A Geographic Information System was used to study the characteristics and distribution of precipitation in southwest of Saudi Arabia based on a number of influencing factors including elevation, mountain slope and orientation, distance from moisture source and equator, wind direction, and geographic location. Results indicate that elevation explains about 25% of variability in precipitation in most of the seasons. Other factors such as aspect, slope, distance from moisture source have different effects but less than the effect of elevation. Finally a polynomial trend surface has been generated to correlate geographic location with precipitation and has shown a coefficient of determination of 70%.