For the past few days I've been carefully watching the development for Friday's coastal storm. The greater than usual uncertainty has been the result of two key factors: 1) the thermal profile is going to right on the cusp between snow and rain, and 2) the system is going to be highly energetic. Robust storms like this one are difficult to forecast since the perturbations enter a non-linear regime meaning small changes in computer model inputs can result in dramatic differences in the forecast. Model solutions have ranged from a non-event to a crippling blizzard of historic proportions.
As the storm draws closer, we're starting to get a better handle on the uncertainty. Nothing in out of the question yet, but here is the general idea...
A cut-off low will approach southern New England and then drift off to the south. The primary impacts of the storm will largely miss ski country. Heavy snow is likely to confined to upstate New York.
Expect strong on-shore flow keeping the thermal profile above bringing heavy rain to southern New England. Snow will be limited to higher elevations, except at the tail end where the snow level may drop enough to bring pasty snow toward lower elevations. Areas like the Monadnocks, southern Greens through the Berkshires, and Worcester hills stand to get higher than forecast snowfall totals if the thermal profile changes much. The Catskills look to be in the best position for reliable snowfall out of this storm. Across southern New England, several inches of rain likely somewhere in a band where the low-level jet is the strongest.
But this really isn't a story focused on rain or snow, at least not on its own. Instead this is going to be a story about severe coastal flooding and widespread power outages. There is still a lot of time for things to change, but multi-day power outages are a strong possibility. To highlight the strength of this storm, check out the pronounced tropopause fold that is forecast. You're probably thinking either a) what the hell is that? or b) cool! but get to the point. The tropopause fold itself is not on consequence to this story, but is a symptom of how robust the storm will be.
A strong low-level jet is going to bring onshore plenty of moisture and exceptionally strong winds just above the surface. It's unlikely, but possible, that this moisture could reach the ground as snow if the thermal profile cools, which would increase the potential for power outages as the pasty snow further burdens trees that will be exposed to damaging winds. But check out that low-level jet...
Wind speeds at 850 mb (around 5,000 feet) are going to be on the order of 100 kts (115 mph) around Cape Cod. Yikes! It's an open question of how much of that mixes down to the surface, but expect widespread wind damage. Also another cool geek note: check out the gravity waves being shed from the exit region of the low-level jet off the coast of Virginia. There's definitely some numerical issues caused by the weather model, but they're just another sign of the energy in the system. Cool.