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100kt flight level wind reduces to roughly 90mph at the surface.
Flight level wind in knots x 1.15 = flight level wind in mph x 0.90 = surface wind estimate. This is for the 700mb or 10000ft flight level. But it is close enough for the lower levels.

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Uh, check your math Danny
100kt = 115mph at flight level...
115mph FL * .9 = 103.5mph at the surface

I always look at flight level winds in knots, and just use the same rough number in MPH at the surface, since .9 * 1.15 is 1.035, which is close enough to 1.