It was only about six months ago when the US Northeast was the focus of our attention in Weather and Climate Discussion. Hurricane Sandy, also known as the “Frankenstorm”, brought a massive storm surge and devastating coastal flooding to the states of New Jersey, New York, and Connecticut. Two weeks ago, the US Northeast was in the world weather headlines again. This time the story was snow. Lots of snow.
The map below shows snowfall totals (from the National Weather Service in Brookhaven, NY). Note that these totals are reported in inches not centimeters. More than 2 feet of snow fell across central Connecticut and Long Island, with a few places reporting in excess of 3 feet. You would need more than a garden trowel to remove this snow from your driveway.
The storm that dumped such an extraordinary quantity of snow was a Nor’easter (which, phonetically, is how a New Englander would say “northeaster”). Nor’easters are named as such because if you were standing on a beach anywhere in New England, you would experience a northeast wind as the storm approached from the south (i.e., you would be located in the storm’s northwest quadrant).
An animation of the satellite imagery over a three-day period is available here: GOES Satellite Loop 8-10 February . A broad region of cloud enters the image from the south on 8 February and tracks northeastwards along the coast. When this disturbance reaches the coastal waters off New England (about halfway through the animation) the storm intensifies and a distinct circulation develops. This is the time when the heaviest snow fell.
Why did the storm intensify? The mechanism of intensification was the same as that for any Nor’easter. Two essential ingredients are required: 1) an incipient low at the surface somewhere along the coast or in the southeastern US; and 2) a cyclonic disturbance in the upper-troposphere located to the west of the surface low. If the surface and upper-level features are configured just right, then the mutual interaction between them will cause both features to amplify via baroclinic instability. Despite what many broadcast meteorologists claim on air, it is not a “collision” between two weather systems that leads to this kind storm. Rather, it is harmonisation.
The image below demonstrates how this process unfolded on 8-9 February. On the morning of the 8th (left column) a surface low was located along the coast on North Carolina (top panel) and a 500 hPa trough was located over Michigan (bottom panel). The surface low was tracking to the northeast, while the upper-level trough was moving eastward. By the evening on the 8th, the two systems were optimally configured for rapid growth — the surface low was positioned just to the east of the upper-level trough. By the morning of the 9th (right column) the surface low had a central pressure of 970 hPa. That’s a deepening of more than 24 hPa in 24 hours. This storm was a “bomb” in the technical sense.
The heaviest snow fell in the northwest quadrant of the storm. A loop of radar imagery over the duration of the event is available here from the National Weather Service. Notice the band of high radar returns over central Connecticut and Long Island, where the highest snowfall totals were recorded. The band remained almost stationary over this region for approximately 12 hours. Within this band, snowfall rates of 1-5 inches per hour were observed, and there were many reports of thundersnow. This type of mesoscale banding is common in northeast snowstorms and may be associated with conditional symmetric instability (or CSI) — a topic for another blog on another day.
So how did New Yorkers cope with the snow? The heaviest snow began at rush hour on a Friday. While many people had the foresight and opportunity to leave work early, many others either could or would not leave early and were caught on the roads. The rate of accumulation was too fast in some places for snow plows to keep roads clear. The coincidence of the rush hour and the snowfall resulted in disastrous consequences on the roads. The photograph below shows an eerie scene on a Long Island highway on the morning after the blizzard. These cars were stuck in the middle of their commute. Some slept in their cars, others were rescued by snowmobile, and apparently the 24-hour Walmart on this highway offered shelter (and low prices) to the stranded.
As eerily peaceful as this scene may look, it is most likely that at least one car horn was still being beeped intermittently when this photo was taken. Hey, this is New York. Whaddaya expect? (photo from TriHamlet News)
