Northern Lights predictions for locations like Tromsø, Norway, and Reykjavik, Iceland, show the Northern Lights on the horizon even at 0 Kp from autumn to spring. At least 1-2 Kp (and higher) will guarantee that the Northern Lights are directly overhead at these locations.
Rovaniemi, Finland, also only needs a Kp index of 1 for visibility of the Northern Lights on the northern horizon.
As far south as Umeå, Sweden, and Trondheim, Norway, you'll need at least 2 Kp to predict seeing the lights on the horizon, or a Kp value of 4 to enjoy them overhead.
And when you are down in areas around the Scandinavian capitals Oslo, Stockholm, and Helsinki, the Kp index has to be at least 4 for Northern Lights visibility on the northern horizon, or 6 for the Northern Lights to take place directly overhead.
In comparison, central Europe requires 8-9 Kp (Very High Auroral Activity) to see the Northern Lights at all.
(Note that the visibility of the Northern Lights depends on local weather conditions. Cloud cover will hide the Northern Lights even if the prediction points to a likely occurrence.)