March Browns usually start to pop off right around the first week of March and generally around the early afternoon regardless of weather conditions.
In the hours leading up to emergence the insect develops a gas bubble in thorax that helps to propel them up to the surface. They release their grip form the bottom and start to drift downstream in the current on their trip to the surface.
Once they reach the surface they secure themselves in the surface film and emerge out of their exoskeleton where they must wait for their wings to dry before they can fly away. This is another stage in which these bugs are extremely exposed to fish. You can expect to see gentle sipping takes from fish when feeding on emergers.
Traditional tight line nymphing or nymphs under an indicator still work for this stage but I prefer to swing emerging patterns. Keeping a tight line casting downstream at a 45 degree angle and letting the current move your flies across the run produces a very similar movement to the insect swimming up in the water column and can be extremely productive when done properly.
If fish are feeding on the surface emergers switch over to a dry fly.