As for KP.... this is the definition in UAV Forecast.. however, I have heard dissenting opinions on how risky it truly is....
The Kp index measures geomagnetic disruption caused by solar activity, on a scale from 0 (calm) to 9 (major storm). The higher the Kp index, the more likely you are to have problems. Kp's of 1-4 are completely safe. At a Kp of 5, you'll have problems perhaps 1% of the time. At a Kp of 6, you'll lose an average of 5% of your locked satellites; at 7, you'll lose roughly one third.
Solar activity interferes with GPS signals in two ways, both due to disruptions in the ionosphere.
(1) It decreases the signal-to-noise ratio and affects carrier frequency, causing the receiver to lose lock on some satellites. Instead of 9 satellites, you might lock only 6, or the number might fluctuate from second to second.
(2) It changes the propagation delay through the ionosphere, making GPS positioning inaccurate even if the receiver has all satellites locked.
Even during a major storm, the extent to which you see these problems will depend on many other factors, including your latitude (worse at high latitudes) and the time of day (worse at night). Sometimes everything will be fine. At other times you'll lose lock completely, or appear to have a lock but the position will actually be wrong by hundreds of feet.
The storms can also interfere with radio control signals, or with the onboard electronics.
So in general it's better to play it safe during solar storms at Kp of 6, even though most of the time you won't notice any issues. At 7 or higher, you're better off sitting inside and flying a simulator. Happy flying!
For current and forecast Kp conditions, as well as the weather, see
http://www.UAVForecast.com.