Another sample Resource: Shipboard forecasting


Index to tips on forecasting.

The General Discussion covers the role of shipboard observations in confirming existing predictions from official sources (fax maps or voice broadcasts) as well as in making your own independent analysis without any other information at all. There are several areas in which we can make observations, after which the task is to put them together for the best prediction. Each of these areas is singled out here and the best possible use of these observations are discussed as well as how they are related.

This information is often covered in more detail in other parts of the course, but this is the place where it is summarized and cataloged for quick reference.

Part of the section on sea state observations is presented below as a sample.

Sea state as a forecaster

Sea state means swells, waves, and ripples. Below are a few notes on each of how these might relate to forecasting underway.
This demo tour includes only the first.

SWELLS

Swells can tell us about far off storm systems since waves generated in a storm generally build to speeds that are faster than the storm itself. When these outrun a storm they organize into swells that can travel a thousand or more miles from the storm and still be detectable as long low swells. The farther they go, the lower and longer they become. Low refers to the height, trough to crest. Long refers to the wavelength, crest to crest.

We will be in some form of swell more often than not, so our job is to sort out the ones that might tell us something about forthcoming changes in the weather.

Swells from a cyclonic storm radiate outward from the center, but distant ones are only felt from the forward side of the moving Low. In the NH, swells are longest and highest in the right forward quadrant of a slow moving Low, with the weakest swells coming from the left rear quadrant. A fast moving Low, however, can bring storm seas with it, without prior warning from advancing swells. Typical wind waves have periods of a few to half a dozen seconds or so, except for very strong winds with fully developed seas. Swells blend into the sequence from a half a dozen or so for close storms and some 10 to 13 seconds or so for typical distant storms, and even longer in rare cases.

Only storm force systems can develop significant swells with periods over some 13 seconds or so and periods longer than 20 seconds rarely develop for even the largest storms. Hence the arrival of a new swell with periods in the range of some 10 to 20 seconds whose direction does not change with time indicates you are in the dangerous sector of a major storm.

Periods longer than this imply a large storm, but one that is very remote from you that probably won't affect you.

Swells from close storms can be up to 10 or 20 feet high. Ones from a distant storm may only be a few feet high.

Remember we are discussing swells here, not waves. So the first task is to be sure you are indeed looking at swells. The key feature of swells is uniform height from one to the next and a very broad width. Waves on the other hand, have many heights and are not wide. The traverse extent of wind waves is only a few wavelengths.

A steady swell direction, with an increase in swell height and a decrease in swell period indicates a storm approaching. If the swell reaches a peak height and then decreases, the storm either died off or turned in direction. Likewise, if the direction noticeably changes in a steady way, the storm is passing to the side of you veering, it is going poleward of you; backing, it is going to the equator side of you.

On the other hand, if a new swell appears and remains constant in swell height and swell period for some 6 hours or more, then it is possibly from a remote system that won't affect you. The flag is up, however, to pay attention.

With weather maps at hand, it is generally instructive to try to identify the source of any new long, low swell that is detected. It is your laboratory homework underway.

When you feel an unusual swell go by, seemingly isolated from the regular wave train, as if some one just dropped a giant rock into the ocean somewhere, then take note of it immediately and try to spot its direction of travel. And then wait for the next. Without special attention, you might miss them since they can be well separated. If you note immediately, however, you can spot it since they are very wide waves, stretching across the horizon, as opposed to wind waves which are only some several times their length in width.

The period of swells is given by L = 5xT² . A period of 13 seconds would correspond to a wave that is 5x169 or 845 feet long or roughly some 300 yards. Think of looking for the next wave 3 football fields away. Generally you can't see the pattern, just feel them individually. Try timing the period to clinch the identification. Remember too that wave speed is determined by wavelength. These long waves will be fast, they will go right by. Remember that wave speed is the same as "hull speed" = 1.34 x square root (L) or something like 3 x T in knots. At a 13-seconds period, the wave is moving at about 39 knots. It is like driving a car over a speed bump when they go by, if not masked by the prevailing wind waves.