Three common mistakes in weather forecasts for yachting and what to do about it
When making a forecast, a weather model takes into account a myriad of facts. Modern models have about 150 only external parameters, i.e. parameters that models can accept or give away. There are also internal model state parameters.
In other words, a weather model is a very complex system that is quite difficult to solve. Hence, they have some peculiarities that need to be taken into account when reading forecasts.
In this post, I will tell about three of the most common mistakes in the weather forecasts for yachting concerning the most important parameter for this kind of outdoor activity — the wind, using Windy.app as an example.
Ilya Drigo, professional meteorologist, developer, and researcher
Wind gusts
What it is: wind gusts are short acceleration of wind speed up to 20–30 seconds. A wind gust can occur when there’s a barrier on the wind’s way, such as an island or a rock in a sea and a building on the land.
The wind “surrounds” the object, the traction increases, and wind speed decreases. After moving past the object it increases sharply.
Wind gusts are stronger in the city than in an open area given the wind speed is the same.
Why weather models get it wrong: Models often slightly underestimate wind gusts or may not show them at all. This is because models generally tend to average the weather forecast, and wind gusts, among others, are very often flattened.
What to do: Be careful when forecasting wind gusts and see how much this or that weather model reproduces the wind accurately. Switch between models more often to see which one is more accurate.
Weak wind
What it is: wind speed or wind velocity is one of three major characteristics of air movement on par with wind direction and wind gusts.
In general, the high wind speed (for a walking person) is considered to be from 25 to 31 mph (11 to 13.8 m/s). These wind conditions are also called “Strong breeze” in the Beaufort Wind Force Scale.
Accordingly, wind with lower speed is considered not strong or not fast, up to very light wind of 1 to 3 mph (0.4 to 1.3 ms/) and its complete absence (calm).
Why weather models get it wrong: This is due to the calculation scheme or the internal mathematics that is embedded in the models. According to different estimates for doing various wind sports and outdoor activities such as yachting, winds up to 3–5 m/s are considered weak. When you see such winds, be careful.
What to do: Configure the Weather Map view in the Windy.app that is comfortable for you using Extra Settings. For example, there are wind barbs — special symbols for wind display on the maps that almost all yachtsmen can understand. You can also include isobars and weather fronts on the map. The latter is updated every 12 hours with each model update, isobars more often.
Local winds
What is it: local winds are winds that occur and are regularly observed in a certain place due to certain local characteristics (usually the terrain).
Why weather models get it wrong: models do not fully take into account terrain features, and many local weather phenomena depend on the terrain. The point is that the phenomenon must be 10 times smaller than the grid size of the weather model (or its resolution).
It is clear that local winds are of the order of tens of kilometers (10 km is 6.2 miles), but models need to solve equations for phenomena of the order of kilometers (1 km is 0.6 mi) grid size. It used to be thought that models did not show local phenomena, but this is not entirely true.
Now there is active work in this direction and modern high-resolution models such as AROME can already show local phenomena such as katabatic winds — dense and cold air currents down slopes, as well as mistrals, breezes, and other local winds. Still, it remains a very challenging place for models.
What to do: You can download an offline weather forecast. Understandably, it won’t be updated, but if you’re going yachting, it is better to have such a forecast than nothing in case of no internet or other connection.
Text: Ilya Drigo, pro meteorologist, developer, and researcher with a Bachelor’s degree in Oceanography, Meteorology and Hydrology from the Lomonosov Moscow State University (MSU), Windy.app’s ex-chief meteorologist. His favorite sports are hiking and sailing. Ivan Kuznetsov and other Windy.app authors contributed to this article
Cover photo: Jeremy Bishop / Unsplash
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