There’s a lot of weather going on this evening and much of it is centered on very small scale features which are difficult to pinpoint. The smaller the catalyst for a weather event, the more difficult it is for the atmospheric models to discern and the less accurate the forecast. Computer models have a limited resolution and they don’t do well with these more innocuous perturbations in the atmosphere.

The computer models will often indicate the idea of snow bands, thunderstorms or a chilling sea breeze, but pinpointing the precise location of these relatively small features can be virtually impossible.

Difficult Forecast
Tonight we have an area where air is meeting from two slightly different directions. When air comes together, it’s called convergence and this often leads to precipitation. Small areas of convergence can lead to limited areas of precipitation, but the precipitation along these lines can be quite heavy.

Norlun Trough
A particular type of surface convergence occurs around a weather feature known as the Norlun Trough. These features are not uncommon, but don’t happen more than one or two times a winter. They are incredibly difficult to forecast. I have seen a foot of snow predicted based on the setup of a Norlun Trough and not a flake has fallen. I have also seen the opposite occur.

If you are curious about the name of this system, the “Nor” comes from Steve NOguieRa, and the “lun” comes from Weir LUNdstedt. In 1993 these two meteorologists authored a paper which described the aforementioned situation that has come to be known as the Norlun Trough.

The bulk of the snow associated with the current trough is forecast to fall from the seacoast of New Hampshire into Maine. There’s a lot of uncertainty in the position of the band, but I don’t see it setting up through Portland. It appears the heaviest snow will either occur west or north of the Greater Portland area. This doesn’t mean there won’t be enough snow to plow, but the chances of a foot or more of snow in your particular area isn’t very high. Tomorrow morning the commute will be impacted by snow at varying intensities across southern and central Maine. Some ares will have a plowable snow while other areas could just have a coating.

As if this isn’t enough to forecast, there’s also some energy moving across the region at about 10,000 feet. This is going to further help produce some snow into Thursday night and even early Friday. The map below gives snowfall totals through Friday morning. Generally, it’s up to a 6 inch snow event with exceptions. Notice I highlighted various areas that might see more than 6 inches of snow. These are the areas if everything came together just right (or wrong), a foot of snow could fall. My confidence is low on this forecast, especially the location of the heaviest snow. Expected the unexpected over the next 36 hours when it come to snowfall.

It turns cold again Thursday night and Friday. It’s a similar type of cold we have seen this winter. The coast will be near zero and inland areas well below and highs Friday will be in the teens. Do whatever you have been doing to keep warm since January.

More Snow Or Perhaps Mixed Bag This Weekend
Saturday night, as milder air streams north, more snow will break out. I expect accumulation during this period. The snow will eventually become wetter and there could be a mix with freezing rain or even a change to rain south of Portland along the coast. The ground is about as cold as it gets this time of year and therefore even if your thermometer reads 33F or 34F, any rain is likely to still freeze on some surfaces. The snow to mix line is going to be another challenge to forecast. Seems like this is the period for having to make tough calls and this means changing forecasts so keep checking back here and on Twitter @growingwisdom.

The image below is one model’s prediction for where it will be raining, snowing or mixed during Sunday morning. Image credit: WeatherBell Analytics.

The warmer air moving over the snow is also going to help form fog. The fog could become quite thick Sunday and depending on how thick this could impact travel. Obviously, any rain is going to weigh down the snow. If you are concerned about your roof I suggest you take off some of the snow by Sunday. Unless you are experiencing problems, you don’t have to remove all the snow. Leaving a foot of snow, even wet, on a structural sound roof isn’t going to cause an issue. If we had only seen a foot of snow this month and it started raining, I’m not sure anyone would be climbing their roof to remove snow.

Below average temperatures resume Monday and Tuesday with another system to watch next Wednesday. The pattern of cold temperatures with an active storm track shows no signs of loosening just yet.

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