Chance of Rain Making Sense of Precipitation Forecasts

Chance of rain, aka the chance of precipitation and probability of precipitation (PoPs), tells you the likelihood (expressed as a percentage) that a location within your forecast area will see measurable precipitation (at least 0.01 inch) during a specified time period. Lets say tomorrows forecast says your city has a 30% chance of precipitation. This does not mean: Theres a 30% chance it will rain and a 70% chance it wontThree out of 10 times when the weather is similar, it will rain  Precipitation will fall 30% of the day (or night)Thirty percent of the forecast area will experience rain, snow, or storms Rather, the correct interpretation would be: there is a 30% chance that 0.01 inch (or more) of rain will fall somewhere (at any one or multiple locations) within the forecast area. PoP Adjectives Sometimes a forecast wont mention the percent chance of precipitation outright, but instead, will use descriptive words to suggest it. Whenever you see or hear them, heres how to know what percent that is: Forecast Terminology PoP Precipitation's Areal Coverage -- Less than 20% Drizzle, sprinkle (flurries) Slight chance 20% Isolated Chance 30-50% Scattered Likely 60-70% Numerous Notice that no descriptive words are listed for probabilities of precipitation of 80%, 90%, or 100% This is because when the chance of rain is this high, its basically a given that precipitation will occur. Instead, youll see words like periods of, occasional, or intermittent used, each conveying that precipitation is promised. You may also see the type of precipitation punctuated with a period;  rain, snow, showers, and thunderstorms. If we apply these expressions to our example of a 30% chance of rain, the forecast could read in any of the following ways: A 30% chance of showers A chance of showers Scattered showers. How Much Rain Will Accumulate Not only will your forecast tell you how likely your city is to see rain and how much of your city it will cover  itll also let you know the volume of rain that will fall. This intensity is indicated by the following terms: Terminology Rainfall Rate Very light 0.01 inch per hour Light 0.01 to 0.1 inch per hour Moderate 0.1 to 0.3 inches per hour Heavy 0.3 inches per hour How Long the Rain Will Last Most rain forecasts will specify a period of time when the rain can be expected (​after 1 p.m., before 10 p.m., etc.). If yours doesnt, pay attention to whether the chance of rain is advertised in your daytime or nighttime forecast. If its included in your daytime forecast (that is, This afternoon, Monday, etc.), look for it to occur sometime from 6 a.m. to 6 p.m. local time. If its included in your overnight forecast (Tonight, Monday Night, etc.), then expect it between 6 p.m. to 6 a.m. local time. DIY Chance of Rain Forecast Meteorologists arrive at precipitation forecasts by considering two things: How confident they are that precipitation will fall somewhere within the forecast area.How much of the area will get measurable (at least 0.01 inch) rain or snow. This relationship is expressed by the simple formula: Chance of rain Confidence x Areal coverage Where confidence and areal coverage are both percentages in decimal form (that is 60% 0.6). In the U.S. and Canada, the chance of precipitation values is always rounded to increments of 10%. The UKs Met Office rounds theirs to 5%.