Flare size is measured from A (weakest) to X (strongest), so this M2.7 was a decent-sized flare, but not crazy. Stronger flares are more likely to cause CMEs, but flare size =/= CME size. In this case, the flare caused a nice CME to lift off the Sun aimed at us.
A flare is easy to see because it's like a camera flash going off on the Sun, especially when imaged in the extreme ultraviolet wavelengths. CMEs are comparably much more faint, so we use special instruments called "coronagraphs" to see these explosions coming off the Sun.
A coronagraph instrument onboard a spacecraft works by creating an artificial solar eclipse so we can see ejections of solar wind coming off the Sun. If the cloud is squarely aimed at us, the CME will expand around the instrument like a halo. The M2.7 flare produced a halo CME.
A halo CME basically has a 100% chance of hitting us, but the question then is "When?" Scientists measure how far the CME travels in the field of view of the coronagraph to build a profile of the CME's speed and directionality. Then, they feed this information into models.
A few different agencies have space weather forecasters who measure and track these CMEs and generate models with predicted arrival times at Earth. Namely, NASA's Moon to Mars office and the NOAA Space Weather Prediction Center.
NASA's M2M office quickly released a model and later refined it to include another smaller eruption in front of the main CME. These two CMEs may combine and arrive sometime around 17 UT on September 1. This is around the same time as NOAA SWPC's model.
Thank you so much Vincent for these wonderful, informative posts! Now that I’m in PA and can’t just see AB on a regular basis like in N Ontario, I’ve been a bit obsessed. I love learning about how to predict potential sightings.
NOAA SWPC released a model predicting the combined fast+slow CME to arrive around 20 UT on September 1. Since the uncertainty of CME models is around +/- 7 hr, this three hour difference isn't a big deal. SWPC also posted a G2-G3 geomagnetic storm watch for September 1-2.
The CME scoreboard on NASA's CCMC website allows different agencies to post their models. You can see all the different arrival time predictions here. They mostly agree on a CME arrival sometime around 17 UT on September 1. A G2-G3 geomagnetic storm is possible.
Science works in universal time (UT), but most of my audience lives in the U.S. or Canada. Accounting for the +/- 7 hr uncertainty in the arrival time of the CME, here are the time zone conversions for many areas around the world.
The HUXt model is bucking the trend. It thinks our CME will get here on September 2 a full day LATER than everyone else thinks. Maybe the model will get an update with new data, but this is interesting. You can read more about HUxt on my website: go.theauroraguy.com/huxtmodel
I don't put all my eggs in one model "basket" but instead try to look at them in aggregate taking into account edge cases like HUXt. It is likely we will see a CME arrive on Spetember 1 around 17 UT, but there is a possibility it comes much later. We will have to be patient.