Our active pattern is about to go into overdrive as a storm will approach from the west tomorrow before rapidly intensifying off the MA coast tomorrow night and into Monday morning. Light ocean effect snow will arrive tomorrow morning along the southern coast before moderate bands of snow arrive tomorrow afternoon associated with the primary storm. It won’t be until late tomorrow night into Monday morning that the secondary low cranks offshore bringing heavy snow, gusty winds, whiteout conditions, and potentially even thundersnow.
As cold high pressure noses in from the north tonight and tomorrow morning, winds will turn east-northeasterly resulting in some ocean effect snows along the coast SW of the midcoast. These will be fairly light and should only amount to an inch or two through the morning hours. The first part of the main event arrives a little later in the morning as the primary low pressure system approaches from the west.
The first impactful round of snow will arrive from SW to NE tomorrow midday. It will be associated with the warm air advection aloft ahead of the primary low pressure system that will be weakening as it moves through NY state. The warm air overrunning the low-level cold air will produce moderate bands of snow for the entire area. Several inches of accumulation are likely to be associated with this. At this time, the secondary low will be in its infancy over the waters S of Long Island.
The real show begins when upper level energy diving SE through the midwest arrives at the coast and enables the secondary storm to explosively deepen. Model guidance suggests the low could deepen 28mb in 12 hours as it moves across Cape Cod and into the Gulf of Maine overnight Sunday and into Monday Morning. It’s during this phase that the real show occurs.
When storms deepen that explosively, a number of really cool things happens. First, it snows really hard. This map from the 18Z NAM shows a band of absolutely insane upward motion on Monday morning. Look for snowfall rates on the order of 2-4″/hour or more in the heaviest mesoscale bands. It’s important to note that it’s impossible to predict where these bands will set up more than a few hours in advance. At the moment, coastal Maine looks like the best bet for one of these bands though it’s far too early to say exactly which towns end up under it. For the science behind mesoscale banding, be sure to check out my post on the matter from our last storm only a couple of days ago!
The second cool thing that happens is that it gets really windy. As the pressure gradient tightens between a low that has the same pressure as a category 2 hurricane and a strong high pressure system nearby, the winds will take off Monday morning. Look for gusts along the coast in the 50-60mph range with 30-45mph winds away from the coast. Just be thankful the storm isn’t another 50 miles west or we’d be getting into the hurricane force winds lurking just offshore. Not a good day to be out on the water! Winds this strong plus the threat for heavier/wetter snow along the coast will mean power outages are likely Monday for coastal communities. If you’re especially vulnerable to power outages, be prepared to be in the dark Monday night!
The other impact of the winds is that they’ll be blowing around the snow both from today and from the storm. Look for visibilities to approach zero Monday morning as the winds develop. The result will be blizzard conditions and a blizzard watch is out for the coast for Monday. Only travel Monday morning if it’s an absolute emergency!
Snow will wind down Monday afternoon as the storm pulls away to the east. While snow will diminish in intensity and eventually taper off, winds will still be going strong resulting in continued low visibility/blizzard conditions through the evening hours Monday. Winds will calm down Monday night into Tuesday morning.
While coastal flooding is always a concern with storms this strong, winds will be mostly out of the north and will probably push more water offshore than onshore in most areas. However, the intense winds will generate very large waves which will likely cause beach erosion and even some minor splashover in the typical spots. High tides to watch will be 12 AM and PM Monday.
So will this snow be cement or fluff? The answer is, for most, fluff, and for some along the coast, a mix. To get fluffy snow, you need cold temps, strong lift through the snowflake production zone (-12C to -18C, wherever in the atmosphere that happens to be) and you need the snowflake production zone (aka dendritic growth zone) to be full of moisture. We have all the ingredients for fluff away from the coast. However, at the immediate coast, temps will begin on the warmer side and as a result, some heavier/wetter snow could kick things off Sunday evening resulting in the threat for power outages there.
Here’s a plot from the 18Z NAM showing various parameters for Portland through time (which moves forward from right to left along the x-axis). Notice the sharp increase in snow ratios (light blue line) going from a typically heavy 10:1 ratio to a very fluffy 20:1 ratio as temps cool from 30F to 20F. Also notice the presence of instability aloft. That combined with tremendous upward motion means we could be woken up by claps of thunder Monday morning. Hopefully we’ll get the continuous thundersnow parts of CT saw with the storm Thursday but even one strike would be cool! Keep your eyes and ears open!
Here’s how much snow I think will fall. QPF values (liquid equivalent) range from a little under an inch north to over 2″ on the coast. Given fluffy ratios away from the coast, that inch could quickly turn into 18 or even 20 but confidence in the full inch falling is low enough for a 12-18″ forecast for the mountains. Along the coast, around 2″ of QPF is forecast but some of that will fall at lower ratios (or else we’d be looking at over 3 feet!) which means amounts will be held back to right around the 2 foot mark. In a band or two near the midcoast, I wouldn’t be shocked to see a 30″ report based on the fluff factor that will begin to set in as the storm wears on. I’ll adjust these numbers with any new data tomorrow morning!
More updates tomorrow both here and on twitter @JackSillin.