|Friday 2:00 p.m. EDT NOAA projection of the path|
of Hurricane Sandy
This storm is getting a huge amount of press coverage, given that it's only a Category 1 hurricane at the moment. It is getting close to full moon, which means high tides, which means greater than average storm surge as Sandy approaches. Surge and flood damage in the south east is already severe. It caused at least 21 deaths as it passed over Haiti, Jamaica and Cuba.
The mysterious thing about this storm is the strong hook to the northwest that you can see in the graphic above starting on Monday. This is very unusual for an Atlantic Hurricane, and even more unusual, apparently, for an October hurricane. This is the tail end of the hurricane season, supposedly. So, what's going on with this storm? It turns out that it's not all that easy to dig out of the WWW and news stories. (At the bottom of this post are some links that I found helpful.) However, a post by Will Komaromi, Ph.D. student at the Rosenthal School of Marine and Atmospheric Science, University of Miami, is excellent, and much of the material below is from that post.
Normally, as an Atlantic hurricane moves north, it encounters cooler water, drier air away from the tropics, and greater wind shear. In other posts, I've discussed the individual effects of these on hurricanes. However, this year, the Gulf Stream along the Atlantic Coast is warmer than usual.
To address this, we need to look at what is happening out west. There is a large cold weather system, currently bringing snow to Colorado, moving across the country, including a deep trough of low pressure (called a 'midlatitude trough'.) This storm is a large low-pressure system, so, very simplistically, it'll suck Hurricane Sandy in to the west. When the two systems interact, there will be a lot of action--thunder, lightning, rain by the bucketful, and snow. Very similar events occurred 21 years ago in the now famous "Perfect Storm" of October, 1991, that became the topic of a best-selling book and movie. There is an excellent graphic on Komaromi's WWW site above, comparing the two systems at the scale of the continental U.S.
Instead of weakening, the hurricane system merges with the mid-latitude trough (including strong jet stream winds that can provide extra energy to the hurricane). Sharp gradients in air temperature and density between the two systems works (creating sharp pressure gradients, known as the baroclinic instability) works opposit of the weaking effects described above. Air moving out (diverging) from the hurricane system at upper levels combines with the baroclinic instability to create a low-pressure eye to the storm. The lower the pressure in the eye, the stronger the hurricane. In the Perfect Storm, the pressure dropped to 972 mb in the eye; Komaromi says that it's possible that the pressure in the eye of Sandy could be as low as 950 mb.
A final factor in this is that the pressure system lying over the eastern North Atlantic, known as the North Atlantic Oscillation, is in a negative phase, meaning that there is high pressure to the north east of the hurricane that will prevent it from curving out over the Atlantic like most hurricanes do. It's helping it to move westward (called retrograde by the meteorologists) right into the east coast of the U.S. All-in-all, it looks like the northeast is in for a prolonged (days) period of severe wind and drenching rains, and that my childhood stomping grounds in western Pennsylvania are going to get a lot of snow from the cold part of this combined system.
I'll join with other meteorologists and bloggers to say stay safe, be prepared to be self sufficient, and to help your neighbors. Especially, check on elderly relatives and friends who may be vulnerable to early winter cold if the power goes out! And, get out and vote early if you live in these regions; in most, early balloting is already underway. Election Day isn't until November 6, but some meteorologists are concerned that this storm could take down powerlines that are going to take a long time to repair, days to even weeks.