How do you pick the right departure date? One of the most critical aspects of planning for an offshore passage is choosing the right weather window for departure. A weather window refers to a period of favourable weather conditions that allow for safe and efficient passage – for either the beginning, or majority, of your passage.

Here, we take a look at the process of selecting the ideal window. We’re particularly focussing on using PredictWind, though the same principles apply to other sources of weather data.

Plan your route

The first step is to plan your route. Take into account seasonal weather patterns and look for past weather data, using tools like Pilot Charts or Historical Weather in PredictWind. These provide years of averaged monthly data for wind speed, wind direction and waves, along with other atmospheric data.

Other sources of information include forums, cruising associations, or rally organisers. Jimmy Cornell’s World Voyage Planner and Ocean Atlas includes historical data on average wind directions and strengths on many popular cruising routes.

One essential planning consideration is ensuring you have enough time and avoiding hard deadlines. If you have less than a week to find a weather window with tough conditions, it’s likely to force you to make a bad decision.

The PredictWind routing function offers different routing options based on calculations using multiple weather models

Monitor the big picture

Once you know approximately when you’re planning to depart, it’s important to monitor the big picture. Look at the global view to understand the larger weather patterns that may affect your passage, paying particular attention to wind, currents, and wave states. For an ocean crossing this may involve studying weather patterns over a longer period of time and running routing simulations to see how the weather develops in different scenarios.

Ocean currents can be caused by many factors such as wind, density differences in water masses caused by temperature and salinity variations, gravity, and events such as earthquakes or storms.

PredictWind offers three different ocean current models – RTOFS, HyCom and Mercator – on a global scale along with high resolution tidal currents for popular coastal areas around the world. The ocean and tidal currents are used in the Weather Routing and Departure planning calculations and data outputs, with warnings showing in the routing when you have a situation such as wind against current. Different current models can also be compared in the maps with your routes overlaid.

Local weather observations can also help identify trends and provide valuable insights into likely weather conditions that will be encountered. These can be found through a variety of resources, such as The National Weather Service in the US, weather buoys, marinas and airports, and ship reports, as well as apps and GRIB files.

The software searches for optimal conditions depending on your boat’s polars and
the forecast

If you’re doing coastal passages, high-resolution modelling in shorter time frames can make a big difference to the accuracy of the forecast conditions. High-resolution modelling gives more detail and accuracy around land formations from having a much higher number of grid points; they’ll also forecast thermal activity (sea breezes) which you don’t see in lower resolution global weather models. A high res model under 8km resolution will start to show these features, with 1km resolution models showing the most detail.

Once your boat is fully prepared and you’ve set up a reliable form of communication and weather reporting for areas out of mobile data range, you can start looking for a more precise departure date. You are primarily looking for a stable pattern without extreme conditions. Having consensus across multiple models, over an extended period of time, gives more forecast confidence.

Planning tools

Then, we can start using departure planning tools. PredictWind offers a Departure Planning tool that provides critical information for any departure time in the next 10 days, with times set by default to one day apart for offshore crossings, but can be as fine as one hour apart for coastal passages. With this tool you can input your vessel’s polars in various conditions and set your departure start time and spacings to get detailed weather and wave forecasts specific to your boat for each departure time.

The routing algorithm is powered by billions of calculations using six global and five high resolution weather models. It provides route outputs showing average, maximum, and lowest wind speeds, wave heights, ocean currents, and vertical acceleration (the rate at which the vertical motion of air masses or water bodies changes over time), as well as expected roll and boat slamming for each departure time. The tool also highlights any extreme conditions, such as wind against current.

Local weather observations can also help identify trends and weather patterns

Wave routing is also critical. Using data (waterline length, displacement, beam and draught) input which you enter when you set up your vessel’s polars, the program creates a 3D hydrodynamic model of your yacht, and calculates how it will behave based on different swell states.

Departure times

PredictWind’s Departure Planning tool provides multiple departure times for analysis. It’s important for each skipper to analyse the data to find the best conditions for your passage.
Consider your yacht and crew’s preferred conditions and sailplan, and look for departure times that offer wind conditions within your desired range. Additionally, pay attention to parameters like roll, vertical acceleration, and boat slamming, as lower values indicate smoother sailing conditions.

PredictWind has indicated limits for roll (4° RMS), vertical acceleration (0.2g) and boat slamming (less than 50% but ideally none).

The Departure Planning tool shows forecast sailing conditions depending on departure date and weather model

Given there’s rarely a ‘perfect’ forecast, picking a departure time involves weighing up all the options and being flexible. Leaving on the back of a front, for example, may mean you get great downwind conditions, but it’s likely that the frontal band, which brings a new wind direction, will leave a messy wave state. So you may have upper-end wave conditions for the first few days, although the excellent wind conditions may outweigh this, and conditions will likely improve as the front moves away over time.

It’s also essential to remember that the wind maps show average windspeed, and a passage that appears fairly benign can take on a far different appearance when you look at the gust maps, which can show windspeed in excess of 40% higher than the averages. It goes without saying to check for any extreme weather warnings on the route.

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Keep monitoring

Once you have chosen a weather window, you need to keep assessing the conditions, ideally every 12 hours after the forecast models update to get new routing data, including wave data. On passage, weather conditions can change quickly. Adapting your route to suit the forecast is essential.

Additionally, utilise functions such as PredictWind’s visual representation of GMDSS weather warnings (text forecast written by a meteorologist giving interpretation and warnings around any developing or existing weather events, transformed into images using AI), which can highlight events forming outside your field of view and help you make informed decisions while under way.

It’s always wise to have a backup plan in case weather conditions do turn unfavourable. Make sure you have alternative routes, and if possible identify some sheltered anchorages you could run to if the need arises.

Ready to go

Experienced sailors often say the best seamanship is not getting caught out in bad weather. But despite the advancements in forecasting technology, sailors can still encounter unforeseen circumstances such as equipment failure or crew illness. No weather model is correct 100% of the time, which is one reason we use multiple models and look for trends and consensus; we expect models to be right 85% of the time.

By carefully planning your route, monitoring weather patterns and utilising departure planning tools to interpret the data, you can give your passage the best possible start.

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