What makes superstorm Sandy so unusual?

National and local forecasters on the East Coast are really in for a challenge this week not only forecasting what's expected to become a monster storm in Sandy but trying to communicate the storm's expected impacts because... there really is no playbook for a storm like this.

"Hurricane" Sandy -- I use quotation marks because it's set to become a different type of storm that won't technically be a hurricane -- is about to both undergo a transformation and follow a path that is exceedingly rare.

Add in the fact that it's aiming for the most populated corridor in the United States and you've got history brewing -- a stitched-together weather monster expected to unleash its power over several states, with predictions in some areas of 12 inches of rain, 2 feet of snow and sustained 40- to 50 mph winds.

But what makes this storm so different from others that have struck the northeast? That area is certainly no stranger to hurricanes nor the infamous "Nor'Easters". But this storm is not only set to combine both kinds of storm into one, but potentially be stronger than either kind that has hit the area before.

Here are some characteristics that make the storm quite unique:

1) The path

Atlantic hurricanes are common in October, but typically what happens is the storms move north up the southeastern coastline, then follow a warm water current that curves to the east, roughly following the curve of North Carolina's southern coastline, and sends it out to sea.

To get the occasional New England hurricanes, you'll have other weather factors in play that prevent the hurricane from turning east and instead send it into North Carolina's coast. The storm then keeps churning north where it might reenter the Atlantic around eastern Virginia, then keep pushing north until it makes landfall somewhere on the New England coast.

But by then, the hurricane has A) been weakened a bit from going over land and B) entered the cooler waters off the Mid-Atlantic and New England coast.

This is roughly what happened with Hurricane Irene, which made landfall in North Carolina's Outer Banks, then went into the New York metro area as a tropical storm. Still a major storm -- especially for beach waves and rainfall. But winds were tropical storm strength.

Nor'Easters typically move up from the Gulf of Mexico and southeastern U.S. where it merges with an inbound arctic system from the northwest to get stronger and can bring gusty northeast winds to the New England coast and heavy snow inland.

This storm is taking a strong Nor'easter type set up, and infusing Hurricane Sandy into the mix.

But in addition to the volatile mix of ingredients, the path of the storm is extremely unusual. Sandy is expected to take the "North Carolina curve" a bit and head to the northeast, dodging North Carolina and not getting the weakening effects from landfall.

But then Sandy is going to run into a big ridge of high pressure to it's east - actually it's a similar "Rex Block" pattern that we had here in late September/early October that blocked all the incoming storms and continued our dry, sunny streak.

Then, as I wrote on Friday, an upper level trough that was here Friday will eventually move east and mix with Sandy to pull the storm back to the west -- very rare! -- and into the oncoming big arctic storm dropping out of Canada.

2) The Storm's Makeup

You've probably heard it called "Frankenstorm" in its early days (now everyone is sticking with Sandy for continuity sake, considering the seriousness and need to have everyone be on the same page) but the moniker was partially due to the calendar (near Halloween) but also because this storm is expected to become a hybrid of other storms.

Sandy is not going to be a traditional hurricane making landfall, but is going to undergo a transformation from a hurricane/tropical type storm to what's called an Extra-Tropical storm (extra in the "exclusion" sense, not "Now with even more tropical!")

The difference is the source of the storm's energy. Hurricanes/tropical storms get their energy from the heat of warm ocean waters. Extra-tropical storms get it from battle between warm and cold air masses (Northwest storms are similar "ET" storms.) Sandy is going to transform from getting its energy from the Atlantic Ocean waters to tapping into the intense arctic air moving into New England from Canada and the Midwest. But in the interim, it's going to draw power from both sources, making it rapidly intensify.

(Incidentally, our great Columbus Day Storm of 1962 also underwent a similar transition from Typhoon Freda to an ET storm)

It also means Sandy won't behave like a typical hurricane. There, you expect the greatest winds to be right around the eye wall, diminishing as you get further away. This storm instead will have an incredible widespread difference in pressure spread out over hundreds of miles, creating a huge cone of wind around the storm. Unlike hurricanes, it won't matter if you suffer a direct hit -- the winds 30 miles form the storm center could be on par or even less than winds 100 miles away. Models also indicate the wind field will stretch hundreds of miles inland -- not just be relegated to the coastal areas (although they will likely have the strongest winds.)

It will also feature the immense storm surge of a strong hurricane on the coast and potential for heavy snows inland around the Appalachian Mountains.

"We may have written the first hurricane forecast article for that includes a heavy snow outlook map," Jon Erdman, senior meteorologist for The Weather Channel, wrote on

Louis Uccellini, the environmental prediction chief of the National Oceanic and Atmospheric Administration meteorologists, estimates 60 million people will feel the storm's wrath somehow: "This storm as it grows and moves back to the coast on Monday and Tuesday, the circulation of this storm will extend all the way from the Midwest, the Ohio Valley, toward the Carolinas up into New England and southern Canada. It's really going to be an expansive storm system."

The transition from hurricane to ET storm is also creating headaches for those crafting warnings for the storm. The National Hurricane Center said in their 5pm EDT advisory Saturday that it was not going to issue tropical storm/hurricane warnings for the northeast and mid-Atlantic as Sandy transforms to a non-tropical storm. Instead, they will let each of the local offices of the National Weather Service issue High Wind Watch/Warnings instead.

That decision has already led to some disagreement within the weather forecasting community:

I completely disagree with NHC not putting up Hurricane warnings for the northeast.#sandy

Jim Cantore (@JimCantore) October 27, 2012

Just another illustration on the communication challenges of this storm.

3) The Storm's duration

With the blocking pattern to the east, Sandy will not just blow through in several hours like most hurricanes do, but will meander around the northeast for possibly 2-3 days. It'll be gradually weakening, but continue to pour rain, heavy winds, and even heavy snow to some areas as it does so.

How does it compare to a Pacific Northwest windstorm?

NOAA forecaster Jim Cisco and others have called this storm unprecedented. Uccellini, who has written histories about winter storms, said the closest analogs are the 1991 Perfect Storm that struck northern New England and a November 1950 storm. But this is likely to be stronger and bigger than the Perfect Storm; it will strike farther south, and affect far more people.

In fact, the location among those with the highest odds for gale-force winds in the country's most populous place: New York City. New York has nearly a 2-in-3 chance of gale force winds by Tuesday afternoon.

I thought I'd try to show some comparisons to the last great Pacific Northwest windstorm: The Hanukah Eve storm of Dec. 14-15, 2006. Although I must caution this is a bit apples-to-oranges, but it's just to give some perspective:

That storm had wind speeds in the Puget Sound interior that roughly ranged from 55-70 mph with similar speeds on the coast. Sea-Tac Airport set its all time highest recorded wind speed gust at 69 mph. More than 1 million people lost power. Strong winds lasted about 8-10 hours with peak gusts about 3-4 hours' worth.

Sandy has potential to have its peak winds last for much longer.

Let's talk pressure difference -- the true driver of winds around here. The greater the difference in pressure, the faster the winds will be. The Hanukah Eve storm set a record for greatest difference in pressure between Portland and Bellingham -- a benchmark for measuring Puget Sound wind storms -- at 23.2 milibars. That is a distance of roughly 250 miles.

Some forecast models indicate about a 20 milibar difference in pressure just over 120 miles with Sandy. Incredible.

Seattle received 1.57" of rain in the storm -- much of it coming in an intense squall ahead of the storm. A wide swath of the northeast is expected to have 4-8 inches of rain with some spots could get 10-12 inches.

Seattle also didn't have to deal with intense storm surge, several hours of pounding waves, or snow either.

Even some of our other windstorms have brought greater winds to the coast -- wind speeds of 70-90+ mph have peppered Northwest history. But here, the coast is rather barren and the small towns on the shores are certainly used to it. With Sandy, you're potentially bringing that kind of winds to Jersey Shore and the rest of the heavily populated coastal regions.

Truly a one-of-a-kind storm

You can see forecasters have a challenge to communicate just how dangerous the storm is turning in to, even if Sandy is "only" a category 1 hurricane or tropical storm in the days leading up to its landfall. The Sandy of today will not be the Sandy of Monday.

They also will have to emphasize this won't be "just another hurricane" or "just another Nor'Easter". A lot of long-time residents there may be thinking they've been through these type of storms before, but this one could truly be something unique to our generation.

The Associated Press contributed to this report.