As our storm today continues dumping heavy snowfall across much of the area, I figured I’d take some time to explore mesoscale banding: the reason why some towns will be buried in snow today and why others right nearby will see much lighter amounts. The atmosphere is set up perfectly for mesoscale banding which makes this a great time to learn about the phenomenon.
Here’s one of the best examples of mesoscale banding you can find brought to you by today’s storm. Snow was coming down at rates of 2-4″/hour over Portland and Lewiston in the heavy bands (purple lines) while at my house in Yarmouth, we got a measly .4″ during the same period (11AM -12PM). Even better, these bands hardly moved throughout the morning and aren’t going anywhere as of this writing at 1 PM meaning there will be intense snowfall gradients as some downs hit the jackpot and others miss big. What causes these intense bands and why do some so close to the jackpot miss out?
The answer lies in the dynamics in the mid levels of the atmosphere, roughly 10,000 feet above our heads. Way up there, a low pressure system is developing and ahead of it winds are shifting from SW to S to SE. This is pushing warm, moisture laden air west into the cold airmass and creating a warm front (frontogenesis- creation of a front). This process involves the sharpening of the thermal gradient as well as convergence, both of which favor strong upward motion focused along the axis of the front itself. Why is upward motion maximized along frontal zones? For that we turn to isentropy.
As the warm front sharpens and develops through the mid levels, air parcels rushing in from the east/south east hit a wall of cold air. The sharper and steeper that wall is (e.g. the stronger the frontal zone), the faster they have to rise as they are bound to their potential temperature surface. Potential temperature is the temperature that an air parcel (basketball sized chunk of air) would reach if it was dropped to 1000mb (air warms as it drops). There are “surfaces” of potential temperature in the atmosphere and air parcels are bound to that surface in that they rise or fall based on the level of that surface in the atmosphere. In frontal zones, the slope of these surfaces become very steep forcing air parcels to rise steeply resulting in intense upward motion.
This whole process is aided by the explosive development of the surface low off the Mid Atlantic coast and by the fact that the explosively rising air can be vented quickly away by upper level divergence. With winds blowing away from each other in the upper levels, a void is left forcing air from below to rise to fill the area of lower pressure. This helps continue the explosive upward motion. The divergence is enhanced by jet streak coupling where the left exit region of one jet streak overlaps the right entrance region of another. Now that we’ve discussed all the things that go into creating heavy snow bands, why are some missing out?
If you look at the radar image at the top of the post you’ll notice that for every powerful snow band, there’s an equally powerful lull. What goes up must come down which results in sinking motion that suppresses snow. That’s why some folks get nailed with the crushing snow bands and others escape with only moderate accumulations.
These bands are notoriously hard to predict and can make the difference between 6″ and 12″ for any one place. That’s why we give you the ranges in snowfall forecasts- you could end up under a heavy band and get the upper end of the range or it could go the other way. Be prepared for both!