North Atlantic storm

30°21.2′N 069°01.1′W

Waves left about 28 Sep 16:48Z, ± 7 h

Placed by 2 buoys, whose own placements lie within 138 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