The residents of the Mount Washington Valley have experienced numerous storms that produce mixed precipitation. Many dread the combination of rain, freezing rain, sleet, and snow during the duration of a low-pressure system. This arrangement produces dangerous driving conditions that are far more hazardous then an ordinary snowfall. The diverse platter of precipitation also fails to generate positive conditions for wintertime sports. Known as the extratropical storm, this low-pressure system is the culprit for many seasonal events that produce mixed precipitation during autumn, winter, and spring.
The term, extratropical storm, may have captured you as being an event that would not occur in North Conway. Such reasoning is sometimes based upon its title reference, tropics. Nonetheless, the low-pressure system is much more common in our region then what is perceived. In fact, extratropical storms dominate the pattern in the United States and many areas of the world outside of the tropics. These storms usually produce the dreaded bag of mixed precipitation not only for North Conway, but also for areas that usually do not receive snow or sleet.
The center of the extratropical storm is an area of low pressure that has winds blowing counterclockwise. These winds are crucial towards further development of the system. For this reasoning, cold air plunges southward from the North Pole as a cold front extends along its leading edge. Warmer air then drives northward ahead of the cold front, which in turn develops a warm front in advance of the storm. The clash of the warm and cold air leads to an extensive breakout in varying precipitation. Likewise with many of North Conway's events, the track of the extratropical storm is a crucial aspect for how much snow or freezing rain the region will receive.
Now that you have become an expert behind the force of mixed precipitation, it is also important to know what mechanisms produce rain, freezing rain, sleet or snow. First and foremost, all precipitation begins as snow. Since warm air is dominant ahead of an extratropical storm, it flows over the cold air near the surface of the earth. Therefore, the result is a layer of warm air (32 degrees or higher) sandwiched between cold air. In areas where the warm air extends towards the surface of the ground, the precipitation will fall and accumulate as ordinary rain. However, if there is a thin layer of cold air upon the surface, the precipitation will refreeze on contact as freezing rain.
On the backside of the extratropical storm, the cold air is much more prominent. Sleet will transpire when the surface layer of cold air supports the refreezing of liquid droplets into ice. As always, if little or any warm air is available, the precipitation will fall towards the ground as snow. All in all, one could say that the extratropical storm is an event of great magnitude in the atmosphere where large domes of air masses battle for dominancy over North Conway.