Cold air is firmly entrenched today but even colder air is on the way. Look for a cold front to smash SE tonight bringing behind it the coldest air all season and the first truly Arctic air we’ve seen all season. As the cold air moves in, an upper low diving to our south will set the stage for a Norlun trough event along the midcoast tomorrow. Beyond that, bitter and dangerous cold arrives Sunday followed by a messy storm midweek next week.
Saturday Norlun Trough
The front gets hung up along the midcoast as the upper low slides to our south. Winds out ahead of the 500mb trough will turn southeasterly and eventually easterly which will provide a mechanism for moisture to be transported into the region. Also, with southeasterly winds aloft, there will be nothing to move the trough once it sets up. Most of us will see the moisture transport aloft in the form of light snow/snow showers during the day tomorrow with light accumulations. The midcoast will be the only area to really capitalize on that. To understand why, read the next paragraph about how Norlun troughs work. If you’re more interested in the forecast, skip on down to the next picture and read about the cold coming Sunday.
A Look Behind The Scenes At Norlun Troughs
Because it’s been so warm recently, the ocean is still quite toasty. This sets up a very large difference in temperature between the warm ocean and the cold air. The warm water will heat the lowest level of the atmosphere creating warm air which wants to rise. Above this very shallow layer of relative warmth lies an entire atmosphere of very cold air which wants to sink. Somewhere in the impenetrable fortress of cold, there will be a weakness. The warm air will rush upwards through this weakness generating tremendous upward motion and leading to warm moist air crashing into cold dry air. This violent collision will produce snow.
As the new trough takes up the energy immediately near it, warm air trapped under the cold air all across the Gulf of Maine will rush in to take its place so it too can rush upwards and cool. As the trough is using up the existing warm air, new warm air will keep being created as the water continues to warm the lowest levels. This warm air will rush to the weakness in the cold and rise up, depositing its moisture along the way. This is what we call a Norlun trough.
One of those will form over the midcoast and deposit heavy snow tomorrow. Totals are likely to come it at around a foot in the core of the trough but it remains to be seen exactly where that sets up. These events are notoriously fickle and can set up wherever they please or not at all. One good example happened just last night when a weak Norlun formed over Portland out of left field causing major issues. Bottom line: expect the unexpected.
Expected snowfall for tomorrow. Upslope snow showers in the mountains are likely to produce 1-3″ in the usual spots. Farther south, a dusting to as much as 2″ is possible with isolated snow showers/flurries. In the core of the Norlun, thundersnow is possible as there will be a solid amount of instability associated with the temperature falling dramatically with height from the warm ocean to the bitter cold air moving in aloft (technical: lapse rates will be very impressive).
NW winds will be howling tomorrow beginning in the afternoon. While blowing and drifting snow will initially not be an issue, by Saturday night, those who saw snow will also see it blowing around leading to more slick spots and reduced visibility. It will also add an extra sting to the air if you happen to be out and about Saturday night.
Dangerous Cold Sunday
Very cold air is still on track to come into the area behind the storm on Sunday. Actual temps will drop well below zero and as NW winds howl at over 25 mph, wind chills will drop even lower. This is the image I used for last night’s post and it still applies as wind chills approach -40 to -50 in the north with -20 to -30 degree wind chills in the south. Bundle up for the Valentines Day dates both Saturday and Sunday nights. Thankfully, the extreme cold is out on Monday.
Messy Midweek Storm
As the cold retreats, we have another storm threat. We are virtually certain to see precip the question is of what variety. Right now, it looks like a mess with all precip types possible for everyone. It is far too early to nail down exact impacts or amounts but total accumulation of whatever falls could be significant. My thinking from yesterday still applies in that uncertainty remains large here. One group of forecast models leans towards a colder, farther east solution with more snow than mix while another camp of models leans towards more rain and mix than snow.
Another storm is on the horizon late next week but we have to get the first two resolved before we can dig into that one. Just know that there is the potential for a storm and as always, ignore any hype you may see.
I’ll have more updates in the coming days as the midweek storm becomes clearer. I’ll also be on Twitter watching the Norlun trough evolve tomorrow @JackSillin.