This past week has definitely not been short of excitement
You are by now all aware of Hurricane Sandy and the damage she did to the north east coast of America, so instead of following the media on their journey of discussing the economic cost, loss of life and relation to the presidential election, this article will be based on the meteorological side of Hurricane Sandy. Sandy was an extremely unique storm in many ways and it has been referred to several times now as a ‘once in a lifetime’ event. The reasons as to why this storm is so unique and why it’s unlikely to be replicated in the near future shall be discussed below.
The 2012 Atlantic Hurricane Season
Let’s start by looking at the 2012 Atlantic Hurricane Season as a whole first to understand the dynamics that have been in place prior to Sandy’s arrival. The season has been an active one, and it started off very early with the formation of Tropical Storm Alberto in late May, Tropical Storm Beryl then formed also in the month of May, prior to the start of the official hurricane season. These storms formed close to the CONUS, with both of the storms making landfall.
As the season progressed however, with the exception of a couple of storms such as Isaac – the systems were typically moving out to sea as the ridge over the North Atlantic remained somewhat weak, allowing the storms to be steered polewards as opposed to running along the bottom of the ridge towards the United States.
This year there was a general struggle with storms to establish themselves properly once they got started. As was seen with Isaac in particular, there was always some sort of problem with the storm, whether it be vertical wind shear or excessive forward speed that was displacing the LLC from the MLC or the dry air that would get sucked into the core… Storms were having trouble getting going, and those that did almost always turned north very quickly and went safely out to sea.
The Model Runs
The model runs began depicting Hurricane Sandy forming very early on, and a few of the models, namely the EURO/ECMWF and the CMC both showed Sandy traversing up around the East Coast and then pulling back in west due to a trough. At this stage the hurricane trackers on the forums and chat rooms were all calling it an impossibility, no one believed that a storm would take that track, and further more the models were depicting Sandy dropping down to sub-940mb pressure by landfall. This was essentially a worst case scenario for the landfalling areas from what the models were showing, one of the users even stated that “If this were to actually occur like the model is showing, I may as well quit weather tracking because nothing will top it”.
While the CMC and EURO seemed fairly set on their idea of a large and powerful storm impacting the north east coast, the GFS on the other hand kept taking Hurricane Sandy out to sea instead of bending it back west like the rest of the models, and it took a while before the GFS started shifting towards the other models consensus of a westwardly turn near the end.
The Birth of Sandy
On 22 October 2012, tropical depression 18 formed in the central Caribbean. The storm’s motion was typically to the north-northeast originally and she looked to pass over Jamaica. Even from early advisories from the NHC, the potential for rapid intensification was said to be high and the models agreed with RI odds up to 6x higher than that of most storms, this was due to a low shear environment and warm sea surface temperatures. It was only a matter of hours before TD18 then became Tropical Storm Sandy, during the next day she continued to intensify and by the time she reached Jamaica she was already a 80mph hurricane. Sandy then underwent rapid intensification prior to her landfall in Cuba, where she hit with winds of 110mph sustained.
After passing over Cuba Sandy began to struggle with a dry air intrusion from the south west, a quad that would ironically later become the strongest of the storm. This dry air intrusion kept convection from forming around the south-west and southern parts of the storm and ended up resulting in what already then looked like a sub-tropical or a hybrid storm, though this was most likely just an illusion created by the eroded convection from the dry air and the fact that she had a large wind radius.
As she moved to the north west and then the north east, despite her appearance not being that good – her pressure continued to drop and the storm began to expand its windfield, a bit like Hurricane Irene. Where central pressures were low, but the winds in the storm were not consistent with what one sees with those pressures. This is because the central pressure can be thought of as a heat source, such as a heater or fire place… When that heat source is in a 2×2 room, all it’s energy will be contained in that space, and the temperatures could reach for example 40’C, where as if you took that same heat source and put it in a 10×10 room, while the source is still emitting the same amount of heat/energy, the overall ambient temperature of the room could only be 25’C due to the area that it has to spread the heat out in.
She then went on to escape much of the dry air as she passed over the warm gulf stream and at times had an eye visible on satellite and confirmed by recon flights. Her pressure still steadily dropping slightly as she moved more towards the north and began her westward turn towards the US coast.
The Interesting Parts
Expanding on the previous paragraphs… Sandy was forecast to become a hybrid storm and not only feed off the oceanic energy but also become baroclonic in nature, where she would also garner strength from the synoptic environment around her. The NHC had originally called for Sandy to become extra-tropical a day or two prior to landfall, but as the storm progressed it was noted by recon that the core temperatures were still decent and that Sandy remained a tropical system with a warm core temperature.
The core temperature was one of the things that made Sandy unique, it’s late October – there’s snow in the northern parts and the sea temperatures have begun to cool… One expects that a storm at such a latitude at this time of the year could only really be an extra-tropical storm. But Sandy kept maintaining tropical characteristics right up until a few hours before landfall.
The synoptic pattern was the main ingredient in making the event so unique… The timing of a large trough over the US approaching from Sandy’s west, while a ridge to her north meant that she couldn’t take the usual route that one would expect for this time of the year, where the storm would move out to sea to the north east. It is not often you will find this setup and one definitely shouldn’t count on it occurring again in the near future.
The pressure gradient and wind field that was discussed earlier was another major interesting factor for Sandy. The storm was forecast from early on in it’s life cycle, by the NHC, to have an excessive wind field and at one stage the tropical storm force winds stretched out over 500 miles in a single quad. The wind field began to contract a little bit closer to landfall.
The nature of the winds at various altitudes was also not regular… While at the time Hurricane Sandy was tropical in nature, recon was finding surface winds that far exceeded the flight level winds, something that is somewhat typical in Nor’Easters and non-tropical systems, but with tropical storms the winds aloft almost always tend to be higher than surface winds.
The fact that Sandy was strengthening as it approached land was also a rare factor. Storms that are making landfall on the CONUS, especially late season or further north end up generally weaken as they approach land, due to dry air being pulled in, lower SSTs or increased shear from other weather patterns. The opposite was true for Sandy and she ended up with a perfect set of conditions when approaching the coast – the shear was lowering, there was baroclonic influence from the synoptics over the CONUS and there was venting of her outflow from the trough.
Sandy was just overall, a unique storm that no one was really expecting at this time of the season… But the models proved themselves well worthy with this one and warnings and media persistence on the event meant people had a chance to get out of it’s path. Whether or not they listened is another story…