One thing I’ve noticed that often occurs before extremely cold/snowy/snow-stormy weather is that the day preceding it is unusually calm and/or warm. Like today, for example. Here it is about 50 degrees with no snow or precipitation, but for the next few days we are set to have temperatures in the teens and snow daily. What causes this ‘calm before the storm’?
The general term is a weather front. The layman’s explanation is that it is the result of one set of atmospheric conditions being pushed ahead by a different set. For example, the fronts that you are seeing are a warm front followed by a more powerful cold front that is pushing the former away so that it can settle in.
That isn’t generally what is meant when people say ‘the calm before the storm’ however. The colloquial terms refers to the fact that storms often accumulate the atmospheric energy around them so the areas in the periphery are calm due to lack of energy. You can also so this in the ‘eye’ of a hurricane which has relatively little energy compared to the bands. People used to think the storm was over when the eye passed over them but it was only half-way over.
Ah, but why the unusually warm weather? Why would it spike to 50 degrees (in January…in New York!) before dropping all of a sudden? A similar instance happened last year in March (ish). It was cold, cold, then cold some more, then one day it was 70 degrees! and then there was a big snowstorm the next day that temporarily crippled the area.
In this particular case (and many others) it is due to the circulation around the low pressure system and the trailing cold front. As the low approaches, the counter-clockwise circulation means air is being pushed northwards from warm areas in the south. When the front moves through, the circulation on the back side comes down from Canada.
Yeah, blame Canada. As usual.