Forecasters commit to numbers before reality arrives. Your turn. No peeking before you've made your call.
A CME leaves the Sun at roughly 2,500 to 3,000 km/s, one of the fastest ever recorded. Earth's orbit is about 150 million km out. Your call: roughly how many hours from launch to Earth's distance? Distance divided by speed; ignore deceleration.
At 2,500 km/s: 150,000,000 ÷ 2,500 = 60,000 seconds, about 17 hours. The real one crossed to 1 AU in under a day and hit the STEREO-A spacecraft, which measured a CME that would have driven a Carrington-class storm had Earth been in its path. Earth had been in that spot about a week earlier.
The uncomfortable lesson: the most dangerous CMEs give the least warning. A three-day forecast window is a luxury of slow storms.
March 2015: a thoroughly ordinary CME leaves the Sun at moderate speed, taking a leisurely two days to arrive. Nothing about the launch looks special. Your call: what storm level would you forecast?
It drove a G4 severe storm, Kp near 8, the biggest of the whole solar cycle to that point. The launch data couldn't have told you. What mattered was the field inside the cloud, which pointed strongly south for hours.
This is the difference it makes. Same cloud, same speed, same arrival. Flip which way its magnetic field points.
The lesson every forecaster lives with: arrival time is predictable days out; storm strength often isn't known until the last hour. Two different forecasts, two different confidence levels.
The sliders run the same drag-based model this site uses for its real forecasts. Launch a CME and watch what the solar wind does to it. The dashed line is where it would be if its speed never changed.