Weather models have limited resolution (for example GFS data on windguru have about 50 km resolution) so the model itself can't predict wind conditions on exact spot, the spot forecasts you see here are interpolated from surrouding model grid datapoints. You see a forecast "in that area" but real wind conditions on your specific spot can be determined by local effects. It can be thermal winds which only appear on small area or another very important feature: local topography.
Very high resolution models can predict local conditions better because they see more detail, but unfortunately it is impossible to cover all world with very high resolution models, there is not enough computer power yet... But never mind, if we know how different weather works on our spot we can at least guess how it will influence local conditions.
The wind modifications are here to help you with local topography effects.
The terrain around your spot can have great impact, light 10 knot forecast can mean great 25 knots if it blows from the right direction. The wind is boosted if it has to squeeze through a narrow gap between mountains or if it blows around a cape or any other significant obstacle. These effects work perfect on many famous spots.
For example in Tarifa where east Levante wind has to squeeze through the narrow Strait of Gibraltar and accelerates, similiar happens between Corsica and Sardinia, on Karpathos in Greece, Sotavento on Fuerteventura and many many places. The opposite can happen too, some directions can work bad on your spot making real wind worse than forecast.
The idea of the "modified wind" is to estimate the difference between model forecast and real wind based on predicted wind direction.
It's easy, if you know how the model works for your spot you can add or substract some % for different directions. Of course it's only very simple method but better than nothing :)
Some spots already have predefined modifications by windguru but any registered user can set his own. You can do it for any spot and of course you can define different modification for every model (high resolution models like MM5 9 km will not need a lot of adjustements but coarse GFS will need more).
Example modification for Tarifa GFS forecast:
You can share your wind modifications with other users, if you think you know the right modification for some spot, do not hesitate, become a guru :) and make it public!
If you are a real expert for some spots and think your modifications should become windguru official, mail me to firstname.lastname@example.org
If there are some available you can see modifications by clicking the [options] in the top right corner of the forecast table:
1... click [options] to open the wind modification select window
2... click [apply] to display the selected modified wind in the forecast
3... click [view] to show the info about this modification
4... (registered user only) click [set as default] to save this modification as your default for this forecast
5... close the select window
(please note that this will only work in modern browser, it was tested on Internet Explorer 6, Firefox and Opera.)
You can edit your modifications in the settings area.