It’s That Time of Year For Snow Maps to Appear. Here’s What You Should Know About Them and Why You Shouldn’t Always Believe Them

Chris Dolce
Published: December 3, 2018

Major winter storms affect the United States every year, and due to their heavy impact on everyone's lives, the biggest question asked is: How much am I going to see?

The eagerness to know how much impact a snow or ice storm will have is understandable, but unfortunately, sometimes that question cannot be answered until within one to three days of its arrival.

This thirst to know what will happen can often be fed by those computer model forecasts of snow amounts that often go viral on social media. Sometimes you might see these maps with pretty blue, pink and purple shadings up to a week or so in advance of a storm's arrival.

(MORE: Winter Storm Central)

Example of a snow model forecast with different colors representing snowfall amounts.

While these maps will never disappear from social media in the winter season, and there is nothing that can be done about it, you can do your part by not buying into what they are showing and also not spreading them like a nasty cold virus.

Need more proof that you should not share snow maps in winter?

This animation illustrates four separate issuances of a long-range snow forecast (beyond 5 days) generated by one computer model over the course of 48 hours.

Sure, there is a consistent signal for accumulating snow east of the Rockies, but notice how the geographic locations and snow amounts from each model run labeled 1, 2, 3, and 4 change drastically over parts of the South, Midwest and Northeast.

In southern Virginia, the range of forecasts produced by this particular computer model is from less than an inch to more than a foot. A similar swing is depicted in southern Missouri, with two versions of this computer model squeezing out more than a half foot of snow and two others with little or no snow.

This type of flip-flopping is commonly seen in long-range computer model snowfall forecasts, further proof the snow maps you may see on social media are an unreliable source of information.

On top of that, those maps often don't take into account how much snow can be produced from the forecast amount of liquid equivalent precipitation, also known as the snow ratio. Snow totals on the maps can also be inflated in areas where sleet is expected rather than snow.

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Long-range snow forecasts from a single computer model that were made at four separate times over a 48-hour period. Notice how the location and amounts of snow change in each version. Darker blue and purple are heavier amounts.

Many times it's apparent to meteorologists there is the potential for a significant winter storm five to seven days in advance. This is diagnosed using some of the same forecast models that generate those viral social media snow maps.

Meteorologists don't simply key on one model, but rather examine multiple numerical models, and their evolution over time, using their expertise on each model's strong and weak points, or biases.

For example, on a Monday morning, there may be indications in forecast model guidance that a winter storm could affect several regions of the United States the following Friday through Sunday. Instead of specific details, meteorologists usually give a head's up that we are watching for a particular storm in a general timeframe. This is typically followed by the caveat that the forecast will change and we will provide more details as they are available.

Part of the reason you can expect forecast changes over time is that weather disturbances triggering the development of a potential winter storm may be thousands of miles away from their future destination. That means they may be traveling over a data-void region such as the Pacific Ocean before they reach the United States.

As a result, numerical forecast models may have difficulty resolving important details of how a winter storm may come together from that incipient disturbance(s). It's a basic concept of forecast models that initial errors in analysis or short-term forecasts amplify with time.

Throw in questions surrounding the location and depth of the cold air source, and there are countless scenarios forecasters are faced with when dealing with a winter storm in the long-range forecast.

So the next time there is a potential for a significant winter storm in your area, resist the temptation to run with the forecast on any snow map(s) you might encounter on social media.

Instead, consider the source. If the person who posted the image isn't a familiar face that you've seen on TV, a trained meteorologist from the National Weather Service or private forecast company such as The Weather Channel, a quick Google search of their name will almost always tell you in less than 30 seconds if this is a reputable source. Trained meteorologists should not be sharing a wild forecast for a week into the future, and if you can't find the person's portfolio and credentials in a web search, that's a red flag.


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