A relatively new term "Space weather" is used in context of solar radiation in space (in our solar system). Radiation can be particles or electromagnetic radiation. It originates from the sun and its flare eruptions. Radiation may have consequences to satellites and their functioning, to astronauts' health and to our atmosphere. In particular, particle radiation causes auroras.
Space weather forecast is needed to get some in advance info whether there is any chance of auroras in the coming nights. For "tourists" Finnish Meteorological Office provides on-line graphs of
current auroral activity on four geomagnetic stations. These graphs describe current geomagnetic activity and this can be used to make a sensible guess what activity might be in the coming hours. However, this is not an aurora forecast.
More serious aurora hunt requires more data. A good starting point is www.spaceweather.com. There one can find real time space weather values and links to more data. More important, one can get verbal forecasts of auroras, and in case of an auroral storm in progress there are alerts of this. However, to make more accurate aurora forecasts, more on-line graphs have to be examined.
A very good verbal description of solar activity, analysis of active solar regions and geomagnetic forecast is given in Solar Terrestial activity report. Site is updated daily (at about 4-7 UT) and in case of something unexpected is going on updates can appear several times during the day. This is a very reliable source to plan night's activities.
In the following I'll give my own routines in forecasting auroral activity.
1) First of all I follow the above mentioned sites to interpret the more detailed data correctly.
2) Real time forecast of geomagnetic activity is provided in Wing Kp Model: ACE-satellite measures particle flow between Earth and Sun. Graph is constructed from this data. The graph is updated every 15 minutes. Time (in UT) is the horizontal axis in both graphs. Lower graph gives the time (in minutes in vertical axis) until the corresponding measurement is supposed to take effect, i.e. based on the particles' speed it is calculated when the measured particle flow is arriving in our atmosphere. Typically model gives a 30-80 minute in advance forecast of geomagnetic activity. Activity forecast itself can be found in the upper graph. Note how this graph is moved to right by the amount given in the lower graph. In this graph vertical axis gives predicted K-index. Value K=5 is marked in red colour. The higher K-value is the more intense auroras are. Geomagnetic activity depends strongly on geomagnetic latitude - a very rough geomagnetic latitude estimate is your geographic latitude. Anyway, when K-index passes - say - value 4, there is a chance of mild auroras at 60N (or at my geomagnetic latitude 56N). Value K=6 means almost sure auroras even this south. K-value alone is not, however, enough to make an estimate of auroral activity.
3) Having checked K-value it is time to pay attention to Planetary magnetic field z-component Bz. It is the topmost, red graph in this ACE-satellite data. Southern (minus) values mean increased possibility of seeing auroras, while northern (plus) values mean supressing possible auroral activity. In general, high K-value (5 and up) and deep (-20 and below) negative Bz mean big auroras. To get some impressions of these combinations, you have to check values against your true aurora observations.
4) A very easy picture of current auroral activity is given in
aurora ovation which is updated every few minutes. This is not a forecast but rather shows the extent of aurora and the approximate southenmost line where you can possibly view current auroras.
5) In Finland final check might be a visit to
Sodankylä Geophysical Observatory and their real time magnetometer reading. If x-component has large drops or jumps (>1000nT), then there is most probably auroral activity even in southern Finland.
There is a lot of space weather related material available. In paragraph 3 above a link to ACE-satellite's 24 hour data was given, but more is available in ACE-data table. (Select one of the plots in the first column.)
You may want to check current space weather values. A useful graphical representation of last 2-3 days' solar activity is in Today’s Space Weather, and these graphs also help in forecasting coming nights' auroral activity.