In the meteorological community, the sharp cutoff of snow nearly two weeks ago was a fun challenge - trying to nail down exactly where significant snow would and wouldn't fall was the biggest concern. Now...nature has added a second challenge - a sharp northern cutoff to the precipitation, plus marginal temperatures that, 24 hours out from the expected change to snow, are so warm it's hard to believe snow could possibly fly.
The temperature map Friday morning poses a big problem for a forecast of snow: where are you expecting to get your cold air? All the way into Canada, cold air is lacking and an incoming cold front is just approaching the Canadian border, but with a very sluggish drop in temperature behind it. So...how could the atmosphere cool enough for snow? The honest assessment is: it may not. This storm will be a matter of quite literally a couple of degrees, and not even at ground level - in fact, we're quite sure any change to snow would begin with temperatures above the freezing mark in the predawn hours of Friday morning.
The possibility of heavy snow has caused Boston Public Schools to close Friday.
The key to a change to snow, however, can be found aloft. As close to the ground as just 500 to 1000 feet, cooler air will be moving in overnight Thursday night, and it's this cold air just off the deck that is the reasoning behind the expected change to snow. In circumstances like this, if snow survives the trip down to the ground it wouldn't accumulate quickly, but would start to stick...and with copious amounts of moisture in the atmosphere and several hours of precipitation left until midday or the early afternoon, eventually, this adds up.
We expect the change to snow to occur from predawn Friday to middle or late morning Friday, from northwest to southeast, respectively. This means the first impact to the morning commute will be in some of the Metro West into Northern Rhode Island and Eastern Connecticut, then the change from rain to snow likely will happen during the morning drive, between 5 AM and 8 AM, from Boston through Southeastern Massachusetts, and a bit later on the Cape. The farther northwest one is, the less total moisture that will be available from the storm...in Southeastern Massachusetts and Eastern Massachusetts, the expectation is for the greatest intersection of moisture and cold air...then on the Middle and Outer Cape, one likely starts to run into enough warm air for more rain than snow, with a burst to snow at the end of the storm. This leaves us with a general pattern that probably looks like this:
Boston Area features quick cutoff of snow the farther northwest one is - it's possible Boston still squeaks up to 5 or 6 inches even with the back edge not changing much:
Southeast Massachusetts likely sees the greatest amounts, with amounts dropping a bit due to more rain on Cape Cod:
Eastern Connecticut sees some snow on the northwest edge of the storm, but Hartford may avoid significant impact:
Eastern Maine may get grazed by this storm late Friday morning, especially right at the coastline:
Changes are still possible, particularly by tightening up the 3 to 7 inch snow increase, carrying the five and seven inch lines farther north and west to be closer to the back edge.