If you have more rain than normal you will likely see more flies. Since pest fly eggs and larvae need to be in a moist medium, if it’s wetter than normal more breeding areas will stay “just perfect” longer for producing lots of flies.
Flies have a staggering reproduction potential with each female fly laying up to 900 eggs. Thankfully due to natural predation (from beneficials like Fly Predators plus beetles, mites, ants, birds, etc.) plus the need for a moist environment for development, only 2-4% of fly eggs generally make it to adults. But a small favorable change to factors affecting their survival rate, like more rain (or decimating the beneficials with pesticides) can mean a huge increase in the numbers of flies. This is one of the reasons why there are “good” and “bad” fly years. Also, it only takes one burst of wet weather, in an otherwise dry year, to yield a larger than normal hatch of flies who then can hang around for nearly a month.
If you do have a rainy spell, by reacting quickly and stopping the extra reproduction before the fly population builds up, it will take less effort and cost than waiting until the flies are intolerable. Often quickly adding extra Fly Predators, plus more traps can bring the pest flies back under control within weeks.
Surprising as it may seem, flies need a place to rest and get out of the heat or cool temperature. Weeds and tall grasses are perfect for this, so if you remove weeds from around buildings you can “encourage” pest flies to hang out elsewhere.