Blizzard conditions along a highway. South Dakota Highway Patrol photo, 2020.
National Weather Service meteorologists from the Northern Indiana office and elsewhere have used the “B word” when writing about the winter storm that’s forecast for Indiana later this week.
For example, a winter storm watch that the Northern Indiana office issued today includes the phrase, “blizzard conditions possible.”
When I was a kid, depictions of blizzards in movies and TV dramas led me to think of a blizzard as a very bad, very dangerous storm that drops huge amounts of snow.
It wasn’t until much later that I learned that the meteorological definition of “blizzard” has nothing to do with accumulation!
What really defines a blizzard is how strong the wind is and how far you can see through the snow and blowing snow. The National Weather Service website defines “blizzard” as “blowing and/or falling snow with winds of at least 35 mph, reducing visibilities to a quarter of a mile or less for at least three hours.”
This means that if the wind is strong enough and the forward visibility is low enough, you can have blizzard conditions without large accumulations!
Travel in northern Indiana could be extremely dangerous Friday, no matter how much snow falls, because strong wind could cause white-out conditions. Making the situation even more dangerous will be extreme cold, which would be especially bad for anyone who slides off a road and gets stranded in a ditch.
It’s wise, therefore, not to focus too much on accumulation forecasts with this storm. The wind, visibility and cold could be dangerous even in areas where accumulation is relatively light.
The biggest reason is that meteorological science isn’t capable yet of forecasting the exact track of a low pressure system four days in advance.
I’m not a meteorologist, but I’ve spent enough time collaborating with meteorologists to develop a fair understanding of their work. So, I’ll try to explain here what I know about the coming storm, based on what the pros have published.
First, it helps to know how low pressure systems affect winter weather. Typically, the heaviest snow falls to the north and northwest of a low pressure system. Areas to the east and northeast usually get mostly rain. Areas to the southeast can get clear skies with little or no precipitation.
The path a low pressure system takes therefore has a major impact on where the biggest winter storm impacts will occur.
The main way meteorologists predict a system’s path is by monitoring the output of special computer programs known as numerical weather prediction models. These programs run on supercomputers, because they ingest massive amounts of weather observation data and apply complex equations in their attempts to create reliable, mathematical simulations of the atmosphere. Several such programs exist, all written differently. It’s not unusual for the various models to produce different solutions, which typically come out every 12 hours.
Meteorologist Megan Dodson at the Northern Indiana NWS office wrote in an Area Forecast Discussion this morning that they’re watching the outputs of at least four different models. They’re paying attention to how much consensus exists between the solutions of the various models and how consistent each model’s output is with previous output from the same model.
As of this morning’s forecast discussion, these models indicated that the low pressure system would move from the southwest to the northeast and that the path it follows would be farther west than previous model runs indicated. The image below, published by the Indianapolis NWS office, shows three possible paths for the system. One model, known as the Global Forecast System (GFS), is an outlier, forecasting the most westward path. If it’s right, and the center of the low stays west of Indiana, the Hoosier state won’t get much snow. Other models forecast more easterly paths.
Graphic published by NWS Indianaolis showing possible tracks of a low pressure system associated with a forecast winter storm
All the models could still be wrong!
Numerical weather prediction is not perfect, but the closer the low pressure system gets to Indiana, the more the models will align and the better the forecasts will get.
As I write this, the system is still over the Pacific Ocean, so it should be understandable that it’s too early for the models to accurately predict how it will track over the Midwest.
For now, it’s important to understand that a winter storm will very likely occur somewhere, but as much as you’d like to know exactly where the most snow will fall (and where the most travel impacts will be), it’s just too early to know.
That said, because it’s also too early to rule out big impacts in any part of northern Indiana, it’s wise to prepare for such impacts. For example, you might shop for Christmas dinner ingredients a day earlier than you originally planned, just in case.
It’s also important to realize that some people who are not meteorologists post official-looking but unscientific forecasts on social media. Often, they choose the single model output that looks the most dramatic (even though it’s an outlier), rather than create legitimate forecasts based on a full understanding of model limitations and model consensus. Often, the main goal of such posts is to create bigger audiences for advertisements. Keep that in mind the next time you see a dramatic snow forecast for any time that’s more than a few days away.
