Below you'll find an explanation for the following points:
- Catastrophic ice is expected for some of the Southeast U.S., resulting in hundreds of thousands without power.
- A large swath of 6"-12" of relatively heavy, wet, snow is expected for interior locations along the U.S. East Coast
- A tight rain/snow line is expected closer to the coast, and it is too early to predict exactly where that line will be
- In New England, even where a change to rain occurs, for most except Cape Cod at least a burst of Thursday morning snow is quite likely
The storm expected to run the East Coast Wednesday through Friday will deliver catastrophic accumulations of ice to the interior Southeast United States, and a large swath of 6"-12" of snow to the interior Eastern United States, with some locally higher amounts.
The abundance of warm air and moisture, flowing atop a shallow cold dome of surface air, will create raindrops that fall into a sub-freezing environment in the Southeast United States - freezing rain. Because it falls as liquid, freezing rain is "plain rain" that will freeze on contact with surface that sit in the below-freezing air, including roads, walkways, tree limbs and power lines. Of course, it's the latter two that lead to power outages, as limbs fall on power lines, or the lines themselves sag and snap under the weight of ice encasement. Typically, ice accretion amounts of over half an inch are sufficient for widespread power outages - in this storm, nearly a full inch of ice is expected to accrete on surfaces, meaning catastrophic impacts to travel and especiallly the power grid. Though freezing rain amounts can vary by microclimate, as one or two degrees makes all the difference, here are the forecast total new ice accumulations for the hardest hit area of the Southeast U.S.:
By Wednesday night, the storm will be rounding the bend, so to speak, heading north up the East Coast. As the incoming warmth and moisture encounters colder air, snow will fall, rather than raindrops, and a large swath of 6"-12" of snow is expected, with some spots picking up more. The most difficult aspect of the storm in these snow areas is exactly where and when a rain/snow line will set up, as warmer air tries to scour out substantial cold in place ahead of the storm. Consider this 3-panel image of computer guidance forecasts, featuring the ECMWF (upper left), GFS (lower left) and NAM (lower right) computer guidance snow estimates. It's worth noting that none of these snow forecasts can be taken literally, but they can serve as a guide...and notice how different they are. The ECMWF solution shows black for areas of 12"+. The big blue areas on the GFS and NAM solutions represent 8" and red areas represent 12". It's quite likely that none of these solutions is completely correct, with the NAM holding the cold more than the others, and the ECMWF farthest west with the snow zone, as it drives warm air inland (click image to enlarge).
At this early juncture, the GFS is a decent middle-of-the-road solution, not only with regard to snowfall, but also with regard to storm track and intensity. That said, it's likely that the colder snow solution of the NAM will have some validity to it, as will the higher precipitation amounts of the ECMWF. In the end, verification will likely incorporate pieces of each guidance solution - a tight rain/snow line nudged a bit farther southeast (somewhere near Boston/Providence corridor? Interstate 95? We shall see), a track near the GFS, and beefed up precipitation amounts from the ECMWF. So, the best forecast is either none of these, or a blend of them, depending upon your perspective, but the bottom line is we have a good foundation to build from and add details as more precise data becomes available.