Here's why your phone app may say rain when WRAL Weather forecasts sun

A common expression you'll hear from friends is "my phone shows rain for next Thursday," but weather enthusiasts know you can't put too much stock in that.

Your phone's default weather app is not always the best way to get an idea of what you'll see in the coming days or even in the hours ahead.

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Most phone forecasts are never looked at by a person. Sure, they may be in the ballpark sometimes, but most of the apps out there are just using a single computer model with no forecasting element going into it. Your phone could show snowflakes at 6 a.m. and a sun graphic by noon.

When you're entrusting your forecast to one model with its own implicit biases and tendencies, that's where things get murky. One of the models, the GFS, which stands for Global Forecast System, is particularly known for predicting "fantasy snowstorms."

"The GFS loves to show a fantasy snowstorm five days to 10 days out," WRAL meteorologist Elizabeth Gardner said. "We look at that and we all get a good laugh. I can't tell you how many fantasy snowstorms it's spit out for us this winter."

And we all know how much snow we've seen this year.

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That's why it's best to trust a meteorologist. WRAL meteorologists can interpret the data, and have an understanding of the local topography and weather patterns to ensure the best accuracy. WRAL meteorologists like Gardner and Mike Maze have been AMS certified and lived in the Triangle for decades.

"Modeling is done by taking the information that we can collect about the weather right now, putting it through a physics equation that describes the atmosphere, then it spits out a lot of different parameters," Gardner said.

WRAL uses an in-house model along with other models that are public domain such as the European (European Center for Medium-Range Weather Forecasts), NAM (North American Mesoscale Forecast System) and the GFS. Meteorologists generally use a mixture of all the models.

Gardner used a recent example where the GFS showed snow coming to North Carolina several days out. It never got a mention on-air because it was an outlier compared to the other models and because it was days away with too many variables.

Here’s an example: A GFS run for the weekend of March 18-19. A run on March 10 shows no snow.

Prior to that, the March 9 morning run had snow in North Carolina.

"The closer we are to today, the more accurate the forecast is going to be. When you push it out to seven days, there are a lot of errors that could have been in the data," Gardner said. "They're not as apparent in the first 24 to 48 hours but those errors will grow the further out in time you get. That's why we look in the seven-day forecast and we tell people 'keep checking back.'"

The bottom line is when a meteorologist puts together a forecast, they're gathering as many models as possible and taking all variables into consideration.

"If we see the GFS showing a snowstorm five days from now, we're not going with that as our forecast because we know in most cases, we get a little bit closer to it, the GFS is going to drop that," Gardner said.