UPDATED9:30 p.m. ET: New computer model runs coming in this evening may change the storm total snow forecast somewhat, with the biggest alterations possible along the dicey northern fringe of the snow shield, near New York City, Connecticut and Boston.

However, the impacts of this storm on the mid-Atlantic region, including Washington, D.C., needs to be especially emphasized. The Capital Weather Gang blog put the upcoming situation this way in a post on Wednesday early evening: "Wherever you are Friday evening, it is quite possible you may need to remain there until Sunday or Monday, or even a bit longer."

Travel in the nation's capital, possibly all the northeastward into Philadelphia and central New Jersey, may become not just treacherous on Friday night and Saturday, but downright impossible.

It could be one of the top three largest snowstorms of all time in Washington, D.C., and Baltimore.

The storm, which the Weather Channel is calling "Jonas," is poised to blast the region with wind-driven heavy snow, sleet and perhaps some rain beginning on Friday and lasting straight through the weekend. The snowfall totals — some are predicting that more than 30 inches is possible — could rival those not seen since 1922, and possibly even breaking all-time records for the nation's capital, though that is far from a sure bet at this point.

Computer model simulations are unanimously showing double-digit snowfall totals for the nation's capital northeast to southern New Jersey. Each model has its own quirks, with some showing a 24-inch plus snowfall maximum on top of Washington and others favoring other locations.

If D.C. were to pick up 30 inches of snow, it would be the biggest snowstorm in the notoriously snow-phobic city's history, and would very likely cripple the region for days. However, other models, as well as historical records of how past storms behaved, show the heaviest snow hitting west of the I-95 corridor, toward higher-elevation areas where there is less of a chance of a mix with sleet or rain.

GFS model snowfall projection, with the highest amounts in the D.C. area, and lower amounts near New York City and Boston. Image: WeatherBell Analytics

What we know

There will be a powerful storm.

The storm will take shape across the southeastern states on Thursday night, tracking into the Mid-Atlantic on Friday. Blizzard watches and winter storm watches are already in effect for the Washington area in advance of the onset of heavy snow on Friday evening.

The snow will be extremely heavy in some areas.

In some places in the Mid-Atlantic, the snow is likely to exceed the rate of 1 inch per hour, and approach 2 to 4 inches per hour at times. Thundersnow — any winter storm enthusiast's dream occurrence — is also a possibility.

This storm is not likely to fizzle out or disappoint snow lovers in the Washington to Baltimore corridor, but...

People should keep their expectations in check. It is possible this will be a top three snowstorm for Washington, but that is not guaranteed. Many factors could come into play and somewhat hold down accumulations. Still, at least 1 to 2 feet of snow look very likely in the Washington area, with the potential for even more.

For historical perspective, consider that Washington has not had a 20-inch or greater snowstorm in nearly 100 years. The largest snowstorm on record for the city was in 1922, when 28 inches fell.

Most of the recent blizzards in the city, including the infamous "Snowmageddon" event of 2010, amounted to about a foot-and-a-half of snow.

Everything you need to know about DC snow. Which graphics will need updating after storm?! https://t.co/APZQYERXYr pic.twitter.com/gA2vg0nRt5 — Ian Livingston (@islivingston) January 20, 2016

This storm will have significant coastal impacts.

Due to strong onshore winds, high surf and an astronomically high tide from a full moon, this storm could do severe damage to some parts of the New Jersey shoreline. On Tuesday, the National Weather Service forecast office in Mt. Holly, New Jersey brought up the "S" word for the Jersey Shore, warning of storm tides possibly "within inches" of Hurricane Sandy levels.

That storm devastated the Jersey Shore, with residents and business owners spending millions in the years since to rebuild.

National Weather Service offices in the New York City area as well as New Jersey are forecasting moderate to major coastal flooding at the time of high tide on Saturday morning and evening. Exactly how high the water gets — and how much damage ensues — is not yet clear, since it depends on the timing of the strongest winds, wind direction and other factors.

'Interesting' Saturday along Del coast. Forecast low tide (black line on right) higher than normal high tide. pic.twitter.com/7rIqm8No65 — Gary Szatkowski (@GarySzatkowski) January 20, 2016

Warmer than normal ocean temps will also promote instability transfer of energy over water; i.e., stronger winds, bigger waves. Science! — Gary Szatkowski (@GarySzatkowski) January 20, 2016

As of Wednesday afternoon, predicted high tides in Atlantic City may peak at about the 10th-highest on record there, while possibly making the top 5 highest on record at Cape May, New Jersey.

Lower amounts of flooding are expected for New York City based on the expected wind direction from the north-northeast, compared to when Hurricane Sandy hit and the winds were pushing more of the water from southeast to northwest.

The current predicted high tides in Atlantic City Saturday evening and Sunday morning would rank in the top 10 highest on record there. Tides are likely to produce moderate to major flooding along the entire coast across at least three high tide cycles Saturday and Sunday, the weather service said.

The unknown

Where the heaviest snowbands will set up.

In storm systems like this one, narrow bands of heavier snow tend to form northwest of the low pressure center, and they help determine who gets the jackpot totals versus areas that see somewhat lower accumulations. Unfortunately, computer model guidance on where those bands will develop and persist is only accurate a few hours in advance, rather than 72 hours ahead of time.

What precipitation type we'll see.

Ocean temperatures off the East Coast are running unusually high for this time of year, partly because December was so warm in this region. It's likely that some coastal locations in the Mid-Atlantic — possibly including Washington, Baltimore, Philadelphia and New York City — could mix with sleet or rain for a time on Saturday. However, this depends on the exact track of the storm, as well as its mid- and upper-level energy higher in the atmosphere. And these details are tougher to pin down until the storm gets closer.

The mixing issue has often appeared suddenly in Washington during past snowstorms, including the blizzard of 2003, when sleet held the city's snow totals down significantly compared to areas north and west of the city.

Berie Rayno, a meteorologist with the private forecasting firm AccuWeather, said the lack of true Arctic air feeding into the storm and presence of milder-than-average ocean waters along the East Coast make the case for not overstating the snow potential for the nation's capital.

“Let’s be careful not to go overboard,” he said. “I just think there is so many things that the models are not going to handle, that I think you've got to be careful along the I-95 [corridor].”

Another southerly shift?

Some computer model runs have shown a tendency to shift the storm southward from previous projections, taking Boston and Hartford out of the main snow shield, and even putting New York City out of the 6- to 12-inch range.

As of Wednesday at noon, many weather forecasters with experience in the Philadelphia to New York region were still calling for at least a moderate snowstorm in the Big Apple.

Updated GEFS 12z forecast for snowfall DC 1 or 2 or 3 feet ... so take the median 18" pic.twitter.com/KbA0ENxrw9 — Ryan Maue (@RyanMaue) January 20, 2016

For example, Kate Bilo, a meteorologist for CBS in Philadelphia, said in a Twitter exchange that the sharp northern cutoff of the storm is "one of the toughest calls with this system," but that 6 to 12 inches up through New York City looks reasonable.

Rayno told Mashable that the southward trend in the model forecasts may very well reverse as the event nears.

“I think the trend south is going to be stopping and there will be trend north as we go through our Thursday and Friday,” he said. There's little question that the New York to Boston corridor will be the "fringe area" for this storm, Rayno said, but that his forecast right now is also for 6 to 12 inches in these areas.

What concerns forecasters

Weather forecasters across the country have been weighing in with each shift of computer model runs.

Much of the discussion reveals how serious of a threat this storm really is. For example, here's how the Weather Prediction Center, a unit of the National Weather Service (NWS), described the storm threat on Wednesday morning. This post applies to anticipated conditions on Saturday. (The ALL CAPS style is from the NWS.)

THE WINTER STORM ALONG THE EAST COAST SHOULD BE WELL UNDERWAY WITH 1 TO 2 FEET OF SNOW EXPECTED FROM THE APPALACHIANS ACROSS THE MID-ATLANTIC AND TOWARD NEW YORK WITH HUGE IMPACTS TO COMMERCE...TRAVEL...AND POSSIBLY LIFE AND PROPERTY.

The NWS in Mt. Holly, New Jersey highlighted the potential for "moderate to major" coastal flooding. The biggest threat for this appears to focus on the Saturday morning high tide.

Here's what one of its forecasters wrote:

THE SURGE GUIDANCE IS SHOWING A SIGNIFICANT WATER RISE SATURDAY, HOWEVER THIS WILL DEPEND ON THE WIND DIRECTION ESPECIALLY AT HIGH TIDE AS A MORE ONSHORE COMPONENT WILL HELP WITH WATER PILING UP. IN ADDITION, HIGH WAVE ACTION INTO THE COAST AND PRECIPITATION WILL TEND TO HOLD WATER AT THE COAST ADDING TO THE WATER RISE. THIS COULD LEAD TO PROPERTY DAMAGE IN SOME AREAS AS WELL AS POTENTIALLY SUBSTANTIAL BEACH EROSION. HE DELAWARE BAY IMPACT FOR MODERATE OR MAJOR FLOODING IS ESPECIALLY HIGHER ON THE DELAWARE SIDE OF THE BAY DUE TO A POUNDING NORTHEAST TO NORTH WIND. THIS POTENTIAL WILL CONTINUE TO BE CLOSELY MONITORED AS THE DETAILS OF THE WINTER STORM BECOME MORE CLEAR.

Rayno said he's concerned that forecasts of more than 2 feet of snow near Washington and Philadelphia may be too high, considering many factors including the forecast track and intensity of the storm.

The Mashable science team will continue to update this story as the forecast changes throughout Wednesday.