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Good news: Irene still struggling to gain strength

August 25th, 2011 - 3:00 pm

I will start this post as I’m going to end it: with a caveat. Hurricane Irene is still a huge threat to millions of people. It could still be a disaster, maybe even a world-historical catastrophe. Everyone in its path should continue to take it EXTREMELY seriously, and PREPARE NOW for a major, destructive direct hit.

Having said all that…

The 5:00 PM National Hurricane Center discussion gives some reason to hope that Hurricane Irene — while still dangerous — may not prove to be quite the monster that we’ve feared. First, a summary of where the hurricane stands now:

THE EYE OF IRENE MOVED OVER ABACO ISLAND EARLIER THIS AFTERNOON. AN AUTOMATED WEATHER STATION ON THE ISLAND MEASURED A MINIMUM PRESSURE OF 950.4 MB AROUND 1700 UTC. THE AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INVESTIGATING IRENE THIS AFTERNOON HAS REPORTED A 700 MB PEAK-FLIGHT WIND OF 99 KT [114 MPH]. ALTHOUGH RECENT AIRCRAFT DATA DO NOT QUITE SUPPORT THE 100-KT [115 MPH] ADVISORY INTENSITY…WE WILL HOLD THE INTENSITY FOR A LITTLE LONGER TO SEE IF THE AIRCRAFT FINDS HIGHER WINDS.

Okay, some translation is needed from geek-speak. It may sound like the NHC is simply quibbling over whether the winds are 114 mph or 115 mph, but that’s not the case. The recon aircraft found a peak flight-level wind of 114 mph. Normally, that translates to something in the ballpark of 100 mph at the surface. (Winds are stronger aloft.) So, in other words, if the NHC was relying strictly on this data, Irene would be classified as a borderline Cat. 1/2 hurricane, with winds somewhere around 100 mph, not a Category 3 with winds of 115 mph.

The central pressure of 950mb is awfully low for a borderline Cat. 1/2 storm — but then, that sometimes happens in very large, sprawling hurricanes, which Irene is. Realistically, Irene has probably been no stronger than Category 2 ever since last night, when it started, or attempted to start, an eyewall replacement cycle. (The 11am advisory suggested that it was unclear if the cycle had actually completed. Irene may be stuck in some sort of eyewall replacement limbo. Unfortunately, those dynamics are very poorly understood, even by the best meteorologists on the planet. Hurricanes are complex and, in many ways, still very mysterious beasts.)

So why is Irene still classified as a Category 3, with 115 mph winds, if it’s an open secret that she’s actually weaker than that? Because the Hurricane Center is very, very cautious about officially downgrading land-threatening hurricanes that it expects to re-strengthen. Frankly, there’s some art as well as science in the sustained-wind estimation process, and the NHC is wont to take a little bit of artistic license at times like these — for understandable reasons. Forecasters don’t want people to “let their guard down,” falsely thinking that a temporarily-weakened storm is no longer a big deal. So they will maintain a storm like Irene at a higher intensity than its likely “true” strength for some period of time, waiting for the expected restrengthening. Eventually, though, the NHC has to bow to the data, and the 5pm advisory is telegraphing that Irene is likely to be downgraded to a Category 2 if it doesn’t get its act together soon.

That brings us to the next encouraging part of the discussion — the forward-looking intensity discussion:

THERE IS STILL SOME TIME FOR IRENE TO STRENGTHEN AS IT WILL BE MOVING OVER WARM WATER AND THE SHEAR REMAINS LOW. HOWEVER…THE UPDATED FORECAST CALLS FOR A LITTLE LOWER INTENSITY OVER THE FIRST 24-36 HOURS. IN ABOUT 48 HOURS…SOUTHWESTERLY SHEAR IS FORECAST TO INCREASE…WHICH SHOULD START A WEAKENING PROCESS. HOWEVER…SINCE IRENE HAS SUCH A LARGE AND INTENSE CIRCULATION…IT WILL PROBABLY BE SLOW TO WEAKEN.

The previous advisory called for Irene to peak at 125 mph, and stay above 120 mph for 24-36 hours. (The amount of time she’s a major hurricane is important, because it means a lot more huge surf and storm surge builds up ahead of the storm, which eventually gets pushed ashore somewhere.) This advisory, by contrast, calls for Irene to peak at 120mph, and to maintain that intensity for just 12 hours, give or take. So she’d be a little less strong, and stay that way for a little less long.

Now, because intensity forecasting is so inexact, it may sound a little silly to obsess over such relatively small variations. But — stop me if you’ve heard this one before — the trend is your friend. And the trend is clearly indicating that the NHC is losing confidence that Hurricane Irene will substantially re-strengthen before shear, cold water, and perhaps land interaction with the Carolinas begin to induce weakening.

Keep in mind, Irene is “really” probably a ~100mph hurricane right now, so strengthening to 120mph (which is what is officially being forecast) would be fairly substantial. If that happens, it would indicate she’s gotten her act back together, and it’s quite possible she might get a good bit stronger still, after all. On the other hand, it’s very conceivable that she might never make it back to “true” Category 3 (115+) status (notwithstanding that she’s officially a Cat. 3 — a “Major Hurricane In Name Only,” or MHINO, for you political types — right now).

Either way, the odds of Irene becoming a monster high-end Cat. 3 or low-end Cat. 4 as she menaces the Carolinas, then weakening only to Category 2 strength before hitting the northeast, have decreased (although that scenario is still possible). Conversely, the odds of Irene hitting North Carolina as a Cat. 2, the mid-Atlantic as a Cat. 1, and New York/New England as a minimal hurricane or high-end tropical storm, or something along those lines, have improved.

Now, again, for the enormous caveat. This does not mean anyone can breathe easy. It is still very possible that Irene could become the monster we’ve all been fearing. The meta-conditions are ripe for it to happen; the jet stream, which can cause a hurricane to explode like jet fuel poured on a fire, is dead ahead; intensification could happen, and happen rapidly. The only question is whether the poorly understood internal dynamics of Irene’s eyewall and core structure will permit it. WE JUST DO NOT KNOW the answer to that question. It could go either way.

Moreover, even if Irene doesn’t become a monster, she’ll do plenty of damage, ESPECIALLY in terms of inland flooding, which may very well end up causing the most damage to life and property. But the wind and surge could be pretty bad too, given the vulnerability of the targeted areas.

So, please, please, if you’re in a potentially affected area, DO NOT GROW COMPLACENT and DO NOT STOP PREPARING FOR THE WORST. Things could still be very bad. But there’s a bit more hope than there was 90 minutes ago.

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