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For those who like to cut to the chase, see numbered points at the bottom of this post. For those who enjoy delving into some of the science of meteorology and exploring the unknown and prediction of the unknown, no need to skip ahead!

As a meteorologist, perhaps the most important note regarding any storm potential is to put the current state of the atmosphere in context - the critical bundle of energy responsible for driving storm development Thursday or Thursday into Friday is currently over the Northern Pacific Ocean and Gulf of Alaska. In fact, here's the satellite image of the storm, taken Monday morning:

The reason noting the location of the instigating atmospheric energy at this juncture is so important, is because the Pacific Ocean is a "data sparse" region - that is, the number and quality of observations over an ocean is far less dense than the observing network over land, which often includes weather balloons, multiple aircraft reports, and plentiful surface observations, to name a few. You lose much of that information over oceans, and this makes satellite interpolation a crucial part of the "initialization" process, determining how strong a storm is, and how fast the wind associated with it may be, both at the surface and aloft. In the world of forecasting days in advance, we often see significant changes in storm timing, intensity and track as the picture becomes clearer to exactly how the storm is operating now - it stands to figure, if you can't accurately decipher what the atmosphere is doing now, you are hard-pressed to accurately determine what it will do in the future.

The inherent uncertainty that goes with limited observation of current data produces inherently flawed, and inherently variable, forecasts the farther out in time one looks. This leads to solutions that can be as wildly different as the ones contrasted below - both forecasts are valid for this coming Friday, January 3 at 1 PM - the left image is a strong blizzard depicted by the European guidance (ECMWF) and the right image is a storm that's already gone after strengthening to our east, delivering a plowable but much lighter snowfall, from the American guidance (GFS) - click to enlarge:

You'll hear some meteorologists and weather fanatics swear by the European guidance, and others swear by the American guidance, and others scattered across the board. There are some differences that are important - for instance, the European Center for Medium Range Weather Forecasting has put a tremendous amount of money and resources into developing their computer guidance products, and the result has been a set of equations that operate on a higher resolution - that is, taking more data into account. Upgrades are scheduled to the American guidance by the National Oceanic and Atmospheric Administration in 2014 that would also provide resolution upgrades to the American product, though that has not happened yet. That said, better resolution - more data - is not always the key to a better forecast. In some instances, more or less data can have a positive OR deleterious impact on the forecast...often it is dependent upon the evolution of the atmospheric pattern. In the end, the best method is to consider the myriad of potential solutions, which includes dozens of different versions of computer guidance, which can all be run in unison to yield varying results, but give a great indication of low-probability and high-probability impacts and events.

So...where are we left for Thursday into Friday? Overall, low-end guidance supports a 6-12" snowfall for Southern New England, with very little in Northern New England. High-end guidance supports a couple of feet for much of New England in an all-out blizzard. The key difference is in handling the Pacific energy as it drops south through the Intermountain Region Tuesday night into Wednesday morning, and that handling is going to be determined by the speed of the upper level wind associated with the storm as it moves from the Pacific Ocean to British Columbia, in Western Canada, early Tuesday. I expect greater agreement on the evolution of the system across the Lower 48 states to come in the Tuesday morning computer guidance run, which means you'll start hearing more certainty Tuesday midday to afternoon, and much more by the time you wake up Wednesday morning. Until that Tuesday midday/afternoon turning point, we're in a position where we know:

It's very likely to snow Thursday in New England, not only owing to the developing storm, but also owing to the predicted northeast wind off the Atlantic Ocean into an antecedent arctic airmass. Temperatures Thursday are unlikely to exceed the teens for highs as snow falls into our arctic air. A six inch or greater snowfall seems quite likely for Southern New England. The average snowfall from similar atmospheric setups in history has yielded an average of 6-10" of snow, so that's a good starting point for Southern New England. Amounts could be much higher, with blizzard conditions realized Friday morning and snow extending well into Northern New England, too, if a stronger of the potential scenarios verifies. Tides will be quite high on January 2nd and 3rd, so if a stronger solution verifies, substantial coastal flooding and beach erosion will be a serious concern, if a weaker solution verifies, some minor coastal flooding still quite likely given persistent onshore wind.

I'll keep you posted on how this evolves, of course.

Matt