Ask Tom: What does a 40 percent chance of snow (or rain) mean? (2024)

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Ask Tom: What does a 40 percent chance of snow (or rain) mean? ›

Probabilities of precipitation enable forecasters to convey certainty (or uncertainty) of the likelihood of precipitation. A 40 percent chance of snow means the forecaster believes you will experience snow 4 out of every 10 times you hear such a forecast (assuming the forecasts verify perfectly).

What does this "40 percent" mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area? The "Probability of Precipitation" (PoP) simply describes the probability that the forecast grid/point in question will receive at least 0.01" of rain.

In our second example in Schoharie County, even though there is just a 40% chance, that thunderstorm can still drop a hefty amount of rain in a short period of time. In another words, a 40% chance of rain is a signal an umbrella wouldn't be a bad idea.

If the chance of rain is less than 50%, there's a greater likelihood of it staying dry than not. For example, a 20% chance of rain means there's an 80% chance of dry weather. On the other hand, an 80% chance of rain means there's only a 20% chance of dry weather.

So when they say 20 percent chance of rain it means 1 out of every 5 times they make that prediction it isn't going to rain. 50% chance of snow means 50% of the time they make that prediction it will snow. Another factor in weather prediction is time out. You must use that in interpreting multiple day forecasts.

An 80 percent chance of rain (or of any other kind of precipitation) means the weather forecaster believes there will be an eight in ten chance (or 80 chances out of 100) of measurable precipitation (0.01 inch or more) in the area under consideration during the time interval that is specified in the weather forecast ( ...

Rain The percentage of rain refers to the probability of rain occurring at the location and time specified. For example, a forecast of 80% rain means that there is an 80% chance of rain. Snow The percentage of snow refers to the probability of snow occurring at the location and time specified.

Closer look at percentages

Let's use 60% as the rain chance in the forecast. That means 60% of the entire area (Central Florida) could see measurable rain at some point in the day, while the other 40% won't. That goes beyond the city that you live in.

When you see a "100% chance of rain," it doesn't mean it's going to pour everywhere, all the time. Think of it like this: the forecasters are super confident that rain will fall somewhere within a certain area, but they're not sure exactly where or for how long.

Like not about the % of area affected, but rather a % of times that it would rain in 12 hours. Like if there's a 40% chance to rain in DC, it means that if you were to run a simulation of that 12 hours 10 times, 4 of those times it'd be raining. Upvote 1 Downvote Share.

Does precipitation mean snow? ›

In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail.

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.

The percentage means How much of your current area will see rain. If you're in NYC and it's says 99% then all of NYC will have rain today. If it instead says 10% then only a small part of NYC will see rain. It's more a measure of how big the rain clouds are.

Probabilities of precipitation enable forecasters to convey certainty (or uncertainty) of the likelihood of precipitation. A 40 percent chance of snow means the forecaster believes you will experience snow 4 out of every 10 times you hear such a forecast (assuming the forecasts verify perfectly).

snow/sleet/graupel despite actual surface temperatures above freezing • With rain events in the summer, there's zero chance of any mixed precipitation, so forecasts are easier when compared to winter events (no forecast is ever easy!).

However, it can snow with ground temperatures in the upper 30s -- or, in rare cases, even well into the 40s -- if the warm layer is very thin, with temperatures quickly dropping below freezing with minimal elevation.

Meteorologists say since the freezing point of water is 32 degrees Fahrenheit, or 0 degrees Celsius, air must be warmer than that for it to melt. On a 40-degree day, an estimated inch of snow is lost. On a 50-degree day, up to 2-4 inches of snow can melt away.

Simply put, the snowfall rate is how much snow falls over a given amount of time. Meteorologists generally talk about snowfall rate as inches per hour.

New snow is composed of a high percentage of air trapped among the accumulated snow crystals. Since the air can barely move, heat transfer is minimal. Fresh, uncompacted snow typically contains 90 to 95 percent trapped air.