Question for McGlups

[BrickHouse]

Why did this long period swell first show out of the south east. (the first long period lines reflected as east/south east on the outer buoys.) Does that have to do with refraction as the swell bends around Nova Scotia? I mean it was generated out of the north east so thats the only explanation I can come up with. Or was there an original south east component to the system as it developed.

Thoughts.
Jamie

[mcglups]

There definitely was not a south east component to the system as it developed but there are a few explanations that can provide insight to what was observed at offshore buoy locations.

1. The NDBC fleet has an algorithm that has a seperation frequency that puts energy on one side as 'swell' and the other side as 'wind waves'. When it makes this split, it also brings along with it the similar split based on directional seperation frequency and this can be a little confusing, especially because we had lingering energy from the ESE at a shorter period, but it still was on the "swell side" of the seperation. Take a look at the raw spectral data for 44008:

http://www.ndbc.noaa.gov/station_rea...?station=44008

There are four frequency dependent parameters that provide the raw data for directional buoys: ALPHA1 (mean wave direction), ALPHA2 (principle wave direction), and R1 and R2 (parameters which describe the directional spreading about the main direction).

When you dig into these details, it is evident that most of the energy was centered around 265 but the spreading was out to 255 to 285 for the energy greater than 12 seconds. When you focus on just the energy greater than 13 seconds, the center is around 267 with a spread of 262-268, give or take a couple degrees.

I wish I had graphics of this stuff, it helps to explain it much better.

2. The fetch area of this system was complex in geometry and intensity. The first zone of fetch was oriented from the ENE to the WSW with a broad zone of 30-40 knot winds and this was at a latitude well north of 40 about 2400 miles out. The second zone of fetch was oriented more along the lines of E (with a touch of N) to W (with a touch of S) with a broad zone of 30-40 knot winds with a small area of 50 knot winds and this was around 39N about 2200 miles out.

The second zone therefor sent a swell that propogated towards us, but did so from a lower latitude. The dispersion of this energy results in some energy moving away from the mean, so it is definitely possible for some energy to arrive (especially at the start of a swell) at an angle that deviates from the mean.

Take a look at the historical quikscat data, use this link and change the julian dates and take a look at the east/north atlantic.

http://manati.orbit.nesdis.noaa.gov/...qscat_day-1.pl

3. The rotation of the earth, good ole' Coriolos Force. Energy moving in the Northern Hemisphere will experience a slight tug to the right. This actually gets blended into #2 when you work the full set of equations, but in general there is a slight (slight) bias of dispersion in the direction of the coriolos force, but it is pretty small. So in this case, a little tug to the right equates to the energy leaking into the 280 range.

Glad you scored some swell. It is a treat to be able to forecast long period energy for the northeast. I sure hope we score some South Atlantic Swells in the coming months, those are the gem swells that undergo all sorts of planetary influence...

Don't be surprised if this swell sticks around longer than I have forecasted on the forecast page, it is just that I think it is going to get hacked by the local storm so I don't want to raise expectations, but you never know, it might last but I'd like to see more energy near the mid-teens.