Why the forecast fails
There are many reason why the forecast fail. If you learn them step by step, you will spend more time on a water and less time painfully waiting for a wind:
Convective clouds are these clouds, that are formed by sun shining on a ground. Maybe they are small in size but they are usually very tall and they contain a lot of energy.
I mean a lot.
Imagine that there is so much energy, that they can lift several tons of water and suck it us high as 12km above the ground, and create hundreds of 20.000Volts flashes.
When they are being formed they suck all the moisture from bellow, sometimes killing all the wind being forecasted.
And when such a cloud decides to turn all that energy into the rain or thunderstorm, they can create hurricane force winds and rain in the place, where sun and calm was forecasted. We are unable to forecast where and when these clouds will hit. The only thing we can forecast is probability, that in some area thunderstorms will be formed, and how severe they will be. On Windyty use CAPE layer that is indication of storms, that can be formed in this area. CAPE index higher than 1,500 can produce large thunderstorms, while 2,500 extreme once.
Mountains and difficult terrain
Earth is not flat. Mountains, rivers, water surfaces, all those are factors that influence computation of forecast model. And since forecast models have certain resolution we simplify all these obstructions so Rocky mountains, that look like this are simplified so computer see it like this. Therefore, and especially in mountain area, models with high resolution can provide much more better results. But event that, weather in some mountain area will be just unpredictable. But some terrain is good, increasing any wind that is forecasted. On Windyty zoom in to see satellite or topographic map, and make your own judgement how the terrain will affect forecast.
When there is low temperature at night and high temperature during the day, with some luck of terrain, the thermic wind can be formed. On some kiting places, this wind is even so regular, that they are occupied by many kiting schools. These thermic winds are not forecasted by computer models. Check you kiting guide, which tell you, at which spot thermic effect works, and how strong it is. But do not forget to check Windyty for weather at the spot, because clouds, even the thin layer of them, rain and low temperature will result, that thermic wind will not be formed. Even warm night can lead to bad thermic effect.
Forecast model fails
Yes this can happen a it happens a lot. On Windyty, we use more forecast models so if you want to be sure, check all of them and make your own decision. And even though all models we have can fail. More important than the computer program, that computes forecast model, are the data, that are put to run the model, because small error in measured data, can lead in big errors in computed results. This process of data gathering is called data assimilation and since ECMWF has most advanced data assimilation method in the world, we recommend checking ECMWF on Windyty.