See the chart above? This was downloaded and fiddled with on June 16th when Tropical Storm Arlene was just getting going. Notice the big yellow dots, right? That's her and a rough plot of where she was.
Also, notice the red dots with the red numbers beside them. Those are four buoys which are of interest to anyone surfing in the Northeast. We use them to check the swell, what's happening offshore.The George's Bank Buoy you likely knew about, as well as the Nantucket Buoy , but here's a couple more of interest: he Long Island Buoy and the Virginia Beach Buoy.
What's so special about them? Well, they not only measure and report the swells, just like their siblings, they also report what direction the swells come from. This is very useful.
Let us say that there is a swell reported from George's Bank, 2 feet (SwH) with a swell period (SwP) of 10 seconds. Well, it's similar at Nantucket, say it's 2.2 feet and 10 seconds. Great. What's it coming from, so that we can figure out if it is from Arlene or the low that just left or what? We know where Arlene is, and where the buoys are, but the little problem we have is like a blind man who can hear a sound but not where it's coming from.
Enter the two directional buoys: At the same time, Virginia Beach is reporting a swell, slightly larger (averaging 2.3 to 3.6 feet) which is coming from the East...look for SwD for the direction. Long Island is reporting a swell, say it's 2.2 to 3.0 feet, coming from the East-Southeast, or a little bit south of east, call it thirty degrees or so. I draw a couple of lines from the buoys, one east, one east-southeast. They wind up...right about on the northwest edge of old Arlene.
Northwest edge? Uh huh. Let's not forget what we see in the Tropical Storm reports: Tropical Storm Force Winds extending out blah blah blah miles from the center. In the case of Arlene, right then, that distance is over a hundred miles. As well, swells take a while to travel from a storm to the shore...so ya gotta take the distance and figure how long it took for the swells to get there, then figure where the storm was. Say, if the storm is around six hundred miles offshore, then it took twenty hours for the swells to get to shore, moving at 30 miles an hour, so you have to figurwe where the storm was twenty hours ago...still with me?
So, what does it all mean? The swell with the ten second period is coming, very likely, from TS Arlene. It's not only coming from the right direction, it's bigger to the south of New England than it is to the north, which indicates a storm that's south of us. The swell period may change a little, but it's kind of like Arlene's fingerprints.. By checking with these buoys and more even further south, you can track a swell all the way up the coast.