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
The prediction of the occurrence of auroras is difficult but we do have some success. However, we cannot say that next week on Thursday evening at 9 pm there will be auroras in the sky. But the prediction is rather good over 2-3 days.
We have developed at the FMI a short-range auroral prediction method basing on the real-time monitoring the space weather in South-Finland. If, for example, the spaceweather conditions in the morning exceed certain limit, we can predict that next evening in South-Finland there will be seen auroras with 75 % probability.
The occurrence of auroras depends on the latitude of the observer. The Northern Lights form an oval band around the magnetic poles of the Earth. At a distance about 2500 km from these poles, the probability for seeing auroras is almost 100 %.
The northern parts of Fennoscandia belongs to the maximum auroral zone. In the coast of Ice Sea in North-Norway you will see auroras almost every evening when the sky is clear enough. When moving southwards, the frequency of auroras decreases. In Sodankylä every second night is an auroral night, in Helsinki every 20th. These are statistical rules giving the average extent of auroras. When the Earth's magnetic field is very disturbed, the auroras can spread all over Europe for a couple of hours.
The best time to see auroras is between 9 p.m. - 1 a.m. local time. The best months are February - March and September - October. During summer months you cannot see aurora due to light nights.