Wish I would have known this years ago.  It's all about the weather and forecast.  An excellent video that explains it well.

https://midcurrent.com/2019/07/30/video-hatch-how-to-predict-a-stonefly-hatch/?utm_source=MidCurrent+Fly+Fishing+Email+Newsletter&utm_campaign=30f6b6d669-EMAIL_CAMPAIGN_2019_07_31_19&utm_medium=email&utm_term=0_8efbf3b958-30f6b6d669-42789169&mc_cid=30f6b6d669&mc_eid=9ff7f7ed4c