UPDATED 9:45 p.m. ET: Forecast models run throughout the day, as well as guidance from National Weather Service forecast offices and private sector meteorologists, all signal growing confidence in a high impact storm from Richmond, Virginia to Washington, D.C., north to central and southern New Jersey from Friday through Sunday.

A tougher forecast exists for whether or not heavy snow creeps northward to New York City and Boston, since these areas will be on the northern edge of the significant precipitation. Forecast amounts in these areas are likely to fluctuate significantly in the coming 36 hours or so, possibly straight on through the storm itself.

Of growing concern to forecasters is the potential, too, for major coastal flooding along the New Jersey and Delaware shorelines, likely extending to southern New England. A combination of high onshore winds, unusually high tides due to a full moon, as well as building waves as the storm slowly moves away, will cause a damaging storm surge along east-facing shorelines.

A potentially crippling — possibly even historic — winter storm is likely to strike areas from Virginia northward to Maine late this week into the weekend, spreading a messy mix of snow, sleet and rain across the metropolitan corridor from Washington, D.C., to Boston.

Areas to the north and west of these cities may pick up several feet of snow, with small shifts in the storm track capable of bringing such totals into the big cities themselves.

In areas that won't see a mix or changeover to sleet or rain, up to 3 feet of snow is possible, based on the consensus of computer models used to help forecast the weather. In the Washington, D.C., area, this storm has the potential to deliver the most punishing blow from a winter storm since the notorious winter of 2009-10. (That winter featured the storm that came to be known as Snowmageddon, which shut down the nation's capital for days.)

The National Weather Service forecast office in the D.C. area has declared this storm to be a significant threat, with the potential to cause "significant travel delays, closures and threats to life and property."

One of the remarkable aspects of this storm is that the major computer models, such as America's Global Forecast System, or GFS, and the European model, which is also known as the ECMWF, have largely been in agreement about this storm's formation and track for more than two days now. This also extends to other major computer models, such as the Canadian model and a model known as UKMET.

Winter storm threat graphic from NWS Baltimore/Washington. Image: NOAA/NWS

This model consensus lends an unusual amount of confidence to weather forecasts compared to what might typically be seen this far in advance. Many of the computer models have been upgraded over the past few years, and are now run on faster supercomputers aimed at capturing more details about the atmosphere. This includes the National Weather Service in the U.S. and other centers around the world.

Winter storm threat now HIGH says @NWS_BaltWash "Significant travel delays, closures & threats to life & property" pic.twitter.com/m3F6KcWSgQ — Angela Fritz (@angelafritz) January 19, 2016

However, the devil is in the details, since this storm will be strengthening as it moves over unusually mild ocean waters for this time of year. This will both aid in its development and could also result in a surge of relatively mild air into coastal areas from Interstate 95 and points eastward, limiting snowfall totals there.

A closer look at the forecasts

To forecast storms like this, weather forecasters focus closely on what are known as ensemble models — or computer models that are run many times with different initial conditions each time.

GEFS ensemble run from Tuesday, Jan. 19, 2016, showing 5.5 day snowfall totals. Image: WeatherBell Analytics

This gives forecasters an idea of how sensitive the forecast is to subtle variations in weather conditions at the current time. For example, ensemble forecasts that all show a snowstorm but differ in the snowfall totals give forecasters more confidence in a snowstorm's occurrence.

So far, very few of the ensemble runs of the main computer models used to forecast this event are showing no storm at all, and many are showing a downright crippling nor'easter from Washington to New York to Boston and points west and northwest.

Upper-level (tropopause) animation shows textbook "cyclonic wrapup" we see w/baroclinic storm development. pic.twitter.com/iPyDM8hcwH — Ryan Maue (@RyanMaue) January 19, 2016

National Weather Service sounds alarm while urging caution

The National Weather Service forecast offices from the D.C. area northward to Boston are making the threat from this storm clear to their audiences, while also cautioning that even a 50-mile shift in the storm track would mean the difference between a primarily white versus wet storm.

Shifts in the potential track of the low pressure will have a large impact on weather Fri-Sat. #njwx #dewx #pawx #mdwx — NWS Mount Holly (@NWS_MountHolly) January 18, 2016

For example, the National Weather Service office that covers the Philadelphia area noted in a discussion on Tuesday morning that a key component of the storm is still over the Pacific Ocean. Until that moves over land, where it can be sampled by weather balloons and commercial aircraft, forecasts have a higher amount of uncertainty concerning details like the precise location of the rain-snow line.

One particular forecast trend to watch is if future computer model runs start shifting the axis of heavy snow southward, which is what happened with several 2009-2010 storms that slammed the D.C. area while sparing the New York to Boston corridor.

Full moon + storm = sizable coastal flooding threat

Depending on the exact intensity, track and speed of the storm, coastal flooding from strong onshore winds will also be a potential threat from the Delaware coast northward into Massachusetts. Winter storms in this region have proved especially destructive to vulnerable low-lying communities, and this one will hit at the same time as a full moon that will elevate tide levels.

The Weather Service in Philadelphia is predicting moderate to major coastal flooding with this storm, and similar impacts could occur in eastern Massachusetts if the storm tracks near what meteorologists call the "benchmark," which is a point at 40 degrees north latitude and 70 degrees west longitude. When storms pass near or over this benchmark, they tend to bring significant impacts to southeastern New England.