Why NYC Flood Maps Got It Wrong

[marmar]

Why NYC Flood Maps Got It Wrong

Brooklyn | 12/05/2012 12:15pm |
Ali Winston | City Limits


via City Limits



[font size="1"]Credit: OEM/City Limits[/font]


When Hurricane Sandy’s storm surge ripped through New York City on the evening of October 29, it exposed seaside houses to devastating waves, basement electrical systems to the corroding menace of salt water and subway tunnels to unprecedented flooding. But Sandy also exposed flaws in the maps New York City uses to order evacuations, and in the models scientists employ to predict the impact of tropical storms.

Using modeling data provided by the National Oceanic and Atmospheric Administration, New York City’s Office of Emergency Management (OEM) divides the city’s low-lying and coastal areas up into three zones according to their risk of flooding. Zone A, demarcated in red on the above map, would be at risk of flooding from any potential hurricane. Zone B, demarcated in orange, could flood during category 2 or higher hurricanes, while Zone C, outlined in yellow, warned of flooding from any category 3 or 4 hurricane that struck just south of New York City.

On the Sunday before the storm hit, Mayor Michael Bloomberg signed an executive order mandating that people living in Zone A leave their homes ahead of the storm. “This is a serious and dangerous storm. For those in Zone A, evacuation is mandatory,” the mayor said at a press conference. He didn’t mention Zone B at all.

2003 Maps for a 2012 Storm

But when the storm hit, Zone B areas like Canarsie, Gravesend, Gerritsen Beach, Bergen Beach, portions of Bath Beach, Mill Basin, Marine Park, Lindenwood, areas of Howard Beach, Springfield Gardens, Brookville, Rosewood, North Woodmere and Woodmere all flooded to varying degrees. In Manhattan, portions of the Upper East Side between the 70s and the 90s near the East River flooded despite their designation as Zone B areas. ................(more)

The complete piece is at: http://americancity.org/daily/entry/why-nyc-flood-maps-got-it-wrong

[xchrom]

1. du rec. nt

[OldDem2012]

2. Actually, the reporter did some VERY weak research. The NYC flood maps are correctly drawn,....

....but the reporting on Sandy's strength was terribly skewed.

The National Weather Service consistently forecast Sandy to be a Category 1 storm at landfall. Sandy was described as a hybrid storm with both tropical and sub-tropical characteristics. On-air weather reporters for The Weather Channel, as well as all other TV stations, continued to repeat that forecast right up until landfall. A few questions were raised by a couple of the hurricane experts about the falling barometric pressure of the storm, but even they continued to stick by the Category 1 forecast. That forecast is what the City of New York used to declare evacuations in Zone A.

When Sandy made landfall almost perpendicular to the New Jersey coast at a forward speed of 25 mph, it was still reported as a Category 1 storm packing maximum sustained winds of 90 mph. What that basically means is that the sustained winds in the storm's strongest quadrant were actually as strong as 115 mph (90 mph + 25 mph forward speed). 115 mph actually falls within the Saffir-Simpson scale as a Category 3 storm (111 - 130 mph). The strongest quadrant of Sandy impacted almost the entire New Jersey coast line northwards into New York City.

Secondly, let's take a look at the minimum barometric pressure of Sandy. The 940 mb pressure of the storm reported at landfall made Sandy a Category 4 storm (920 - 944 mb) on the Saffir-Simpson scale. Let me repeat that....based on the minimum barometric pressure, Sandy could have been classified as a Category 4 storm. But, once again, the weather forecasters failed to sound the alarm as Sandy approached the New Jersey shoreline.

Finally, let's look at the storm surge. Sandy produced a storm surge of 13 feet along the New Jersey and New York coastlines. That is the storm surge associated with a minimal Category 4 storm on the Saffir-Simpson scale (13 - 18 feet). The weather forecasters knew in advance, and reported on air, that the storm surge was going to be in the range of at least 8 - 11 feet. According to the Saffir-Simpson Scale, that projected surge falls between a Category 2 and Category 3 storm, but Sandy continued to be forecast by the NWS as a Category 1 storm at landfall.

Despite all of the facts at their disposal about the intensity of Sandy, the NWS forecasters continued to stick by their Category 1 forecast. The authorities in New York City acted on that advice and evacuated Zone A in response to a Category 1 storm.

The NYC flood maps aren't wrong or outdated, but the NWS weather forecasting for a storm like Sandy needs to closely examined and changed where necessary.

NYC Evacuation Zones

Hurricanes and Tropical Storms