I've been meaning to bring this up in the blog, but I feel like the posts are going to get inordinately long (I mean even longer than the long that they are now)... So, in lieu of tangents that take away from the constant demand to know more about what's going to happen next, I thought it a good idea to stick this educational nugget on its own, in case you're interested.
This post will talk about how and why a hurricane might come on-shore like the way the models are projecting, despite the most common meteorological expectation is that it continue harmlessly out to sea.
As of this posting, here is the projected storm track of Hurricane Sandy, together with what climatology would say the storm "should" normally do...
|NHC - Hurricane Sandy - Projected Track|
When forecasting hurricanes going up the eastern seaboard, the normal rule of thumb involves Cape Hatteras, NC (the eastern most tip of the outer banks). If the hurricane's center passes to the west of the town, the storm will go inland; if it passes to the east, the storm will end up curving out to sea and only affect coastal/surf/shipping lanes; if it goes right over it, then it will hug the coast before slamming into New England. In our situation right now, the conventional wisdom would have Sandy passing well out to sea and only amount to white-caps on the coast. However, all the computer models and indeed the official NHC forecast does not have this.
So what's going on here?
The answer does not lie in what's going on at the surface, but instead aloft. Perhaps the best indications come with those oh-so-confusing 500mb charts that we sometimes use. Since this is a blog post focused only on this issue, I can expand the dialogue here... so let's look at the first map:
|GFS - 500mb Chart - Saturday Morning|
For those of you who haven't seen a map like this, allow me to explain it a little bit. The lines on this map represent the altitude (in 10s of meters, or decameters) above sea-level at which the 500mb pressure level is reached. I wouldn't worry about this technical definition though, other than to know that on this map we can use these lines to identify highs and lows just like we would at the surface.
As far as the red and purples go, this is also pretty important. The Earth is spinning at a constant rate, and this spin by itself can generate twist in the atmosphere, as well as send air moving in an initial direction (typically west-to-east in our latitudes, and east-to-west in the southern hemisphere). Within that general spin, there are always little eddies of concentrated twist that appear due to small-scale features we call "natural variability" or "weather". On this map, we refer to this as "vorticity". The vorticity can be "positive", or enhancing relative to the spin of the Earth; or, it can be "negative", or diminishing relative to the spin of the Earth. Here, the reds represent areas of positive vorticity, and the purples are areas of negative vorticity. If you think of it in terms of topography, for the best storm development, you always want to look at places where you're going uphill, as in toward increasing vorticity, rather than downhill toward the negative. Are we good there? If not, feel free to ask questions in the comments below.
Okay, so now I'm going to re-present the same map, except with a few additions...
|GFS - 500mb Chart - Saturday AM|
Each one of these Xs represents a local maximum of positive vorticity. We call each of these a "vortmax" for short. They typically follow the main airflow on the map, and enhance/agitate the air wherever they go. If you follow along the black lines, keeping the lower numbers to the left, you can trace the general flow of wind (on this map it's generally west-to-east with a big dip in the Northern Plains). Hurricane Sandy (at the bottom right) also has a heckuva lot of positive vorticity (naturally), but it's trapped within the circulation of the system and isn't going anywhere.
So these Xs will be moving and eventually coming around the base of this trough you see in the Northern Plains. The first vortmax is responsible for scaring up our scattered showers we'll be watching today, but will also enhance how far the trough digs behind it. It will have an additive effect.
|GFS - 500mb Chart - Saturday Evening|
By Saturday night, the first vortmax has rounded the base of the trough, and done its job of enhancing it. Notice the trough is started to get more pointed, just itching to spill down through Minnesota, etc. But there are still two more vortmaxes to go. Meanwhile, Sandy is still wandering northward toward the Carolinas, prevented from racing off to sea due to just enough of a blocking pattern to its north.
|GFS - 500mb Chart - Monday Afternoon|
It takes a 1-2-3 Punch, but the third vort-max is the charm, and you can see now that the enhancement of the large trough has now caught the outflow of Sandy, and it is becoming part of the system. Even though a hurricane is a big deal, look at the relative sizes of the two in these maps above-- the deepening trough by far dwarfs the pinpoint prominence of Sandy.
Earlier today, I described it as akin to a "bad mattress" :-)
Poor old Sandy gets into one side of the bed, but the big, lumbering trough gets into the other. In the situation of a bad mattress-- Sandy will sink to the middle. (Am I the only one who's had a bad mattress before?)
So...now what? Well, now that the trough has also captured the hurricane, you can see that this particular vortmax now has more ground to cover, and an even broader trough to bend around (and enhance). So it can actually bend the hurricane inward as it rotates around the base of this new "Frankenstorm"-y trough...
|GFS - 500mb Chart - Tuesday Afternoon|
So now this monster vortmax is rotating around this new combo storm-- Well actually, they are both rotating around eachother, which by itself is quite a sight (and a horrific math problem if you ever had to crank it out on paper thankgodforcomputers). This process actually has a name: the "Fujiwhara Effect", named after the guy who first described it in the 1920s. Normally it can refer to how hurricanes would revolve around each other without combining, but here it can also describe how the center of Sandy becomes entrapped within this deep trough and is forced to wobble around in the middle while a steady stream of juicy vorticity dances around it. If all these models prove correct, we will see this storm appear nearly stationary for more than 24-hours over land, but will actually be doing small loop-de-loops as tremendous upper air energy interacts.
Here's a video showing the Fujiwhara Effect when it comes to two tropical systems dancing around each other...
I will be expecting to see such looping around with this "Frankenstorm" in the coming days. But remember, the actual 'catch' of the hurricane won't even happen until Monday. So we still have plenty of time to continue to monitor the models, evaluate their accuracy to date, and see what happens.
Again, if you have any questions or comments or complaints or suggestions, or anything else. Please feel free to ask them :-)