North Atlantic storm

45°29.3′N 049°53.7′W

Waves left about 23 Sep 11:46Z, ± 6 h

Placed by 2 buoys, whose own placements lie within 239 nmi of it

Each buoy places the storm from how its swell's period shortened as it arrived: the longest waves travel fastest. Fronts are drawn at deep-water group speeds along great circles, and stop at land. How it is worked out