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If my reading of the Battery Park tidal gauges (charts here and here; raw data here) is correct … and if my understanding is right that a water level (i.e., the red line on the charts) higher than 10 to 10.5 feet would cause a major flooding event in New York City that would inundate the subways and swamp low-lying areas of the city … it appears to me that, unless the storm surge (i.e., the green line on the charts) slows down from its current pace of increasing approximately 1 foot per hour, a significant New York City flooding event will begin sometime in the 5:00 to 6:00 PM hour local time, probably closer to 5:30 or 5:40 PM, but in any event well before the 9:00 PM high tide (and thus, presumably, continuing unabated for several hours).
Caveat: I’m neither a meteorologist nor a hydrologist. But I can do arithmetic, and this is what I’m seeing.
If you are in a flood-prone area in New York City, and you are reading this, and you can get safely to higher ground, DO SO IMMEDIATELY!!!
I hope and pray that everyone who needs to be on higher ground is already there, and that this is solely a property damage event, with no loss of life.
The same goes, of course, for New Jersey and Long Island and Connecticut and everywhere else. I’m just focusing on New York City here because, well, it’s New York City. But the suffering is going to be widespread.
UPDATE, 4:21 PM: Literally as I was publishing this, the surge’s acceleration slowed down from ~1 foot of additional surge per hour, to ~1/2 foot of additional surge per hour. (That’s on top of the normal increase in water level due to the tide coming in, i.e., the blue line.) But this deceleration only delays the inevitable. Unless the storm surge (green line) not only slows its increase, but reverses itself and begins going down — which presumably won’t happen until the winds from Sandy shift, which does not appear imminent — the NYC flood will begin no later than 7:00 PM, and likely sooner.
UPDATE, 5:15 PM: Knock furiously on wood, but it appears that the storm surge stopped rising at 4:40 PM, and may now be very slowly decreasing. (The overall water level is still going up, but the storm surge — the difference between a “normal” tide and the surge-driven tide — is now stalling or dropping.) This is probably related to Hurricane Sandy’s acceleration toward the coast; the storm made a further left turn, and is about to make landfall. It is now possible that NYC will be spared the major flood that appeared imminent 90 minutes ago. However, it is going to be a very, VERY close-run thing, and a significant flood is still entirely possible. As it stands now, the current 6.6-foot storm surge would, without any further increase, be enough at high tide (which would be 4.74′ today with no surge) to push total water levels to 11.34′. So we need the surge to drop somewhere between 1 and 1.5 feet in the next few hours.
UPDATE, 5:44 PM: The surge is plateauing, not dropping, so for now, we’re still on course for the water level to reach 10 feet around 6:30 PM, and 10.5 feet around 7:00 PM, peaking at 11.4 feet just before 9:00 PM — if the surge doesn’t start to drop by then. That timing assumes the surge holds steady at 6.7 feet; the projected water level rise is due solely to the “normal” tide coming in.
New York City may well still flood, but at a minimum, if this trend holds, the flood will be less severe and less long-lasting than if the surge had kept accelerating, as it likely would have if Sandy hadn’t taken the left turn and sped up toward the coast.
UPDATE, 6:16 PM: It now appears the surge (green line) has resumed increasing, albeit at a much slower rate: 0.02′ per 6-minute update on the tidal data, or 0.2′ per hour. If that continues, the total water level will top 10′ very shortly — at around 6:20 PM — and will top 10.5′ at around 6:45 PM.
UPDATE, 6:36 PM: The water level at the Battery has topped 10 feet and will soon reach 10.5 feet, with 2 1/2 hours until high tide. I assume that the flood is underway, though I haven’t seen confirmation of that yet.
UPDATE, 7:10 PM: Parts of New York City are, indeed, flooding. Here are two photos by Julian Ehrhardt, apparently in Brooklyn:
Parts of Lower Manhattan are reportedly flooding, too, and power has been shut off because of the flooding. See my Twitter feed for the latest.
UPDATE, 7:49 PM: The surge is accelerating again, increasing at a pace of more than 1.5 feet per hour over the last 30 minutes. Surge is now 8.41′, water level is 12.75′. If this surge pace continues, the water level will be above 15′ by high tide, more than 10′ of it from the surge alone.
Again, see my Twitter feed for the latest.