There is two kind of Northern lights prediction: statistical and real-time ones. The former are based on a large amount of observations of Northern lights at different latitudes during several years. From these statistics we can say what is the probability of the occurrence of Northern lights during the course of year.
According to the statistics compiled by the Finnish Meteorological Institute, four nights out of five are illuminated by Northern lights in Northern Lapland (Kilpisjärvi-Utsjoki area) providing that the sky is free enough from clouds. On the coast of the Arctic Ocean in North Norway (e.g. in Tromsö) one can see Northern lights almost every night. Even in South Finland, say Helsinki, one can see them but much more seldom; in Helsinki only one night out of 20.
The latter predictions are based on a space-weather monitoring system either by ground-based devices or satellites watching the space-weather conditions around the Earth. When the monitoring devices (e.g., magnetometers or particle detectors in a satellite) show certain deviations from the normal situation, one can expect that a space weather storm is approaching in a few hours.
The skill of real-time predictions is, unfortunately, rather low; we cannot make predictions for several days like forecasting normal weather. The reason for this shortcoming is that we do not fully understand the complicated processes, starting from the Sun and ending in the near-space of the Earth, involved in the cosmic ignition of the Northern lights. Therefore, more space-research is needed for better forecasts.
More about spaceweather www.spaceweather.com.
AurorasNow is a service which helps watching auroras in Finland