Norlun Trough: An axis of shifting winds on the NW side of a low. They are known to bring very localized, very heavy bands of snow so that one town gets buried and the next barely receives an inch. These features are wicked fickle and are very hard to forecast. 30 miles can make the difference between 1″ and 11″ and it’s hard to tell where exactly they’ll set up.

Here’s an excerpt from a post back in 2016 where I discussed Norlun troughs in a bit more detail. Note that this forecast info is not current, but is generally representative of what forecasts for Norlun troughs look like.

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.