Mark Fischenich: Ask Us: Chance of rain mainly explained

Mark Fischenich, The Free Press, Mankato, Minn.
·4 min read

Sep. 25—Q: Hello Ask Us Guy,

My question is about every Minnesotan's favorite topic: weather! The National Weather Service and other forecasters use percentages for an area's chance of precipitation. I'm wondering what a 70% chance of rain, for example, actually means. Is there a 70% chance of rain falling somewhere within city limits? Is there a 70% chance of it raining at a singular site used as a barometer for the city? For all of our planning purposes, it'd be helpful to understand how forecasters arrive at their percentages and what the percentages should tell us.

Thank you,

A: Over the years, Ask Us Guy has wondered about the meaning of those percentages, too. And beyond the ones mentioned by the reader, there's at least a couple of other ways the number could be interpreted. A person could assume that rain will fall 70% of the day or that 70% of the forecast area will get wet.

And Ask Us Guy would throw in one other question. Sometimes the NWS will include rainfall amounts in the forecast for the following day or two, something like "Chance of precipitation is 80%. New rainfall amounts of one-half of an inch to one inch, except higher amounts possible in thunderstorms." Does that mean that there's an 80% chance of getting at least a half-inch of rain?

First, the "what" of those percentages — which the Weather Service meteorologists refer to as "Probability of Precipitation" or PoP for short and normal people typically translate to "chance of rain" or "chance of snow."

"The 'Probability of Precipitation' (PoP) simply describes the probability that the forecast grid-point in question will receive at least .01 inch of rain," according to the NWS.

So that covers the first part of the question. The percentage only refers to the odds of getting a hundredth of an inch of rain, which isn't much rain but enough to leave someone wet if they're out in it. For snow, it appears the minimum threshold is a tenth of an inch when predicting a chance of snow.

Now, onto the "where" of the question ... .

The NWS isn't just being pointlessly jargony when it mentions "the forecast grid-point in question." The Weather Service doesn't issue forecasts for cities or counties. It issues forecasts for 2.5-kilometer squares covering the entire United States.

That's not immediately apparent when looking up a forecast on the NWS website. Type in "St. Clair," which is less than one square mile in size, and you'll get a forecast. Type in "Mankato," which is just under 20 square miles, and you get a single forecast. Type in "Minneapolis," nearly 58 square miles, and you also get just a single forecast.

But if you look at the map on the right side of the forecast page, there's a little green 2.5-kilometer square showing the area that the forecast actually applies to. For St. Clair, the forecast covers the entire town from Horseshoe Lane on the northwest side to Hiawatha Drive on the southeast end.

For bigger cities, though, there are multiple forecasts. In Mankato, the one that pops up initially is for a 2.5-kilometer square that includes downtown, parts of adjacent neighborhoods and much of lower North Mankato.

But the Sibley Park neighborhood is in a different forecast grid-point. So is the Tourtellotte Park neighborhood. In all, more than 12 Weather Service forecasts are required to cover every part of Mankato.

So there could theoretically be a dozen unique forecasts for various parts of town, and there are differences.

For instance, downtown Mankato frequently is forecast to have warmer temperatures than other areas of town. It might not be enough to get hilltop residents snowbirding to a timeshare on South Front Street for the winter months, but the forecast low there was a balmy 46 degrees for Sunday night compared to a frigid 45 on Augusta Drive on the northeast side.

That's the deal with the precipitation chances, too. The NWS doesn't predict the likelihood of rain falling on Mankato. It predicts the likelihood of rain falling on a specific 2.5-kilometer section of Mankato.

As for the way the percentage is determined, it's based on a simple formula of confidence and coverage. If meteorologists believe with 40% confidence that various weather factors will produce precipitation during a forecast period and they believe that the rainy or snowy weather conditions will cover 80% of the forecast area, 40% times 80% is 32%. So there would be a 32% chance of precipitation.

Contact Ask Us at The Free Press, 418 S. Second St., Mankato, MN 56001. Call Mark Fischenich at 344-6321 or email your question to mfischenich@mankatofreepress.com; put Ask Us in the subject line.