Very Active Pattern Incoming

Hello everyone!

We’re now locked into what looks to be an extremely active pattern for cold and snow with today’s system wrapping up tonight, a clipper system bringing light to moderate snow Saturday, a coastal storm potentially bringing a major snowstorm Monday, and another coastal storm threatening in about a week. I’ll go over what we know, what we don’t know, and some of the mechanics behind each storm below. Welcome back to winter!

HRRR Model Showing Snow Winding Down This Evening. Credit: Weatherbell
HRRR Model Showing Snow Winding Down This Evening. Credit: Weatherbell

The current storm will wind down this evening as the coastal storm moves offshore. Here’s a simulation of what that will likely look like from the HRRR model. There was plenty of mesoscale banding today, for some of the science behind that phenomenon, check out my post from this afternoon. An additional several inches of snow are expected before things wind down later this evening. Gusty NW winds will bring in very cold temps for tonight and tomorrow resulting in very low (subzero) wind chills and the continued threat for blowing/drifting snow.

18Z NAM Showing Light Snow Saturday Afternoon. Image Credit: Weatherbell
18Z NAM Showing Light Snow Saturday Afternoon. Image Credit: Weatherbell

Light snow will develop early Saturday morning and will continue through the day. Snow will be light for most but a little coastal front/mesoscale low pressure system will likely enhance snowfall along the coast. Most folks will end up with 1-3″ though the coast is likely to see 2-4″. With the fluff factor in place due to cold air, there’s an outside chance someone ends up with 6″ but that seems unlikely at the moment. Snow will taper off Saturday night.

12Z GFS Showing An Upper Level Setup Favorable For Rapid Intensification Monday. Image Credit: Accuweather
12Z GFS Showing An Upper Level Setup Favorable For Rapid Intensification Monday. Image Credit: Accuweather

Our next system will take shape Sunday night into Monday. Considerable uncertainty still exists though if current forecasts from guidance verified, we would be in for a major snowstorm. All the typical threats are looking likely with heavy snow, high winds, and coastal flooding all on the table. This storm has the potential to rival some of the blizzards of the past few years so stay tuned to the latest updates as confidence increases. Of course it wouldn’t take much of a change in the timing of a couple disturbances to result in a much more benign event though at the moment that looks unlikely.

Depending on the intensity of the system, snow could linger into Tuesday before moving out. We’ll have a break for most of the day Tuesday before another storm threatens for Wednesday into Thursday. With such a chaotic pattern, it’s far too early for amounts but it’s definitely something to keep an eye on as our active pattern rolls on!

Various Teleconnections Showing Active Weather Continuing For The Next 2 Weeks And Possibly Beyond. Images From CPC.
Various Teleconnections Showing Active Weather Continuing For The Next 2 Weeks And Possibly Beyond. Images From CPC.

Looking farther out into the future, the pattern across both the tropics (MJO, top left) and the Northern Hemisphere (PNA, AO, NAO, right column) show a pattern favorable for New England snowstorms though at least the next two weeks. While it’s impossible to pin down individual threats past 7 days and it’s entirely possible that for any given threat the pieces don’t come together just right for a major (or even minor) storm, it does look like we’re in for a good little ride as we let the rest of February go by. Buckle up and enjoy the show!

I sadly will be preoccupied tomorrow evening with a most unworthy pastime (5 hour indoor track meet, ugh!) but I’ll have more this weekend.

-Jack

Why Is It Snowing SO Hard?

Hello everyone!

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.

Radar Showing Intense Banding Late This Morning. Image Credit: COD
Radar Showing Intense Banding Late This Morning. Image Credit: COD

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?

700mb Analysis Showing Intense Convergence And Frontogenesis Setting Up Mesoscale Banding Environment. Image Credit: SPC
700mb Analysis Showing Intense Convergence And Frontogenesis Setting Up Mesoscale Banding Environment. Image Credit: SPC

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.

Cross Section Across Coastal New England For This Morning Showing An Elevated Warm Front. Image Credit: Accuweather
Cross Section Across Coastal New England For This Morning Showing An Elevated Warm Front. Image Credit: Accuweather

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.

300mb Analysis Showing Intense Divergence Aloft. Image Credit: SPC
300mb Analysis Showing Intense Divergence Aloft. Image Credit: SPC

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?

mesoscale-banding

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!

-Jack

 

Quick But Intense Storm Today

Hello everyone!

Today will feature cloudy skies, cold temps, and, depending on location, heavy snow. For all the details on specifics/dynamics etc. check out Tuesday evening’s update and for a more general forecast overview, look at last night’s post. Here’s the barebones snapshot. Snow will move in this morning from SW to NE in time for the tail end of the AM commute (if you’re unfortunate enough to have to go to work, go early. you’ll want to leave early too). Snow will become heavy in the afternoon hours with rates of 1-3″/hr at times. Winds will be increasing during the day out of the north so expect blowing and drifting to further reduce visibilities this afternoon. Blizzard conditions are expected along the coast though their duration will be too short for official criteria (need 3hrs+ of 35mph winds and 1/4mi visibility). Snow tapers to snow showers this evening before everyone is dry tonight. Temps will be falling through the 10’s into the single digits.

-Jack