As the clock counts down to Tuesday, when a massive East Coast storm will take shape, forecasters are still trying to nail down the crucial details of what will likely be the most intense storm of the long 2013-14 winter season, including whether heavy snow and damaging winds will affect New York and Boston.

As of Sunday afternoon, the greatest threat for heavy snow and damaging winds appeared to be from eastern Long Island to Cape Cod and the islands of Martha's Vineyard and Nantucket, with lighter snows and blustery conditions to the northwest of those areas. However, there are important caveats to that forecast.

Here is what we know: A storm will take shape beginning on Monday night into Tuesday morning off the coast of Virginia. It will move north-northeast, gathering strength at an extraordinarily rapid pace, until it closely resembles a full-fledged late winter hurricane.

Image: Weatherbell.com

By Tuesday evening, the storm center may be located due east of Atlantic City, N.J., as it undergoes a process that meteorologists call "bombogenesis," which is defined as a storm whose minimum central air pressure drops by at least 24 millibars within 24 hours. However, this storm is expected to greatly exceed the pressure free-fall that would qualify it as an East Coast "bomb" of a storm, the National Weather Service said in a technical forecast discussion on Sunday morning.

According to the NWS, forecast models show the atmospheric pressure at the center of the storm plummeting by a whopping 40 millibars in just 18 hours, which forecasters called "quite remarkable." (In general, the lower the pressure, the stronger the storm.) Unusually warm sea-surface temperatures, which may be due to both manmade global warming and natural variability in ocean currents, are present off the East Coast. These waters may add fuel for the storm, enabling it to intensify more than it otherwise would have.

Hurricane-force winds (74 miles per hour or greater) are likely off the coast of New England, with massive waves posing a dire risk to mariners, and threatening the east-facing Cape Cod shoreline with beach erosion.

In forecasting Northeast snowstorms, weather forecasters use the benchmark geographical coordinates of 40-degrees North longitude and 70-degrees West latitude as a benchmark for determining how far north and west snow will fall. A storm that tracks over or just northwest, or inside, of the benchmark will bring significant impacts to Boston and New York. However, a storm that passes southeast, or just outside, the benchmark will typically reserve its harshest lashing to far southeastern New England, including Cape Cod.

So far, nearly all computer models bring the storm just outside the benchmark, but close enough to spread heavy snow, damaging winds and high surf into southeast Massachusetts and Rhode Island. These impacts would then move northward into northeastern Maine, before the storm goes on to clobber Nova Scotia, Canada, with the equivalent impacts of a full-fledged winter hurricane that could have a minimum central pressure lower than 950 millibars. (In case you don't speak "weather geek," that is only four millibars higher, or weaker, than Hurricane Sandy was when it made landfall as a post-tropical cyclone in New Jersey in 2012.

The big question for meteorologists now is whether the storm comes close enough to the coast, or is structured in such a way that allows heavier bands of snow to spread further northwest into the New York City and Boston metro areas. One particularly sophisticated computer model, run by the European Center for Medium Range-Weather Forecasts in Reading, England, shows just such a scenario occurring. The NWS forecast office in Boston said that if this model were to prove correct, it would mean "an epic snowstorm/blizzard for eastern New England."

Right now, any slight jog to the west in the storm track, even by just 50 to 75 miles, could mean the difference between a few inches of snow in Boston and New York and a full-fledged, but short-lived, raging snowstorm.

The reason why computer models — which are based on the physical laws governing the atmosphere and oceans, and are run using thousands of weather observations from land, sea, air and space — are still wrestling with this storm is that in order for it to form and intensify rapidly, several pieces have to fall into place at exactly the right place and time. Some of these ingredients, including an area of atmospheric spin that is pushing south across central Canada on Sunday, have been moving above areas that don't have a lot of weather observations. This forces the models to do more guesswork than usual regarding how the storm will come together in the end.

By Sunday night, meteorologists should have more observational data on this and other key players in the forecast, and this may make computer-model projections converge more closely on a particular forecast for the storm. So, if you're in New York or Boston, you might not want to let your guard down quite yet.

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