What a Marine Forecast Tells You…And Why It Matters

Here is today’s marine forecast for my home coastline, the north end of Oregon: A FRONT OFFSHORE WILL WEAKEN FURTHER AS IT MOVES ONTO THE SOUTH WASHINGTON AND NORTH OREGON COASTS LATER THIS AFTERNOON…AND ACROSS THE OREGON CENTRAL COAST LATER THIS EVENING. THEN HIGH PRES WILL SIT OVER THE NE PAC FOR TONIGHT AND MON. WILL SEE A PATTERN CHANGE THIS WEEK AS HIGH PRES STRENGTHENS OVER THE INLAND PAC NW AND A BROAD AREA OF LOW PRES SITS WELL OFFSHORE. S wind 15-20 knots, veering to SW in the afternoon. Gusts to 25 kt. Wind waves SW 4 ft at 5 seconds. W swell 11 ft at 12 seconds.

©istockphoto/nightman1965

Marine forecasts can sound like the cryptic predictions of the Oracle of Delphi. What does it all mean? And should you care?

Why You Should Care
I’ll answer the second question first. If you’re going out in the water in any form—kayak, sailboat, offshore cruiser, charter fishing, yacht, dingy or surfboard—the marine forecast is critical. It tells you whether it’s safe to go out or not and what conditions to expect. If you’re just playing on the beach, it can still tell you where you might want to go. It could also tell you whether it’s a good day for a beach campout…and at which end of the beach you should set up your tent.

What Marine Forecasts Are and Aren’t
You’ll note that this forecast doesn’t tell you whether its sunny or rainy. That’s because it’s for mariners and wind and swell are what really matter. If you want to figure out if its going to rain, you’ll either need a traditional forecast, or better yet, learn to interpret the marine forecast. I know that on the West Coast, south wind means clouds and rain. The word “front” is an obvious clue. When the wind rolls around to the northwest, the rain usually stops and it becomes sunny, but windy in the afternoon.

Wind
A marine forecast does tell you a lot about is wind: not just strength, but direction and changes. If you’re camping on the beach in a south wind of 15-20 knots (1 knot is slightly faster than 1mph) it’s going to be windy enough that your face will become sandblasted. You’ll want to set up shop at the south end of the beach, where you’re protected. Surfers love offshore wind (blowing from land to sea) for steepening waves. Kayakers hate offshore winds because if something goes wrong, the wind is pushing you out to sea instead of back to shore.

Swell Versus Waves
Both swell and waves are caused by wind; as they saying goes, waves are caused by wind here, and swell is caused by wind somewhere else. Swells are the ripples in the ocean caused by a storm on the other side of the world. And with swells size isn’t everything. Swell also has direction and period. West swell off 11 feet at 12 seconds is a severe pounding. Coming from straight west, the waves will pound directly into a beach instead of wrapping around headlands.

Period is how much time there is between swells. The shorter the period, the closer the swells are together and the more choppy and chaotic the sea will be. The longer the period, the more space between breaking rollers…but each one packs more punch when it breaks, whether you’re a surfer or a golden retriever chasing a stick in the waves.

Now add the wind waves on top of the swell. 4-foot wind waves on top of 11 foot swell makes for combined seas of 15 feet. 10-15 knots of wind gusting to 25 knots will have wind-whipped spray called “liquid smoke.” A great day for winter storm watching. The kayak will stay home.

Pressure, Coming Down on Me
The best way to track to know what’s coming is to watch changes in air pressure. When the pressure rises, generally it means clear skies and northwest winds…the higher pressure, the higher winds. Low pressure means rain, south to southwest winds, and generally less beach lounging and more storm watching.

All Weather Is Local
But all weather is ultimately local. The forecasts above cover huge areas—half the state of Oregon and westward 60 miles, for instance. You can still find a protected cove when it’s windy. And if the forecast is for light wind, it can still whip up blowing sand if you’re camped in a spot where the wind funnels.

For a more precise forecast than these big areas, look at reports from ocean buoys, which give you actual conditions for specific spots. As I write this, the buoy 18 miles west of the Golden Gate is showing 7.5 ft swell 12 seconds from the WNW, with rising barometric pressure. The weather on Bay area beaches should be improving. Find a buoy near you. Chances are the conditions there will find their way to you soon, if that’s the way the weather moves.

If you’re just going out for a walk, you can stick to the simplest forecast: look out the window. But if you’re planning a larger outing or just want to seem like a wise old salt, learn what the marine forecast is telling you.