I've flown the trench in August quite a few times doing sheep hunts. Even in August the "typical " weather is so variable that you need to be very flexible with your timing. I've had heavy wind, rainstorms, and scorching hot and dead still on the same trip. For fuel planning, plan for stiff headwinds whichever direction you're traveling. I gathered from another thread this is a ferry flight with a plane you're not terribly confident in. Is it on floats? On floats you can at least land on Williston...probably. Getting a plane safely secured to shore without incurring damage is another matter. On wheels your emergency landing options are pretty sparse.