50% Chance of Anything

Posted: November 30, 2009 in ALL TOPICS, WEATHER WORDS

Chance,  Possible, and Probability:   Will it rain or Not!?!?

What does it mean if a forecast calls for a 50% chance of rain?   I have always maintained this is a confusing concept and it’s the main reason that I will rarely if ever use a percent chance in a forecast.  Instead I will use terms like slight chance, showers possible, or rain likely to give the viewer better guidance on what to expect.   However, when 50% chance is used it is not describing a flip of the coin probability.  Instead,  it means there is a 50% chance that a given location within a forecast area will receive measurable rain. Technically, this “probability of precipitation” is defined as the likelihood (expressed as a percent) of a measurable amount (at least 0.01 inch) of liquid precipitation during a specified time period at any given point in the forecast area.   

Amy Freeze in Norman, OK

Amy Freeze in Norman, OK by the Dual Pole Research Radar

A 20% chance of showers?  This means that of the last one hundred times that these conditions existed in this area, it rained twenty times.  A 20% chance that part of the area will receive rain.  According to The National Weather Service, Probability of Precipitation, or POP, is defined as the likelihood of occurrence (expressed as a percent) of a precipitation event at any given point in the forecast area.  The equation used to arrive at POP contains a value for “Percent of Areal Coverage,” so the result of the equation produces a number that also expresses the percent of areal coverage.

Here’s a bit more about how % chances calculated:    Computer models also forecast the probability of precipitation (also known as “POP”) for 6 and 12-hour periods, which some forecasters will blindly pass along to the viewer without explanation and can be very confusing to viewers.  While the math behind the POP is complex, the computer model takes into account the amount of moisture at different levels of the atmosphere, whether air is expected to be rising or sinking, and other meteorological factors. 

The National Weather Service says, “The probability of precipitation is the likelihood of measurable precipitation (0.01 inches or greater) for a specified forecast time period, and occurring at any point for which the forecast is valid.  The probability gives the odds of any one place in the area covered by the forecast getting wet, whether it’s from rain or snow.  The 0.01 inches or greater comes from the fact that any less rain or water from melted snow or ice can’t be measured. If the bottom of the rain gauge is wet, but the water isn’t deep enough to measure, that’s called a “trace’ and really doesn’t count.”

When the NWS forecasters assign a precipitation probability, it  shows their confidence in the forecast, how much of the forecast area is likely to have precipitation, and low long the precipitation is expected to last.  The important point is that the odds are for the rain or snow to fall on any place in the area covered by the forecast.  This means, that the probability could be low as 30% but you have hard rain for a few hours. In this case, the forecast isn’t wrong, you’re just one of the unlucky people who happened to be in the small part of the region that got wet that day. Or, if your lawn and garden needed the rain, you were one of the lucky ones. And, your friend a couple of miles away who needs the rain can’t complain about the forecast not working out. Rain did fall on at least one place in the area. 

Regardless of its accuracy, a weather forecast fails if the user does not understand the forecaster’s words!!  The proper interpretation of a 30 percent chance of rain (assuming the forecast verifies perfectly) is that you will have rain on your head three out of ten times that you hear such a forecast.  A couple of scenarios could play out:   the weather forecaster may believe rain will cover 100 percent of the area if the rain arrives, but his/her confidence that it will arrive is only 30 percent. Alternatively, the forecaster might have great confidence that rain will occur, but he/she believes it will be scattered showers affecting only 30 percent of the area. Regardless of the forecaster’s rationale, the meaning for you is always the same: The chance of rain on your head is 30 percent.
Chief Meteorologist Amy Freeze

The Percentage of Probabiliy (POP) can be described in words by forecasters by following these guidelines, if you can see the percentages the definitions might also clear the air:

    10% probability: Slight chance, isolated or none.
    20% probability: Slight chance, isolated
    30-50% probability: Chance,scattered
    60-70% probability:Likely, numerous

If the probability is 80% or above the forecast will be categorical, such as “Rain this afternoon.”

Amy Freeze at Soldier Field

Amy Freeze at Soldier Field Forecasting for the Chicago Bears

Chief Meteorologist Amy Freeze
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