More Snow On The Way As Norlun Trough Sets Up Tomorrow

Hello everyone!

Special update this evening to cover our next storm threat that is likely to bring snow to the region tomorrow afternoon into tomorrow night. This will be a Norlun trough type event which are notoriously fickle and localized. This is the type of situation not dissimilar to Sunday when York got 21″ while the Portland area picked up between 1 and 3″. Right off the bat, there is bust potential. Don’t be surprised if you get far less snow than expected because the heavy band set up 10 miles away. That being said, let’s dive into the mechanics of this event.

12Z RGEM Model Showing A Complex Snow-Maker Across The Region Thursday Morning Credit: Weatherbell, Additions By Me.

Let’s start by looking at some of the basic players on the field so to speak. A wave of low pressure will develop too far offshore to bring any impact on its own but upper level energy will help to create an inverted, or Norlun, trough extending towards the coast. A Norlun trough is basically an axis of shifting winds, in this case ESE winds
ahead of the trough and NW winds behind the front. The ESE winds bring in the moisture and when they run into the cold NW winds, upward motion is created and thus precip occurs.

Another interesting feature most models are hinting at is the development of a mesolow along the coastal baroclinic zone. This is basically a ‘mini storm’ that forms along the natural frontal boundary between the bitter cold continental airmass and the somewhat less cold marine airmass. What this little ‘mini storm’ does is help to focus precip where it forms which would result in heavier bands of snow wherever it develops. Right now, the area with the best chance for seeing mesolow enhancement is east of Portland.

Upward Motion
12Z GFS Showing Intense Lifting Associated With The Trough And Mesolow. Credit: Accuweather Additions By Me.


Another supporter of heavy snow bands comes from the intense upward motion brought on by the mesolow and the wind shift line. The GFS model (shown) suggests the most intense upward motion to be between Portland and Brunswick and inland from there while the NAM has it farther south over York county and points inland.

NAM upward motion
12Z NAM Showing Intense Lifting Farther South. Credit Accuweather


The NAM shifts the heaviest precip south into York County similar to Sunday’s storm.

CMC upward motion
12Z CMC Showing Intense Lifting Farther North. Credit: Accuweather


The CMC model puts the heaviest precip back into the Midcoast. All these maps are to say: there is bust potential here. As with all Norlun trough events, the location of the heavy band is very hard to predict until right when the snow starts falling. Right now, I think the best threat for being under the heavy band for the longest period of time exists north of Portland. That being said, York County still has the potential to hit the jackpot though their chance is much lower. All areas should see at least some snow with most coastal and inland areas seeing at least some heavier snow. For the mountains, this is mainly a snow shower type event.

Upper level map
12Z GFS Showing Various Upper Level Components. Credit Accuweather.

The upper air map for Thursday shows several interesting things. First notice the closed upper low (black lines) over Ontario and Upstate NY. This is the trigger for the mountain snow showers. Where there are upper lows, there are snow showers. Second notice the trough is tilted ever so slightly negative (blue line). This helps to turn winds ahead of it southeasterly which draws in Atlantic moisture.

Finally notice the batch of energy over eastern MA (Green circle). This is the trigger for the mesolow. This is the wild card here as how strong that energy is will dictate how powerful the mesolow and the trough will be. Stronger energy would mean more heavy snow while weaker energy would mean lighter snow. Remember the last time the upper level energy was a little too weak? That would be Sunday and it had disastrous consequences.

snow map 2-17


Here is my thinking as to snowfall amounts. Expect a coating-2″ outside of coastal Maine where the mesolow and trough axis will enhance things. For coastal Maine, the bottom line looks to be 2-5″ but I really want to stress that where the trough sets up will get a decent amount more than that. A 4-8″ event is likely where the trough sets up but where that happens is far too uncertain to pinpoint. Hopefully that becomes more clear in the coming days.

Another storm is looking more and more likely over the weekend, more details to come on that one later on.


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