Furthermore, officials didn't detect this CME before it hit Earth, with the wave of charged particles from the Sun being described as a "stealth" CME. So, why didn't they see it coming? Space weather forecasters explain that CMEs can come in different variations, and these fainter but no less powerful blasts can be difficult to observe. For example, some of the more dense objects on the Sun may obscure viewing of these CMEs, while others may travel at a much slower speed than what is usual, slipping under the radar of forecasters.

"These nearly invisible storms launch much more slowly than eruptive CMEs and are very difficult to observe leaving the sun's surface without specialized training," said U.S. space weather forecaster Tamitha Skov to Space.com.

https://www.tweaktown.com/news/90850/earth-unexpectedly-rocked-by-the-most-intense-blast-from-sun-in-6-years/index.html