Why are winter storm forecasts all over the place?

Snow? Ice? How to Make Sense of Winter Storm Forecasts

By Andrea Thompson edited by Claire Cameron

A major winter storm is bearing down on the United States, with dozens of states and more than 160 million people likely to be affected by snow, ice and extreme cold from Friday through next Monday.

So far, forecasters have an idea of ​​what will happen when the storm arrives: Some areas are expected to see a foot of snow or more, while others will experience freezing rain that covers everything it touches in a dangerous coat of solid ice. And some areas could see more modest snowfall or perhaps just a cold, soggy day.

But forecasts about where the weather will hit have changed from day to day and even hour to hour. What gives?

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In short, weather forecasts are incredibly difficult to get 100% in advance, especially in winter weather. The reasons for this are complex. But knowing what to look for in the forecast as winter storms like this weekend approach can help you be better prepared.

What caused this weekend’s winter storm?

First things first: why is so much of the United States facing such a powerful winter storm? Essentially, the cause is arctic air rising south over the contiguous United States, just as an area of ​​low pressure moves east, drawing moisture from the south. When wet and cold meet, the result is a combination of snow, sleet and freezing rain.

But those are just the basics. Several features of the atmosphere are essential to understanding this particular storm. A key question for forecasters is whether a blob of Arctic air heading south encounters the area of ​​low pressure moving east, meeting in such a way that they “phase,” essentially combining into a larger, stronger storm. If this happens, it will strengthen an area of ​​high pressure that extends off the coast of the southeastern United States, which will push warm southern air further north and also shift snowfall northward.

To further complicate matters, there is an area of ​​high pressure located above Alaska that influences how everything downstream moves.

Long story short, there are a lot of moving parts coming together to create this storm. And “the more complex the flow at the intermediate and upper levels. [in the atmosphere] In other words, the harder it is for models to predict it,” says Alan Gerard, meteorologist and CEO of Balanced Weather.

What do weather models show?

If you’ve been paying attention to the forecast ahead of the weekend, you may have noticed that different forecasts rely on different models, such as those used by the U.S. National Weather Service and the European Center for Medium-Range Weather Forecasts, respectively. These models are tools used to predict the weather based on various measurements of the atmosphere, temperature and other factors, and they can have very different results. However, generally speaking, the European model outperforms the others in terms of predictive accuracy.

Weather models are the product of human ingenuity and decades of scientific research, but they are not perfect. They’re not supposed to be. To create these models, scientists must approximate – essentially, make a good estimate – of much of the physics of the atmosphere. Different weather services make distinct choices about their model priorities based on the types of weather they typically encounter: the US, for example, cares more about modeling tornadoes, but the UK Met Office doesn’t really need them because they are so much rarer there.

Data is also important for these models: the more detailed and precise the data, the better the predictions will be. A key source of information is twice-daily balloon launches from most U.S. weather offices scattered across the country. These provide a timely “snapshot” of the atmosphere. But some weather offices in Alaska have been unable to conduct regular launches recently due to staff shortages and equipment problems. This could affect the quality of data going into the models, which is particularly important for this storm due to the high pressure area over Alaska.

Still, it’s helpful to have a range of models with their own strengths and weaknesses, because they give a more complete picture of plausible scenarios, says Gérard. “The sum of all the parts gives the best answer,” he says.

Indeed, the different national models converge as the storm approaches – in this case, they all start to show what the European model has predicted all along: that the phases that worry meteorologists will indeed occur.

Either way, this storm is going to be complicated

As models begin to align with each other and meteorologists get a clearer idea of ​​the storm’s track and timing — and who will receive snow or ice — there will nevertheless be small quirks of the storm that could derail even the best forecasts.

Heavy rain and snow often arrive in what are called “bands.” It’s not easy to predict exactly where these bands will form, even on the day of a storm. And where the bands form can mean the difference between, say, 10 inches of snow in one place and only a few inches, or none, a few miles away. And storms are, ultimately, very dynamic environments. Tiny changes in temperature or airflow in the upper atmosphere can have serious consequences on the ground.

So if you’re one of the tens of millions of people in the United States wondering what to expect this weekend, here are some general things to keep in mind. Now that we’re within a day or two of the storm’s expected start, forecasts are becoming more accurate, but don’t be surprised if things change by next Sunday. Instead, keep an eye on them and prepare for risks.

As Gérard points out, meteorologists can’t tell you much in advance. “We predict the future. Have any of you tried to predict the future recently?” said Gerard. “I find it amazing that we are able to do as well as we can.”

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