Wildcards abound, making it impossible to give specific forecasts this far. By Wednesday and especially Thursday, the details will become clear, when the precipitation will start and stop at specific locations, who will see rain and snow, and we can begin to predict how much will fall.
However, some of the most populous cities in the country may experience snowfall or a combination of snow, ice and rain.
Here are some plausible scenarios, what we see and what the consequences might be.
Currently, there is a “short wave” or aloft cold air, low pressure and vorticity pocket south of the Aleutian Islands in the northeast Pacific Ocean. That system will dip southeast into the lower 48 in the coming days, bringing rain to the Southern Plains and Deep South on Friday, then forming a low pressure area near the mid-Atlantic coast by the weekend.
That low pressure system will ride north parallel to the eastern seaboard. In the east, well along the coast, warm, moist air moves north around the Gulf Stream, triggering thunderstorms. Some of that moisture then pinwheels west and northwest around the low pressure center and falls as snow on the cold side of the storm. It will be at least 50 to 100 miles west of the storm's track.
Low pressure system tracks near or far from the coast can have major impacts on snowfall. Low pressure tracking along the coast or inland will pull enough warm air westward for heavy rain along the I-95 corridor, with heavy snow mostly confined to the mountains. But if the low passes a little further out to sea, it will allow enough cold air to be in place for Washington, Baltimore, Philadelphia and New York to receive their first significant snowfall in two years.
The worst storms will arrive Saturday through Sunday. For the Piedmont of western North Carolina, it will begin before dawn on Saturday. Later in the early morning to late afternoon, it will expand across the Mid-Atlantic and reach the Northeast on Saturday evening.
As the rain-snow line could well cross the I-95 corridor from Washington to New York, possible scenarios include mostly rain, mostly snow, or mixed precipitation. That is why specific snow totals cannot be planned so far in advance.
Providence, RI; Boston; And Portland, Maine, has better snow prospects compared to I-95 cities to the south.
The heaviest snow, more than a foot, will likely fall in the Appalachians, where the forecast is more confident.
Models are in unusually strong agreement that a low pressure system will form somewhere near the mid-Atlantic coast on Saturday. With sufficient moisture, significant accumulations of rain, snow, or both can occur over the region from Virginia to Maine.
We know that the air ahead of the storm will not be particularly cold, meaning it will not be a blizzard. What's more likely is that snow will fall inland, especially at higher elevations where it will be cold.
Much to the dismay of forecasters, the forecast for what kind of precipitation will fall in major cities is very uncertain, especially from Washington to New York.
An upper-air disturbance over the ocean has not yet moved into the Pacific Northwest. Once it's done, the National Weather Service can launch weather balloons into it. That's how they find important data about its shape, strength and movement, which can be fed into computer models to better simulate the coming storm.
We don't know where the storm will go, and then we can't predict where the rain-snow line will be. That means forecasters are essentially guessing specific rain or snow totals for large population centers along I-95.
We don't know how strong.50-50 less” — or a low-pressure area near Newfoundland near 50 degrees north latitude and 50 degrees west longitude — will set up. The depression causes cold air to circulate southward ahead of the storm, along with a high pressure area over southeastern Canada. Models do not make this pair of pressure systems particularly strong, limiting the amount of cold air. That's why there isn't much ice east of the mountains in the mid-Atlantic.
We don't know where the western edge of the precipitation shield will establish. In storms like this, the western edge is usually sharp. It's still unclear where significant snowfall will break up in northern and western New England.
Putting it all together, it's too soon to know what will happen with this storm.
For many, this is the first solid chance of snow in two-plus years, following a mild winter.
We will continue to share updates and fine-tune the latest forecasts.
