At times, the precipitation will not reach the ground until the lower air becomes saturated through this addition of water vapor. Radar will overestimate precipitation in these cases. This is most common in the desert southwest, the interior west and in winter on the leading edge of winter storms, when the moisture moves over cold, dry arctic or polar air. Radar will also overestimate precipitation when hail, sleet or melting snow is occurring. This is because these precipitation types are very good reflectors of radar waves.
Radars have some difficulty in mountain regions.
Properly positioned the radar may show this enhancement. Often however, radars underestimate the rainfall and snowfall in the mountains. In other cases, the radar may show false high returns. This occurs when the radar s microwaves bounce off the mountains themselves and either appear to be precipitation, or enhance the return from real echoes. Precipitation will be overestimated at times due to this effect, sometimes very considerably.