Ice? snow? How to understand winter storm forecasts
A major winter storm this weekend will bring brutal cold and chill to millions, but why is it so hard to decide who will get the snow, ice or rain?
Manhattan during a blizzard.
Bill Hornstein/Getty Images
A major winter storm is coming to the US, with dozens of states and more than 160 million people expected to be affected by snow, ice and extreme cold from Friday through next Monday.
Forecasters so far have some idea of what will happen when the storm hits: Some areas will see a foot or more of snow, while others will experience freezing rain that will blanket everything it touches in a dangerous blanket of solid ice. And different areas may see more light snowfall or perhaps just a cool and muggy day.
But predicting which places will be affected by which weather is difficult due to day-to-day and even hour-to-hour variations. What did you give?
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In short, weather forecasting is incredibly difficult to get 100 percent ahead of time, especially for the winter season. The reasons are complex. But knowing what to look for in the forecasts ahead of winter storms like this weekend can help you better prepare.
What’s causing this weekend’s winter storm?
Let’s start at the beginning: Why is so much of the US hit by such a powerful winter storm? Essentially, it’s caused by Arctic air moving southward over the contiguous US, just as a low pressure area is moving eastward, drawing moisture from the south. When moisture and cold temperatures meet, the result is some combination of snow, sleet and freezing rain.
Minimal wind chills expected across US
But that’s just the basics. Several characteristics of the atmosphere are important to understanding this particular storm. A key question for forecasters is whether a gust of southward-moving Arctic air encounters an eastward-moving low pressure area, meeting in such a way that they “phase” — essentially combine into a larger, stronger storm. If this happens, it will strengthen the high pressure area hovering off the coast of the southeastern US, which will push warm air from the south northward and also shift snowfall northward.
Making matters more complicated is an area of high pressure that is located over Alaska and is affecting how everything is going on downwind of it.
To make a long story short, there are a lot of moving parts coming together to create this storm. “The more complex the mid- and upper-level flow[in the atmosphere]is, the more difficult it is for models to predict it,” says meteorologist and CEO Alan Gerard. balanced weather.
What do weather models show?
If you’ve been paying attention to the forecasts ahead of the weekend, you may have noticed that different forecasts depend on different models, such as those used by the US National Weather Service and the European Center for Medium-Range Weather Forecasts, respectively. These models are tools employed to predict weather based on various measurements of the atmosphere, temperature, and other factors – and their results can vary greatly. However, broadly speaking, the European model outperforms the rest in terms of forecast accuracy.
Weather models are the result of human ingenuity and decades of scientific research, but they are not perfect. They are not meant to be. To create these models, scientists have to guess at a lot of the physics of the atmosphere – essentially making a good guess. Different weather services make different choices about what will be prioritized depending on what kind of weather their models typically deal with – for example, the US cares more about modeling for tornadoes, but the UK Met Office doesn’t really need it as they are so rare in that country.
Areas colored red and orange are more likely to receive more than six inches of snowfall.
Data are also important for these models: the more detailed and accurate the data, the better the forecast. A major information source is twice-daily balloon launches from most US weather offices spread across the country. These provide a “snapshot” of the environment in time. But some Alaska weather offices haven’t been able to conduct regular launches recently due to staffing shortages and equipment problems. This could affect the quality of data going into the models – data that is especially important for this storm because of that high pressure area over Alaska.
Still, it’s useful to have a range of models with their strengths and weaknesses because they give a more complete picture of plausible scenarios, says Gerard. “The sum of all the parts gives you the best answer,” he says.
Indeed, as the storm approaches, different national models are coming together – in this case, they are all starting to show what the European models have largely predicted: the phase that meteorologists are worried about will actually happen.
No matter what, this storm is going to mess up
As models begin to agree with each other and meteorologists have a firm idea of what a storm’s path and timing will be – and who will get snow or ice – there will still be small quirks of the storm that can throw off even the best forecasts.
Heavy rain and snowfall often form in what are called “bands”. Exactly where those bands will form is not easy to predict, even on the day of the storm. And there can be differences in where the bands form, for example, 10 inches of snow in one location and only a few inches or even nothing a few miles away. And hurricanes are, ultimately, highly dynamic environments. Small changes in temperature or airflow in the upper atmosphere can result in large consequences on the ground.
Areas colored red and orange have a higher chance of freezing rain of at least a quarter inch.
So if you’re one of the millions of people in America wondering what’s in store for them this weekend, here are some general things to keep in mind. Now that we’re less than a day or two away from the storm’s anticipated start, forecasts are becoming more accurate, but don’t be surprised if things change between now and next Sunday. Instead keep an eye on them and be prepared for the risks.
As Gerard explains, there’s only so much that meteorologists can tell you ahead of time. “We are predicting the future. Have any of you tried to predict the future lately?” Gerard says. “I find it amazing that we’ve been able to do as well as we can.”
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