A transition zone of temperature and/or moisture and/or wind in the atmosphere. Fronts, drylines, and outflow boundaries are examples.

A radar reflectivity echo which is linear, but bent outward into the shape of an archer's bow. The strongest straight-line winds often occur near the "crest" or apex of the bow.

Key structural features include an intense rear-inflow jet impinging on the core of the bow, with book-end vortices on both sides of the rear-inflow jet, behind the ends of the bowed convective segment. Bow echoes have been observed with scales between 20 and 200 km, and often have lifetimes between 3 and 6 h. At early stages in their evolution, both cyclonic and anticyclonic book-end vortices tend to be of similar strength, but later in the evolution, the northern cyclonic vortex often dominates (in the Northern Hemisphere), giving the convective system a comma-shaped appearance (see comma echo). Tornadoes sometimes occur, especially with the cyclonic vortex.

(Also called convective cell.) In radar usage, a deep, moist, convectively-induced local maximum in precipitation density that undergoes a life cycle of growth and decay.

The rising portion of the reflectivity maximum is indicative of updraft, and the later descending portion is indicative of a precipitation downdraft. Cells in ordinary convective storms last from 20 to 30 min, but often form longer-lasting multicell convective storms. Cells in supercell storms are more steady and last considerably longer.

See also thunderstorm cell.

A region of relatively cold air surrounded by warmer air. It is represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.

In the same direction as the shear vector within a specified layer.

In the same direction as the wind flow, or toward the direction in which the wind is moving.

An organized lifting zone of cumulus and towering cumulus clouds, connected to and extending outward from the mature updraft tower of a supercell or strong multicell convective storm.

The flanking line often has a stair-step appearance, with the tallest clouds adjacent to the mature updraft tower.

A rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam). However, the actual time threshold may vary in different parts of the country. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters.

The leading edge of a mesoscale pressure dome separating the outflow air in a convective storm from the environmental air.

This boundary, which is marked by upward motion along it and downward motion behind it, is followed by a surge of gusty winds on or near the ground. A gust front is often associated with a pressure jump, wind shift, temperature drop, and sometimes with heavy precipitation. Gust fronts are often marked by arcus clouds.

A plot representing the vertical distribution of horizontal winds, using polar coordinates. A hodograph is obtained by plotting the end points of the wind vectors at various altitudes, and connecting these points in order of increasing height. Interpretation of a hodograph can help in forecasting the subsequent evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs. non-splitting storms, tornadic vs. nontornadic storms, etc.).

The level at which a parcel of air lifted dry-adiabatically until saturated and saturation-adiabatically thereafter would first become warmer than its surroundings in a conditionally unstable atmosphere.

On a thermodynamic diagram the level of free convection is given by the point of intersection of the process curve, representing the process followed by the ascending parcel, and the sounding curve, representing the lapse rate of temperature in the environment. From the level of free convection to the point where the ascending parcel again becomes colder than its surroundings the atmosphere is characterized by latent instability. Throughout this region the parcel will gain kinetic energy as it rises.

The lower portion of the troposphere. No distinct limit is set, but the term can generally be applied to the levels 850 mb and below.

Relatively strong winds concentrated within a narrow band found in the lowest 2–3 km of the troposphere.

At night, sometimes called a nocturnal jet. In the United States, it often refers to a southerly wind maximum in the boundary layer, common over the Plains states at night during the warm season (spring and summer).

A procedure used to forecast the short-term (3-6 hour) motion of Mesoscale Convective System (MCS) centroids which builds on the long-established observation that MCS motion is a function of both the advection of existing cells by the mean wind and the propagation of new cells relative to existing storms. MCS centroid forecast motion is calculated to be the vector sum of the mean cloud-layer wind (typically assumed to be the 850-300 mb layer) and the mesoscale beta element (MBE) propagation component.

The MBE propagation vector is dictated by the location of the maximum cold-pool gust front convergence in the presence of conditional instability. For an upwind (downwind) propagating MCS, the magnitude and direction is assumed to be equal and opposite (identical) to that of the low-level jet (typically assumed to be the 850 mb wind).

A subset of mesoscale convective systems (MCS) that exhibits a large, circular (as observed by satellite), long-lived, cold cloud shield with the following physical characteristics:

A: Cloud shield with continuously low infrared (IR) temperature ≤ -32°C must have an area ≥ 105 km2.

B: Interior cold cloud region with temperature ≤ -52°C must have an area ≥ 0.5 X 105 km2. Duration: Size definitions A and B must be met for a period ≥ 6 h. Maximum extent: Contiguous cold cloud shield (IR temperature ≤ -33°C) reaches maximum size.

Shape: Eccentricity (minor axis/major axis) ≥ 0.7 at time of maximum extent.

Terminate: Size definitions A and B no longer satisfied. MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity (usually at night), the primary threat shifts toward heavy rain and flooding.

A complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, produces a contiguous precipitation area on the order of 100 km or more in horizontal scale in at least one direction, and normally persists for several hours or more. An MCS exhibits deep, moist convective overturning contiguous with or embedded within a mesoscale vertical circulation that is at least partially driven by the convective overturning. MCSs may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and MCCs (among others). MCS often is used to describe a cluster of thunderstorms that does not satisfy the criteria of a mesoscale convective complex (MCC).

The middle portion of the troposphere. No distinct limits are set, but the term can generally be applied to levels between 700 mb to 500 mb.Compare low-level. See also upper air.

(Also called multicell, multicell thunderstorm). A cluster of ordinary cells and/or supercells at various stages of their life cycle in close enough proximity to at least share a common precipitation area and cold pool (gust front).

New cells are generated primarily by either low-level convergence along a preexisting boundary or by lifting at the leading edge of the system-scale cold pool that was produced by the previous cells. The cells move roughly with the mean wind. However, the storm motion usually deviates significantly from the mean wind due to discrete propagation (new cell development) along the gust front. The multicellular nature of the storm is usually apparent on radar with multiple reflectivity cores and maximum tops. Lifetime may be several hours.

1. The component of convective storm motion that does not lie along the passive steering layer flow. It is the result of new cell development and old cell dissipation.

2. The transmission of electromagnetic energy as waves through or along a medium.

Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance within the atmosphere. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.

A line of active thunderstorms, either continuous or with breaks, including contiguous precipitation areas resulting from the existence of the thunderstorms.

The squall line is a type of mesoscale convective system (MCS) distinguished from other types by a larger length-to-width ratio.

(Also called steering currents, steering flow). A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.

Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.

In the opposite direction as shear vector within a specified layer.

The change in the wind's direction and speed with height. It plays a critical role in the evolution of deep, moist, convection.

A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation.