If you've looked at the the forecasting models for Hurricane Sandy, like the ones above on Weather Underground, you've no doubt noticed the hard left turn in the track that has meteorologists expecting an impact in the northeast U.S. It almost looks like an afterthought. At 35°N latitude it looks like a sure bet for "out to sea" and then, suddenly, the fake out move back to the coast. So what's going on there?
And what's more, what's this business of it lingering for so long? The AP today explains that it just won't leave, quoting NOAA forecater Jim Cisco, "It's almost a weeklong, five-day, six-day event."
The National Weather Service, in coining the "Frankenstorm" designation for Sandy characterizes the whole system as a "hybrid vortex" that includes an "amplifying polar trough" and a "high degree of blocking from eastern North America across the entire Atlantic basin." High degree of blocking?
Luckily, a line from yesterday's Metropolis blog at the Wall Street Journal (which is going with "snor'eastercane") gave me a lead on what to look up...
"An increasingly negative NAO pattern is forecasted to become much stronger than normal, which will bring a near halt to the normal west-to-east movement of storm systems across the U.S. That more than anything else will open the door for Sandy to be sucked back towards the East Coast."
NAO is North Atlantic Oscillation.
It turns out there is a semi-permanent low pressure spot around Iceland called "the Icelandic Low" and there's a corresponding semi-permanent high pressure spot around the Azores called "the Azores High" (ahem... middle of the Atlantic, kind of the same latitude as Portugal).
A positive NAO means the Icelandic Low is especially low while the Azores High is especially high. When that's the case, cold air over Canada moves right across toward Iceland (via the prevailing westerlies, which for some reason I associate with Vikings). BUT, we're looking at an increasingly negative NAO that is expected to be especially strong. Negative NAO means that low over Iceland is not so low and the high over the Azores is not so high, so "strong" in this case means much less contrast between the low and the high.
This is where things get a little less intuitive for me, but apparently without the strong low pressure to motivate those westerlies, cold Arctic air just builds up over Canada. I keep seeing the word "blocking" but it seems more passive than that. A better way of saying it is that it "decreases the pressure gradient across the North Atlantic. This decreased pressure gradient results in a slackening of the westerlies."
Anyway, that cold air makes its way down across the U.S. to the southeastern states via lower-than-average pressure (this is the trough part of the explanation) and the northeastern states end up with an especially snowy winter.
What you don't usually have is a hurricane sitting right in the middle of it all. But knowing the dynamics at play, it's easier to see how Sandy is hooking back to land and lingering for lack of any strong winter westerlies to push it away.
If I may add, separately, it's interesting to see the relationship between the North Atlantic Oscillation and the Arctic Oscillation. Last time I took a deep dive on weird weather (See: Why is Seattle getting all of my snow?), Arctic Oscillation figured prominently in that explanation as well. It's weird that we are generally familiar with the Jet Stream and El Nino and La Nina (and newly, a Derecho) but we don't often hear about these oscillations even though they seem especially relevant.
It's worth pointing out that we do sometimes hear about the Bermuda High. As far as I can figure, the Bermuda High is the same semi-permanent high pressure that defines the Azores High, and what it's called depends on where it is over the Atlantic.
Not linked in the text but also helpful, Wikipedia on NAO, which includes perspective on the effect on weather in Western Europe.
As ever, your expertise in this is likely greater than mine, so any further insight you can offer is appreciated. Weather is so complicated, with so many variables, I realize it's simplistic to single out this particular feature. I'm particularly frustrated that I can't find a map of pressure over the Atlantic that clearly shows the North Atlantic Oscillation. I get this far and then I'm not sure what I'm looking at anymore. Is that because Tony is swirling around out there?
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Oh Will, way to give us a Moment of Geek on the Weather! XOXOXOX!
Cool post. I enjoyed it. Thanks.
This doesn't have the ocean winds but does show over land: http://hint.fm/wind/ You can see the winds in the northeast going inland instead of out to sea. Not sure if that's the pattern that is caused by this.
Visual of NAO effects.
Run to the store and get bread and milk.
That puppy is heading over my neighborhood.
Yay.
Not.
Don't forget the ding dongs.
Or the bourbon.
Sharp turn in the other direction? I think this hurricane got a case of the Romnesia.
+1 Internets for you.
I am so FUBAR here in RI - almost every model has a direct hit to us.
The blocking high pressure system in this case can be seen most easily in the upper portions of the atmosphere. Here's a map approximately half-way up 'into the sky' showing the air movement. It is a forecast of the air flow around midnight on Tuesday.
http://mag.ncep.noaa.gov/GemPakTier/MagGemPakImages/gfs/06/gfs_atlantic_096_500_vort_ht.gi
The dark lines--labelled with numbers like 576 and 570--can be visualized as a topographical map representing elevation. So high values show the shape of 'hills' (or higher pressure), conversely lower values show depressions (lower pressure). The wind at this height in the atmosphere flow parallel to these lines as shown by the funny looking 'barbs'-- clockwise around high pressure, and CCW around lows. Regions of high 'spin' (vorticity in meteorology) appear as solid colors of yellow and orange. The location of Sandy is pretty obvious, south of Cape Cod.
With a little imagination you can see that "hill" (high pressure) in the top center of this image with winds flowing clockwise around it (the barbs help to visualize it). This high is blocking any further northward movement of Sandy and is pushing it to the west.
The usual disclamer: this is a forecast from a weather model to illustrate the complex flow patterns surrounding Hurricane Sandy. Don't use this image to make critical decisions.
Missing the 'f' at the very end of that link. Correct link below.
http://mag.ncep.noaa.gov/GemPakTier/MagGemPakImages/gfs/06/gfs_atlantic_096_500_vort_ht.gif
Liarwatha Warren is going to be out all weekend with her goofy line she's used throughout the senate campaign, "The middle class all across this state is (going to) get HAMMERED!" "Any body seen my fake Indian Head dress?"
They're listening. Yes, the people who own the intellegient horses. The ones that have no use for bayonets.
Frankenstorm? That was just one too many new words, we will need to revise the whole Dictionary again. There's quite an extensive new list this year, Romnesia was the latest entry, now this one. We are celebrating a new tradition here, the sighting of the great wall of Haboob. All the women from our tiny village commense to taking our tops off and dancing. They can hear the chanting from miles around, Haboob, haboob. This may all be blown out of proportion, Sandy is probably just experiencing a little PMS. I remember Fox news was quick to point this out for us, only women cry and only women bleed. It's okay, there, there, give me a break.
What I wonder is whether this may be related to the warming of the Arctic and whether this portends a shutting down of the thermohaline circulation. I realize they work on different timescales, but it seems to me that a weakening t.c. could reduce the Icelandic low.
Call me geeky too....I'm out to get the application for my MAIL-IN ballot today! I live in Jersey, just in case we get hit hard, have some power outages or worse.....
There are going to be a lot of wet pumpkins and soggy candy :( Thanks for the Update! and-- I just filled out my Florida Absentee Ballot--- DONE! going out in the mail in the AM!
Thanks Will !!!
Well done, Will! Looks like another late-season wind-and-rain event will put my sump pumps to the ultimate test. If the power stays on, that is. At least this old house has enough steel beams (boy, was I surprised to see those babies!) to endure the last two 'canes, not the mention the "little" Virginia earthquake without damage. You know, the quake that shut down the Washington Memorial and a nuclear power plant. Time to wash everything in sight, move every last piece of loose anything out of the garden, give the kittehs spanking clean litter boxes, and nose the Prius tight into the carport. At least this one will provide some daytime experiences. Hard to feign sleep with all that screaming wind, waiting for the next 50-year-old tree to figure out where it's going to fall.
Look, I know people will be seriously endangered by this storm, as will I and my neighbors. (My Southern Maryland county cares an awful lot about that, being as how it was nearly destroyed by a brutal tornado in 2002.) But like most scientists, I get a thrill out of Big Nature. Physics is beautiful, even when it's destroying things. Used to tell my students that, if we knew another 10-km-sized meteor was headed to earth (a la Chicxulub), I'd just throw out a towel in the back yard, lay on my back with a cold Diet Coke (tm) in hand, and observe the fireworks.
Oh, and early-vote on Sunday afternoon. Natcherly.
What both the media and the political campaigns need to emphasize is that all the people in the northeast where this storm is likely to hit need to get to the polls ASAP because a power outage could really suppress the vote on election day.
I had to call the Obama campaign headquarters this morning about a contribution that I tried to make yesterday that did not seem to go through and I suggested that the President include this as a reason to vote early in his radio addresses. I hope Rachel and the other MSNBC shows will pick this up and encourage the voters to get to the polls before the storm comes in.
(I live in Colorado and have already cast my vote for Obama and our Demo. Congressional candidate.)
That's a little extreme of the Romney campaign to control weather like that and send a hurricane up here to the mostly blueish northeast to suppress democrat voting, don't ya think? Shows the lengths they will go to.
Totally not serious folks! But I wouldn't put it past Faux news to use this as "evidence" that god is on the side of the GOP.
Be safe, friends and neighbors.
Are you making fun of me Roz?
Will et al. --
Just a word on how some counties are doing it right with regard to early voting and possible hurricane infringement:
My State Voting Board is deferring, of course, to the county Boards of Election. My county already has a very useful and effective Notification System, for phones, e-mails, tweets, etc., of weather, power, major traffic, and other short-term emergencies, and our Public Information Office has express plans to send out any info re changes/cancellations of our early voting on that CNS. They even have plans to move the early voting site to the County Emergency Management Office, if the original site is threatened. They'll close up only if the election personnel must go home to shelter in place.
I do love Charles County, Maryland! Not even Hurricane Sandy will keep this ol' varmint from casting her (early) Blue votes!