Conditions are right for dangerous weather to form. Get ready, know where you will go, and keep checking for warnings.
Warning
Dangerous weather is happening or about to happen. Act now.
Advisory
Weather that is a nuisance or could be hazardous if you are not careful, like dense fog or a wind advisory.
Tornado Emergency
The highest alert the National Weather Service issues. A large, damaging tornado is confirmed and headed for a populated area. Take cover immediately.
PDS (Particularly Dangerous Situation)
Extra wording added to a watch or warning when forecasters expect an unusually dangerous event, such as a major tornado outbreak.
Considerable and destructive damage threat
Tags on severe thunderstorm and tornado warnings. A destructive severe thunderstorm warning means winds of 80 mph or more or hail at least baseball size, and it sets off phone alerts.
Radar indicated vs. observed
A radar-indicated tornado warning means radar shows rotation that could produce a tornado. Observed means a tornado has been seen by spotters or confirmed on radar.
Extreme Wind Warning
Issued when the core of a strong hurricane will bring sustained winds of 115 mph or more. Treat it like a tornado warning.
Flash Flood Emergency
The highest flood alert. Severe, life-threatening flooding is happening in a populated area.
Radar
Reflectivity
The standard radar picture. Brighter colors mean heavier rain or hail.
Velocity
A radar view showing wind moving toward (green) or away from (red) the radar. Bright green right next to bright red is a sign of rotation.
Hook echo
A hook shape on the back side of a storm on radar. It often marks where a tornado could form.
Debris signature (debris ball)
Radar picking up things lofted by a tornado, like roof pieces and trees. It means a tornado is on the ground causing damage.
Couplet
Strong inbound and outbound winds side by side on velocity radar, showing rotation in a storm.
Bow echo
A line of storms bent into a bow shape. It usually means strong straight-line winds.
QLCS (squall line)
Quasi-Linear Convective System: a long line of storms. Tornadoes in a QLCS can form quickly and are often hidden by rain.
Storms
Supercell
A thunderstorm with a rotating updraft. Supercells produce most strong tornadoes and large hail.
Mesocyclone
The rotating part of a supercell, usually a few miles wide.
Wall cloud
A lowered cloud under a storm’s updraft. Rotating wall clouds can produce tornadoes.
Rain-wrapped tornado
A tornado hidden by heavy rain, so you can’t see it coming. Common in the Southeast, which is why you should never wait to see a tornado.
Straight-line winds
Damaging winds that blow in one direction, not in a circle. They can cause as much damage as a weak tornado.
Downburst and microburst
A sudden, powerful rush of air down out of a storm that spreads out when it hits the ground.
Derecho
A long-lived, widespread windstorm from a line of storms, causing wind damage along a path hundreds of miles long.
Severe thunderstorm
A storm with winds of 58 mph or more, hail 1 inch across (quarter size) or larger, or a tornado.
Ingredients
Dew point
The temperature the air would have to cool to for dew to form. It measures moisture: above 60° is humid enough to fuel storms, and above 70° feels oppressive.
Instability (CAPE)
How much energy the atmosphere has for storms to grow. Higher CAPE means stronger updrafts.
Wind shear
Winds changing speed or direction with height. Shear helps storms rotate and organize.
Helicity
A measure of how much spin is available in the low-level winds for storms to turn into rotation.
Cold front
The leading edge of cooler air. Storms often form along it as it pushes through.
Cap
A warm layer of air above the ground that can hold storms back. When it breaks, storms can develop quickly.
Outlooks
SPC risk levels
The Storm Prediction Center ranks severe weather risk from 1 to 5: Marginal, Slight, Enhanced, Moderate and High. Even a Marginal or Slight risk can bring a tornado.
Mesoscale discussion
A short note from the Storm Prediction Center explaining what forecasters are watching in an area, often issued before a watch.
Heat index
What the temperature feels like when humidity is added in. A heat index of 105° or more is dangerous.
Wind chill
What the temperature feels like on your skin when wind is added in.
Hard freeze
Temperatures of 28° or colder for several hours, cold enough to kill most plants and burst unprotected pipes.
Flood stage
The water level at which a river or creek starts to flood land and roads.