You check your weather app and see a snow icon later in the week. Your local TV meteorologist mentions "snow is possible by Friday" in their 7-day forecast. An alarming social media post shows a model forecast snow graphic seven days away.
Perhaps questions like these then race through your mind:
-"How much snow will we get?"
-"Will this mess with my morning drive? Should I work from home?"
-"Will the kids have off school?"
Then, once the storm is over, you end up with more or less than the last forecast you remember.
There are many important aspects to any snowfall forecast beyond the basic "X to Y inches of accumulation are expected."
So, let's lay out some key things you should keep in mind the next time snow is in the forecast.
1. We Typically Can't Predict Precise Snow Totals Beyond a Couple of Days
You might be frustrated when meteorologists say "this snow forecast is highly uncertain" or "it's too soon to tell how much snow will fall."
We understand completely. But we're not holding back information.
It turns out, snow forecasts are usually challenging, even for the most experienced meteorologists.
The system responsible for the potential snowstorm several days away may currently be over 1,000 miles away as a jet-stream disturbance over the Pacific or Arctic Ocean.
Or it might depend on the potential interaction of the northern (polar) and southern (subtropical) jet streams.
If computer forecast models don't accurately represent where and how strong those distant jet disturbances are, they probably won't correctly forecast the evolution of the snowstorm several days away.
Moreover, a small change in the amount of moisture translates into a much bigger change in the amount of snowfall. If you expect a quarter-inch of rain and get an inch, you may not see that as a forecast bust, but if you expect three inches of snow and end up with a foot, that's a different story.
So meteorologists frequently can see the potential for a snowstorm, say, somewhere in the Plains or Northeast up to a week out, and specify generally what time period it may occur. But it's just too soon to pinpoint any exact snowfall totals for specific cities.
And in a few cases, the potential snowstorm indicated by forecast models a week away evolves to a relative nothingburger by the time it moves through.
(WINTER WEATHER 101: What You Need to Know)
2. That Model Snowfall Graphic You Found in Social Media Isn't Necessarily a Forecast
Meteorologist Chris Dolce wrote extensively about this topic.
You might follow someone on social media who likes to post colorful maps of model snowfall forecasts, sometimes over a week ahead of time.
They're pervasive on the internet these days. Sometimes the snowfall totals on those model graphics are attention-grabbing.
I'm not a mechanic, nor a doctor, so I can't look under the hood of my vehicle nor look at an X-ray and tell you what's wrong. Similarly, you shouldn't assume the person posting that splashy model snowfall graphic is a meteorologist.
There are many computer forecast models, and while some might have a good handle on the forecast, others may not. And those model forecasts could show important trends that can't be detected by looking at a single model at one particular time.
Professional, experienced meteorologists are trained to assimilate and interpret a wealth of weather data, including many different weather models, learning what they do best and what they struggle with, to produce a forecast.
Make sure your source of weather information, particularly on social media, is a trusted source, such as your local National Weather Service office, a private forecasting company such as The Weather Channel, or a local TV meteorologist.
If it's not a trusted source, don't blindly share a clickbait model snowfall forecast graphic.
3. The First Snowfall Forecast You Hear a Few Days Out Will Probably Change
Don't assume the first snowfall forecast you heard two days ago is gospel. Check your forecast frequently leading up to the storm for important changes.
Even when we're close enough to begin issuing specific snowfall forecasts, there can be considerable remaining question marks.
In general, snow falls to the north and northwest of the track of the surface low pressure center – the red "L" you see on weather maps.
Changes in the track of this low, even 50 miles or less, can shift the area of snow, ice and rain. If this precipitation footprint is over major cities, that can change the forecast for tens of millions of people.
One example of this is shown in the animation below, where different low tracks can mean the difference between rain and then a little snow near the Northeast Seaboard, or heavier snow.
Examples of two possible scenarios where small shifts in track and the amount of cold air available can result in different impacts.
Sometimes, changeovers from rain to snow and vice versa are also tricky.
In November 2018, one such predicted changeover didn't occur as quickly as expected, leading to a snowy mess of an evening commute in New York City, rather than just a rainy drive home.
In early-fall or late-spring storms, temperatures may be more marginal to support snow, and even if snow does fall, it might not accumulate as much on pavement.
4. Snow Can Be Highly Variable Over Short Distances
Pay attention to areas around your location on a snowfall forecast map. If totals are much higher or lower, there's boom or bust potential in your snowfall forecast.
Larger areas of snow often have embedded, smaller bands of much heavier snow. These bands may only be 5 to 10 miles wide and can produce snowfall rates of over 1 inch per hour, sometimes accompanied by lightning.
One example below was from a March 2017 Northeast snowstorm, indicated in the radar loop by the thin band of darkest green.
Except for lake-effect snow situations, it's not possible to forecast exactly where these heavier bands will set up in a snowstorm. However, meteorologists will typically spell out the potential for these heavy bands in their overall storm forecast.
On the other end of the spectrum, sometimes there's a sharp cutoff to snowfall on the northern edge as dry air intrudes into the storm. Such was the case in March 2018 over Indiana, where parts of the Indianapolis metro area picked up 8 inches of snow, but areas just north of the city picked up little or nothing.
The edge of Winter Storm Uma's snow cover near Indianapolis, as seen from the window seat of a flight on March 25, 2018. Yellow arrows highlight the boundary marking the extent of snow cover that fell from Uma.
5. Timing, Temperatures and Winds Matter, Too
It's not all about forecast amounts. There are other key items to keep in mind with any snow forecast.
Timing can be as important as snowfall totals.
When will the snow start and end? Will it fall during rush hour on a weekday or on a weekend? That could mean the difference between a nightmare commute and a relaxing Saturday morning at home, hot chocolate in hand, watching the storm outside.
Also, pay attention to temperatures during and after the storm.
Will temperatures be warm enough to melt the snow as it falls on roads? This is often a consideration in more southern locations and also early in the fall or later in spring.
Will colder air eventually turn that slush or melted snow to stretches of ice the following night or morning?
And in more northern locations where road salt is heavily used, will the cold plunge behind the storm be so cold to render road salt ineffective? In general, once temperatures dip into the teens or colder, road salt can't melt much ice.
Finally, check your wind forecast during the storm, especially if you have plans to drive.
Wind-driven snow can reduce visibility, particularly in rural areas and open country, leaving you disoriented, even if you're only a few miles from home. Drifting snow can make roads impassable, stranding drivers.
Strong winds are also important in fall and spring snowstorms.
When heavy, wet snow causes tree limbs and power lines to sag, strong winds can be the final straw that triggers more widespread power outages and tree damage.
In general, checking frequently with a trusted meteorologist for forecast updates should keep you in the know the next time snow is in the forecast.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.
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