In the Willamette Valley, some claim the best MB hatches come off during the worst weather. I call B.S. on that one!
I've tried fishing during cold, torrential rains and thunderstorms and didn't catch a damned thing.
I do the best during the MB hatches when it is a rare, warm, no rain day with a mix of sun and clouds.
in this part of Oregon, the MB hatch runs from late Feb through early May so the window of opportunity is large enough that you really don't need to suffer to fish during a MB hatch... unless you really, really think the weather needs to suck for a hatch to come off.... which is a bald-faced lie!!!
(sorry, I can't abide by the crummy weather = MB hatch philosophy that many flyfishers around here live and die by)
The few times I've observed good March Brown hatches on the Yakima River (few because I normally fish well down in the canyon which is apparently not the kind of water most favored by the of the March Brown), I've been mostly impressed with the sped with which the emergers rise through the water column. In moderately deep water (say 5-6 feet) the trip from the bottom seems to take only a matter of seconds (less than 10). Sometimes it takes longer than that for the dun do be ready to leave the surface and fly away (this seems to depend primarily on temperature and humidity, taking longest when the weather is cold and/or wet).