Q:

What does "spot shower" mean in weather forcasting?

I saw "spot shower" on a weather forecast here. It reads: 

Muggy with Spot Shower This Weekend

What does "spot shower" truly mean?

A:

"Spotted showers" is American English. In British English we call them "scattered showers". They are meteorological terms.
Scattered showers are rain showers expected only to affect 10-50% of the named area (eg Boston).
"Isolated showers" are those that affect around 5% or less.

A:

Meteorologists use a handful of set terms to describe weather conditions, such as partly cloudy or mostly sunny. 
When it comes to rain, two commonly-used terms are scattered showers and isolated showers. As one website explains: 

isolated and scattered showers do indeed have different meanings.
The term "isolated" refers to showers that are few and far between .. displaying between 10% to 20% coverage.  In other words, when the forecast calls for "isolated" showers, only 10% to 20% of the forecast area will receive measurable rainfall within the forecast period – most neighborhoods stay dry.
"Scattered" refers to the range of 30% to 50% coverage.  So, even with "scattered" showers, half or less of the neighborhoods are expected to "get wet." 

That same website goes on to say:  

I've taken to using the term "spotty" for rain chances under 20%, although the "purists" will say that the term has not been "approved."  But I think it provides a mental picture that DOES distinguish it as being even less likely than "isolated."

So, it looks like the term "spot showers" is being used by some weather forecasters to say that the chance of rain is low and will only affect a small percentage of the forecast area.

