Weather Forecasts and Interpretation

When my student “skipper-of-the-day” is discussing the weather as part of presenting the passage plan, my usual reaction is:“Where did you get this information from?”

The standard answer is: “An app” or “the internet.”When asked about the reliability of this information, “I don’t really know” is the slightly disappointing but common response.

This is a problem. Weather is a critical input during all phases of passage planning:

• Appraisal – shall we go or shall we stay?

• Planning – shaping a route and timing based on the forecast

• Execution – how do we sail the plan safely?

• Monitoring – when do we need to adapt or abort?

Simply reading wind direction and speed off a screen is not enough. Good seamanship requires understanding where forecasts come from, what they can and cannot tell us, and how to interpret them in context, while continuously combining them with our own observations.

Observation: The Skipper’s First Weather Model

Before opening any app, the skipper should look outside.

Observation is the starting point of situational awareness and remains the most immediate and reliable source of information we have. What matters is not only what the weather is doing now, but also what the signs are for change.

Questions every skipper should be asking:

• Actual wind speed and direction

• How it is changing over time (logbook)

• Whether the wind is backing or veering

• Whether it is steady or gusty

• Visibility trend

• Development of the sea state

• Cloud type and evolution

• Barometric pressure trend

Traditional observations used by seafarers for centuries remain valid today, such as:

• Red sky at dawn – unsettled weather approaching

• High, thin cirrus (“mares’ tails”) – a low-pressure system is arriving

• Rapid vertical cloud build-up – squalls or thunderstorms possible

• Sudden calm after steady wind – often precedes strong weather and a significant wind shift

• Birds flying low – falling pressure and deteriorating weather

  
  

What Is a Weather Model?

A weather model is a mathematical simulation of the atmosphere. It divides the world into a three-dimensional grid and calculates how air pressure, wind, temperature, and moisture evolve over time. The model calculates conditions at grid points, not at your exact position.

The Pillars Behind Every Weather Model

1. Observation

All models depend on real-world measurements from land stations, offshore buoys, ships, weather balloons, and increasingly from satellites. If observations are sparse (open ocean, remote areas), forecast confidence is lower.

2. Historical Data

Models are trained and tested against decades of past weather patterns. This is why they perform well in typical situations and less well in unusual ones. The availability of historical data (for example UK waters versus the Indian Ocean) directly affects forecast quality.

3. Science

Meteorology is a rapidly developing science. Large-scale drivers such as El Niño and atmospheric oscillations were only properly understood in recent decades. As scientific understanding improves, so do the models.

4. Computing Power

Forecasting skill is closely tied to computing power. Faster computers allow:

• Finer grid resolution

• More complex physics

• Larger ensemble forecasts

New AI-based models are already showing impressive results, but no model removes uncertainty, especially regarding wind strength and timing at sea.

Resolution in a Weather Model – and Why It Matters

Resolution refers to the distance between grid points and defines the size of the three-dimensional grid cells for which a forecast is calculated.

In a model with a 25 km resolution, the atmosphere is averaged over each 25 × 25 km grid cell. Within that entire area, the model produces one single forecast value for wind, pressure, and other parameters. Every location inside that grid cell is assumed to experience the same conditions.

Global models, such as the widely used American GFS, are designed to cover very large areas. They perform well when the atmosphere is relatively uniform and when no major features disturb airflow — for example over the open ocean, far from land.

Problems arise when local features exist inside a grid cell. Mountains, cliffs, coastlines, headlands, and narrow channels strongly influence airflow, acceleration, and turbulence. A low-resolution model cannot “see” these features and therefore smooths them out.

Only high-resolution models (typically 1–3 km) can represent small-scale phenomena highly relevant to sailors, such as:

• Sea and land breezes

• Wind acceleration around headlands

• Shelter and gusting close to the coast

• Local convergence and divergence zones

• Thunderstorms and squall lines

Between grid points, the model interpolates values, assuming smooth changes that often do not exist near coasts.

Key lesson:A low-resolution forecast shows the general picture; a high-resolution forecast reveals the essential details for a safe coastal passage.

25 KM RESOLUTION VS. 1 KM RESOLUTION

Grib Files

A GRIB File is a standardised format used to distribute weather model output and display it in weather apps and chartplotters, using Wind Barbs:

Accuracy of Weather Models

Some models perform better than others, but no single model is consistently the most accurate. Key factors influencing forecast accuracy include:

• Model resolution

• Availability of historical data

• Atmospheric stability (unstable air leads to squalls and gusts that are hard to predict precisely)

• The chaotic nature of the atmosphere — small uncertainties in initial conditions grow over time

• Time horizon (skill is high in the first 24 hours and drops significantly beyond 5–7 days)

• Local effects such as mountains, cliffs, and sea-surface temperature

  
  

The Critical Role of Official Maritime Forecasts

GRIB files and weather apps are useful tools, but must never be used in isolation. One of the most important and often neglected sources of information is the official maritime forecast issued by national meteorological institutes.

These forecasts are written by human forecasters, not generated automatically. They combine:

• Multiple weather models

• Observations from ships, buoys, and radar

• Local climatology

• Human pattern recognition

This interpretation is invaluable.

The British Shipping Forecast – A Gold Standard

The British Shipping Forecast, issued by the UK Met Office and broadcast by the BBC, remains one of the most practical and safety-focused forecasts available.

It provides information that GRIBs often do not:

• Wind direction and force (Beaufort scale)

• Sea state

• Visibility

• Significant weather (rain, squalls, thunderstorms)

• Pressure trends

These directly affect safety, comfort, collision risk, sail handling, fatigue, and whether a passage is sensible at all.

The forecast issued by the UK Metoffice for today, covering the island of Wight, showing unfavourable, and potentially dangerous conditions

The Windy App is producing a very different picture for today. A different wind direction, and no sign at all of a Southwesterly Gale (Force 8)

Why Sea State and Visibility Matter More Than Wind Speed

A GRIB showing 15 knots tells only part of the story:

• 15 knots against strong tide can produce short, steep seas

• Residual swell can make conditions uncomfortable or dangerous

• Poor visibility greatly increases collision risk

• Fog can turn a routine passage into a high-stress exercise

These factors are explicitly addressed in official maritime forecasts and often absent from GRIB displays.

Weather and Passage Planning – A Practical Guide

For my passage planning, I use a structured, layered approach.

Appraisal – Shall We Stay or Shall We Go?

I start with the big picture:

• Wind visualisation apps to see system movement

• Synoptic charts to identify pressure systems and fronts

If the big picture does not look right, the passage stops here.

Planning

1. Model comparison: compare several forecast models to assess consistency and uncertainty. When models disagree, I assume greater uncertainty and plan conservatively. Predictwind is my favourite tool, allowing for easy comparison between models in both spot and map views.

2. Local maritime forecast:For Spain and the Canary Islands, I always consult the official maritime forecast issued by AEMET. Sea state, visibility, and warnings often outweigh wind speed.

3. Offshore passages: For longer routes (e.g. Lanzarote–Madeira), I use weather routing, as provided by LUCKGRIB as a planning aid — never as an instruction.

Execution

On the day of departure:

• Check updated forecasts and compare them with earlier versions

• Note changes and update timing

• Observe actual conditions, clouds, pressure trend

• When possible, look beyond the harbour wall

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Monitoring

During the passage:

• Continuously observe wind, sea state, clouds

• Monitor VHF forecasts and warnings (Channel 16)

• Update forecasts and compare them with reality

  
  

The Seamanship Rule

Carry on when conditions match expectations — but change the plan early when they do not.

Good passage planning is not about sticking to a plan.It is about recognising when assumptions are no longer valid and decisions must be made.

This is where forecasts, observation, and experience come together — and where good skippers distinguish themselves.

Comparing Weather Models – Strengths and Limitations

One of the major strengths of PredictWind is that it provides access to several different weather models. This allows skippers to compare forecasts rather than blindly trust a single output.

No model is “best” in all situations. Each is built differently and performs better under certain conditions, regions, and timeframes.

ECMWF (European Centre for Medium-Range Weather Forecasts)

Strengths

• Widely regarded as one of the most accurate global models

• Excellent representation of large-scale pressure systems and fronts

• Strong performance in the 2–7 day range

Limitations

• Global resolution limits coastal detail

• Can smooth out gusts and local acceleration

Best used for:Understanding the overall synoptic pattern and medium-range planning.

GFS (Global Forecast System – USA)

Strengths

• Free and widely available

• Frequent updates

• Useful for identifying trends and changes over time

Limitations

• Low resolution (25 km)

• Can underestimate wind strength in some situations

• Less reliable or unreliable near complex coastlines

Best used for:Cross-checking trends and spotting model agreement or disagreement.

PWG (PredictWind Global)

Strengths

• Tuned specifically for marine use

• Often more responsive to stronger wind scenarios

• Useful counterbalance to GFS and ECMWF

• 1 km resolution deals with coastal and land effects

Limitations

• Based on the GFS model

Best used for: Coastal and near-offshore passage planning where local effects matter.

PWE (PredictWind European)

Strengths

• Higher resolution than global models

• 1 km provides good and usually accurate representation of coastal effects

• Often very useful for short-range forecasts (0–3 days) in European waters

Limitations

Shorter forecast horizon

Best used for:Coastal and near-offshore passage planning where local effects matter.

ICON (German Weather Service)

Strengths

• Modern global and regional modelling system

• Often performs well with wind direction changes and pressure evolution

• Good alternative perspective to ECMWF and GFS

Limitations

• Like all global models, limited resolution near the coast

• Local effects still smoothed

Best used for:Comparing synoptic evolution and timing of systems.

UK Met Office Models

Strengths

• Very strong regional models for UK and nearby waters

• Excellent handling of frontal systems and wind shifts

• Closely aligned with official maritime forecasts

Limitations

• Limited geographical coverage

• Less relevant outside NW European waters

Best used for:Coastal and offshore planning in UK and adjacent sea areas.

Spire (Data-Driven Model)

Strengths

• Uses large volumes of satellite and observational data

• Often reacts quickly to changing conditions

• Can perform well in data-rich regions

Limitations

• Less transparent methodology

• Performance can vary by region

Best used for:An additional independent perspective, especially when models disagree.

ARPEGE (France – Global Model)

Strengths

• Global model operated by Météo-France

• Good representation of large-scale pressure patterns and frontal systems

• Often performs well in European and North Atlantic waters

Limitations

• Global resolution limits coastal and small-scale detail

• Like other global models, local effects are averaged out

Best used for:Understanding the synoptic situation and medium-range planning, particularly when sailing in or near French forecast areas.

AROME (France – High-Resolution Regional Model)

Strengths

• Very high resolution (typically 1–2.5 km)

• Excellent representation of:

  • Coastal acceleration

  • Sea and land breezes

  • Convective weather and thunderstorms

• Particularly strong close to land

Limitations

• Short forecast range (usually 36–48 hours)

• Limited geographical coverage (based on covering French territory and surroundings

• Less suitable for offshore planning

Best used for:Short-range coastal decision-making, harbour departures, and understanding local wind behaviour — especially in summer and in complex coastal terrain

AI-Based Weather Models (Emerging)

Strengths

• Extremely fast computation

• Increasingly good pattern recognition

• Already outperforming traditional models in some scenarios

Limitations

• Limited track record

• Still rely on the same observational data

• Do not remove uncertainty or local effects

Best used for:Early scenario exploration, not final decision-making.

The Key Lesson for Skippers

The real value of comparing models lies not in selecting one “best” model, but in comparing several.

• When models broadly agree → confidence increases

• When models disagree → uncertainty is high

• When uncertainty is high → plan conservatively