The percentage often seen next to a snowflake icon on a weather app is officially known by meteorologists as the Probability of Precipitation, or PoP. When applied to winter weather, PoP represents the likelihood of snow falling within a specific time and area. Misunderstanding this single percentage can lead to poor planning, as it does not communicate everything a person needs to know about an impending winter event.
The Official Definition of the Percentage
The Probability of Precipitation is not a simple guess; it is a precisely calculated value derived from a formula. This percentage is the product of two distinct factors that forecasters weigh when developing their outlook. The formula is expressed as PoP equals C multiplied by A, where both C and A are represented as decimals or percentages.
The “C” factor stands for the forecaster’s confidence that measurable precipitation will occur somewhere within the designated forecast area. This area is typically a county or a metropolitan region, and the time frame is usually a 12-hour period. The “A” factor represents the percentage of the forecast area that is expected to receive measurable precipitation during that same time frame. Measurable snow is defined by the National Weather Service as an accumulation of 0.01 inches or more.
To illustrate, if a forecaster is 100% confident (C=1.0) that snow will fall, but they only expect it to cover 50% of the entire county (A=0.5), the resulting PoP is 50% (1.0 x 0.5 = 0.5). Conversely, if the forecaster is only 50% confident (C=0.5) that snow will occur, but believes that if it does, it will cover the entire area (A=1.0), the PoP remains 50% (0.5 x 1.0 = 0.5).
This calculation means a 50% chance of snow does not mean there is a 50% chance that it will snow at your house. Instead, it indicates a blend of forecaster certainty and the expected coverage across the entire region. The PoP is a statistical tool that allows meteorologists to communicate the likelihood of a weather event based on their models and experience.
What the Percentage Does Not Indicate
While the Probability of Precipitation is a valuable tool, it intentionally excludes several pieces of information a person might assume it contains. First, the percentage does not communicate the intensity of the snowfall. A 90% PoP could refer to a light, scattered flurry or a heavy, blinding blizzard, as this single number does not indicate how hard the snow is expected to fall per hour.
Furthermore, the percentage provides no information about the duration of the event. A 60% chance of snow could mean brief flurries lasting only an hour, or it could signify continuous snowfall stretching across an entire day. Forecasters communicate duration through the specific time ranges listed in the text forecast, not through the PoP value itself.
Most importantly, the percentage is completely separate from the expected snow accumulation. A high PoP of 80% means snow is very likely, but that snow might only amount to a trace, or less than 0.01 inches, which is the threshold for measurable precipitation. To understand how much snow will fall, one must look specifically at the accumulation forecast provided by the weather service, which is usually given in inches.
Interpreting the Forecast for Your Location
For someone planning their day, the PoP needs to be used in conjunction with other forecast elements to be truly useful. A high PoP, such as 80% or 90%, generally means that a winter event is almost certain to happen somewhere in the area. In this scenario, the “C” (confidence) factor is likely very high, and the “A” (area) factor suggests widespread coverage, making it highly probable that your specific location will be affected.
Conversely, a lower PoP, like 30% or 40%, requires a more nuanced interpretation, as it could result from different combinations of the C and A factors. A 40% PoP could mean forecasters are highly confident (C=0.8) that snow will occur, but only in a small, isolated 50% of the area (A=0.5, resulting in 40%). This suggests a risk of localized, heavy snow, but most people will see nothing.
The 40% PoP could also result from low confidence (C=0.4) that a major storm will hit, even if it would cover 100% of the region (A=1.0, resulting in 40%). This scenario suggests a low-probability, high-impact event that necessitates monitoring for updates.
When the PoP is high, the focus should shift immediately to the accumulation and intensity forecasts to determine the severity of the expected snow. When the PoP is low, the prudent action is to check the forecast again for changes, especially if the potential accumulation is high. Understanding the PoP as a blend of confidence and coverage allows for better decisions regarding travel and planning.