Scattered? Isolated? 40%? What does this mean?

Often times you'll hear meteorologists throw out the terms "A chance for an isolated thunderstorm;" "Scattered thunderstorms throughout the area;" "A 50% chance of rain."  But, have you ever wondered what exactly that means?  It's time to clear up any confusion some may have.

"Isolated" is a term meaning widely scattered throughout an area (in our case, the 22 counties covering eastern IA and western IL) over a period of time (generally less than 20%).  Scattered generally means 30% to 50% of the area has the chance of receiving precipitation.  Now, you may be asking about the percentage that you often see on our 7 Day Forecast:

Those percentages are the "POP" or "potential of precipitation."  This is basically a simple math problem.  It is the level of Confidence multiplied by the amount of area the rain will cover.  For example, a 30% chance of rain comes about with 60% confidence rain will occur over 50% of the area (0.60 * 0.50 = 0.30 = 30% ).

Some people may think that because there's a 30% chance of rain, it won't be heavy or there won't be severe weather.  That's not necessarily the case.  A great case is in an "isolated" case.  Especially out in the Plains it's very easy to have an isolated storm pop up and turn severe within 20 or 30 minutes.

Hopefully this was a good lesson for you!