A wind forecast comes from the analysis of three things: gradient of air pressure, the Coriolis effect and friction. The Coriolis effect is a result of Earth's rotation that tends to deflect wind to the left and is fairly easy to calculate. The pressure gradient is the change in air pressure over distance and time and is also fairly easy to model mathematically. The tricky part is friction.
Depending on how well it lines up, wind aloft can increase surface wind by dragging it along. However, there can be a great deal of variety in the efficiency of that upper wind energy being translated down to the surface. Columns of warm, rising air called thermals affect how the wind mixes together aloft and at the ground. Localized, small-scale effects such as precipitation, clouds, humidity and air temperature can impact the speed and direction of the wind in ways that are sometimes difficult to forecast.