Visibility of tornadoes is a function of 4 primary factors which are vegetation cover, rainfall rate in immediate vicinity of tornado, time of day, and air quality.
Large trees restrict the viewing of tornadoes. In the Southeast U.S. and other regions with large trees, tornadoes can not be seen beyond the canopy of the trees. Regions with small trees or few trees make tornado observation significantly better. Examples of regions with small or few trees include the high plains and the western Great Plains.
The type of supercell associated with a tornado is important in determining the visibility through rainfall. LP (low precipitation) supercells provide the greatest visibility due to the lack of rainfall in the immediate vicinity of the tornado. HP (high precipitation) supercells commonly have rain-wrapped tornadoes. Often an observer can not tell the difference between a rain-wrapped tornado and a heavy rain curtain until the tornado or rain curtain is right on top of them. Many motorists have been killed by rain-wrapped tornadoes. Classic supercells are in between LP and HP. Rainfall may wrap into the tornado in a Classic supercell situation, but not as much as in a HP situation. Classic supercells will have a hook echo on Doppler radar while HP supercells look more like a kidney bean.
The time of the day is critical for tornado observation (specifically day versus night). Obviously, nighttime tornadoes are more difficult to view than daytime tornadoes. There tend to be more daytime tornadoes in the high and Great Plains than in the Eastern and Southeastern U.S. This is often because storms and storm complexes form in the west and move east.
Preexisting visibility is the forth factor. Examples of preexisting visibility include smoke, haze, steam fog, and pollution.
Visibility of tornadoes is maximized when there is no vegetation, the supercell is an LP or Classic supercell, a daytime occurrence, and little preexisting visibility problems. Tornado observation is minimized by tall trees, HP supercells, nighttime tornadoes, and pre-existing obstructions to vision. The best place to view a tornado at a considerable distance is in the Great and High Plains, while the Southeast U.S. is the worst. I have lived in Oklahoma and Mississippi and without a doubt tornado chasing in Oklahoma gave a much better terrain and environment to chase tornadoes. Many of the tornadoes in Mississippi occur at night and are associated with HP supercells, not to mention the continuous forest of trees we have here.
More people are killed by low visibility tornadoes for several reasons, a few are: more difficult to spot in the field, rain hides the dangerous tornado, and darkness hides the tornado. HP supercells, night time supercells, and supercells in a forested environment are best monitored using the NWS Doppler radar (and other radars) along with what trained spotters can pick out under the adverse circumstances. Although the tornado is hidden or partially hidden from surface observers, the radar can pick out a circulation that may indicate a tornado (strong gate to gate shear, strong mesocyclone).