Radar School › Lesson 4
LESSON 4Motion
You will be able to predict where a storm's core will be in thirty minutes: start from its recent motion, and know when to trust the straight line and when a storm is about to bend it.
A good radar reader does more than identify a storm. They can tell where it is heading. Motion is not a storm type. It is a property every storm has, and one of the most useful things you can learn to read.
Everything in this lesson describes motion in two different ways:
Storms are named for where they are going. A storm moving east is an eastbound storm.
Wind is named for where it comes from. A west wind is blowing from the west toward the east.
Both conventions are standard, so a west wind and an eastbound storm are traveling the same way.
Start with a straight line
The simplest short-term forecast is to assume a storm keeps moving the way it has been moving. That is called extrapolation. It is also the basic idea behind many short-term radar nowcasts.
It will not predict a storm suddenly strengthening, weakening, splitting or changing direction. But over short periods, recent motion is one of the best places to start.
The farther ahead you look, the less you should trust a straight line. Storms can speed up, slow down, turn, merge, split, grow or disappear. That is when you need more than motion alone.
Storms move two ways at once
When the straight line goes wrong, this is usually why: storms are doing two separate things simultaneously.
Translation is the whole cell riding the mid level wind. Same storm, new place. This is the part that straight line extrapolation handles fine.
Propagation is different and it is the part that catches people. New cells build on one flank of a system while old cells die on the other. No individual cell went anywhere unusual, but the system as a whole shifted toward the building side.
A squall line can move southeast while every single cell inside it moves northeast. Nothing is behaving strangely. Cells travel northeast with the wind, and the line keeps building new cells on its southern end, so the line as an object migrates southeast.
If you extrapolate the cells you get the wrong answer. If you extrapolate the system you get the right one.
Supercells cheat
A supercell often does not move with the steering wind.
The reason is genuinely interesting. A rotating updraft in sheared flow experiences a sideways pressure force, and in the Northern Hemisphere the right moving version of the storm is the one that gets reinforced while the left mover usually falls apart. The result is a storm tracking noticeably right of everything around it, sometimes by twenty or thirty degrees.
Deviation on its own is not proof of rotation. Terrain, a boundary, or a merger will all bend a track. It is a strong hint worth acting on, not a verdict.
So if you are watching a line of cells all drifting northeast and one of them is quietly angling east southeast instead, that storm is telling you something about itself before its hook ever forms.
Training on training
The worst flooding on a radar screen usually comes from a cluster of storms that is not going anywhere, and the reason it is not going anywhere is pure motion. Here is the mechanism.
When cells propagate in the direction opposite to their translation, the two effects can cancel. Cells form on the upwind end, travel downwind, decay, and are replaced by new cells forming in the same starting spot. Each individual cell moves quickly. The system does not move at all.
The ground underneath gets hit by cell after cell in sequence, like watching train cars pass a crossing, which is why it is called training. Hours of heavy rain on one narrow strip while three miles away it stays dry. The giveaway on radar is that the system is not moving even though everything in it clearly is.
How to track the motion
- Loop at least four frames. A frame is one radar update, usually four to six minutes apart, so four frames is about twenty minutes of history. Two frames give you a direction. Four give you whether that direction is changing, which is the part that matters.
- Track a feature, not the blob. Pick the brightest core or a distinctive notch and follow that specific thing. Blob edges expand and contract for reasons unrelated to motion.
- Ask whether the system is doing the same thing as its parts. If new cells are appearing consistently on one flank, the system is going somewhere the cells are not.
- Check for deviation. A cell angling right of its neighbors is organizing. Extrapolate its own recent track, not the group's.
- Extrapolate with a curve when the track is curving. Storms turn. Thirty minutes is long enough for that to matter.
Chase the core
Four real storms, four real radar loops. Watch each one, call where its core lands in about thirty minutes, and see your pin against where it actually went.
Want to get more motion practice? This 30-minute prediction is the core of RadarWrangler, a free live storm chasing game built on real radar. Keep chasing →
A squall line is moving southeast, but every individual cell inside it is moving northeast. Is something wrong?
Nothing is wrong. Cells ride the wind northeast while the line keeps building new cells on one flank, so the system as an object migrates somewhere no single cell ever went. Extrapolate the system, not the cells.
Among a line of cells all drifting northeast, one is angling east southeast instead. What does that suggest?
It is organizing. Supercells deviate to the right of the steering wind because a rotating updraft in sheared flow gets reinforced on its right side. The deviation often shows before a hook does.
What is the minimum number of frames worth looping before you call a storm's motion?
Four. Two frames give you a direction. Four tell you whether that direction is changing, which is the part that actually matters over a thirty minute forecast.