Weather Whys--Norlun Trough May Bring a Foot of Snow

After another mild stretch this weekend, Southern New England is bracing for a bitter blast of cold air and snow.

The next batch of snow arrives first as snow showers Monday evening, and becomes a steadier and heavier snow by Tuesday morning.

The snow comes courtesy of a developing coastal storm passing just offshore. However, a Norlun Trough extending outward from the storm will provide a focus for heavy coastal snows.

A trough is simply a finger of low pressure that ends outward from a main storm center (dashed black line in image). The "Norlun" name comes from two meteorologists who studied heavy New England snow events associated with these trough features.

They found that these troughs extended out from coastal storms and can result in extremely heavy snowfall along the coast. In March 1992, parts of coastal Maine picked up 1-2 feet of snow in just 8 hours. A year later, in 1993, parts of the Cape saw 12-20" in 6 hours.

Troughs produce intense snow squalls with snowfall rates of 3-4"/hours at times. They are able to effectively focus and lift moisture in an unstable atmosphere.

But the prolific snows that can result from the Norlun Troughs are very localized, and forecasting where the trough sets up is exceptionally challenging.

So, by Tuesday morning, the question of where the trough sets up is crucial. Right now it looks like the trough will first clip the Outer Cape before spinning up towards the North Shore, the Seacoast of New Hampshire, and Southwest Maine. That's where 6-12" is likely. Elsewhere, expect a general 3-6" of snow (highest totals closest to the coast). But, if that trough sets up in a different location, all bets are off.

Stay tuned to HinghamWeather.com and my twitter account (@MichaelPageWx) for the latest on the Norlun Trough position.

-Meteorologist Michael Page