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Weather Nerd

[NOTE: For the very latest information, check my Twitter feed, and the other sources listed below.]

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As of 11:00 AM EDT, Irene’s maximum sustained winds are down to 105 mph, making it a mid-range Category 2, and the hurricane is officially no longer expected to re-strengthen at all. The forecast calls for the status quo through landfall in North Carolina, followed by weakening to 100 mph (low-end Cat. 2) and then 85 mph (mid-range Cat. 1) as the storm moves up the coast toward Long Island. If the track shifts slightly left, weakening would presumably happen faster over land. Either way, NYC & environs are now likely looking at Category 1 winds at most. (And maybe not even that, as Dr. Jeff Masters explains below.)

As I just tweeted, Irene is reminder of how much mystery remains in the science of hurricane intensity forecasting. All of the meta-conditions were ripe for her to become a monster. But disruptions in the storm’s own internal structure — the least well understood part of a hurricane — have prevented Irene from getting her act together well enough to take full advantage of the favorable environment. Those fears of a Category 4 monster were not unjustified hype. There was every reason to believe they’d be realized. They just…weren’t. So it goes with hurricanes sometimes. (Phew!)

Anyway, although we can now almost assuredly remove “world-historical disaster” from the list of realistic possibilities, this remains a serious situation for North Carolina and for the Mid-Atlantic and Northeast, including the New York area. Dr. Jeff Masters elaborates:

With its eyewall collapsed and just 24 more hours over water before landfall, it is unlikely Irene will have time to build a new eyewall and intensify. The storm is too large to weaken quickly, and the best forecast is that Irene will be a Category 2 hurricane at landfall in North Carolina on Saturday, and a rapidly weakening Category 1 hurricane at its second landfall in New England on Sunday. However, since Irene is such a huge storm–tropical storm force winds extend out up to 290 miles from the center–it has set a massive amount of the ocean’s surface in motion, which will cause a much larger storm surge than the winds would suggest. At 9:30am EDT this morning, a wind analysis from NOAA/HRD indicated that the potential storm surge damage from Irene rated a 5.1 on a scale of 0 to 6. This is equivalent to the storm surge a typical Category 4 hurricane would have. While this damage potential should gradually decline as Irene moves northwards and weakens, we can still expect a storm surge one full Saffir-Simpson Category higher than Irene’s winds. Since tides are at their highest levels of the month this weekend due to the new moon, storm surge flooding will be at a maximum during the high tidal cycles that will occur at 8 pm Saturday night and 8 am Sunday morning. At those times, Irene is expected to be near the NC/VA border, then close to Long Island, NY, respectively. Thus, storm surge damage rivaling that experienced during Hurricane Isabel in 2003 is likely in northern NC, southern Maryland, and up Chesapeake Bay on Saturday night. It looks like Irene will pass New Jersey during low tide, which may limit the storm surge inundation to 3 – 6 feet there. Coastal New England from New York City to Massachusetts may also see storm surges characteristic of a Category 1 hurricane during Sunday morning’s high tide, even if Irene has weakened to a tropical storm. I continue to give a 20% chance that a storm surge high enough to over-top the Manhattan flood walls and swamp the New York City subway system will occur on Sunday.

Significant wind damage, outside of North Carolina, is unlikely, Masters explains: “the eye of the storm will be just offshore, and the I-95 corridor from Virginia to New Jersey will be on the weak (left) side of the hurricane,” so with “almost all of the hurricane’s winds are on the right side of the storm…there will be likely be no hurricane-force winds on the left side of Irene” by the time it reaches Virginia and points northward. “Sustained winds should stay below 74 mph (hurricane force), and wind damage will be similar to that wrought be some of the strongest Nor’easters of the past 20 years, from Virginia northwards to New York City.” That said, tree damage will be much worse than in a Nor’easter, because “the trees are in full leaf during hurricane season, and catch the wind much more readily than during the winter. Tree damage will very heavy, and we can expect trees in regions with saturated soils will fall over in high winds onto power lines. Irene is likely to cause one of the top-five most widespread power outages in American history from a storm.”

Meanwhile, the biggest threat of all may be inland flooding:

In addition to storm surge, flash flooding and river flooding from Irene’s torrential rains are the main threats. The hurricane is expected to bring rains in excess of 8″ to a 100-mile-wide swath from Eastern North Carolina northwards along the coast, through New York City. The danger of fresh water flooding is greatest in northern New Jersey, Southeast Pennsylvania, and Southeast New York, where the soils are saturated from heavy August rains that were among the heaviest on record. New Jersey has had its 6th wettest August on record, with most of that rain falling in the past two weeks. Expect major river flooding throughout New Jersey the Delmarva Peninsula, and regions near New York City, as Irene’s rains run off the saturated soils directly into the rivers. In general, the heaviest rains will fall along the west side of the hurricane’s track, and the greatest wind damage will occur on the east side.

Also, the possibility of re-strengthening, although no longer expected, cannot be totally ruled out until Irene hits North Carolina. So stay tuned, and continue to prepare for the worst.

I won’t be able to update this blog as much today, but stay tuned to my Twitter feed for the latest. Other sources for information:

• The National Hurricane Center, with major updates, including new forecast tracks, coming out at 5:00 and 11:00 EDT (AM and PM), and minor updates at 2:00 and 8:00 (again, AM and PM).
• For computer model graphics, Ryan Maue’s Twitter feed and his excellent website.
NOAA satellite images
Doppler radar images
• Weatherblogger Dr. Jeff Masters
• Weatherblogger Eric Berger
• Weatherblog FLhurricane.com

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