This complex subject deserves a bit of explanation, and that begins with describing the differences between tropical and non-tropical low pressure systems.

A tropical cyclone is a low-level circulation that gets its energy from ocean heat. It's a little bit like a whirlwind on a parking lot that gets its energy from the heat rising from asphalt.

A non-tropical low-pressure area is a circulation through many layers of the atmosphere that gets its energy from atmospheric dynamics.

Think of the atmosphere as a broad river of water, with a series of currents. The currents develop eddies, or swirls. A swirl near the top of the water sometimes gets the water swirling underneath it, until its swirling from the bottom to the top. This is something like our atmosphere.

When you hear the common term "upper-level low," it's referring to one of these swirls, counterclockwise in our hemisphere, riding along in the atmospheric currents several miles high. An upper-level low is typically a pocket of cold air, which helps trigger precipitation as warm, moist air rises into it and condenses.

Under the right circumstances, the upper-level low can get air swirling in the lower layers, and a surface low pressure area can be born, typically along a boundary between warm and cool air at the surface.

This low pressure setup differs dramatically from a tropical cyclone, which begins at the surface and builds upward in a warm, humid environment. A tropical cyclone also differs in that, rather than having an area of low-pressure in the layers above it, it is capped by a calm area of high pressure aloft.

A strong low-pressure aloft deters tropical cyclone development largely because it produces strong winds that can shear the tops off the tropical cyclone's developing squall and eye wall clouds. But the calm air of the high pressure aloft allows the tropical cyclone to build undeterred.

So how can a tropical cyclone end up changing into a strong non-tropical low?

Well, most of of the time, it doesn't happen. Most tropical cyclones are destined to rain themselves out somewhere inland, or get blown apart by the atmospheric winds aloft over the continent. Hurricane Dennis this past summer had nothing to steer it anywhere after landfall, so it slowly rained itself out over the Tennessee and Ohio valleys. Hurricanes Katrina and Rita were two of the mightiest storms ever to prowl the Gulf of Mexico, but once they got into the atmospheric flow over the continent, they were quickly ripped apart into just a few wet wisps of wind.

Sometimes, the moisture from a tropical system will get entrained into an existing non-tropical storm system. The tropical circulation dies, but the storm's moisture lives on to rain or even snow on people far away. Wilma was something like this.

But in a few cases, the tropical cyclone is in the right place at the right time to combine with a frontal boundary and connect with an upper-level low, and its circulation is the genesis of a new non-tropical surface low. The wind field weakens some and broadens out, and the center of the storm converts from being "warm-core," with an area of warmer, very moist air near the center, to "cold-core," with chilly winds rotating near its center.

A few extreme cases like this become very powerful non-tropical storms. Hurricane Hazel converted into a strong inland low in 1954, bringing near-hurricane-force winds to parts of Virginia and all the way into Canada. The 1900 Galveston Hurricane did the same, bringing damaging winds days later to Chicago. And of course there was the "Perfect Storm" that absorbed a hurricane's circulation and became a historic and deadly menace.

Just because a storm isn't tropical doesn't mean in can't be fierce. Folks on Cape Cod and Cape Hatteras will tell you a violent nor'easter is quite often more frightening than many hurricanes.