The weather models are hinting at a potentially significant winter storm in the eastern United States this weekend, but they're vague on the small details like "what will happen?" Washington D.C., for instance, could see either one or fourteen inches of snow, or rain, or sunshine. Welcome to winter!

Coastal Lows

The uncertainty with this (and most) storms is what track it will take once it hits the Atlantic Ocean. Lows that track up the East Coast are tricky. During the cold months, there are three scenarios that can play out that determine what kind of precipitation will be seen in the I-95 corridor. The first is that the low tracks either over land or directly over the shoreline. When this happens, the low kicks warm air too far inland for any of the major cities to see snow. Any snow that does occur falls at the end of the storm and doesn't amount of much (if any) accumulation.

The second scenario is the classic storm that produces major snowfall from D.C. to Boston. In this setup, a coastal low stays far enough offshore to keep warm air confined to the coast, while producing a heavy shield of precipitation in the cold air that sits along and west of the interstate. These storms are the memorable ones.

The third scenario is a low that tracks so far offshore that only some areas immediately along the coast see precipitation. When there's enough cold air present, these storms can cause some heavy snow along the beaches (especially from the Outer Banks up through the Delmarva Peninsula).

The single biggest issue with this system—aside from the fact that we're more than five days away from the event—is which track the storm will take.

The Setup

This is what the Vincent van GFS model thinks the jet stream will look like on Friday night. The European model has just about the same thing, but with the jet stream smudged around in a slightly more organized pattern. When we look at the upper-levels, we like to see big, arching ridges or deep, looping troughs to give us an idea of what will happen at the surface. The above image shows a giant mess. In fact, it's so much of a mess that this particular run of the GFS model (7 AM EST) doesn't predict much more than some light rain.

The upgraded GFS model, often called the "parallel GFS" because they still run it alongside the current GFS, does show a storm forming. The parallel model shows a low pressure system developing in Texas on Friday morning, rapidly shooting across the Deep South and reaching the Outer Banks by Sunday morning. This solution produces some ice along the Ohio River Valley, as well as some pretty heavy snowfall in the northeast, possibly threatening places like New York City and Boston.

The European model has been the most bullish about the storm, as for the past couple of days it has consistently shown a significant storm developing next weekend. Of course, I can't actually show you that, because it's also against the ECMWF's usage license to distribute the images. When it comes to the weather, Europeans do that whole "money-hungry capitalist" act better than Americans. (For good reason, too—it costs $90 million a year to run the ECMWF.)

Anyway...this morning's run of the Euro for Sunday morning shows a broad trough situated over the East Coast with a strong jet streak over South Carolina. The result is a strengthening low sitting off the coast of the Delmarva Peninsula, but too far offshore to produce heavy snow in the big cities. The past couple of runs have shown beaucoup snow from Arkansas to Maine, and it's healthy to be skeptical when it starts showing enormous snowfall totals like that, especially so far in advance.

Case in point: Last night's run showed about fourteen inches of snow falling in Washington D.C., compared to this morning's run which showed the city barely cracking an inch. Last night's run also showed all rain east of a line from Asheville to Charlottesvile, while this morning's run clobbers cities like Greensboro, Richmond, and Raleigh with a healthy coating of snow. Talk about consistency!

The event is still a workweek away, so the models are going to waffle on what they think will happen.

What's going to happen?

It's simply too early to tell. Weather sites (including the National Weather Service) and meteorologists on social media are raising the alarm right now because 1) it looks like some kind of storm will form, and 2) betting against the European model usually doesn't end well.

It's funny—we've spent the past year telling people not to place much stock in the models beyond five days, especially during the winter, only to have to write this stuff today. I saw someone on Twitter last night say that the weather media has to address this stuff so far in advance because if we don't, they'll get it off the "black market" from a bunch of hype-driven twerps.

At this point, if you live anywhere from Arkansas to Maine, it's good to keep in mind that it's December and there's a chance for snow. Or rain. Or bright sunshine. Something could happen. Always be prepared for the worst, and make sure you pay close attention to forecasts in the coming days.

[Images: AP, TwisterData, WeatherBELL]

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