Radar School › Lesson 7
LESSON 7You will be able to place any storm on the ladder based on strength and organization, and recognize the signatures that make a forecaster reach for the warning button.
Everything on a radar screen sits somewhere on one ladder, and the rungs are not about how much rain is falling. They are about how organized the air is. Organization is what lets a storm last, and lasting is what lets it do damage.
The bottom four you only need to recognize.
From here the rungs get a full entry each, because from here the details start to matter.
Several cores at different ages, moving as a pack.
A clump of cells living and dying together. Some bright and growing, others fading. No single cell lasts an hour but the cluster can roll for most of a day, because new cells keep forming along the cluster's own gust front as old ones collapse.
A group of storms organized around one shared advancing line.
A squall line is made of many thunderstorms working together. Individual cores may be separated or merged. The spacing is not the giveaway. The organization is.
Look across the system. If you can trace one common leading edge through the storms, you are looking at a line. The tell: one shared advancing front, often with a hard leading edge and a softer area of precipitation trailing behind it.
Sometimes part of the line surges forward into a bow echo.
That forward bulge can be associated with damaging straight-line winds.
A multicell cluster can also look stretched out. Do not count gaps between cores. Ask whether the storms share one advancing front. If they do, think squall line. If they are growing and merging as a broader clump, think cluster.
A thunderstorm built around a rotating updraft.
Ordinary thunderstorms often weaken when their downdraft cuts off the warm air feeding them. Strong wind shear can keep the updraft and downdraft separated, allowing the storm to stay organized for hours.
That shear can also make the updraft rotate. A persistent rotating updraft is called a mesocyclone, and it is what makes a supercell a supercell.
On velocity, look for organized rotation: neighboring inbound and outbound velocities arranged as a couplet. Rotation is the confirmation.
Many of the hook echoes and tornado-warned storms you will see later are supercells. But neither a hook nor a tornado warning is required for a storm to be a supercell.
A hole in the rain with an arm curled around it, on the storm's rear right corner.
The most famous shape in weather, and the one people most often fail to find. Some of that is that the version in the textbook is rarer than the textbook lets on. Most of it is that almost everybody is hunting for the wrong feature.
A hook is a curl of rain, and a curl is a hard thing to pick out of a screen that is already full of rain. Locating the notch is helpful: a bite of weak or empty echo cutting in toward the bright core. Your eye finds a gap far faster than it finds a shape, and it does it without being told what shape to expect.
The notch is also the more meaningful of the two. It is the inflow, the place air is being pulled into the storm. The hook is only the rain that has been dragged around it.
The search, in order, and it is the order that does the work.
The same five steps, drawn one at a time on two real supercells, ending with a full spiral wrapped around lofted debris. Flip the storm: the search never changes.
Hooks vary enormously, and this is worth seeing rather than being told. Some are broad and obvious. Some are a narrow finger barely curving, sometimes called a pendant. Some storms that produce tornadoes never draw one at all. What they share is position, not shape.
Supercells run along a range. At the dry end the storm is small and nearly rain free and may never draw a hook at all. At the wet end there is so much rain that it wraps all the way around the rotating part, and what you get on radar is a kidney bean or a comma instead of a clean hook.
Several clues lining up in the same place.
A Tornado Warning is not a radar signature. It is a decision made by a National Weather Service forecaster when a tornado is occurring or considered imminent, using radar, spotter reports, and the storm environment.
This is a historical training example, from a past storm with no injuries or damage, not a current warning. In a real radar app, a warning polygon appears as a colored outline on the map within seconds of NWS issuing the warning, and disappears once the warning expires or is cancelled. Click the polygon to see the warning.
The storm may occupy only part of the polygon. The polygon extends downstream because the storm is moving.
The polygon is a forecaster's warning area, not another radar product.
DOW7 · Reflectivity · 5 Jun 2009 UTCFor a classic supercell, the evidence you have learned begins to stack up:
No single clue automatically means tornado. The more of them line up in the same place and persist together, the stronger the evidence becomes.
A debris signature is about as close as radar gets to confirming that a tornado is on the ground. But its absence does not mean there is no tornado.
A debris signature is not a forecast. It is confirmation that damage is already happening. If you ever see one over a populated area, you are watching people have the worst day of their lives in real time. That is the appropriate weight to give it.
Work left to right. Find the hook on reflectivity. Confirm the couplet on velocity and apply the Green and Red test. Then find the debris hole on CC.
Reflectivity says this storm has a shape consistent with rotation. Velocity says it is genuinely rotating and tells you how hard. CC says the rotation has reached the ground and is picking things up.
The reason this progression matters is that plenty of storms produce the first without the second, and a meaningful number produce the second without ever producing the third. Each step is real evidence, and none of the earlier steps is a guarantee of the later ones.
A core so bright it hurts, with a thin spike pointing away from the radar.
Over 60 dBZ means hail is very likely in the sample. Over 70 means large hail is close to certain.
Two honest caveats. Hail melts on the way down, so what is aloft may arrive smaller or as rain. And there is no direct relationship between dBZ and hail size. Reflectivity says something big is up there, not how big. That gap is exactly why ground reports from spotters matter.
Hail cores can produce a thin spike pointing directly away from the radar. It is called a three-body scatter spike.
The extra travel time fools the radar into placing an echo farther away than it really is.
The spike is fake. The hail that caused it is real. Seeing one is a strong clue that large hail is present.
If your app has CC, check the streak. A three-body scatter spike often has very low CC, helping separate the false echo from the precipitation that produced it.
Five real storms, one signature each. Three marked spots at a time, pick the one feature that makes the call.
A narrow finger of weak echo extends outward along the radial from behind an intense core. What is it?
A three body scatter spike. Energy bounces off large hail, down to the ground, back up into the hail, then home. The detour takes time and the radar turns time into distance, so it plots an echo where nothing is.
Correlation coefficient reads very low in a small area inside a tight velocity couplet. What does that mean?
Debris. Correlation coefficient measures how similar the targets in a sample are. Rain is uniform and reads high. Lumber, insulation and soil are nothing alike, so they read low. It means a tornado is on the ground right now.
Which combination should worry a forecaster most?
The stack. Any single one of those signatures is weak evidence. Shape, rotation, position and persistence all agreeing in the same storm is what a warning is built on.
