When it comes to showers and storms, we use probability of precipitation to develop the rain or snow "chances" (ex: 20%, 50%). However, the terms isolated, scattered and numerous are based off of those chances.
The term "isolated" refers to showers that are few and far between. WDBJ7 Meteorologists see isolated between 10% to 20% coverage.
"Scattered" refers to the range of 30% to 60% coverage. So, even with "scattered" showers, half or less of the neighborhoods are expected to "get wet."
When it comes to 60% and above, the terms most frequently used are "likely" and "numerous." At 90% to 100% coverage, we often simply say something like widespread or just rain or snow..