Glossary

Back Door Cold Front: A cold front that approaches from the east. This is a typical situation in New Mexico as cold air filters down through the Great Plains and enters the state through the northeastern corner. Often, they stall at the mountains, turning into a stationary front and then mixing out completely.

Back-Sheared Anvil: Parcels of air travel upwards at high velocity during storm development. When they reach the Equilibrium Layer, they lose buoyancy and are carried downwind in the average thunderstorm. In a strong thunderstorm, some of the parcels are carried UPwind, against the shear, producing a smaller, but distinct anvil shaped cloud behind the storm.

Capping Inversion: Typically, the air cools as you rise through the atmosphere. However, if a capping inversion is present, there is an area in the lower troposphere where the air warms with height. This means that as a parcel of air rises, it encounters warmer air. If the air at this level is warmer than the air in the parcel, the parcel will begin to sink. Areas with a capping inversion are also said to have Convective Inhibition (CIN or CINH).

Cold Air Advection (CAA): Colder air is migrating into an area. This can be seen on the 850 mb upper air plot as an area where winds blow across a temperature gradient from colder to warmer, transporting cooler air into an area with warmer air. Strong CAA ends up as a cold front on the surface map. CAA can wreak havoc for winter weather predictions, migrating into an area of precipitation, changing a rainy day to a winter weather situation.

Convective Inhibition (CIN or CINH): CIN can be thought of as “negative CAPE.” It is where buoyancy is lost, often due to a capping inversion. In this region of the sounding, the parcel is cooler than the environment, so that it wants to sink.

Correlation Coefficient: This is a product from a dual-polarization Doppler radar. It compares vertically polarized and horizontally polarized reflections. If something appears the same size in both directions, the object is likely round (CC=1, 100%, or red on the image). If something appears different sizes in one direction or the other, chances are it is not round (CC < 1, 100%, or blue, green, yellow, etc). If round objects are being detected, they are likely rain or hailstones. If oblong objects, (such as sheet metal and boards) are being detected, it could be that a tornado or high winds are kicking up debris in this area.

Dewpoint Depression: This is the difference between the temperature and the dewpoint. Low dewpoint depressions mean that the dewpoint and temperature are close together. When this happens, the relative humidity is high, and clouds (and storms) are more likely. On the upper air sounding, the dewpoint depression is the distance between the red and green traces at any given pressure level.

Discrete-Mode: Storms are forming as individual storms, rather than as clusters or lines. These are the best type for chasing, as you are more likely to be able to see all of the storm structure. Also non-discrete cells tend to interfere with each other, and often reduce the tornado potential.

Gustnado: A brief rotation on the ground that corresponds to the edge of the outflow region of the storm (sometimes called a gust front). These gustnadoes may extend from cloud to ground, and may be reported as tornadoes to the National Weather Service if this is the case. Most of them are weak, but they have been known to cause minor damage.

Hook Echo: A hook echo is a reflectivity signature on Doppler Radar that is shaped like a hook, or like a claw. Often, this can be an indication that the storm is rotating (becoming supercellular). As a hook echo forms, it can indicate that precipitation is being pulled around a center of circulation. It does not always mean that there is a tornado; its absence does not necessarily mean that there is no tornado, either. There is a correlation, however, and given other evidence, a hook echo can be an indication that a tornado has either formed or is in the process of forming.

Low Precipitation (LP) Supercell: An LP supercell is a rotating storm (supercell) that is dropping very little precipitation. It is a narrow, stripped down version of a supercell, typically missing some features as compared to a classic supercell. They typically have higher bases, and may lack a wall cloud. On radar, the hook echo may not be present, as there is little precipitation to wrap around the mesocyclone. Only around 5% of LP cells produce tornadoes. However, an LP cell can still produce very large hail, in spite of its name. Furthermore, the LP cell may produce strong downdrafts, particularly if the “LP” nature is due to evaporation.

Outflow Boundary: When a storm produces precipitation, or even virga, air is cooled. This cooled air sinks and spreads out once it reaches the surface of the Earth. The boundary between this cooler, sinking air acts as a cold front, and is often referred to as an “outflow boundary” as it is dependent on the outflow of a storm.

Overshooting Top: During a strong thunderstorm’s development, parcels are carried upwards at high velocity. They lose buoyancy at the Equilibrium Layer (EL), but, if they were travelling fast enough, they still have kinetic energy that must be dissipated. The parcels travel past the EL, producing a bulge in the top of the anvil cloud, visible by the naked eye and satellite imagery.

Precipitable Water: This is the total amount of moisture in the atmosphere as integrated from the surface to 300 mb. The higher this number, the more moisture is present. However, this cannot be used to determine rainfall or snowfall amounts, as not all of this moisture condenses and falls as precipitation. Also, this measure is for a vertical column over a specific area- as the system moves, a new vertical column will be over that area, and more moisture may fall. The total rainfall amount depends on system movement and a lot of other variables.

Rear Flank Downdraft (RFD): The Rear Flank Downdraft is where precipitation becomes wrapped around a rotating storm (mesocyclone). On radar, this is what helps to form the classic “hook echo” that indicates the storm is rotating. On the ground, a clear slot may develop on the updraft side of the wall cloud. Tornadoes are more likely if the RFD is warmer than the environment, and the RFD itself may be what spawns the tornado, though this is still being researched.

Staccato Lightning: Lightning that does not stay connected for very long. All lightning is fast, but staccato lightning is even faster. Often, it is associated with strong updrafts in supercells. The lightning path does not stay open for long, perhaps due to near-storm environmental changes. The storm makes up for the short-duration lightning stroke by striking more often, and these storms often look like a strobe light.

Visible Satellite Imagery: This is an actual photo from a satellite as to what the Earth looks like from above. At night, this image is black (because that is what the Earth looks like at that time!) It shows visible clouds, but clouds must be distinguished from fog, snow and ice, and so on.

Warm Air Advection (WAA): The opposite of CAA. This is when warm air migrates into a colder air. It can be shown on the 850 mb chart as winds blowing across a temperature gradient from warm to cold. These are especially important for forecasting severe weather in the spring, as areas with warmer temperatures will be a possible area for convection.

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