North Atlantic storm

40°33.1′N 062°26.2′W

Waves left about 25 Sep 10:28Z, ± 2 h

Placed by 5 buoys, whose own placements lie within 172 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