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Weather Nerd

Sandy’s Surge Was NOT Unexpectedly High

October 31st, 2012 - 12:31 pm

An Associated Press story this morning propagates a total falsehood that’s gaining in currency:

With all the planning, and all the predictions, planning big was not big enough. Superstorm Sandy went bigger [than previous storms] — a surge of 14 feet.

“Nobody predicted it would be that high,” said ConEd spokesman Allan Drury.

[FRIDAY UPDATE: Mayor Bloomberg has now repeated the same claim, asserting that "no one expected" a surge as high as what occurred.]

THIS IS UTTERLY, COMPLETELY AND DEMONSTRABLY FALSE. The Associated Press must issue a correction and retraction immediately (as basically the entire story is premised on the falsehood), and the journalistic repetition of this factually incorrect statement needs to stop NOW.

First of all, Sandy’s “surge” in NYC was not 14 feet; it was 9 feet. 14 feet was level of surge + tide. More on that in a moment.

Secondly, the storm surge that occurred — 9 feet — was predicted, well in advance, by the computer models and the National Weather Service and countless others. In fact, you need look no further than this blog’s wee-hours Saturday morning update, titled in part “NYC In Peril”:

[W]e could be looking at a 6-10 foot storm surge in NYC, plus astronomical high tide — as opposed to the 3-5 foot surge in Irene — if Sandy slams New Jersey from the east.

Did you get that? A 6-10 foot storm surge plus high tide? That’s what I wrote at 1:08 AM Saturday, more than 64 hours before landfall.

The “plus high tide” part is critical. To explain why, let’s go over some terminology. Here are NOAA’s definitions of the terms storm surge and storm tide:

Storm Surge: An abnormal rise in sea level accompanying a hurricane or other intense storm, whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic tide from the observed storm tide.

Storm Tide: The actual level of sea water resulting from the astronomic tide combined with the storm surge. Most NWS flood statements, watches, or warnings quantifying above-normal tides will report the Storm Tide.

To repeat: the “storm tide” is the total water level, caused by the “storm surge” plus the regular (“astronomic”) tide.

Sandy’s storm tide (or total water level) was 14 feet because the 9-foot storm surge peaked at high tide — and Monday’s high tide at Battery Park was to be 5 feet even without any surge. Now, remember those definitions. 5 feet is “the level that would have occurred in the absence of the cyclone.” 14 feet is “the astronomic tide combined with the storm surge.” 9 feet is the storm surge alone. 9 + 5 = 14.

This is no mere semantic distinction. It completely obliterates the entire notion that Con Edison, or Mayor Bloomberg or anyone else, can reasonably claim they were unprepared for a “14-foot surge” that was higher than anyone predicted. Here’s what the National Weather Service predicted at 11:23 PM Saturday, almost 48 hours before the storm hit:

TIDAL DEPARTURES…1 TO 1 1/2 FT ABOVE ASTRONOMICAL TIDES SUNDAY NIGHT…2 TO 4 FT ABOVE ASTRONOMICAL TIDES MONDAY MORNING…AND POTENTIAL FOR 5 TO 10 FT ABOVE MONDAY NIGHT INTO TUESDAY MORNING. THE HIGHER END OF THE RANGE RELEGATED TO THE MOST FLOOD PRONE COASTAL LOCATIONS.

So the National Weather Service predicted, as of Saturday night, a storm surge of 5 to 10 feet “above astronomical tides,” with the higher end of the range (closer to 10 feet) quite possible in New York Harbor specifically. Since astronomical high tide is 5′, that means NWS was necessarily predicting a potential storm tide of 10 to 15′ if the surge happened to arrive at high tide, which was obviously possible. (Hence the widespread pre-storm concern about “astronomical high tides” making matters worse.)

The NWS forecast was revised upward at 1:29 PM Sunday:

TIDAL DEPARTURES…BETWEEN 2 TO 3 FT ABOVE ASTRONOMICAL TIDES [SUNDAY NIGHT] DURING HIGH TIDE WITH LOCALLY HIGHER VALUES…3 TO 4.5 FT ABOVE ASTRONOMICAL TIDES MONDAY MORNING…AND POTENTIAL FOR 6 TO 11 FT ABOVE MONDAY NIGHT INTO TUESDAY MORNING. THE HIGHER END OF THE RANGE RELEGATED TO THE NEW YORK HARBOR…WESTERN LONG ISLAND SOUND AND THE LONG ISLAND SOUTH SHORE BACK BAYS.

So, roughly 30 hours before landfall, that’s a prediction of a storm surge 6 to 11 feet “above astronomical tides,” again with the higher end of the range (closer to 11 feet) possible in New York Harbor.

To review:

New York Harbor got a storm surge 9 feet above astronomical tides.

Forecasters predicted a storm surge 6 to 11 feet above astronomical tides, leaning toward the higher end of the range (i.e., 9 to 11 feet) in New York Harbor specifically.

And these people have the audacity to claim “nobody predicted” the surge that occurred?!?

Any public official who says the surge was “unexpected” or higher than predicted is either criminally incompetent, or blatantly lying, or both.

(More information and outrage after the jump.)

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