I don't think they will swarm until a good flow,then they will backfill the broodnest with nectar and pollen before leaving.
They will backfill the combs in the broodnest before they swarm and then take most of it with them.
Early Signs to watch for signaling swarming:
1. Lack of empty combs, and all combs being filled.
2. Backfilling of broodnest other than at summers end.
3. No available space--forgot to super or at least pull full frames and replace with empty frames.
4. Sudden increase in burr comb building.
5. Choked brood nest--little or no arc of honey around top of frames, also laying eggs in burr comb.
6. Bearding outside of hive even on moderately warm days.
7. If you wait until you see a queen cell you've waited 3 weeks too long.
Notice that most of those swarming signs can be prevented by keeping an open brood nest and timely supering. Note also that burr comb can be much more than an irritation. Proper ventilation plays its part too.