Olympian, America\u2019s Cup winner, and founder of PredictWind, Jon Bilger, explains how to find the right weather window for cruising sailorsReefed down in the windy conditions. Photo: Richard Langdon\/Ocean Images 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 \u2013 for either the beginning, or majority, of your passage.
\nHere, we take a look at the process of selecting the ideal window. We\u2019re particularly focussing on using PredictWind, though the same principles apply to other sources of weather data.
\nPlan your route
\nThe 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.
\nOther sources of information include forums, cruising associations, or rally organisers. Jimmy Cornell\u2019s World Voyage Planner and Ocean Atlas includes historical data on average wind directions and strengths on many popular cruising routes.
\nOne 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\u2019s likely to force you to make a bad decision.
\nThe PredictWind routing function offers different routing options based on calculations using multiple weather models
\nMonitor the big picture
\nOnce you know approximately when you\u2019re planning to depart, it\u2019s 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.
\nOcean 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.
\nPredictWind offers three different ocean current models \u2013 RTOFS, HyCom and Mercator \u2013 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.
\nLocal 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.
\nThe software searches for optimal conditions depending on your boat\u2019s polars andthe forecast
\nIf you\u2019re 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\u2019ll also forecast thermal activity (sea breezes) which you don\u2019t 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.
\nOnce your boat is fully prepared and you\u2019ve 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.
\nPlanning tools
\nThen, 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\u2019s 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.
\nThe 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.
\nLocal weather observations can also help identify trends and weather patterns
\nWave routing is also critical. Using data (waterline length, displacement, beam and draught) input which you enter when you set up your vessel\u2019s polars, the program creates a 3D hydrodynamic model of your yacht, and calculates how it will behave based on different swell states.
\nDeparture times
\nPredictWind\u2019s Departure Planning tool provides multiple departure times for analysis. It\u2019s important for each skipper to analyse the data to find the best conditions for your passage.
\nConsider your yacht and crew\u2019s 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.
\nPredictWind has indicated limits for roll (4\u00b0 RMS), vertical acceleration (0.2g) and boat slamming (less than 50% but ideally none).
\nThe Departure Planning tool shows forecast sailing conditions depending on departure date and weather model
\nGiven there\u2019s rarely a \u2018perfect\u2019 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\u2019s 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.
\nIt\u2019s 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.
\nArticle continues below\u2026<\/p>\n
\t\t\t\t\t\t\tSqualls and gusts: How to predict and cope with them<\/p>\n
\t\t\t\t\t\t\tSqualls are the school bullies of the ocean. But, like all bullies, your best bet is to outsmart them \u2013\u2026<\/p>\n
\t\t\t\t\t\t\tWeather expert Chris Tibbs on how to predict and cope with fog at sea<\/p>\n
\t\t\t\t\t\t\tRadar and AIS help to make navigating in fog safer, but it is still an unnerving experience to sail in\u2026<\/p>\n
Keep monitoring
\nOnce 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.
\nAdditionally, utilise functions such as PredictWind\u2019s 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.
\nIt\u2019s 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.
\nReady to go
\nExperienced 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.
\nBy 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.<\/p>\n
If you enjoyed this\u2026.<\/p>\n
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The post How to find the right weather window appeared first on Yachting World.<\/p>\n","protected":false},"excerpt":{"rendered":"
Olympian, America\u2019s Cup winner, and founder of PredictWind, Jon Bilger, explains how to find the right weather window for cruising sailorsReefed down in the windy conditions. Photo: Richard Langdon\/Ocean Images How do you pick the right departure date? One of … <\/p>\n