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Sunday, June 18, 2017

The Confusing Chance of Rain Percentages

     So exactly what do those percentages mean? Does that 50 percent chance of rain Monday mean maybe it will rain or maybe it won't, or does it mean something else? 
     When they say there’s a 50 percent chance of rain some people think that’s means 50 percent of the area will see rain. Some people think it means it will rain half of the time. Some people think it’s the odds of seeing rain. Actually, the last option is closer to the truth, but not quite. 
     The correct terminology is Probability Of Precipitation or POPS to the weather forecasters. POPS is a mathematical calculation which is:  the forecast area of coverage by the rain multiplied by the confidence in the forecast. Thus: Chance of rain = coverage x confidence
     So, if the rain is expected to cover half the area and the forecasters are 100 percent sure there will be MEASURABLE rain, the formula is: 
50 percent (Coverage) x 100 percent (Confidence) (0.50 x 1.00 = 0.50) or 50 percent. 

Example 2: the predicted coverage is 50 percent, but the forecasters are only 50 percent confident that is correct. The formula is therefore: 50 percent (Coverage) x 50 percent (Confidence) (0.50 x 0.50 = 0.25) or 25 percent. 
Example 3: the predicted coverage is 100 percent (there will be a widespread rain) but the forecasters are 25 percent confident. The formula is: 100 percent (Coverage) x 25 percent (Confidence) (1.00 x 0.25 = 0.25) or 25 percent. 

     Note that measurable rain is 0.01” or more, so the AMOUNT or rain is not a factor! That 50 percent could be 0.01 inch or it could be 1.5 inches! 
     As used on television, when the forecaster says there will be a 25 percent chance of rain, they mean for any given point on the map. That means if you don't leave your backyard there is a 25 percent chance you'll get rained on. But, it you travel from home to work and back your chance of getting rained on increases as you move around. For example, for every, say, 5 miles you travel, the odds of getting wet go up. 
     As mentioned, in the equation the Chance percentage has nothing to do with the amount of rain and nothing is factored into the chance of rain for how fast it falls or for how long. That means if the chance of rain is only 10 percent, you could experience enough rain in a short time to get a flash flood. On the other hand, a 100 percent chance of rain could mean a light drizzle for a couple of hours.

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