Marine Intelligence Center · Education

Marine Data Center

Wind speed is only one piece of the water. Learn how fetch, wind direction, tidal current, wave period, residual sea and forecast uncertainty combine to shape conditions on the Strait of Georgia.

Education — not a go/no-go tool. This page explains marine-weather and sea-state concepts. It does not replace official marine forecasts, proper training, vessel-specific operating limits, local knowledge or the judgment of the vessel operator.
Fetch Wind Against Current Waves & Period Forecasts Observations Forecast Models Local Factors Glossary
01 · Building the Sea

Fetch

The same wind speed can produce very different water depending on how much open water the wind has travelled across, how long it has been blowing, and what the sea was doing beforehand.

What is fetch?

Fetch is the distance wind blows over open water in a reasonably consistent direction. More fetch gives the wind more opportunity to transfer energy into the water and build waves.

Wind duration matters too. A fresh wind that started recently may not yet have built the same sea as the identical wind blowing for many hours.

LAND → wind → → → → → → → LAND
     small waves → larger developed waves
     short fetch     longer fetch

Why direction matters

A station reporting 12 knots does not tell the entire story. If that wind is aligned with a long stretch of open water, it can build a more developed sea than a similar wind blowing across a short, sheltered fetch.

This is why marine interpretation should consider speed + direction + duration + geography, not wind speed alone.

A calm observation at one protected location does not necessarily describe an exposed route farther offshore.
02 · When Water Meets Wind

Wind Against Tide & Current

People often say “wind against tide.” More precisely, the important interaction is wind acting against the direction of the moving water.

Wind with the current

When wind and current move broadly in the same direction, the wave pattern can be less compressed than when the current opposes the wind. That does not automatically mean calm water—wind strength, fetch and existing swell still matter.

WIND    → → → → →
CURRENT → → → → →

~~~~~~~~  ~~~~~~~~  ~~~~~~~~
less opposing compression

Wind opposing the current

An opposing current can shorten the apparent wavelength and make waves steeper. In the wrong combination, relatively ordinary wind can produce a noticeably rougher and less comfortable sea.

WIND    → → → → →
CURRENT ← ← ← ← ←

/\/\ /\/\ /\/\ /\/\ /\/\
shorter · steeper · more abrupt
Tidal current is a modifier—not a replacement for looking at wind, waves and actual observations.
03 · Reading the Water

Waves, Swell & Period

Wave height gets most of the attention, but period and wave character can be just as important to how conditions actually feel aboard a small vessel.

Wave Height vs Wave Period
Wave height describes vertical size. Wave period is the time between successive wave crests. Two seas with similar wave heights can feel completely different: longer-period waves may be broad rolling swells, while short-period waves can be steep, closely spaced and uncomfortable.
Wind Waves vs Swell
Wind waves are generated by the local or nearby wind. Swell is wave energy that has travelled away from the area where it was generated. A location can therefore have light local winds while still experiencing significant residual or incoming wave energy.
Residual Sea
The wind can fall faster than the sea does. After a prolonged blow, waves may remain after the local wind has diminished. This is one reason a reading of 5 knots does not automatically mean flat water.
Wave Direction
Direction matters relative to the vessel. A head sea, following sea, beam sea and quartering sea place different demands on the boat and operator. Vessel size, hull form, loading and speed all affect how those conditions are experienced.
04 · Reading the Forecast

What the Numbers Actually Mean

A forecast is an estimate of future conditions over an area and time period—not a promise that every location will experience the same number.

Sustained Wind & Gusts

Sustained wind represents the underlying wind regime over a defined averaging period. A gust is a shorter-lived increase above that background wind.

A forecast of light sustained wind with occasional stronger gusts is therefore different from sustained wind continuously near the same higher speed.

A gust number should be treated as context. Sea state, frequency of gusts, direction and the sustained wind remain important.

Forecast Ranges

Marine forecasts often use ranges because conditions vary across both space and time. “10 to 20 knots” does not mean every point in the forecast area will continuously experience 20 knots.

Timing language matters: increasing, diminishing, becoming, veering and backing can be as operationally important as the headline wind number.

05 · Forecast vs Reality

Observations

Forecasts tell us what may happen. Observations tell us what instruments are actually measuring at specific locations right now or recently.

One Station is One Location

A buoy or land station is extremely useful, but it samples its own environment. Terrain, islands, exposure and local fetch can make conditions at another part of a route different.

The best picture usually comes from comparing multiple relevant observations with the forecast and the geography between them.

Trends Matter

A single reading is a snapshot. Several hours of history can reveal whether wind is building, diminishing, oscillating or changing direction.

That trend can help explain the sea state you may encounter even after the latest wind number has dropped.

06 · Looking Ahead

Forecast Models & Uncertainty

Different forecast systems solve the atmosphere in different ways. Agreement between them can increase confidence; disagreement is itself useful information.

Deterministic Models
A deterministic forecast is one model run producing one projected evolution of the atmosphere. The Marine Intelligence Center can compare multiple model families rather than assuming a single model has the answer.
Ensemble Forecasts
An ensemble runs many related forecasts with small differences in starting conditions or model configuration. The spread between members helps reveal uncertainty. A tight cluster suggests a more stable signal; a wide spread means the future is less settled.
Model Agreement
High agreement means the available guidance is telling a relatively similar story. Low agreement means there are meaningful differences. Low agreement does not mean “bad weather”—it means the forecast deserves less confidence.
Why Long-Range Forecasts Change
Small uncertainties grow with time. A forecast several days away is best treated as planning guidance. As departure approaches, higher-value information becomes available: updated forecasts, recent trends, official observations and finally the actual conditions on the day.
07 · Strait of Georgia

Local Crossing Factors

Geography matters. Conditions can change across a relatively short route because islands, shoreline orientation, open-water exposure and local wind effects are not uniform.

Lee & Exposure

The lee side of land can temporarily reduce wind or wave exposure, while leaving that shelter can expose a vessel to a much more developed sea. A calm departure point is therefore not proof of a calm crossing.

The Strongest Section

Route-based forecasting asks a better question than “what is the weather at the dock?” It considers whether one portion of the crossing may carry stronger wind or a different direction than another.

Recent Wind History

What happened overnight can matter the next morning. A strong wind that has only recently diminished may leave residual waves even though the current observation looks benign.

The Boat Matters

There is no universal wind number that makes every crossing safe or unsafe. Vessel size, hull, loading, route, operator experience and passenger comfort all matter. Conditions suitable for one vessel may be unsuitable for another.

08 · Quick Reference

Marine Glossary

A plain-language reference for common terms used in marine forecasts and the Marine Intelligence Center.

FetchDistance wind travels over open water while blowing in a reasonably consistent direction.
GustA short-lived increase in wind speed above the sustained wind.
Sustained WindThe underlying wind speed averaged over a specified observation or forecast period.
Sea StateThe overall character of the water surface, including waves, swell and their interaction.
Wave PeriodTime between successive wave crests, normally expressed in seconds.
SwellWave energy that has travelled away from the weather system or wind field that generated it.
Residual SeaWaves that remain after the wind that generated them has weakened or changed.
LeeThe sheltered side of land relative to the wind.
WindwardThe side facing or exposed to the incoming wind.
Slack WaterA period when tidal current is weak or changing direction; it does not necessarily occur exactly at high or low tide.
Flood CurrentTidal current associated with the incoming/rising phase in a local current regime.
Ebb CurrentTidal current associated with the outgoing/falling phase in a local current regime.
Head SeaWaves approaching generally from ahead of the vessel.
Following SeaWaves travelling generally in the same direction as the vessel.
Beam SeaWaves approaching approximately from the side of the vessel.
Model AgreementHow closely different forecast guidance agrees on the expected conditions.
EnsembleA collection of related model forecasts used to explore possible outcomes and uncertainty.
VisibilityThe horizontal distance at which objects can be clearly identified, affected by fog, rain, haze and other conditions.
Pirates of Perdition Marine Data Center · V1
Built as a companion to the Marine Intelligence Center. For operational decisions, consult current official marine forecasts and observations and use appropriate seamanship and vessel-specific judgment.