Precipitation Probability

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PRECIPITATION PROBABILITY The probability of precipitation forecast is one of the most least understood elements of the weather forecast. The probability of precipitation has the following features: ..... The likelihood of occurrence of precipitation is stated as a percentage ..... A measurable amount is defined as 0.01" (one hundredth of an inch) or more (usually produces enough runoff for puddles to form) ..... The measurement is of liquid precipitation or the water equivalent of frozen precipitation ..... The probability is for a specified time period (i.e., today, this afternoon, tonight, Thursday) ..... The probability forecast is for any given point in the forecast area To summarize, the probability of precipitation is simply a statistical probability of 0.01" inch of more of precipitation at a given area in the given forecast area in the time period specified. Using a 40% probability of rain as an example, it does not mean (1) that 40% of the area will be covered by precipitation at given time in the given forecast area or (2) that you will be seeing precipitation 40% of the time in the given forecast area for the given forecast time period. Let's look at an example of what the probability does mean. If a forecast for a given county says that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point in the county from noon to 6 p.m. local time. This point probability of precipitation is predetermined and arrived at by the forecaster by multiplying two factors: Forecaster certainty that precipitation will form or move into the area X Areal coverage of precipitation that is expected (and then moving the decimal point two places to the left) Using this, here are two examples giving the same statistical result: (1) If the forecaster was 80% certain that rain would develop but only expected to cover 50% of the forecast area, then the forecast would read "a 40% chance of rain" for any given location. (2) If the forecaster expected a widespread area of precipitation with 100% coverage to approach, but he/she was only 40% certain that it would reach the forecast area, this would, as well, result in a "40% chance of rain" at any given location in the forecast area. .

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