Yesterday's gigantic badass solar flare is today's Aurora Borealis.
Folks in the upper Midwest and Canada could see the Northern Lights directly overhead; those throughout the US may be able to see them on the northern horizon, if it's clear.
From the Geophysical Institute's extremely-useful Aurora Forecast (emphasis mine):
Forecast: Auroral activity will be high. Weather permitting, highly active auroral displays will be visible overhead from Inuvik, Yellowknife, Rankin and Igaluit to Juneau, Edmonton, Winnipeg, Thunder Bay and Sept-Iles, and visible low on the horizon from Seattle, Des Moines, Chicago, Cleveland, Boston, and Halifax.
It is difficult to predict what the K index will be during night of the 8th in North America, but it is reasonable to expect K=6 which would put the aurora over Milwaukee, and visible on the northern horizon on a line from Portland Ore, southern Nebraska, southern Indiana, to Washington, DC.
Go to the auroral forecast page at: http://www.gi.alaska.edu/... and watch the Short Term (1 hr) forecast. It will increase when the arrival of the disturbance is one hour away. It will also show you whether you can see the aurora from your vantage point, if you choose the map that shows your location.
These videos show what happened yesterday on the surface of the Sun:
(h/t to Bad Astronomy)
According to Space Weather, this was just an M2-class solar flare—nowhere near the X classification, which is the highest—but it still put on quite a show... and the show may go on tonight, if the Geophysical Institute is to be believed.
I've seen the Northern Lights once in my life—the solar storm in late March/early April 2001 was big enough that we could see a faint red glow in the sky in Grand Rapids, MI, where I went to college. Even that was incredible; it created a lifelong dream in me to go north and see the Northern Lights at their most spectacular, in Canada or Scandinavia.
Some aurora pictures I found (Creative Commons licensed) on Flickr:
NickRussil
zhengxu
If you're in the northern US or Canada and it's clear tonight, look up—you may just see one of nature's most spectacular features.