Excellent explanation on why storm Sandy was the worst!


Sep 1, 2011
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Weather Journal: What Made Sandy’s Flooding So Bad?

By Eric Holthaus

"National Hurricane Center to the Hydrometeorological Prediction Center. The HPC’s latest track forecast for Sandy calls for the storm’s center to dawdle for another full day over parts of western Pennsylvania and far western New York State, until Wednesday morning or so. Sandy’s winds are still creating tropical storm-like conditions as far west as Chicago’s Indiana suburbs. Waves on Lake Michigan are as high as 22-24 feet today with sustained winds in excess of 40mph. Meanwhile, it’s still snowing in West Virginia, which could receive another 12-18 inches by Wednesday night. A heavy batch of rain is also expected on Tuesday and Wednesday for parts of Maine and northern New England where an additional two or three inches could fall.
In the tri-state area, the worst winds and coastal flooding from Sandy are now over. Still, that doesn’t mean damaging weather has completely subsided. Late Tuesday morning, wind gusts above tropical storm force were still commonly being recorded across Long Island and coastal Connecticut. Bridgeport, Conn., Farmingdale, N.Y., and even spots within New York Harbor are still showing frequent winds in excess of 40mph. High tide crested midday Tuesday on Long Island sound – after this tide cycle is over, water levels should be back to near-normal across the tri-state.
At first glance, it looks like Sandy’s record-setting tidal flooding came in at the top end of pre-landfall forecasted ranges, which was six to 10 feet in Lower Manhattan. Peak surge in Lower Manhattan was 9.23 feet at 9:24 p.m., which when added with the 4.65-foot-high tide led to sea levels of 13.88 feet. The previous record sea level at Battery Park, set during Hurricane Donna in 1960, was 10.5 feet. Since 1851, only a foot and a half separated the previous top 10 coastal floods in New York City, which includes some of the biggest storms to ever hit the region (Donna, Irene, the ‘Perfect Storm’, and several particularly intense nor’easters). Sandy’s new coastal flood record for New York City is more than three feet above the previous record — more than double the range between the previous top 10. I don’t anticipate Sandy’s record being broken for quite a while.
What’s more, according to NOAA tide gauge data in Battery Park, peak surge arrived within 24 minutes of high tide — within 3% of the peak of the 12 hour tide cycle — further exacerbating flooding in New York City. Such a perfect alignment of surge and high tide was feared ahead of time as a worst case scenario for Sandy by the National Weather Service.

What caused Sandy’s record flooding to be so severe?
1) Stronger storm
Sandy’s center made landfall near Atlantic City, N.J. with a minimum pressure of 946mb. That’s in line with some of the top end forecasts of the storm’s strength and intensity issued days in advance. That extremely low pressure reading makes Sandy the most intense hurricane (or “post-tropical cycloneâ€) to make landfall north of North Carolina since at least 1851 — tied with the Long Island Express of 1938.
Why the name change? Technically, in the hours before landfall, the National Hurricane Center determined that Sandy had switched over its primary source of energy to the clashing of the cold and warm air that gave it its hybrid “Frankenstorm†features. Since, by rule, hurricanes must derive most of their energy from warm ocean waters, Sandy was deemed “post-tropical†and the name switch was applied. This is also directly related to the National Hurricane Center’s decision to not issue traditional hurricane warnings for Sandy — for fear this switch in name just before landfall would generate concern that Sandy’s threat had diminished. How to handle warnings for future storms like Sandy will be an active topic of discussion among meteorologists for quite some time.
2) Bad Timing
Just before landfall, Sandy’s forward speed picked up pace towards the New Jersey coast. Weather Journal warned that due to Sandy’s quicker than anticipated approach to the coast there was an increasing chance that peak surge may line up with the peak of the high tide — boosting the flood risk for New York City. Tragically, it is now appears that is exactly what happened.
Owing to the region’s unique geography, storm surge also built up quickly in western Long Island sound. The tide gauge at Kings Point, N.Y. on the north shore of Long Island recorded the highest surge values in the tri-state: 12.65 feet — but those values came almost precisely at low tide. Water levels in Long Island Sound peaked a few hours later at 14.31 feet, only slightly higher than those in Lower Manhattan. By that point, surge had fallen to about 8.5 feet.
3) Landfall location
Sandy made landfall near Atlantic City — about 100 miles south of New York City. Because hurricane circulate counter-clockwise, this meant that the worst storm surge was directed toward New York Harbor. Since winds were from the east (with the storm to the south), surge also poured down the East River from Long Island Sound, further boosting water levels in the city. It was this second wave of surge that likely contributed to the top-end result of coastal flooding in New York City.
Over its lifespan, Sandy set several important records: just before landfall it was the most intense hurricane (as measured by its minimum central pressure of 940mb) ever recorded north of North Carolina (even including those that never made landfall), its potential wave/storm surge destructiveness (peaking at 5.8 on a 6.0 scale) was ranked by NOAA to be higher than any other hurricane in the Atlantic or Gulf of Mexico since at least 1969, and it was also at one point the largest hurricane ever recorded (since 1851) as measured by the diameter of its gale force winds (945mi). Though National Weather Service forecasts for Sandy appeared to have been very good, Sandy’s impacts — at least relating to storm surge — came in at the top end of anticipated ranges. It’s quite likely that Sandy’s mix of ingredients maximized damage for New York City in a way that no other hurricane has anywhere in the United States for at least the last 150 years.
I’ll have a full county-by-county recap of forecasts versus actual landfall conditions in my next post.

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