Hurricane Sandy
Mentioning Hurricane Sandy to anyone will likely recall memories of a huge "Superstorm" that battered New York City and the Mid-Atlantic states of the USA. That's exactly what happened late on October 29th and into the early hours of October 30th 2012. Sandy developed as a hurricane in the Caribbean on October 22nd, before heading north-east and eventually slamming into the US east coast a week later. The meteorological conditions that led to Sandy's reintensification over the Gulf Stream and evolution into the largest hurricane ever recorded in the Atlantic are truely remarkable. Such conditions allowed Sandy to become a hybrid strom, fueled by both the warm sea surface and strong upper-level divergence - from a meteorological standpoint, Sandy was the Perfect Storm.  

Satellite image of Hurricane Sandy as it tracked up the US east coast. Note the size of the storm: Sandy was the largest Atlantic hurricane on record. 
Credit: NOAA/ NHC. 

Meteorological history
Hurricane Sandy was the 10th named tropical cyclone of the 2012 Atlantic Hurricane season. An area of convection in the Caribbean became organized enough by 12:00 UTC on 22nd October to be named by the National Hurricane Center as Tropical Depression Sandy. The cyclone moved northeastwards towards Jamacia, where it made landfall as a Category 1 hurricane, and then rapidly intensified over the warm sea on its northward path to Cuba. Here Sandy made landfall as a Category 3 major hurricane, with a minimum central pressure of 954 hPa and maximum sustained winds of over 100 knots (115 mph). 

Considerable damage was caused in Cuba: although the mountainous terrain rapidly weakened the storm. Sandy further weakened as it crossed into the Atlantic Ocean, due to strong vertical wind shear - which essentially prevented the storm from maintaining a coherent warm core structure that is needed for tropical cyclone development. As the storm tracked forther north - parallel to the east coast of the US, it began to undergo extratropical transition, with its main energy source switiching from the warm sea surface to baroclinic processes, from October 26 - October 29th. 

By 00:00 UTC on the 29th, Sandy began to make an unusual left turn towards the New Jersey Coast and underwent a complex set of dynamic and thermodynamic processes to reintensify and reach a secondary wind maximum of 83 knots (96 mph) by 12:00 UTC on the 29th, as well as reaching a minimum central pressure of 940 hPa by 18:00 UTC that day. The US National Weather Service confirmed that the storm became fully extratropical by 21:00 UTC on the 29th, shortly before making landfall just prior to 00:00 UTC near Brigantine, New Jersey.

Track of Hurricane Sandy, indicating the stage of development of the storm. Sandy reached a minimum central pressure of 940 hPa just before making landfall in the USA. 
Credit: NHC 

Hurricane Sandy was the strongest storm of the 2012 Atlantic Hurricane Season. Sandy initially had devistating impacts in Jamacia - where damage of thousands of homes was estimated at $100 million USD. Damage was especially severe in eastern Cuba, where the storm made landfall as a major hurricane. Eleven people were killed there, with more than 226,000 homes damaged and at least 17,000 completely destroyed by the high winds. Total loses were estimated at $2 billion USD, making Sandy one of the costliest hurricanes in Cuba's history. Heavy rains also caused severe flooding and damage in Haiti, where 54 people were killed and over 27,000 homes flooded. Total loses here were estimated at over $750 million USD. 

The effects of Hurricane Sandy in the United States were enhanced by the occurrence of some of the highest spring tides of the year. The track of Sandy resulted in a worse case scenario for storm surge conditions in coastal regions from New Jersey to Connecticut, including New York City. The slow movement of Sandy and persistent strong winds towards the coast (on the north side of the storm), combined with the storm surge occuring near the time of high tide, contributed to record tide levels for many locations along the Atlantic Coast. Sandy made landfall with maximum sustained winds of 80 mph, and brought a record storm surge to New York City: a maximum of 12.65 ft (3.86 m) above normal tide levels was recorded at Kings Point on the western end of Long Island Sound, as well as record wave heights. 

Estimated storm surge inundation (feet above ground level).
Credit: NHC 
Low-level winds gusted to over 90 mph at landfall and rainfall of over 12 inches (205 mm) was reported in Maryland, which exacerbated coastal flooding as the storm surge prevented rivers from efficiently draining this excess water into the ocean. Sandy also produced record early-season snowfalls in the Appalachian Mountains - of over 3 feet (91 cm) in some locations. Overall, 147 direct deaths across the Atlantic basin have been attributed to Sandy, with 72 of these in the United States, as well as $50 billion USD in damage in the US alone. Hurricane Sandy was the second costliest Atlantic hurricane ever, with total losses estimated at  $71.4 billion USD. 
Surface pressure analysis overlaid on GOES East Visible Satellite image valid for 15:00 UTC October 29, 2012.
Credit: NHC. 
GOES East Visible Satellite image valid for 15:00 UTC October 29, 2012.
Credit: NHC. 

Synoptic intensification prior to landfall
Although Sandy started life as a late-season hurricane in the Carribbean, the storm ultimately made landfall in New Jersey as an intense extratropical cyclone. Many significant (and unusual) meteorological and oceanographical factors contributed to the hurricane's extratropical transition and reintensification.

Although the storm was still a tropical cyclone at the start of the 29th October, its strength and track was beginning to be governed by a strong, negatively tilted, upper-level trough to the west and a large stationary, blocking upper-level ridge to the north-east in the northern Atlantic. The position of this blocking high was very unusual for late October, at which time of year this region of the world tends to produce a continuous conveyor belt of extratropical cyclones that track westwards towards Europe. Positive vorticity advection ahead of the upper-level trough encouraged strong dynamical ascent, and the subsequent movement of Sandy towards the north-west (towards the region of strong ascent). The eastward expansion of the upper-level blocking high caused the upper-level trough to become extremely negatively tilted, which moved the strongest region of dynamical ascent closer to the hurricane, therefore encouraging the westward turn of Sandy towards the US east coast. 

500 hPa geopotential height in black, surface pressure in white, and the 1000 - 500 hPa geopotential thickness in the coloured fill. 
The view is at 00:00 UTC on the 29th October for North America. The strongly negatively tilted trough is centred over the US Midwest, with the large upper-level ridge clearly visible to the north-east of Sandy.

Although the tropical warm core temperatures associated with Sandy were cooler that compared with other tropical cyclones, Sandy maintained its warm core well into the midlatitudes - almost to a latitude of 40˚N. The strength of the warm core was significantly enhanced as the storm passed back over the very warm waters of the Gulf Stream (approaching 30˚C), as it made the left turn towards the United States. This helped the storm to reach it's secondary wind maximum and pressure minimum just prior to landfall. Interestingly, at this point the storm was a truely hybrid monster, with a band of thunderstorms close to the storm centre, deriving energy from the warm sea surface, and a large area of storm-force winds and cloud surrounding this that had charateristics of an extratropical cyclone - deriving energy from large-scale baroclinicity.  

The US east coast is a natural baroclinic region, especially in the autumn and early winter, when the temperature difference between the cold continental USA and warm waters of the Gulf Stream is very large. A stationary front was draped across much of the eastern portion of the USA as during the 27th and 28th October, oriented north-east to south-west. As Sandy approached, the front was dragged into the storm as a cold front, ultimately wrapping cold continental air all around the storm's warm tropical core. This enhancement of low-level baroclinicity signalled the further extratropical transition of the storm as it approached the coast, and also provided further instability that allowed the storm to intensify as it moved closer to land. 

Timeseries of the minimum central pressure of Hurricane Sandy. An initial minima occurred when the storm made landfall in Cuba as a category 3 hurricane (25th October), with a secondary (more intense) maxima occuring before the storm made landfall in New Jersey (30th October). 
Credit: NHC.
The final reason for Sandy's intensification lies in the positioning of the upper-level jet stream. A large jet streak was present downstream from the base of the trough over the Midwestern USA. At the right jet entrance of the cyclonically curved jet streak, there exists enhanced ageostrophic forcing for ascent throughout the atmosphere. As Sandy moved north east, this right jet entrance promoted enhanced ascent to the west of Sandy's location over the Gulf Stream - resulting in favourable conditions for cyclone development, and representing another reason for the sudden left turn and reintensification that Sandy undertook. To the south-east of the storm was a secondary jet streak, whose left jet exit was located somewhat close to the storm, also a region for large scale ageostrophic ascent and extratropical cyclone development. This jet streak likely played a secondary role in the reintensification of Sandy before landfall. 

A combination of factors helped this superstorm to reintensify and grow in size before making landfall in New Jersey: the warm waters of the Gulf Stream, a large and strongly negatively-tilted upper level trough to the north-east and ridge to the north-west, plentiful low-level baroclinicity and the presence of coupled jet streaks were all in existence to create the Perfect Storm.  

NOAA Storm Prediction Centre 300 hPa charts displaying the wind field (shaded regions indicate speeds above 50 knots), streamlines and upper-level divergence (in yellow contours) from 00:00 UTC on the 28th to 12:00 UTC on the 29th October 2012 at 12 hour intervals.
Credit: NOAA/ SPC.
The above charts give a good indication of how the 300 hPa wind field evolved during the passage of Hurricane Sandy up the US east coast. The yellow contours indicate where large-scale ascent is located, and thus the preferred location for the movement of Sandy (as well as being driven by the upper-level wind field, a storm will tend to propagate towards regions where ascent is favoured, and thus where lowering of the surface pressure is also favoured). Throughout the 28th October, the 300 hPa flow over Sandy was typical of that for a tropical cyclone: with a large area of anticyclonic outflow from the top of the storm as a result of the thermal high that developed aloft. As the storm approaches the mid-latitude jet stream, notice how this divergence is modified to look more like that associated with an extratropical cyclone - indicative of the hybrid nature of the storm at this point.