September keeps August's swell and adds August's weather. The southern season is running out but has not stopped, the hurricane season is still at its peak, and the rains are at their heaviest on both coasts.
That combination is why September is the month to hold dates loosely. A week here can produce the best three days of anybody's year and can also produce five days of grey water pushing out of the rivers with nothing rideable in front of it. Neither outcome is unusual and neither is forecastable a month ahead.
The compensation is that nobody else is here. September is between the northern summer holidays and the winter season on both coasts, so a line-up that holds thirty people in December holds five, and the prices reflect it.
