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 PRECIPITATION CHANCE

METEOROLOGIST JEFF HABY

A precipitation chance is a probability that precipitation will occur at a particular location in the forecast area. This chance ranges from 0% to 100%. A 0% chance indicates there are not adequate mechanisms in order to generate precipitation. The troposphere is too stable to generate precipitation. A 100% chance guarantees precipitation will occur. A 100% chance will be issued when it is already precipitating or mechanisms are in place to guarantee precipitation. The precipitation chance is often rounded to 10% increments. For example, you will see forecast values such as 40%, 60% and 80% chance and not chances such as 67% and 94%. The closer the chance is to 100%, then the more likely precipitation is. The closer the chance is to 0%, then the less likely precipitation is.

One aspect of precipitation chance is a time frame. Generally this time frame is divided into 12 hour time frames which are generally a daytime time frame and a nighttime time frame. Thus, when assessing precipitation chance it is important to note the day of the week and time frame the forecast is being made for.

Another aspect of precipitation chance is the location. Generally the location is for a specific location such as at a measuring station at an airport. If a person lives close to the measuring station then the precipitation chance forecast is fairly reliable for that person. However, if a person lives a significant distance from the measuring station, there can be synoptic and mesoscale factors that can make the precipitation forecast different for the location that is not close to the measuring station. Examples of influencing factors include elevation changes, proximity to a body of water, closer or further to an uplift mechanism, and land surface changes. Various factors can either increase or decrease the precipitation chance for a person that does not live relatively close to the measuring station.

There are several misconceptions about precipitation chance. Below are a few examples. The first two are the most common misconceptions.

Misconception 1: Precipitation chance is the percent of the forecast area that will get precipitation on that forecast day (i.e. 40% means 40% of land area in forecast area will get precipitation today)

Misconception 2: Precipitation chance is the percent of the time frame in which precipitation will occur (i.e. 20% means it will be precipitating 20% of the time during the day)

Misconception 3: Precipitation chance is the probability the forecast will be right (i.e. 80% means there is an 80% confidence the forecast will be right)

Misconception 4: Precipitation chance is the percent of weather forecasters and computer forecast models that think it will precipitate (i.e. 60% means 60% of forecasters and weather models think precipitation will occur)

Precipitation probability is the probability of any particular point location within a forecast area receiving measurable precipitation in a given time period. This forecast area needs to be relatively small since as mentioned synoptic and mesoscale conditions can vary with distance from a particular official measuring station. Probability of precipitation is determined by two parameters: The probability that any precipitation will occur in forecast area AND the predicted areal coverage of precipitation if precipitation does occur. Thus, areal coverage is just one aspect of precipitation probability. The chance that any measurable precipitation will occur in the first place within the forecast area must also be considered. When referring to probability of precipitation it is most accurate to say, "There is a ___% probability of precipitation that any particular measurement station in the viewing area will get precipitation". For example, when averaged over many 30% probability of precipitation days, a particular station should have precipitation 30% of the time if the forecaster or computer model is accurate.

Suppose during the course of a year that a Dallas measurement station has a 30% probability of precipitation on 100 individual days. If the probability of precipitation prediction is fairly accurate over the long term, it should have precipitated at the Dallas station on about 30 of those days. Thus, think of probability of precipitation in terms of a long term average prediction. Often, people assume a 20% chance of rain means 20% of forecast area will get rain on that one day. This is often not the case for any one day. Often no precipitation occurs at all in the forecast area on days with a 20% probability of precipitation since one component of probability of precipitation is the chance that any precipitation will develop.