Jon Bilger on monitoring weather while on passage and making good decisions when it comes to weather forecasting

While it’s essential to monitor the conditions when choosing the best weather window for departure, equally important is being well-versed in obtaining and interpreting weather data while offshore – keeping up with the latest evolving weather movements enables you to position yourself to take advantage of favourable winds and avoid dangerous conditions.

Getting weather data while on passage can be done via SSB radio or satellite systems like Iridium, Inmarsat, and Starlink. The crucial aspect is the device’s ability to transfer meaningful data, particularly weather files in GRIB format and weather routing files. Certain satellite devices, such as Inreach, Zoleo, and Spot, cannot transfer weather data files. These SBD (short burst data) devices have limited functionality despite their popularity, and cannot handle voice communications either.

Reliability is paramount, especially in adverse conditions when weather updates become most critical. The connection should be available at any time, ensuring you never rely on outdated data: twice-daily weather data retrievals are sensible. Timing your analysis and planning with weather model updates will further enhance your decision-making process.

Get into a routine of monitoring long- and short-range weather at sea. Photo: Sophie Dingwall

Specialist software

Offshore weather demands specialised software so you don’t have to be constantly connected to the web. The software should be capable of saving data offline, reading, and compressing GRIB format files while efficiently managing your downloads based on your connection type’s speed and bandwidth.

The PredictWind Offshore App allows you to select weather models, GRIB resolution, time step, and the number of days for each GRIB. You can customise parameters such as wind, pressure, rain, CAPE (Convective Available Potential Energy), wave heights, gusts, temperature, and cloud cover. Ocean and tidal currents can also be added, but their large file sizes require careful management.

Article continues below…

These GRIBs form the foundational data for offshore weather, but to truly harness their potential, additional tools and datasets are crucial. Weather routing, wave routing, ocean data, AIS, graphical GMDSS, and associated warnings significantly enhance the interpretation of this data. You can also include parameters such as observations, satellite imagery, and tracking data.

Different scenarios

Weather routing is at the core of weather monitoring while on passage. This gives you the information to allow you to plan and adapt your strategy. A weather routing algorithm calculates your route to your destination using the performance of your boat in any wind speed or angle.

A smart weather routing tool will also use currents, along with wind waves and swell. The PredictWind weather routing tool has these features, and routes are calculated in the PredictWind cloud. This saves the user time and a massive amount of data. In the cloud, the route calculation involves billions of computations, exploring every conceivable scenario to provide you with the most optimal route based on six weather models, three wave models, and three ocean current models.

Compare models for consensus – a divergent route will also indicate where models don’t all align

This wealth of data is derived from the latest model run, ensuring the highest resolution at all times. In contrast, attempting to download and process this vast amount of data onto the boat might consume over an hour, and still the results may not match the efficiency achieved using the cloud-based system.

Short- and long-term analysis

There are two main types of analysis you can conduct from the data you receive during offshore sailing. The short-term focus involves examining the weather conditions for the next 6-12 hours, 12-24 hours, and up to three days in detail, covering a smaller area. This analysis is done every 12 hours to keep track of the immediate conditions.

For a longer-term outlook you focus on the next 3-10 day range covering a wider area. This provides insights into the weather trends you might encounter during this extended period.

For short-term routing, you should look for key waypoints: identify crucial points on the route, taking note of wind direction changes and trends. Add these waypoints to your plotter or navigation software for reference.
Then download weather routes for all six models (always download them all). Choose the specific GRIB files and parameters you need for analysis. To manage file size, decrease the resolution to 50km or 100km and select a smaller area for downloading. The minimum GRIB parameters include wind, pressure, rain, and 3-5 days of data. If file size permits, you can also add CAPE, gust, and possibly wave features.

Besides weather routes, you should also download GMDSS text and graphical data, along with AIS data to view shipping and fishing vessels within 330 miles of your position. Once the download is complete, analyse the data by quickly checking the graphs to get a rapid overview of the current situation.

Routing display shows parameters that are available to download for PC, tablet and mobile devices

Route conditions

When assessing route conditions, first observe whether the trend in wind speed is increasing or decreasing and whether the wind direction is veering or backing. Consistent trends across most weather models provide a higher level of forecast certainty, while divergence requires further investigation on the maps.

Next, quickly check the average wind speed to ensure it falls within your comfort level, taking into account the true wind angle you’re sailing. Afterwards, focus on the wave data, examining metrics such as roll, vertical acceleration, and boat slamming. Ideally, you’d want the roll to stay below 4°, vertical acceleration below 0.2g, and no boat slamming, as 50% slamming could damage your boat.

With this initial information in mind, you can now make informed decisions. If any levels exceed your comfort zone, consider how best to mitigate them. You might want to think about alternative courses to avoid adverse conditions, slowing down to wait for weather changes, or prepare the boat and crew for the conditions ahead.

To assess the conditions along your route, refer to the wind tab, which provides a comprehensive view of each model’s conditions for your routes. Pay particular attention to warnings such as thunderstorms, lightning, gusts, wind against current, vertical acceleration, roll, slamming, and wind chill. The table displays the parameters responsible for these warnings and their timing.

For a broader perspective of the entire route, a quick look at the summary tab can be beneficial, especially for longer-term planning.

Moving on to the wave tab, you’ll find a more detailed breakdown of the data observed in the graphs. This section offers a clearer understanding of elevated levels of roll, vertical acceleration, and slamming. The wave heights and primary, secondary, and tertiary swell states provide additional insight into different directions, sizes, and wave periods.

In cases where you don’t have a GRIB file for currents, you can use the tables to access current speed and direction at any time. Alternatively, you can tap on specific points on the weather routing map to view current details at those locations.

Diving into observations will give you a more comprehensive picture of actual conditions

Model consensus?

When evaluating the map view with the overlaid routes, it’s crucial to look for consensus across the models. You can easily spot this by examining the tracks of the different routes over the next 12 hours. If there’s an outlier – when a model shows a different route – investigate the reason.

This often occurs in light winds or when a front or weather feature crosses your route. For instance, a rain band associated with a wind direction change might indicate a passing front, causing variations in routes. In such cases, choose the route that closely matches your current conditions and timing.

Additionally, keep an eye on the CAPE parameter because elevated CAPE levels suggest an intense event. That should prompt you to brief the crew to prepare for reducing sail, and consider any other safety precautions.

If you’re looking at the timing of a change, then look closely at the wind, gust and rain parameters across the models. If you saw warnings in the tables, the PredictWind router shows these on the maps at the corresponding time along the routes.

Handling large files

Handling large current GRIB files can be a challenge via offshore connections. However, there’s a strategy to mitigate this issue. Download the currents GRIBs before you leave, as they don’t change as rapidly as atmospheric conditions. Even if you’re using slightly older data for visualising the currents overlaid with weather routes, they can still provide valuable insights.

In the map view, focus on identifying instances when wind is against the current. Weather routing warnings typically pick up on this, but the map view allows you to visualise the timings when you might enter current zones. By flicking through different current models like Mercator, Hycom, and RTOFS, you can identify any differences in positioning and strength between the models.


The longer term view

Typically, the longer-term view only requires attention every 24 to 36 hours, unless specific weather features are forming and warrant closer monitoring. For this planning, start by enlarging your GRIB area well beyond your passage length and consider which side of your route the weather is approaching from. Choose a 100km resolution and download just one model – ECMWF is my recommended choice for longer-term analysis.

Next, narrow down the parameters to wind and pressure only, while obtaining more days of data. Ensure the download also includes GMDSS text and graphical data.

Weather routes are still important, as they allow you to track any low-pressure systems or extreme weather events that might develop over time. This broader view provides insight into the bigger picture of weather patterns forming.

To identify potential weather events more accurately, refer to the GMDSS graph, which indicates everything from troughs and fronts to gale areas and hurricanes.

For a more in-depth analysis, interpret the GMDSS text, which has been professionally crafted by a meteorologist, to investigate any specific events of interest.

This longer term analysis will help you make better informed decisions for a more efficient and more comfortable passage, and either avoid or prepare for heavy weather.

Just bear in mind that this isn’t an exhaustive methodology and there are various ways to utilise this weather data. With practice, you’ll likely develop your own methods and processes over time.

If you enjoyed this….

Yachting World is the world’s leading magazine for bluewater cruisers and offshore sailors. Every month we have inspirational adventures and practical features to help you realise your sailing dreams.
Build your knowledge with a subscription delivered to your door. See our latest offers and save at least 30% off the cover price.