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The Perfect Storm, October 1991
"The Perfect Storm" is one of those clichéd phrases the media loves to use whenever a good gust of wind or a few flakes of snow blows through the Megalopolis these days. The real Perfect Storm was an unnamed hurricane (one that the National Hurricane Center didn't identify as a hurricane until nearly the end of its life cycle) that formed off the coast of New England on October 28, 1991. The fourth hurricane of the '91 season clocked maximum winds of 75 miles per hour and had a minimum central pressure of 972 millibars. It was dubbed "The Perfect Storm" after an interview between the author of the book of the same name and a meteorologist who worked on forecasts for the storm when it occurred.

The Perfect Storm was unusual in that it formed under rare meteorological conditions. Not only did it move from northeast to southwest -- the opposite is usually true for storms in this part of the world -- but the strong pressure gradient (rapid change in pressure over a short distance) from the storm interacted with a strong pressure gradient with a high pressure system to its north, creating an enhanced area of very tight pressure gradients, and therefore, very strong winds. This area of strong winds was focused over New England, which is why it was so damaging.

While New England has experienced countless numbers of storms in the 21 years after The Perfect Storm, none have come close to doing as much damage as the original storm did.

Until now.

Enter Sandy.

Formation


The tropical wave that went on to form Sandy, as seen on October 20, 2012 when the NHC gave it a "high" chance of developing into a tropical cyclone.

As we approach the waning days of hurricane season in the Atlantic basin, we start looking closer to North America for tropical cyclone development. The prime area for development this time of year is in the Caribbean Sea, and that's just where Sandy got its start.

A vigorous tropical wave (a trough, or a generally broad, elongated area of lower air pressure) formed in the Caribbean Sea around the 20th of October. The National Hurricane Center, as well as the meteorologists who run the various weather models, picked up on the system and began tracking it. When they began using the weather models to run solutions (scenarios) on this system, the results were unsettling -- in 10 days the models brought soon-to-be-Sandy dangerously close to the east coast and slammed it into the Mid-Atlantic as a major hurricane. Given that the accuracy of model runs quickly deteriorates more than 5 days out, let alone 10 days out, it was something worth watching, but nothing to be alarmed about. (Full disclosure: I discounted it outright at the time, since models tend to do this kind of thing all the time and I figured it was another fantasy product from the GFS)


GFS run from October 21, showing Hurricane Sandy hitting New Jersey 10 days later. The 10 day GFS almost never verifies, and given that the situation was so complex and rare, the accuracy of this run was a rarity in and of itself.

The system was organized enough to be classified as Tropical Depression 18 on Monday the 22nd, and the initial NHC forecast turned the system into a Tropical Storm and brought it over Jamaica, Cuba, and the Bahamas before bringing it into the Western Atlantic Ocean.


Tropical Depression 18 was in an area of the atmosphere very conducive for rapid strengthening -- warm water temperatures, low wind shear, and ample moisture -- and it did just that. 48 hours from the time it was first classified as a Tropical Depression, Sandy had strengthened into a Hurricane as it was bearing down on the Jamaica. By this point, the NHC reiterated the fact that the long-term track forecast was very uncertain given the spread in the models. Many of the forecast models were continuing to bring Sandy into the Mid-Atlantic as a powerful hurricane, while just as many others were trying to send it out to sea.

The meteorologists at the National Hurricane Center were having none of this.

Forecasting The Track

Meteorologists like to use something called a "spaghetti model plot" as a quick gauge of where the latest weather models think a tropical system will go. As dozens of weather models predict where the center of a tropical cyclone will go, the spaghetti model chart plots all of these forecast paths onto one map. The result looks like spaghetti, and when they're in good agreement, the lines will generally be bunched up and go the same direction.

This was the spaghetti model plot from the evening of October 22, 2012:


They seem to be in agreement that the system will harmlessly curve out to sea, right? Well, not quite. The spaghetti model plot doesn't show every weather model, and many of the models used in everyday forecasting (GFS, NAM, ECMWF/Euro, etc.) continued to show Sandy following the east coast and slamming into land somewhere between Maryland and Connecticut.

A weather model uses lots of data to compile its forecast, right down to stuff you hadn't even thought of, like evapotranspiration (moisture evaporating out of plants) and how much solar radiation is reflected off the tops of clouds. Much of the initial data ingested by weather models is from real-time surface weather observations and radiosondes -- weather balloons. Twice a day (at 12z and 00z, roughly 8AM/PM EDT), every balloon launch site in the world (200+, roughly 80 of which are in the United States) releases a weather balloon to gather wind, temperature, pressure, and dew point/moisture information about that section of the atmosphere.

The weather models use this information to pretty accurately predict how weather features will form and move. Sometimes, though, this data isn't enough. When it comes to hurricane forecasting, the solution to model inconsistency can be as easy as releasing more weather balloons.

And that's exactly what the National Weather Service ordered every launch site in the nation to do. Instead of twice a day, they were ordered to release a weather balloon four times a day (every 6 hours instead of 12) in an effort to gather more information to feed into the weather models. This is the same process used during Hurricane Irene in 2011. It worked then, and thankfully it worked now.

The results were phenomenal. In just two days, the weather models (and subsequently the National Hurricane Center's forecast) pretty accurately pinpointed Sandy's landfall. This was the spaghetti model chart two days later, on the evening of the 25th:


And here was the National Hurricane Center’s forecast 5 days out, pinpointing a forecast landfall pretty darn close to where it actually came ashore (3 miles south of Atlantic City, NJ):


Why it hooked into the coast

The atmospheric circumstances under which Sandy made its unusual left hook into the coast were unique, to say the least. There were three major players involved in making Sandy turn northwestward into New Jersey:

1)    The trough (the Arctic airmass) over the United States served to “suck” Sandy towards it.

2)    A region of high pressure well north of New England served as a roadblock to prevent Sandy from continuing to parallel the coast and move north.

3)    A low pressure center northeast of Sandy prevented it from turning out to sea at the last minute.


These three atmospheric roadblocks were the reason why Sandy turned northwest into New Jersey instead of hugging the coast and hitting Maritime Canada, or turning east out to sea.

Energy

Sandy started out in a very favorable environment for strengthening, and the system took full advantage of this potential. It went from a newly-formed Tropical Depression to a 75 mile-per-hour hurricane in just 48 hours. What took meteorologists off guard is that the system kept strengthening, even when it was over the island of Jamaica (as I joked on Facebook, the pot smoke over Kingston relaxed the laws of physics and meteorology to make this possible).

By the time Sandy reached Santiago de Cuba (on the southeastern side of the island), it had a whopping 110 MPH winds and a minimum central pressure of 957 millibars. This part of the storm’s life was crucial in that it ensured Sandy’s pressure was deep enough to sustain itself over the mountains of Cuba and give it a head start on strengthening once it reached the Gulf Stream in the Atlantic.

Once Sandy reached the Bahamas, it began battling wind shear and dry air, which kept the system from strengthening too much once it reemerged over open water. It slowly weakened to an 80 MPH hurricane, but the pressure kept going down.

This is where the concept of air pressure and something called “baroclinic forcing” comes into play.


Temperature analysis the evening of October 25th, showing the Arctic air moving east across the country.

Baroclinic forcing is the energy a weather system receives when warm air and cold air mix with each other. Sandy’s timing was almost perfect with the timing of a very powerful cold front moving across the United States. The air was Arctic, and really cold. Cold Arctic air and warm tropical air colliding was a recipe for strong baroclinic forcing, and allowed Sandy’s pressure to deepen considerably.

Hurricanes translate their air pressure to winds. Most of the time a tighter pressure gradient (the more rapidly air pressure changes over distance) translates into stronger winds. But hurricanes have two options:

1)    Condense the energy they receive from air pressure into a tightly wound, intense core of winds around the center of the cyclone.

OR

2)    Instead of using this energy to form an intense but small core of strong winds, use the energy to spread the wind field out over a wide area, but with weaker winds.

Sandy approached the east coast with an air pressure similar to that of a Category 3 hurricane. However, it used its energy to spread its wind field out to the second largest area ever recorded in the Atlantic basin – 520 miles from the center to the edge of the tropical storm force winds.


Extratropical Transition

Hurricane Sandy’s transition into an extratropical cyclone is the other piece of the puzzle as to why it was so massive.

The United States is usually affected by two types of low pressure systems: tropical cyclones and extratropical (or mid-latitude) cyclones. Tropical cyclones form (wait for it…) in the tropics, so they have warm air throughout (called a warm-core), and their main energy source is intense thunderstorm activity around the center of the system. Extratropical/mid-latitude cyclones, on the other hand, are cold-core systems that generally get their energy from divergence (winds spreading out at the upper-levels of the atmosphere, creating a lifting motion that leads to low pressure at the surface) caused by jet streams (fast-moving river of air 20-30,000 feet above ground level) and upper-level troughs (elongated areas of lower pressure).

The purpose of hurricanes is to transfer energy from the lower latitudes to the upper latitudes. One of the ways they accomplish this is by dispersing energy as they move northwards through extratropical transition. This is the process wherein a tropical cyclone begins to ingest colder air, and the center of the system becomes cold-core. When the system becomes extratropical, it allows for temperature advection…essentially, the formation of warm and cold fronts. After a storm undergoes this extratropical transition, it can become massive. Hurricane Sandy’s transition to an extratropical cyclone, along with its very deep air pressure, allowed the wind field to grow into the second largest ever recorded in the Atlantic basin.

Storm Surge

Arguably, the biggest story out of Sandy (as with Katrina and many other intense hurricanes) is the sheer size of the system’s storm surge. If you think back to Hurricane Katrina, the reason why its storm surge was so bad was because the system was massive. It took up almost the entire Gulf of Mexico at one point, and its enormously intense wind field pushed an historic storm surge into parts of the northern Gulf Coast.

Sandy’s wind field wasn’t as strong as Katrina’s, but the surge was unprecedented along this stretch of the east coast. Sandy's storm surge was caused by four main reasons:

1)    The strong, widespread winds pushed a large “bubble” of water towards the coast. Think of when you spill something on the counter and go to wipe it up with a paper towel or a Shamwow – the water is spread out until you start pushing it with the rag, at which point it starts to pile up. That’s what happens with storm surge.

2)     The storm surge hit at high tide.

3)     The high tide was already higher than normal because it was a full moon.

2)    As NYFM demonstrated the other night, the storm’s very low air pressure resulted in a 5% increase of the already-existent storm surge, which translated into an extra foot of water in some places.

As this “bubble” of water approaches the coast, it piles up in bays, inlets, and rivers, where the narrowness of the waterway acts to amplify the surge. The NHC issued fairly accurate storm surge predictions ahead of the storm, which I illustrated on a map a few days before the storm hit:


The surge reached a record high of 13.7 feet in Battery Park in lower Manhattan during the height of the storm, and devastated hundreds of communities from North Carolina to Massachusetts.

Blizzard



One of the many unusual parts of Sandy was the incredible amount of snow it brought to parts of the Appalachian Mountains as the system made landfall. The frigid Arctic airmass that served to strengthen Sandy also allowed much of the precipitation on the western side of the system to fall in the form of snow in the mountains. Given that the precipitation was decidedly heavy and wet, snowfall was through the roof in places. 40 to 50 inches of snow fell on parts of the highest ridges in West Virginia, with several inches of snow falling as far west as central Ohio.

Climate Change

One of the biggest criticisms of my diaries is that I don’t incorporate a discussion on climate change into my writing. Well, here it is, so pay attention. While I can’t point to any one specific thing and say “This, this right here, this is because of climate change,” I can talk about ways in which climate change could have made this storm worse, and would make storms worse in the future.

Out of all the major severe weather that impacts the United States, hurricanes are likely going to be the most visibly impacted by our changing climate. Warmer sea surface temperatures will directly translate to more energy for hurricanes to strengthen. The entire area over which Sandy traversed was above average, save for a part of the Bahamas over which Sandy wound up weakening a bit:


As climate change triggers more swings in air temperatures, the odds of hurricanes strengthening due to baroclinic force would go up.  

Rising sea levels would have made the devastating surge in New Jersey and New York (among other areas) even more devastating.

In short, a changing climate will create more opportunities for a “superstorm” to develop. It might be a while before conditions align just perfectly for this type of scenario to happen again, but it’s something to seriously consider as we move forward. We no longer have to use simulations to see how devastating a 13 foot storm surge would be for New York City.

Summary

We've just witnessed (and many millions just lived through) what will be known as the new Perfect Storm, the standard by which future eastern storms will be measured. For decades we've speculated "what if" and "when," and that time came and went. The effects of the surge along the New Jersey coast, and especially in New York City, were beyond what many predicted. We need to study this experience not only for meteorological purposes, but to better prepare ourselves and our cities for when (not if) a storm of this magnitude strikes again. It may not be for decades, but history repeats, especially as the climate changes.


Freedom Tower lights up the city in its moment of despair.



Follow me on Facebook and over on Twitter for more weather information, updates, and the occasional funny picture.
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