An East Coast storm is looming this weekend. A slug of moisture will dump moderate to heavy precipitation — some wet and some white. Major cities such as Washington, Baltimore and Philadelphia will probably walk a tightrope between a waterlogged rainstorm and plowable snow. Boston and New York City may be gassing up the snowblowers.

Plentiful wild cards remain for the Interstate 95 corridor. Farther to the northwest, more significant accumulations are probable. Despite the buzz on social media, the onset of precipitation won't come for another three days. Plenty can change between now and then, meaning it's important to check back for frequent updates and not consider today's forecast "locked in."

The National Weather Service will probably begin to draw up winter storm watches by late Wednesday or early Thursday for areas where the potential for heavy snow is highest. That's around the time most meteorologists can, with some accuracy, take a first stab at putting specific numbers on a map for projected snowfall totals.

For now, let's run through what's changed since our previous forecast assessment, review what we know and don't know about the system, and identify reasonable expectations.

The basic summary

An upper-air disturbance swinging southeast over the Lower 48 will rendezvous with moisture over the southern Plains on Friday. As it progresses eastward, it will spawn a new area of low pressure in the Mid-Atlantic that will intensify upon riding northward off the Virginia coast. Precipitation will surge north ahead of the system. Some will pinwheel back onto the west side of the system, which is where northerly winds will draw cold Canadian air south and manufacture snow.

The path and strength of the system remains in question, which is why it's difficult to nail down what type of and how much precipitation will fall east of the mountains.

What's new with the forecast

Models continue to struggle with the strength of the storm that will form in the Mid-Atlantic and slip south of New England. The American GFS model has a slightly farther offshore and weaker low. The European model, meanwhile, depicts a closer pass of a stronger low-pressure system, but closer passes usually mean more warm air pulled inland from off the ocean and more rain than snow near I-95. However, the stronger storm system in that scenario would lead to heavier precipitation, which could help cool the lower atmosphere and increase snow in some areas.

Both main weather models now agree on a sharp transition zone between rain and snow near Interstate 95 from roughly Washington to just south of New York City. Along and east of I-95, more rain than snow is probable. Thirty to 60 miles to the northwest, chances increase for significant snowfall.

Models are converging on the idea of an initial period of snow as the storm begins Saturday in portions of the Mid-Atlantic, mainly west of I-95. It might last only a few hours, but snow could fall heavily enough to disrupt travel for a time. Thereafter, precipitation may switch to rain in lower elevations.

Timing and possible totals

Heavy rain and thunderstorms will be present across portions of the Deep South and Southeast on Friday night. As precipitation creeps northeast into the Carolinas and southwest Virginia, some freezing rain is possible in the higher terrain. Moisture will slide atop a shallow lip of freezing air bleeding down the mountains.

By daybreak Saturday, snow will be coming down in eastern West Virginia and much of central and western Virginia. It's possible that Washington, Baltimore and Philadelphia will see a few hours of snow, perhaps briefly heavy, before a shift to sleet and rain during the afternoon. That's not set in stone, though.

Then the snow works into the Northeast by Saturday evening and southern New England overnight. New York City could see several inches of snow, but a possible switch to sleet and rain may limit amounts there as the rain-snow line flirts with the city. Farther inland, more snow is expected. The National Weather Service office serving New York City writes confidence in accumulating snow is highest across the Lower Hudson Valley and southern Connecticut.

There will be a sharp western cutoff to the snowfall in central to western Pennsylvania and in the southern Hudson Valley. In southern New England, rain and sleet may mix with snow inside of Interstate 495, but snow is probable, especially northwest of the Mass Pike and Interstate 84.

Boston and Providence could see several inches of snow before precipitation moves off Sunday afternoon or evening, though the Weather Service office serving the region writes that "specific details are still hazy." Providence may see less snow than Boston, especially if the European model projecting a stronger low is correct, and more moisture is drawn back toward the coast.

The storm center may remain too far east for significant snow in most of New Hampshire, Vermont and interior Maine; coastal Maine has a strong chance of snow Sunday.

The "sweet spot" of moisture overlapping with cold air may not be very wide — only 50 miles or so — but is most probable along the eastern slopes of the Appalachians, where there's the potential for 6 to 10 inches of snow and the possibility of locally higher amounts.

Since the system will be moving quickly, precipitation amounts won't be enormous compared with other systems.

Washington may end up on wet side of rain-snow line

All indications are that Washington will be sideswiped by the system's wintry limb. While plenty of precipitation will come down (the equivalent of 0.75 inches or so of rain), temperatures at the mid-levels will warm as the storm slides by. That's why, after a brief period of wet snow Saturday morning into the afternoon, sleet and then rain will begin to fall.

"The forecast for I-95 and points east is pretty well locked in as a mostly rain event that could start as a little snow with little or no accumulation," said Wes Junker, the Capital Weather Gang's winter weather expert. "West of the city accumulations will be dictated by how far the rain-snow line migrates west and on precipitation intensity. The highest probability of meaningful snow will be near and in the mountains."

It's not out of the question that Washington's western suburbs will see an inch or two of snow before the changeover to sleet and rain.

Rather than quickly changing to rain Saturday afternoon, snow could continue over the Potomac Highlands and areas northwest of Montgomery and Loudoun counties into Saturday evening. This could be a storm in which Leesburg and, especially, Winchester see substantially more snow than areas within a one-county radius of the District.

Precipitation should end in the Washington area well before daybreak Sunday. The timing of the storm has sped up, meaning confidence has increased that Sunday will be dry aside from the chance of a passing rain or snow shower.

Wild cards

A few uncertainties remain - namely the specific track and strength of the low-pressure system. That will probably be better pinned down Wednesday night or Thursday, though. The parent upper-air disturbance is moving ashore in the Pacific Northwest, and that means meteorologists can launch weather balloons into it. That greater data sampling will make for more accurate forecasting and modeling.

It's also unclear how strong the "50-50 low" will be. That's a low-pressure system that will set up near 50 degrees north latitude, 50 degrees west longitude - near the Canadian Maritimes. That will swirl down cold air, but probably not enough for a big snowstorm for most of the I-95 corridor.

Jason Samenow contributed to this report.