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The science of Sandy: how a Category 1 storm can panic a nation

Another x-post from BSR.  Wow, it almost looks like I'm turning this into a meteorology blog...

While I'm sure that you are all enthralled by the "in-depth" coverage of our presidential race, a few of you might have noticed that there's a giant hurricane barreling down upon the Northeast.  Amongst the tweets, mudslinging, and poll "results", Hurricane Sandy has quietly gained strength, becoming a hurricane with some real destructive force over the last couple of days.

So what is it about this particular hurricane that's got people so worried?  Certainly, it's not the kind of "mega-storm" that we tend to associate with mass evacuation and destruction: Sandy is only a Category 1 hurricane.  And yet, a wide range of meteorologists and hurricane experts have urged extreme caution on par with other hurricanes that we still reminisce about with anxiety today.  Why?

Well, one question you might ask is: "Why is this hurricane happening in the first place?"  As I'm sure you've all noticed, the large majority of hurricanes occur around latitudes of zero: warm, tropical regions like the Gulf of Mexico or the Caribbean.  However, Sandy is arcing its way up the east coast, and looks to make landfall somewhere in the northeast of America.  Why might this be?

It turns out that (very generally), there are two kinds of forces that mold hurricanes: barytropic and baryclonic.  Broadly speaking, these describe the extent to which there is variability in the atmosphere around the hurricane itself.

Barytropic atmospheres are more-or-less uniform across a large area.  Think of the "tropics", where the temperature tends to be the same muggy and warm across many days.  In these areas of the country, and in these weather systems, there are no large difference in temperature or pressure from one region to another.

This is in contrast to baryclonic, which describes a weather system that is governed by changes in air pressure.  This is pretty much how weather works everywhere outside of the tropics - with cold fronts, warm fronts, and storm systems that get pushed around by differences in air pressure.

Since Sandy has made her way outside of the tropical region of Earth, she must now succumb to the baryclonic pressures that all storm systems are subject to.  Unfortunately, these atmospheric pressures allow Sandy to carve a path straight to the Northeast of the country.

See that low pressure region to the northwest of Sandy (labelled "trough")?  And see that higher-pressure region to the northeast (labelled "block")?  This difference in pressure is basically doing two things:  one is preventing the hurricane from spinning off into the Atlantic ocean, and the other is sucking it in towards America as it moves further north.

So given that these pressure systems are pulling Sandy towards the northeast, why does this particular scenario spell a serious destructive force?  Well, baryclonic  conditions not only guide a storm in a certain direction, they can also introduce energy into the storm.  This is how the destructive winds of tornadoes gather.

In this case, as Sandy gets closer to the low pressure region to the northwest, she will undergo a transition from a so-called "tropical" (warm-core) storm, to an "extra-tropical" (cold-core) storm.  This means two things:

1. The storm will begin to spread out over a larger land area.  This is fairly common for storms that leave the tropics, but it's especially important here because of the huge number of people that live in the northeast.  It's one thing for damaging winds to take down power in a single city, it's another for them to disable one of the most densely-populated regions in the country.  As an example, here's a projected size of Sandy (from 1 day ago).  Whoa that's big.

2. The low-pressure zone will intensify winds near the northern tip of the storm.  The low-pressure zone acts as a "sink", or "trough," into which winds want to flow (as though they were water flowing down a hill).  A steeper pressure gradient means that the storm's winds will pick up more speed as they get closer to the low-pressure region.  This means that the storm surge near the northern half of Sandy will be particularly worrisome.

So, what does all of this mean for those of you living in the northeast?  Should you reinforce your windows?  Stock up on food?  Put up sandbags around your house?  Well, right now it's hard to say.  Undoubtedly, this will be a catastrophe that will impact the lives of millions of Americans.  However, it's also not the kind of "blow your house away" hurricane that strikes fear into our hearts when we see those black and red flags go up.

As with anything meteorological, any prediction of damage is just that: prediction.  However, many of the best weather blogs suggest that we can expect to see the most damage coming from the storm surge north of Sandy's eye.  For example, check out these predicted surge levels for Atlantic City, NJ:

The reason that these predictions rise and fall over time is because of the natural tide motion that we experience year-round.  It's easy to forget about in the context of a massive hurricane, but these tides add several inches to the coastal sea levels, which means a few extra inches of overflow once the seas start to pour into city streets.  Right now, projections suggest that Sandy will hit its peak at low tide, but keep an eye out in case she slows down and looks to hit during high tide.

As you can see, the projected amount of flooding is alarmingly large for cities and towns along the Atlantic Coast.

So, moral of the story:  if you're living inland in the northeast, be wary of a lot of rain, and some damaging winds.  Power will likely be out for quite some time, and your morning commute won't be the same for several days at the very least.  If, however, you live on or near the Atlantic coast, you should take serious precautions in the event of a massive storm surge of water.

Sandy is a powerful storm that is only made more ferocious by a number of weather patterns working in tandem to both draw her to America and to strengthen her power as she makes landfall.  These kinds of storms are truly rare, and so we should be prepared to deal with the significant damage that will likely ensue.  I'd urge you all to contact your loved ones in the northeast, and to keep them in your thoughts as the sit out this massive storm.  More importantly, we should all be prepared to assist the northeast in whatever way possible once the air has cleared.

Note: If you'd like to follow along with the latest news of Sandy, or just want to read about some really interesting weather phenomena, there are a bunch of great weather blogs that I used to pull together much of this information.  In particular are Jeff Master's Wunderblog (from and The EPA Weather Blog. Picture of Sandy from space courtesy NASA TV.