Today will feature mainly cloudy skies, cold temps, high winds, and snow as winter finally shows up in a real capacity at least for a couple days. Look for highs today to struggle into the single digits above zero north and the 10’s south late morning before dropping below zero area-wide tonight. Lows tonight will be dangerously cold with actual temps of 15 to 25 below north and 5 to 15 below south teaming up with 25mph NW winds to create downright dangerous wind chills. Wind chills in the mountains will range from -30 to -50 (colder at elevation, the summits above 4,000 feet willl see even more extreme wind chills) while the coast and foothills see wind chills ranging from -20 to -40.
Across the midcoast, things get really interesting today as a Norlun trough develops. 6-10″ of snow is likely wherever it sets up and thumps though it remains to be seen exactly where that is. I went into detail about the forecast and also about how Norlun troughs work in my blog post last evening which includes my snowfall forecast. Farther west, snow showers and flurries are possible today but accumulation will remain under an inch with the exception of the mountains where a few inches of upslope are possible.
I’ll have more updates on the cold tonight and a messy storm threat for midweek this evening.
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
12Z GFS Showing The Setup For Tomorrow. Image Credit: Weatherbell
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
Here is a graphical explanation of a Norlun trough I did for UpPortland. Notice that tomorrow the trough will be over the Midcoast rather than Portland. All rough image design credit goes to me (and Frontpainter for the map)
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.
A Cross Section Of The Atmosphere Showing The Dynamics Involved With The Trough. More Rough Image Design By Me.
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. Notice The Sharp Cutoff Which Leads To High Uncertainty From Brunswick On East To Rockport.
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.
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
12Z GEFS Showing The Range Of Possible Outcomes And The Lack Of Confidence Regarding The Forecast At This Point. Image Credit: Weatherbell
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.
After a nice little storm today, we turn sunnier and cooler tomorrow as low pressure moves east. Another low is already developing in the Gulf of Mexico and will be moving offshore well to our SE Monday. Normally, this would be a slam dunk miss but a strong upper low will be driving SE and there’s a lot of energy on the leading edge. Surface low pressure will develop over the Great Lakes in response to the upper low and a surface trough will develop between the Great Lakes low and the offshore low. This sets up a tricky forecast for Monday as this trough could take on some Norlun traits leading to narrow and possibly heavy snow bands.
12Z GFS Showing A Very Favorable Setup For A Norlun Event. Image Credit: Accuweather
Looking at the 500mb setup, there are a lot of things going for a Norlun event. A strong upper low is moving in from the NW and on the leading edge, a strong peice of energy is moving right over us. Also, being ahead of the upper low, the flow aloft is divergent. To put this in simpler terms, the air at 20,000 feet is spreading out. This leaves a void that air from below must rise to fill. Think of what happens when you pull your hands apart on the surface of a lake or bath. Water rises to replace the water you pushed to the side. This rising motion helps generate precip. This is how everyone will see a coating to an inch or two of snow tomorrow night into Monday morning. The real fun is along the southern coast with a Norlun trough.
At the surface a storm will blast by well SE tomorrow. Another weak wave will develop over the great lakes. The link between the two will manifest itself in the form of a Norlun trough extending NW-SE across the area. Everyone sees light snow from synoptic (large) scale divergence aloft but any moderate snow amounts will be dominated by a mesoscale (small) norlun trough. These troughs generate narrow, heavy snow bands (think lake effect type) that can drop moderate to heavy accumulations in one town while the next town over sees some flurries and maybe a coating. My point here is that uncertainty is quite high.
Graphic Showing How A Low Forms Off The Maine Coast With Strong West Winds. Credit: Me
Another thing to watch out for with Norlun type events is the threat for a mesolow to develop. Mesolows are caused by rising air along and near the trough and also westerly winds accelerating and veering as they move from a high friction environment (over land) into a low friction environment (over water). Imagine the right front tire of your car running into a patch of ice. As that tire moves from a high friction environment (pavement) into a low friction environment (ice) it accelerates while the left front tire is stuck on the high friction pavement and can’t move as fast. The result is your car spinning out. The same thing happens with air moving from over land to over water. Above I’ve drawn up a quick graphic showing how west winds and a cold airmass emerging over water can generate a low. The divergence aloft and strong upper energy aids in this process. This doesn’t happen behind every cold front because often the air is and sinking. The air monday will be rising due to the approaching upper low/divergence which will help with the development of the low.
12Z CMC Showing The Potential For A Mesolow Along The Coast Monday Afternoon. Credit: Weatherbell
The 12Z CMC shows this potential well. Should this occur, most of the coast could see a moderate snowfall and amounts would be slightly less dependant on the fickle Norlun. While most models don’t show this, based on warm waters (still over 40 degrees!), rising air ahead of the upper low, the strong energy aloft, and the movement of the surface airmass, I think there is a good chance some sort of mesolow develops and strengthens tomorrow afternoon.
Here are my thoughts for snow. This is tentative as Norlun troughs are notoriously fickle. I’ll have another update tomorrow morning.
Snow is currently in progress over areas NE of Portland with the heaviest precip centered to the east of Brunswick. Expect light to moderate snow to end before dawn for areas to the west of Bath with snow sticking around through around 9 AM for areas to the east of there.
After snow moves out this morning, NW winds will take over and we will begin the upslope/downslope cycle where the mountains squeeze out the moisture in the air causing clouds and flurries while the foothills/coast sees all the remnant dry air and thus clearing. The moral of the story – mountain clouds/flurries, coastal sun. Temps will be in the 20’s for most with some 30 degree readings possible along the coast.
Special update this evening to cover another Norlun trough type setup that looks to bring another round of light snow tonight and into early tomorrow morning. While the heavy snow bands usually associated with Norlun troughs will remain mostly offshore, an axis of shifting winds with reasonably impressive instability and some lift too will make an appearance mainly east of Brunswick tonight into tomorrow morning.
18Z NAM Showing A Distinct Wind Shift Line (Orange Arrows/Blue Dashed Line) Over Midcoast Maine At 7 AM Tomorrow. Credit: Weatherbell
A low pressure area that brought snow and ice to the southeast US will move too far offshore to bring major impacts though a Norlun trough, basically a wind shift line, will extend to the NW of the storm bringing coastal areas some snow. Snow will be concentrated between 11 tonight and 9 tomorrow morning.
12Z GFS Showing Intense Upward Motion Over Midcoast Maine At 7 AM Tomorrow. Credit: Accuweather
While snow will mainly be light, especially west of Brunswick, some moderate to even heavy bursts of snow are likely across Midcoast areas. The map above shows intense upward motion across Midcoast areas tomorrow morning signaling bands of heavy precip are possible. The areas most at risk for heavy precip will be east of the Bath/Brunswick area.
Here is the expected snowfall from this event. Not a lot, but the driveway will once again have to be cleared especially over eastern areas.
@SurfSkiWxMan@capecodweather , it's a hybrid NORLUN.. impressive lift/instability..no question there will be +SN, question is for how long?
In case you were wondering if this event was a Norlun trough, Weir Lundstedt, one of the two ‘discoverers’ of the Norlun trough (he represents the ‘lun’ part of Norlun) confirmed yesterday evening this did in fact look to be a hybrid Norlun event.
What a change over the past few days! First we were looking at the potential for a foot of snow from a powerful ocean storm and now we are looking at 6″ from its wicked inverted trough. Remember that inverted or norlun trough from earlier this winter? They are impossible to predict where they will set up until they actually do set up. Now I am thinking it will set up right near Portland or maybe a little south. Either way, there should be snow breaking our today from “ocean effect” or lake effect snow but over the ocean. Ocean effect snow tends not to be as heavy as lake effect and that is what we will see today.
There is a developing low off the Carolina coast that is moving ENE. This low will skirt too far offshore to have too much of an effect. its norlun or inverted trough however will. This troughs are notoriously impossible to predict as we saw earlier this year. Right now my thoughts are that it will dump a sufficient amount (6″) to many across southern Maine. My snow map is below.
Snow map.
Check out my other two sites for town by town forecasts for the snow. Those will be updated with the 18z runs tonight. They can be found at the right of my page.
-Jack
Models now agree this morning that the comination or phasing of these two storms will occur too far to the south and east to give us much snow. I expect a widespread 4-6″ from an inverted trough. Remember what happened with that last time? We all expected 8-12″ but we got 0. These things are absolutely impossible to predict so confidence is super low. Snow map coming with 12z models midday.
The area of low pressure will develop and move to the ESE today into tonight. It will then rapidly deepen South of the Maritimes. The inverted or norlun trough will hang around behind giving us minimal snow. Check back later for the snow map.
-Jack
Forecasterjack 