Interpreting NOAA Marine weather forcasts.

[Joe CD MS 300]

What is the distinction between "seas and "swell" in the recent forecast below? I was under the impression that "seas" in most cases were usually attributable to "swell". Why the special mention?



STONINGTON ME TO MERRIMACK RIVER MA OUT TO 25 NM-
917 AM EDT MON SEP 11 2006

... SMALL CRAFT ADVISORY REMAINS IN EFFECT THROUGH THIS EVENING ...

REST OF TODAY... NE WINDS 10 TO 15 KT WITH GUSTS UP TO 20 KT. SEAS 4 TO 6 FT.

TONIGHT... N WINDS 10 TO 15 KT WITH GUSTS UP TO 20 KT. SEAS 4 TO 6 FT. PATCHY FOG AFTER MIDNIGHT.

TUE... N WINDS 10 TO 15 KT...DECREASING TO 5 TO 10 KT IN THE AFTERNOON. SEAS 4 TO 6 FT.

TUE NIGHT... SW WINDS AROUND 5 KT...BECOMING NW AFTER MIDNIGHT. SEAS 4 TO 6 FT...MOSTLY DUE TO SWELL.

WED... W WINDS 5 TO 10 KT...BECOMING SW IN THE AFTERNOON. SEAS 4 TO 6 FT...MOSTLY DUE TO SWELL.

WED NIGHT... S WINDS 5 TO 10 KT. SEAS 4 TO 6 FT...MOSTLY DUE TO SWELL.
THU... SW WINDS 5 TO 10 KT. SEAS 3 TO 5 FT. A CHANCE OF SHOWERS.

FRI... W WINDS AROUND 5 KT. SEAS 2 TO 3 FT...SUBSIDING TO 1 TO 2 FT AFTER MIDNIGHT.

[mahalocd36]

Swell is "the rising and falling movement of a large area of the sea as a long wave travels through it without breaking".

In this case, caused by Hurricane Florence on the US east coast.

It's got a long period (distance between the crests) as opposed to local wind-driven or wake-driven waves which cause choppy "seas".

[Mitch F]

The voice of NOAA speaks in his own language. When the coastal Maine forecast says "partly cloudy" what he means is "some clouds possible with a chance of lightning, snow, bright sunshine, hail, sleet, rain, wind, brimstone, tornados, and locusts."

[Neil Gordon]

The voice of NOAA speaks in his own language.

I like the girl voice better.

[Oswego John]

Mitch F,

I got a big chuckle out of that. Very humorous. Ha ha.

O J

[Stan W.]

I suspect NOAA just feels compelled to offer some explanation when it forecasts winds of only 5 to 10 knots while at the same time forecasting 4 to 6 foot seas.

[Al Levesque]

I think the distinction between 4 to 6 foot seas and 4 to 6 foot swells is important. The seas would be hard to sail against and would slow the boat considerably while the swells would only raise and lower the boat with much less, if any, effect on speed.

[Tom Keevil]

The forecasts on the Oregon coast usually include "wind waves" and "swell" as part of the forecast. e.g. tonight's forecast is "N WIND 20 TO 25 KT...EASING TO 15 KT AFTER MIDNIGHT. WIND WAVES 5 FT. NW SWELL 6 FT AT 13 SECONDS." On occasion, especially when it is pretty nasty they will report something like "COMBINED SEAS 26 FT" without separating out the wind waves and the swell.

[Efrost]

As explained above, a swell is a long period wave in the ocean which results from distant energy source such as a Hurricane. Seas are shorter lenght waves produced from local energy sources, i.e. wind. You can predict seas by knowing the wind speed and the fetch (distance the wind blows over the water) and the time length that the wind blows, until you have a fully arisen sea. Then no matter how much longer the wind blows it will not get any higher.

As the wave from the local energy source spreads out and moves away from that source the energy spreads out and swells form from the wind induced waves (seas). For total sea ht, Seas should normally be considered to be super-imposed on top of the swell.