aurora forecast

The solar wind excites the atoms on the ionosphere releasing diferent ligth colors depending on the emission spectrum. Oxygen releases green and red. Nitrogen glows blue and purple. Hydrogen also releases blue.
The K_p index is an excellent indicator of disturbances in the Earth's magnetic field. The K_p index data of this site is provided by NOAA.
An aurora is the glowing evidence of the particles from a geomagnetic storm colliding with the atmosphere. During a clear night could be possible to witness those phenomena from latitudes like:

K_p 0:	Barrow (US), Reykjavik (IS), Inari (FI)
K_p 1:	Fairbanks (US), Rovaniemi (FI)
K_p 2:	Anchorage (US), Tórshavn (FO), Oulu (FI)
K_p 3:	Calgary (CA), Ålesund (NO), Jyväskylä (FI)
K_p 4:	Vancouver (CA), Stockholm (SV), Hobart (AU)
K_p 5:	Toronto (CA), Edinburgh (GB), Devonport (AU)
K_p 6:	New York (US), Hamburg (DE), Christchurch (NZ)
K_p 7:	Nashville (US), Brussels (BE), Melbourne (AU)
K_p 8:	San Francisco (US), Paris (FR), Sydney (AU)
K_p 9:	Monterrey (MX), Oviedo (ES), Ushuaia (AR)