I have a similar fence
board opposite board with a softer curve.
I cut mine using a circ saw, 5 3/8" blade Dewalt battery powered.
I used the 3 nail flexible strip method to establish the curve. 2 nails go at the opposite ends of the fence where you want the curve to begin. A long flexible strip of wood is placed under both the nails. The 3rd nail in the center, is pushed up until the desired arc is achieved. Then you take the vertical measurement at the center from the horizontal and all the rest of your arcs will be the same..... more or less. Draw the arc on the pickets, remove the strip and nails and make your cut. The smaller blade will make a large radius arc very easily.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 06-30-2013 at 10:15 PM.