Hello everyone!

Another nor’easter is in the forecast for Thursday and, as per usual, the forecast is shaping up to be a tough one. We know there will be enough cold air for precipitation to remain all snow, but it’s less clear how much moisture will be able to make it to our area given that the track of the storm is expected to be a little farther south than what would usually be considered a blockbuster for ME/NH.

This post will be divided into two parts, an in-depth analysis of some of the meteorological processes and thinking underlying the forecast, and then a quicker rundown of what to expect. If you’re looking for the latter and less interested in the former, scroll down to the second set of large bold lettering.

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Forecast Analysis

As I write this on the afternoon of Tuesday 12/15, I’ve been carefully watching water vapor satellite imagery.

The swirl of primary interest to us is that which is currently located over Oklahoma. Unlike many of our big snowstoms, this is the only disturbance that will drive our snowfall though other swirls on the map here will play supporting roles. If we had another disturbance diving southeast out of Northern Canada, this would be a much different conversation. But as it is, we missed our shot with one of those yesterday. To get a sense of what can happen when an Arctic disturbance gets involved, check out that storm near Newfoundland!

Here’s a look at what I’m seeing when I look at that loop. The features of most importance to our forecast are highlighted with the various colorful annotations. That giant storm over Newfoundland shows up well and has dragged an intense shot of Arctic air into the region on northwesterly winds today. In the other corner of our image, a collection of Pacific disturbances is sliding onshore over British Columbia. They will eventually be forced to turn southeastward by NW flow around a ridge west of California.

Finally, our primary disturbance is clearly visible over Oklahoma, and it’s already giving us some clues about its structure when it arrives over the East Coast. Check out the dry slot wrapping around the southern side of the low. This feature is typical for upper-level lows, and it will remain a player in the forecast as the low heads east-northeast over the next couple days, even as it starts to tap the moisture-rich subtropical jet noted over the Gulf of Mexico.

To more carefully investigate the structure of this upper-level low and its accompanying dry slot, let’s zoom into the southern Plains and look at radar imagery, surface weather observations (little icons), and upper-level heights (orange lines).

The center of the ULL when I made this graphic was over the NE TX Panhandle according to the height analysis shown here. Radar imagery and surface observations show precipitation focused along and just north of the upper-level low’s track. Remember this is because the dry slot is located south of the low and is inhibiting the development of precipitation.

Tonight, the upper-level low will shift off to the east and weaken a bit as it starts to encounter large-scale confluence (a convergence of upper-level winds somewhat analogous to merging highways) between the storm over Newfoundland and a ridge over the Bahamas.

By preventing the upper-level low from moving too far north, this confluence will set a northern limit on snow Thursday. Figuring out exactly where that northern limit is will be the challenge occupying myself and many other forecasters over the next few days.

One key part of the puzzle is a subtle secondary disturbance diving southeast into the Dakotas tomorrow morning. This disturbance is part of that collection we saw over British Columbia on the WV maps above. Because every action has an equal/opposite reaction, the more this secondary disturbance can dig south, the more our upper-level low can move north.

For those who follow along with weather model guidance, you’re probably aware that over the course of the day today, most model guidance has been trending towards a storm track farther NW.

As this animation shows, this trend is due in part to new forecasts showing that Dakotas trough digging farther south tomorrow.

Whatever happens with that secondary disturbance, our primary disturbance will reorganize a bit as it moves over WV/MD tomorrow evening but its overall trend is towards weakening as it moves from the Mississippi Valley towards Nova Scotia.

This is a big reason to be skeptical of some of the really big snowfall totals that you might see floating around on social media, be it for our area (ME/NH) or points farther south. It’s hard to get widespread 18-24+” of snow in the Northeast, and it’s really hard to do that if you don’t have a rapidly intensifying storm, much less one that is weakening aloft.

While this system does not have the right upper-level configuration to drive widespread extremely heavy snow, the ingredients will come together for some heavy snow starting tomorrow evening.

At the jet stream level, this is so close yet so far to being a classic “coupled jet” pattern favorable for rapid cyclogenesis. A cyclonically-curved jet streak extends from Georgia up into the Mid Atlantic while its anticyclonically-curved counterpart extends from Quebec City east into New Brunswick. Jet streaks are just pockets of enhanced winds embedded within the larger jet stream and because they don’t move forward as fast as the air within them, they stir up some circulations in the lower atmosphere to make sure no one part of the atmosphere “runs out” of air.

The portion of these circulations we’re interested in are the horizontal transportation of low-level air from regions outside the jet entrance/exit regions and the subsequent vertical transport of that air up towards the jet stream level. In this case, the air we’re transporting horizontally is coming from the Gulf Stream and thus is rich in subtropical moisture. When that air is then lifted up towards the jet stream, it cools and the moisture it contains condenses/freezes.

This forecast radar map for the same time as that jet stream map confirms the anticipated consequence of these processes.

By tomorrow evening, steady snow will be falling across much of southern New England and southern/central NY.

We can also see this process at work by examining forecasts for slightly lower in the atmosphere (5,000 feet vs 35,000 feet). You can see the wind barbs at this level showing the low-level jet we inferred by analyzing the jet stream pattern above. You can also see that low level jet drawing subtropical moisture northward around the southeastern flank of the low near New York City. Our area will be the battleground between that subtropical moisture and extremely dry Arctic air to the north.

One way of visualizing that battle is a cross-section of relative humidity tomorrow evening shown below.

Note that near point A (north of Bangor), there’s a layer of very dry air from around 750 hPa (12,000 feet ish) to around 950 hPa (1000 feet ish). Even though snowflakes are able to form above that, they sublimate as they fall through that deep layer of dry air and thus don’t reach the ground. As you move farther SW down the cross section towards point B, you can see that dry layer shrink until eventually it disappears. For somewhere like Bangor, it will take a long time for that layer to disappear (if it does at all) so most of the storm’s snow will be lost to evaporation. For spots near point B (southwestern NH), this dry layer will be overcome fairly easily so snowfall totals will be considerably higher.

As the storm moves by on Thursday, bands of moderate to heavy snow will be focused by a process known as frontogenesis (the creation of a new front). The problem though is that by the time the storm gets here, the front will be basically created already and thus frontogenesis, having done its job, will be packing up and leaving. You can sort of see this happening in the loop above where frontogenesis is outlined in purple contours. Remember that aloft, the storm will be weakening on approach to Maine and exactly how quickly that happens will determine how much snow we get.

Whatever snow we do get looks to be quite fluffy thanks to excellent snow growth conditions.

The quintessential six-sided dendrites that pile up most efficiently like to form in moist conditions between -12 and -18C where air is rising quickly. The more of the atmosphere you have satisfying those conditions, the fluffier your snow will be (cold temps near the surface help too). Here, our DGZ (Dendritic Growth Zone) extends from around 5,000 feet aloft to around 16,000 feet which is unusually deep. Moisture and lift appear to be available in plenty throughout this layer, so we should be good to go for fluffy snow across the area as the coastal front shrinks away towards Cape Cod.

The trough axis will cross the region Thursday afternoon and sinking motion behind it will put an end to precipitation from west to east.

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Forecast Overview

Snow will arrive from southwest to northeast tomorrow night after a chilly and mostly cloudy day.

As the storm moves northeast on Thursday, snow will become steadier and heavier at times across the area. The best chance for heavy snow will be in southern and western parts of the region, and the model depicted above may be overstating the northwestern extent of steady/heavy snow on Thursday. This will be a fairly fast-moving system and snow will come to an end from west to east Thursday afternoon/evening.

How much snow will there be?

Here’s my best guess. Highest totals will be in southeastern areas and lowest totals will be up north, but how high and how low those totals are is a tricky question to answer. In a bit of an experiment, I’ve included “boom” and “bust” forecast scenarios for each region along with estimated probabilities that each of those scenarios will occur. Note that the risk of a “boom” (more snow than expected) is higher in the south while the risk of a “bust” (less snow than expected) is higher in the north where Arctic air may well prove to be more stubborn than we currently think.

I’ll have another update on the forecast for this storm tomorrow.

-Jack