They are normally very good weather forecasters and there is normally several fine days after they start flying.
I once did a house swap with a family in Austria, they had an ant colony in the kitchen which they had left being rather amused by the neat liitle trail down the wall and across the kitchen floor. However, one day we arrived back and found the entire kitchen completely full to the brim with flying ants. They were everywhere, never seen so many.
Last year I managed to stamp on them as they came out and this year we have very few in the garden. I have loads on my allotment so would really like to deal with them this year, they seem to farm blackfly.