AlexF,
Sometimes when you expect to see lots of St. Elmo's Fire you simply don't see anything at all. On the other hand, on a flight out of London a few years ago, we had lots of St. Elmo's Fire despite no returns from any weather on the radar. As a result, I don't think St. Elmo's Fire is something you can predict with any certainty.
Whilst you're investigating St. Elmo's Fire why don't you also have a look at 'plasma balls' which are seen following a lightning strike on an aircraft. They are usually about 12" in diameter and travel down the cabin from nose to tail at about waist height. Just like St. Elmo's Fire they are the result of a gas being turned into a plasma (ie. an ionized gas) by a high voltage.