Our first big nor’easter with blizzard potential since the epic winter of ’14-15 is becoming more and more likely Thursday night into Friday. Low pressure will develop off the Carolina coast and move steadily north-east while intensifying rapidly. The result will be heavy snow, rain, and wind by the time Thursday night rolls around. The storm will have two phases: light to moderate precipitation midday Thursday through Thursday afternoon and heavy precipitation Thursday night into Friday. The first round will be a result of warm air advection ahead of a weak storm in Quebec and the second will be the explosively developing coastal storm.
Snow will move into the area from SW to NE Thursday afternoon as an occluded front moves in from the west. At this point, the coastal storm will still be well SW of the area and snow will be light to moderate. The PM commute Thursday will be slick and slow but not awful. The coastal storm will begin to rapidly intensify around this time as well. As far as temps go, weak CAD is expected as warm air tries to move inland and runs into cold air trapped by the mountains. This interaction will create a coastal front with the shorelines seeing above freezing temps and light rain and the coastal plain/foothills/mountains seeing below freezing temps and snow.
As the evening wears on Thursday, the storm will begin developing explosively off the Mid Atlantic coast. A band of very intense snow will develop to the west of the storm and that band will be right over our area. Exactly where it sets up will determine who gets the most snow but for now, signals are very strong for intense snow early Friday morning. To understand just how strong this band is likely to be, it’s imperative to look at dynamics in the mid levels of the atmosphere.
Intense mid level dynamics will drive this heavy band as warm moist air over the Gulf of Maine clashes with cold air from Canada. As the low offshore deepens between 20 and 30mb in 12 hours (!!), lift will become even more enhanced as surface level dynamics contribute to heavy precip. Why is the intensity of the precip so important? The harder precip falls, the faster the air it falls through cools. To understand this, we need a little chemistry.
As snowflakes melt, the water they contain goes from solid (frozen) form to liquid form. This phase change requires outside energy to energize the water molecules into moving around more, thus changing the phase from solid to liquid. Where does this energy come from? The air around the melting hydrometeors (a fancy word for water (hydro) that falls from the sky (meteor) ). What happens when you remove energy from the air? It cools because by definition, air with less energy is colder. If you have tons and tons of snowflakes melting at the same time, the temperature will continue to cool and eventually, the entire column will cool below freezing and snow can reach the ground. This process is aided by the intense lifting as air rises, cools, and is replaced at the surface by colder air moving in from the NW.
Another thing to keep in mind will be snow ratios. While near surface temps don’t necessarily argue in favor of fluffy snow, a few other factors may tip ratios closer to 15:1 rather than 10:1. With 1.25-1.75″ of liquid falling, this difference can be substantial. We already know about the very intense lift but notice the two red lines that run horizontally across the graphic (x-axis is time). Between these lines is the vertical zone where snowflakes are formed. To get the best high-ratio snow, you want a deep dendritic growth zone that’s fully saturated and has tons of lift.
We have the lift and the moisture but not the depth as you can see by looking at the proximity of the lines delineating the DGZ. This means that we won’t see the 20:1/30:1 fluff of ’14-15 but we’re also unlikely to see 6:1/8:1 slop. The exception to this may be along the immediate coast where warm air could be too much to overcome in terms of ratios. This will be something to watch closely in the coming days.
The sounding for Portland Thursday night off the 12Z NAM shows a lot. First, the warm layer near the ground is less than 2000 feet deep. This means that with intense lift and strong dynamic cooling, getting the column subfreezing won’t be unattainable. The sounding shows that the DGZ previously discussed won’t be super deep (only about 7,000 feet) but it will be almost completely saturated and, as discussed above, will have plenty of lift. The final interesting point of note is weak instability above the coastal front structure which means that thundersnow is not out of the question. We’ve discussed the light snow at the onset, some of the impressive mid level dynamics that will drive heavy precip, and explored the dendritic growth zone for clues to the snow:water ratios. Now, it’s time for some wind and water.
With a storm deepening at such incredible rates (several mb/hr), wind will naturally become an issue. As is typical, the coast and the high elevations will be ground zero for the heaviest wind as they have less friction to deal with. Wind gusts along the coast of over 50mph are likely with the greatest wind potential along the midcoast. Farther inland, gusts to 35 or 40mph are expected with any exposed elevations seeing stronger gusts.
Here’s a different view of the winds for Portland showing very intense winds lurking just off the surface. Any band of heavy precip could pull some of those winds down through momentum transfer and as a result, strong winds are expected along the coast. With lots of snow and strong winds comes the risk of power outages. This risk will be fairly uniformly distributed with the strong winds being the issue along the midcoast and the heavy snow more of an issue inland. Be prepared for a night or a couple of days in the cold if the power does go out!
As far as coastal concerns go, this storm looks pretty benign. Tides are astronomically low and winds are mostly parallel to the coast. However, along the midcoast, winds will have more of an onshore component and rough seas could result in some minor splashover in especially vulnerable areas. This risk will subside as winds turn offshore Friday morning.
Here’s my snowfall forecast. The trickiest forecast is for the coastal plain outside the midcoast. Should the dynamic cooling be slightly more intense, the low track slightly farther east, or the coastal front set up a little farther east, totals could be higher. If any of those things happens in reverse (weaker, farther west, etc.), lower totals can be expected. Confidence is high in a big event up in the mountains and a few folks are likely to pick up 2 feet. While those amounts will not be widespread, I highlighted the area where I think it’s most likely. Otherwise, most inland/mountain communities can expect a solid 12-18″ snow. The other high confidence area is the midcoast/peninsulas where light accumulations are expected as most of the precip will fall as rain.
More light snow is likely for the second half of the weekend before more mixed precipitation in a week or so. More on that as we get closer.