Weather

Here's why it's difficult to know if it's going to snow or not

All of the meteorologists at WRAL want to let you know when snow is in the forecast as soon as possible. But this is tricky.

Posted Updated

By
Elizabeth Gardner
, WRAL meteorologist
RALEIGH, N.C. — Is it going to SNOW?
You hear that question over and over all winter across our area. Snow is a high-impact event for us because we don’t see it often. It closes schools and keeps us out of work. All of the meteorologists at WRAL want to let you know when snow is in the forecast as soon as possible. But this is tricky.

During our winter season, our computer models often show the potential for snow seven to 10 days in the future. And often by the time we get to three to four days out, the models have dropped snow from their forecasts. We want to let you know how to plan for snow, but we don’t want to cry wolf.

Why do the computer models spin snow up so often? To understand this, let me explain a little about how they work.

Weather observations are collected across the globe as often as every 20 minutes. That data is fed into a supercomputer that puts it through a physics equation that describes the atmosphere. So the computer sees what the weather is doing right now and, based on our understanding of the atmosphere, shows us what will happen in the future.

There are some issues with this.

  1. Amount and quality of observations are inconsistent. In some parts of the world, observing sites are sparse, and there is a lot going on in the atmosphere that we can’t sample.
  2. Models make certain assumptions about the atmosphere. More often than not these assumptions are true. But some patterns are more difficult to predict than others.
  3. Computer models are run on a grid. A model that runs seven to 10 days in the future is run on a fairly large grid due to the amount of data that must be processed. There may be small weather features that the large grid misses. Over time that error may grow – meaning the forecast gets increasingly inaccurate.

For these reasons it still takes the human brain of a meteorologist to analyze our computer models to give you the most accurate forecast. So when you see a snowflake on a forecast seven days out, it’s time to take note and keep checking with us for updates.

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Three to five days in advance, we will show you the different scenarios that the approaching system may bring. Two to three days in advance, we will talk about how much snow may fall where you are. We will continue to update that all the way through the storm with continuing coverage.

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