Hello and good morning everyone!

It is, for those of us who love snow especially in large quantities, Christmas Eve. Anticipation will be building today as high clouds thicken up, winds start to flip around to the north, and a few appetizer flakes begin to fly. Mostly though it will just be quiet as a cold front slips through. High temps will range from around 20 up in the north to the low/mid 30s right along the coast.

The rest of this update will go into fairly great detail regarding the forecast for tomorrow’s blizzard.

If you love snow, the satellite loop below is what you dream of every August while you wipe sweat off your brow at the mere thought of moving.

If you don’t dream in satellite imagery, here’s why this loop is showing such a potent setup. First notice the two swirls diving southeast through the plains, one just about to enter Minnesota and the other crossing into Missouri. These disturbances are joining forces with another that’s a bit harder to see over Texas, and will help produce exceptional rising motion in the atmosphere over the East Coast.

That rising motion is already apparent in the brightening of the clouds on the loop (not in the real atmosphere, just how the colors work here) over the East Coast. That southern band of gradually brightening values from Texas to NC is also hinting at a deep reserve of rich tropical moisture. Any time you have powerful lift, deep moisture, and strong upper level disturbances joining forces, you know you’re in for a show.

A show is exactly what we’re going to get as low pressure emerges off the Florida coast in just a couple hours before racing northeast. The storm’s pressure will drop about 50mb in just over 24 hours which is approximately double the criteria established for being dubbed a “bomb cyclone”. By the time the storm crosses the Gulf of Maine tomorrow evening, it will have a pressure similar to what we’d expect to see in a category 2-3 hurricane. Though because our storm’s power comes from clashing airmasses, its energy will be spread over a wider area and thus winds will be a bit lower (though not by that much!).

As far as timing goes, look for north winds to pick up overnight and start shifting around to the northeast. The airmass just to our north is Grade A Canadian Arctic air – far northern Maine (outside the area for which I forecast) will drop into the 20s below zero tonight – which means that precipitation will initially struggle to push north as the first volley of snowflakes evaporate before reaching the ground.

We’ll have to wait until a few hours after sunrise for the show to really start.

Meteorologists quantify the clashing of airmasses by computing a parameter known as frontogenesis, shown in pink on the map above. This parameter, meaning literally “the creation of a frontal boundary between two airmasses”, points us straight to the heaviest snow bands and can give us a clue about their intensity. Tomorrow morning, frontogenesis values will be as high as the atmosphere can ever push them in a band extending from the storm center somewhere off New Jersey up towards the Gulf of Maine. In a more practical sense, that means that we’re in for some very, very, heavy snow.

Though there’s still some uncertainty, all indications are that we will find ourselves on the cold side of the storm’s warm front, certainly at the surface and probably through the rest of the lower atmosphere too. This is important because snowflakes like to grow best under the conditions found west of the warm fronts aloft. To get the picture-perfect six-pointed dendrites that pile up quickly in powdery drifts, you need a large portion of the lower atmosphere to be between -12C and -18C, as well as saturated and you need to have air rising rapidly through that layer. All those boxes are going to be checked along the Maine coast (as well as the foothills) tomorrow, especially during the afternoon.

The net result of all this is that for a period tomorrow, snow will fall extremely heavily and will accumulate rapidly. Snowfall rates of 2-4″ per hour are expected along the coast and southeastern foothills. As you get farther into the mountains, and away from the storm’s center, you’ll start running short of the moisture and lift needed to maintain the most prolific snow banding. That said, snow will still be pretty heavy in the mountains especially tomorrow evening.

The peak of the storm will come tomorrow from the late afternoon into the evening as the storm reaches its peak intensity east of Cape Cod. From a meteorological perspective, there’s essentially nothing you could do to make this storm more ferocious. It’s of the same kin that unleash fabled gales on the Viking coasts of the far North Atlantic. It will have a region of calm conditions at its center, somewhat analogous to a hurricane’s eye (though caused by different forces) flanked by hurricane-force winds of 75-85mph.

At some point in its journey, the storm will attempt to turn back to the northwest and perhaps even west, executing the rare loop maneuver we associate with New England’s most famous storms. The biggest question at this point is whether the loop occurs, and if so, where it might take place. This is important because whoever is sitting 50-100 miles west/northwest of the storm as it completes this loop will see the heavy snow band stall overhead. At the moment, it looks like the best shot of a loop sits between a point somewhere south of Block Island RI (at the western range of possibilities) and somewhere east of Cape Cod (at the eastern range of possibilities). That puts the lucky pivot point, where I expect 3-4 feet of snow to fall, just to our south somewhere between Providence RI, Sandwich MA, and Boston. However, if the storm decides to make it a little farther up the coast, this zone could extend into southern coastal Maine.

So what do I think is most likely?

Here’s my best shot as of this morning. Every line and every point was agonized over but at some point you just have to say this is my best guess and see what happens. There will be a very sharp gradient in totals over our area. I think a solid foot and a half to two feet (or, possibly, more) is a good bet along the southern coast into possibly western portions of the Midcoast. East of that, I’m worried about warm air at 10,000 feet or so, not necessarily because I think we’ll change all the way over to sleet (though at Rockland you never know) but because even marginally below-freezing air at that level cuts way into snow ratios.

To the northwest, it’s all about the gradient between the extremely intense snow band and intense sinking motion (no snow) to its west. I think the band will eventually push far enough into the mountains to give most of ski country a fluffy 6″ or so. In the foothills, you’re closer to the storm but I’m worried about downsloping cutting into totals a bit especially south of Mount Washington.

If you’re going jackpot hunting, eastern MA is the place to be for this one. I tweeted last night that I do believe somebody somewhere will get over three feet of snow from this storm given the moisture, cold air, and dynamics involved.

Another key aspect of a storm this intense is the wind. Thankfully we won’t see the core of the wind from this storm and the north/northeasterly direction bodes well for both keeping speeds down a bit and keeping the trees up (remember it’s southeast that’s the Achilles heel of our trees). That said, the coast should have no problem gusting above 50mph which, combined with the heavy snow, will lead to whiteouts. Blizzard warnings have been posted accordingly along the coast, though conditions just inland won’t feel that much different.

Things will settle down overnight tomorrow as the storm roars into Nova Scotia. By Sunday morning, we’ll just be left with some flurries and patchy blowing snow.

I’ll be riding this one out from Cape Cod so in the likely event that I become disconnected from the power grid and/or internet, my next update will be Monday morning.

-Jack