Glossary

The terms used across the guides, in plain language. Link directly to any definition with its anchor, for example /glossary/#tds.

Reflectivity & echoes

dBZ: The unit of radar reflectivity (decibels of Z). Logarithmic: each 10 dBZ step is roughly ten times more returned energy. ~35 dBZ is where rain gets heavy; 50+ is thunderstorm-core territory. → How to Read Weather Radar
Base reflectivity: Reflectivity from a single radar tilt, usually the lowest (0.5°). The product of choice for storm structure near the ground.
Composite reflectivity: The strongest return found at any altitude in each column. Reveals cores suspended aloft, at the cost of smearing structure.
Bright band: A ring or stripe of inflated returns where the beam intersects melting snow. Wet snowflakes mimic giant raindrops, spiking reflectivity 5 to 15 dBZ. → What Radar Can't See
Virga: Precipitation that evaporates before reaching the ground. Paints on radar while the surface stays dry, and it is common when low-level air is very dry.
Anomalous propagation AP: False echoes caused by a temperature inversion bending the beam into the ground. AP sits still while real weather moves.
Cone of silence: The unsampled cone directly above a radar site, where the dish cannot point. Storms passing overhead partially vanish.
Attenuation: Weakening of the radar signal as it passes through heavy precipitation, making echoes behind an intense core look weaker than they are.
Echo tops: The maximum altitude at which a threshold reflectivity (typically 18 dBZ) is detected. A quick proxy for storm depth and updraft strength.

Velocity & rotation

Radial velocity: The component of motion directly toward or away from the radar, the only motion Doppler radar can measure. Green = inbound, red = outbound. → Doppler Velocity, Explained
Storm-relative velocity SRM / SRV: Velocity with average storm motion subtracted, making rotation stand out. The product to use when hunting couplets.
Velocity couplet: Adjacent maxima of inbound and outbound velocity at the same range: the radar signature of rotation.
Mesocyclone: A supercell's persistent rotating updraft, typically 2 to 6 miles wide. Necessary for most significant tornadoes, but most mesocyclones never produce one. → Supercells & Mesocyclones
Tornadic vortex signature TVS: A very tight, intense, gate-to-gate velocity couplet indicating possible tornado-scale rotation. Still rotation aloft at beam height, not a confirmed tornado.
Zero isodop: The line of zero radial velocity, where wind blows perpendicular to the beam. Its shape encodes how wind veers or backs with height.
Range folding RF: Ambiguous returns the radar cannot place in range, shown as purple/gray. Missing data, not a signature.
Divergence signature: Outbound directly downrange of inbound, meaning air spreading apart at the surface. The signature of a downburst/microburst.

Dual-polarization

Differential reflectivity ZDR: Horizontal minus vertical return. High = big flattened raindrops; near zero with high reflectivity = hail. → Dual-Pol Radar, Explained
Correlation coefficient CC / ρHV: How closely the horizontal and vertical returns from one pixel agree. Pure rain and dry snow sit about 0.97 to 1.0, mixed or melting precipitation lower (wet snow down to about 0.88, hail roughly 0.85 to 0.95, down to about 0.75 for very large hail), and a collapse below about 0.8 to 0.9 on top of a velocity couplet inside real reflectivity means tornado debris.
Specific differential phase KDP: Phase-based measure of liquid water content, largely blind to hail. The most honest heavy-rain and flash-flood product.
Tornadic debris signature TDS: Collapsed CC (usually below about 0.8 to 0.9) co-located with a velocity couplet inside real reflectivity (NWS Albany uses 35 dBZ or more), with ZDR near zero: lofted debris, as close as radar gets to confirming a tornado. Appears only after damage has begun, and only if the debris reaches beam height.
ZDR arc: A band of enhanced ZDR along a supercell's forward flank, created by size sorting in strong low-level shear. A sign of strong low-level shear, not a tornado signal on its own.

MRMS & derived products

Multi-Radar Multi-Sensor MRMS: NOAA system fusing ~180 radars plus gauges, lightning, and models into a seamless ~1 km national grid updating every ~2 minutes. → MRMS, Explained
Rotation tracks: Accumulated maps of azimuthal shear showing where rotation has traveled: the "did it pass over my house?" product.
Maximum Estimated Size of Hail MESH: MRMS estimate of maximum hailstone size, painted as swaths along storm paths. Widely used for damage assessment. An estimate, and by design a high one: Witt and colleagues fit MESH so that about three quarters of observed maximum hailstones are smaller than the estimate. Comparisons with hail reports find every version least reliable as stones approach 4 inches (Wendt and Jirak 2021).
Quantitative precipitation estimation QPE: Gauge-corrected rainfall accumulation over rolling windows. The flash-flood product: flooding is about totals, not instantaneous intensity.
Azimuthal shear: The rate of change of velocity across the beam, a pixel-level measure of rotation strength that feeds rotation tracks.

Storm structure & types

Supercell: A thunderstorm organized around a persistent mesocyclone. Responsible for most significant hail and most strong tornadoes. → Hook Echoes & Tornado Signatures
Hook echo: A curl of precipitation wrapped around a mesocyclone by the rear-flank downdraft. Rotation made visible on reflectivity.
Bounded weak echo region BWER: A mid-level pocket of weak returns inside strong ones, marking an updraft so intense precipitation cannot fall through it.
Rear-flank downdraft RFD: Descending air wrapping around the back of a mesocyclone; sculpts the hook echo and plays a central role in tornadogenesis. Downdrafts only slightly cooler than the inflow air favor tornadoes; cold, rain-chilled ones tend to kill the circulation.
Inflow notch: An echo-free wedge on a storm's flank where warm, moist air feeds the updraft. A crisp notch signals a healthy storm.
Quasi-linear convective system QLCS: A squall line. Main threat is damaging straight-line wind; brief tornadoes can spin up along surging, bowing segments.
Bow echo: A line segment bulging forward under strong outflow winds: the classic damaging-wind signature. The apex and its northern flank deserve attention. → Squall Lines, Bow Echoes & Derechos
Derecho: A widespread, long-lived windstorm from a fast-moving line of storms. SPC requires a damage swath at least 250 miles (about 400 km) long with gusts of 58 mph or more along it and several of 75 mph or more.
Training: Cells repeatedly forming and passing over the same area, like train cars over one crossing. The classic flash-flood setup.

Storm ingredients

Convective available potential energy CAPE: The energy a rising parcel of air gains by being warmer than the air around it, in J/kg. The fuel for thunderstorm updrafts. Comes in surface-based, mixed-layer and most-unstable versions, which lift different air. → CAPE & Instability, Explained
Convective inhibition CIN: The energy needed to push a parcel up through a layer where it is cooler than its surroundings, usually a warm layer aloft called the cap. A little holds storms back until lift arrives; a lot prevents them.
Cap: A warm, often dry layer of air above the surface that stops rising air from reaching the height where it would rise on its own. On a Skew-T, the temperature trace kinks to the right.
Lifting condensation level LCL: The height where a rising parcel cools enough to saturate: cloud base. Low LCLs, humid air near the ground, favor tornadoes in supercells.
Level of free convection LFC: The height where a lifted parcel first becomes warmer than its surroundings and starts rising on its own.
Equilibrium level EL: The height where a rising parcel cools back to the temperature of its surroundings. Roughly where a storm's anvil spreads out.
Lapse rate: How fast temperature falls with height, in °C per km. Steep lapse rates, cold air over warm, make the atmosphere more unstable.
Downdraft CAPE DCAPE: The energy a sinking parcel gains as rain evaporates into it and chills it. Larger values point to stronger downdrafts and damaging wind.
Precipitable water PWAT: All the water vapor in the column above a point, as the depth of water it would make if it all fell. High values for the place and season are the shared ingredient of flash floods. → Flash Flooding
Skew-T: A chart of temperature and dew point through the depth of the atmosphere, from a weather balloon or a model. The lines of equal temperature are tilted 45°, which is the skew. → How to Read a Skew-T
Bulk shear: The difference between the wind at the top and the bottom of a layer, direction included, usually 0 to 6 km. About 35 to 40 kt and up is the usual supercell guide. SPC calls it bulk wind difference. → Wind Shear & Helicity, Explained
Storm-relative helicity SRH: How much of the spin in the low-level wind a moving storm can draw into its updraft, in m²/s², usually over 0 to 1 km or 0 to 3 km. Depends on the storm's motion.
Effective inflow layer: The layer a storm can actually feed on: where a lifted parcel has at least 100 J/kg of CAPE and CIN no worse than 250 J/kg. If it starts above the ground, storms are elevated.
Right mover: A supercell that turns to the right of the mean wind, usually favored in the U.S. because it draws in the most low-level spin. Its twin after a split is the left mover. The Bunkers method estimates both.
Hodograph: The wind at many heights drawn as one curve, from the tips of the wind arrows. Its length is the shear; its curvature is how the wind turns with height. → How to Read a Hodograph
Streamwise vorticity: The part of the horizontal spin in the wind that points along the air flowing into a storm, like the spiral on a thrown football. Streamwise spin enters the updraft rotating, which helps a supercell.
Critical angle: The angle between the storm-relative wind at the ground and the 0 to 500 m shear. Near 90° means the near-ground spin points straight into the storm. A weak clue on its own: a larger later study found little difference between tornadic and non-tornadic supercells.
Updraft helicity UH: A forecast model's measure of how strongly the updrafts of its simulated storms rotate, usually 2 to 5 km up. Long streaks are the model's supercells, not a tornado forecast.
Significant Tornado Parameter STP: An SPC composite multiplying CAPE, cloud base height, low-level helicity, deep shear and CIN, each divided by a typical value. Assumes a supercell exists; most significant tornadoes in SPC's sample came with values above 1. → SCP & STP, Explained
Supercell Composite Parameter SCP: An SPC composite of most-unstable CAPE, effective helicity, effective shear and CIN. Values above 1 favor supercells.
Energy Helicity Index EHI: CAPE times storm-relative helicity, divided by 160,000. 0 to 1 km values above 1 to 2 have been associated with significant tornadoes in supercells.
Significant Hail Parameter SHIP: An SPC composite for hail 2 inches or larger. Values above 1 favor significant hail; it is not a hail size forecast.

Hazards

Downburst: A strong downdraft that hits the ground and spreads out as damaging straight-line wind. The damage fans outward. → Squall Lines, Bow Echoes & Derechos
Microburst: A small downburst, about 2.5 miles (4 km) across or less. Brief and intense, and a serious hazard to aircraft.
Gust front: The leading edge of rain-cooled air spreading out from storms. It lifts warm air ahead of it, building new cells. A shelf cloud often marks it.
Rear inflow jet: A stream of air flowing into a line of storms from behind and descending toward the ground. It pushes part of the line forward into a bow.
Bookend vortex: A rotating circulation at the end of a bow echo. In the Northern Hemisphere the northern one is usually stronger and can curl the line into a comma shape.
Hail growth zone: The layer of a storm between about -10 and -30 °C, where supercooled water is plentiful and hailstones grow fastest. → Hail on Radar
Three-body scatter spike TBSS: A spike of weak false echo pointing straight away from the radar behind a storm core, caused by the beam bouncing between large hail and the ground. A sign of large hail aloft.
Flash flood: Flooding that starts within about six hours of heavy rain, often within three. Small streams, low roads and city streets flood first. → Flash Flooding
Flash flood guidance FFG: The estimated rain, over one, three or six hours, needed to start flooding small streams in an area. It changes with how wet the ground already is.
Excessive Rainfall Outlook ERO: The Weather Prediction Center's chance that rain will exceed flash flood guidance within 25 miles of a point: Marginal at least 5%, Slight at least 15%, Moderate at least 40%, High at least 70%.
Geostationary Lightning Mapper GLM: An optical sensor on the GOES satellites that maps total lightning, in-cloud and to the ground, from flashes of light at the cloud top. → Lightning, Explained
Lake-effect snow: Snow from narrow bands that form when very cold air crosses a large, unfrozen lake. A common rule is a lake surface at least about 13 °C warmer than the air at 850 mb. → Lake-Effect Snow
Fetch: The distance wind travels over open water. A longer fetch gives lake-effect bands more heat and moisture.
Warm nose: A layer above freezing aloft in winter that melts falling snow. Whether the melted drops refreeze below it decides sleet or freezing rain.
Freezing rain: Rain that falls through a layer of below-freezing air near the ground too shallow or too mild to freeze it, then freezes on contact.
Sleet: Snow that melts in a warm layer aloft, partly or fully, then refreezes into ice pellets in the cold layer below before landing.
Snow-to-liquid ratio: Inches of snow per inch of melted water. 10 to 1 is the old rule of thumb; Baxter and colleagues found an average nearer 13 to 1 over much of the country, with wet snow lower and fluffy lake-effect snow often 20 to 1 or more.
Eyewall: The ring of tall thunderstorms around a hurricane's eye, with the strongest winds and heaviest rain. → Hurricanes: Structure, the Cone & Model Tracks
Saffir-Simpson Hurricane Wind Scale: Hurricane categories 1 to 5 by peak 1-minute sustained wind, from 74 mph. Wind only: it leaves out storm surge, rain and the storm's size.
Forecast cone: The NHC's probable path of a tropical cyclone's center, sized so two thirds of its past five years of track errors fall inside. Not the storm's size; impacts reach outside it.
Storm surge: The rise of seawater pushed ashore by a storm, above the normal tide. NHC maps it as feet of water above ground. It caused about half of U.S. tropical cyclone deaths from 1963 to 2012; in the decade since, rain flooding has caused more.
Rapid intensification RI: An increase in a tropical cyclone's maximum sustained winds of at least 30 kt (about 35 mph) in 24 hours, NHC's definition.

Alerts & forecasting

Storm Prediction Center SPC: The NWS center that issues convective outlooks (risk levels 1 to 5) and severe thunderstorm/tornado watches. → Watches, Warnings & Outlooks
Watch: Conditions are favorable for severe weather over a region for the next several hours. Be ready; storms may not have formed yet.
Warning: Hazardous weather is occurring, imminent or expected. Tornado, severe thunderstorm and flash flood warnings are drawn around the storm as a polygon; longer-fused warnings, such as winter storm or hurricane warnings, cover counties or forecast zones. Act on it.
Advisory: A lower tier than a warning: weather that is a nuisance or a hazard if you are careless, such as a Winter Weather Advisory or a Flood Advisory.
Impact-based warning tags IBW: Lines at the bottom of an NWS warning. A source tag (RADAR INDICATED or OBSERVED) says how the threat is known; a damage threat tag (considerable, destructive for severe thunderstorms, catastrophic) says how bad it may be. On a severe thunderstorm warning, considerable means 1.75 inch hail or 70 mph wind and destructive 2.75 inch hail or 80 mph, the only tier that sets off Wireless Emergency Alerts. TORNADO...POSSIBLE on a severe thunderstorm warning means a spin-up could happen. → Watches, Warnings & Outlooks
Particularly Dangerous Situation PDS: Enhanced wording for unusually extreme, life-threatening events. Used on tornado watches, on severe thunderstorm watches for high-end wind, and on tornado warnings as the considerable damage tier, for a strong tornado on the ground. Rare and serious.
Tornado Emergency: The catastrophic tier of a Tornado Warning: a violent tornado, confirmed by reports or radar-detected debris, with a severe threat to life and catastrophic damage, usually in a populated area. Exceedingly rare.
Flash Flood Emergency: The catastrophic tier of a Flash Flood Warning: a severe threat to life and catastrophic damage from flash flooding that is happening or about to happen. → Flash Flooding
Snow squall: A short, intense burst of heavy snow and strong wind that drops visibility suddenly, often to near zero, usually for under an hour. The NWS can issue a Snow Squall Warning for it; warnings tagged significant set off Wireless Emergency Alerts. Deadly on highways.
Wireless Emergency Alerts WEA: The phone broadcast system that pushes the most dangerous warnings automatically: tornado warnings, destructive severe thunderstorm warnings, flash flood warnings tagged considerable or catastrophic, snow squall warnings tagged significant, and dust storm, extreme wind, hurricane, storm surge and tsunami warnings.