"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.
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,
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