Radar works by particles in the air reflecting the beam back to the source and is a ‘line-of-sight’ technology. If there are mountains between the radar source and the observer, the radar beam has to be tilted upwards at an angle to ‘see’ the observer’s weather. Any weather occurring below this tilted beam will be missed.
This is less important in the summer months because the clouds and storms tend to be taller. In the winter, the satellite infra-red view is far more instructive.