Unsettled Pattern Of Cold, Snow And Mixed Precipitation

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.

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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.


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. However, there’s enough uncertainty in the position of the band that eastern areas of Massachusetts are vulnerable to more snow, especially Essex County.

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 overnight and into Thursday. The map below gives snowfall totals through Thursday evening. Generally, it’s a 2 inch snow event with a few exceptions. Notice I highlighted much of Essex County. Here is the area if everything came together just right (or wrong), more snow could fall. My confidence is low on this forecast, but I thought it was prudent to mention it.


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

Mixed Bag This Weekend
Saturday night, as milder air streams north, more snow will break out. I expect some accumulation during this period. The snow will eventually change to sleet and freezing rain and then all rain. The ground is about as cold as it gets this time of year and therefore even if your thermometer reads 33F or 34F, the rain is likely to still freeze on some surfaces into the first part of Sunday. Along the coast this freezing rain won’t last long at all, but west of Route 495, there could be icing for several hours.


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, the 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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