind the wall cloud. It soon disipated into thin air.

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The air mass thunderstorm is a common and usually non-severe phenomenon that forms away from frontal systems or other synoptic-scale disturbances. They form where moist and unstable conditions exist in the atmosphere. Air mass thunderstorms are usually produced in areas of very little vertical shear. As a result, the threat for severe is small. When they do reach severe limits, the thunderstorms may produce brief high winds or hail which develop because of high instability. These storms are know as pulse severe storms. Although several storm cells can develop, each individual cell lasts about 30-60 minutes and has three stages.
Cumulus Stage: Graph , Picture
Starts with a warm plume of rising air.
The updraft velocity increases with height.
Entrainment pulls outside air into the cloud.
Supercooled water droplets are carried far above freezing level.
Mature Stage: Graph , Picture
The heaviest rains occur.
The downdraft is initiated by frictional drag of the raindrops.
Evaporative cooling leads to negative buoyancy.
The top of the cloud approaches tropopause and forms anvil top.
Dissipating Stage: Graph , Picture
The downdraft takes over entire cloud.
The storm deprives itself of supersaturated updraft air.
Precipitation decreases.
The cloud evaporates.
(Pics and Graphs from Steve Davis WSFO Milwaukee/Sullivan)
As wind shear organizes the convection, new thunderstorms form as a result of parent thunderstorm outflows converging with warm, moist inflow creating new updrafts. Multicell storms can form in a line known as a squall line, where continuous updrafts form along the leading edge of the outflow, or gust front. Multicell clusters indicate new updrafts are forming where the low-level convergence is strongest, usually at the right, or right-rear flank of existing cells.
Thunderstorms that organize in response to synoptic scale forcing usually need:
- warm, moist air at low levels- cool, dry air at upper levels- upper-level divergence (above 500mb)- a synoptic scale disturbance
In these conditions, thunderstorm formation is probable. Synoptic scale vertical motions tend to create favorable conditions for thunderstorms, but thunderstorm initiation is usually a result of mesoscale forcing. Increasingly favorable vertical wind profiles may lead to a greater possibility of supercell development rather than multicell storms. The development of squall lines, or more commonly storm clusters, when thunderstorms do develop is virtually guaranteed in association with synoptic scale forcing.
Multicellular storms consist of a series of evolving cells. At low levels, cooler air diverging from the downdraft intersects the inflowing air along a gust front, creating a region of strong low- level convergence favorable for new updrafts. It is the presence of vertical wind shear that results in the "tilting" of the updraft and downdraft. Because of the tilting, the less buoyant downdraft air will not destroy the updraft and hence deprive itself up supersaturated updraft air. In any case, the movement of multicell storms systems is determined by combining the new cell development with the mean winds. Each individual cell typically moves with the mean winds, while new cells develop where the inflow meets the outflow, hence, in the region of strongest surface, or low-level, convergence.