Pictures from around western Colorado and eastern Utah
"A Chance of Showers and Thunderstorms":
What does that mean, exactly?
Throughout the year, weather forecasts are filled with terms like "20%", or "slight chance", or "scattered" when precipitation is on the horizon. Many days may go by with with these words stated, and yet, sometimes, many a rain gage remains dry. This is because shower and thunderstorm activity, especially in the arid West, tends to be a "hit and miss" phenomenon. You may receive no rainfall, while your neighbor down the road was drenched.
Photo of various rain gauges
Rain Gages used by the NWS
Thus, the concept of Probability of Precipitation (PoP) was created. Various terms are used to predict the likelihood of precipitation falling at a given site. Technically, the Probability of Precipitation (or "pop", as forecasters routinely call it) is defined as the likelihood of occurrence of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) falling during a specified period of time at any given point in the foreast area. Measurable precipitation is defined as equal to or greater than 0.01 inch (0.2 mm). Normally, the period of time is 12 hours, unless specified otherwise.
With only a 20% chance of showers, your location may be the only one hit, while the majority of the surrounding area remained dry. If precipitation forecasts call for "likely", it's possible that everyone around you but You will receive measurable rain. But, in essence, the larger the percentage, the greater amount of precipitation coverage is expected across a specific area, or "Forecast Zone" (and vice versa). (To see which Weather Forecast Zone you reside in, click here ).

At times, NWS forecasters will use "occasional" or "periods of" to describe a precipitation event that has a high probability of occurrence, though precipitation during that time may not be continuous.

Specific correlations between Probability of Precipitation and terminology is as follows:
PoP (Percent)Expression of Uncertainty Areal Coverage
     
10% Slight Chance Isolated or Few
20% Slight Chance Widely Scattered
30-50% Chance Scattered
60-70% Likely Numerous
80-100% None Used (Categorical) None Used (Categorical)

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