There is nothing like a Norlun trough In the past fourteen years that I have been forecasting, with all of the weather phenomena that go on in this state, there is nothing trickier to predict than a classic Norlun (inverted) trough. Personally, I'd rather figure out the thermodynamics of junk storms. I've lived in this state for 56 years. I've seen these turn into epic busts, and I have watched these turn into mini-blockbusters. They are fingernail biters; they keep forecasters up all night, which, for reference, I woke up at 11 PM last night and have to work the day job. The coffee pot is my IV, along with adrenaline, since Christmas is here.  While Norlun troughs are tricky to predict snowfall amounts, there is a climatological history with these. The area that typically gets them is highlighted in orange on the impact chart. There have been a few York County and DownEast area set-ups in the past as well, but the region of Portland to Lewiston to Augusta to Belfast is where they tend to line up. There could be books written on these. The main ingredients are a trough backing in from the northeast, running up against the ridge, trying to shove its way in. Along the trough, an ocean storm develops, and the interaction syphons moisture toward the shorelines under a southeast wind. With cold air firmly in place over the region thanks to the trough, a high snow-to-water ratio event occurs that can deliver a fluffernutter. Go get some bread, peanut butter, and a jar of Fluff, and celebrate as the snow flies. Have one ready for your plow guy when he comes to clear you out. How much fluff depends on the development of a weak low over the Gulf of Maine... and whereIMPORTANT: This type of setup can turn into a gong show of ideas, depending on where you shop for weather information. Most weather apps are trash here, given how they are wired. This is an event that requires human interpretation and local knowledge. I am using the Canadian short-term idea as a reference point. I am not saying it is entirely correct, but in my view, given the dynamics, it appears more realistic than other guidance given by climatology.  Tuesday 1 PM (18z) to Wednesday 5 PM (22z) - The emergence of the trough has slowed down, which will allow for the Tuesday evening commute to be completed with relatively minor impacts to the roads. Light snow is expected to move northeast as a ridge tries to push a warm front into the area. Overnight is when it gets interesting: the trough backs in, creating the inverted "Norlun" aspect, and a weak surface low forms along the shoreline. The wind picks up around it, and it picks up juice from the offshore low that the ridge hauls in from southeast to northwest. This is where the snowfall rates pick up. This 10:1 snow-to-water ratio idea, if it verifies, could actually be 15 to 20:1, which would bring 50-100% more snow per hour than what it depicts. If all the ingredients continue to work off of each other as ideas indicate, the snow will nuke potentially 2+" per hour around the weak surface low, which will continue in the mid-morning hours on Wednesday. As the ocean storm moves farther out to sea, well south of Newfoundland, the moisture hose shuts off, and snow tapers from northwest to southeast in the morning, ending along the shorelines by mid-afternoon to sunset. With the weak shoreline surface low, the wind picks up...  Tuesday 1 PM (18z) to Wednesday 7 PM (00z Thursday) - Looking at wind gust potential in this loop, which shows the southeast push developing overnight, then with the weak low developing, it shifts the wind to the northeast by Wednesday morning. This will blow around the fluff and may cause localized whiteouts where heavy snow has fallen. This will make it a bit of a challenge for road crews to keep up with for a time and may affect travel on the 95. 295, and Route 1 corridors through early afternoon. As the offshore ocean low moves east and the weak Gulf of Maine low collapses, wind will diminish, giving the region a peaceful Christmas Eve night.  Unless guidance is selling a Porsche with a Yugo engine, which is always the risk with Norlun dynamics, a good portion of the southwestern areas are on track to pick up at least half a foot of snow. With it blowing around, given the fluff factor, it may be challenging to measure. Where the weak shoreline low develops is the ballgame here. These types of inverted troughs are a challenge. Over the years, I've seen areas get an inch or two, and a couple of miles down the road, a foot or more.  The other piece in this for bust potential is how far inland the moisture from the southeast extends. Guidance is a bit more juicy as we get closer, which has allowed for an increase in potential snow further inland. The bottom line here: any deviations in the development and timing of the offshore low, in conjunction with the trough backing in and the ridge moving in, will change the outcome to either more of a boom or a bust. You can't say you haven't been warned.  Outlook from Christmas into the weekend Wednesday 7 PM (00z Thursday) to Sunday 7 PM (00z Monday) - Snow showers appear likely for Christmas day, which may drop an inch or two over the mountains and north and bring flakes elsewhere. Expect a breezy and bitterly cold day on Friday. Another system works in from the west Friday night and may bring snow showers to southwestern areas overnight into Saturday morning. The next system of widespread concern comes from Sunday into Monday. Will it be a junky mess or more of a snowy affair? Stay tuned.  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Mike Haggett
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