[NOTE: Follow me on Twitter for the very latest on Isaac. Also, live tidal data here.]
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Category 1 Hurricane Isaac has made landfall in Plaquemines Parish, Louisiana, near the Mouth of the Mississippi River. Here’s a screen grab of the WunderMap radar:

[UPDATE: I may have spoken too soon. Sure looked on radar like it had made landfall, but now it seems to be stalling or moving very slowly just offshore, and maybe strengthening a bit in the process.

Stay tuned.]

For the very latest, follow me on Twitter.

Also, see below or click here to keep watching the storm surge rise via tidal gauges.

Lots and lots more tidal and lake/river gauges can be found here.

[NOTE: Follow me on Twitter for the very latest on Isaac. Also, live tidal data here.]
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After confounding storm-watchers by refusing again and again since yesterday evening to officially declare Tropical Storm Isaac a hurricane when there seemed to be ample evidence for an upgrade — particularly this morning at 11:00 AM Eastern, when recon data showed flight-level winds of 101 mph, surface winds (via a dropsonde) of 81 mph, and a barometric pressure of 976 mb — the National Hurricane Center finally pulled the trigger in a special 12:20 PM EDT update:

…RECONNAISSANCE DATA INDICATE ISAAC FINALLY ACHIEVES HURRICANE STATUS…

REPORTS FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INDICATE THAT MAXIMUM WINDS ASSOCIATED WITH ISAAC HAVE INCREASED TO 75 MPH…120 KM/H. ON THIS BASIS…ISAAC IS BEING UPGRADED TO A HURRICANE.

So, there you have it. Hurricane Isaac, a Category 1, bearing down on southeastern Louisiana, on what would be a catastrophic track if this were a major hurricane. Thank goodness it’s not; thanks goodness for dry air. That said, Isaac may yet strengthen a little more before landfall tonight. But the main threats will be storm surge and rainwater flooding, and perhaps inland wind and widespread power outages (particularly if Isaac does intensify just before landfall, which would imply that it will remain strong over land for a longer period of time).

To watch Isaac on satellite and radar, and to track the surge with live tidal gauge data, view the post below.

And again, for the very latest, follow me on Twitter.

UPDATE, 2:30 PM: All along the Louisiana and Mississippi coasts, the water is rising as the storm surge comes in — even as it “should” be receding, now that we’re past high tide in most places. Take, for instance, the data from the tidal gauge in Shell Beach, Louisiana (which is located here, for reference). The red line is the water level; the blue line is what the water level “should” be doing, absent Isaac. The green line represents the storm surge (i.e. the red line minus the blue line).

The next high tide throughout most of these areas is around midday tomorrow. (The Gulf of Mexico has diurnal tides.) On the projected track, Isaac will continue piling water onto the shore throughout the ~24-hour period between now and then. If the surge continues to prevent the tide from going out, as it’s doing right now, that next high tide could be a significant problem.

I’ve been playing down the storm surge threat a little bit, comparing it to the “hive, not pimple” phenomenon of Ike and Irene, whose surges were underwhelming as compared to the dire projections. But the trajectory of the green surge line on graphs like the one above, combined with the sheer amount of time Isaac has left to keep driving the surge on shore, has me concerned now.

[UPDATE: The National Weather Service New Orleans office is definitely concerned too. They're labeling this as a potential "extreme" surge flooding event across much of southeastern Louisiana with surge levels of 9 feet or more, which is defined as:

Life threatening flooding possible in areas outside hurricane protection levees and in areas around Lakes Pontchartrain and Maurepas. Sections of west Jefferson, east St Charles and lower Lafourche hurricane protection levees could be over topped. Areas outside of hurricane protection levees will be severely inundated. People not heeding evacuation orders in single family, one or two story homes could face certain death. Many residences of average construction directly on the coast will be destroyed. Widespread and devastating personal property damage is likely elsewhere. Vehicles left behind will likely be swept away. Numerous roads will be swamped. Some may be washed away by the water. Entire flood prone coastal communities will be cutoff, perhaps for more than a week. Water levels may exceed 9 feet or more behind over topped levees. Significant storm surge flooding will move well inland especially along bays and bayous. Coastal residents in multi story facilities risk being cutoff for a week or more.

I'm not sure what "could face certain death" means -- are they saying it "could" happen, or that it's "certain" to happen? -- but there's obviously a high level of alarm. Note, though, that the above is definitional language for an "extreme" event of 9-15 feet. It wasn't written specifically for this storm. Still. Bad news.]

The water level is also rising in and near Lake Pontchartrain. (I can’t embed that graph, alas. But go look at it!) FWIW, Katrina caused these gauges to get up in the 9 to 12 foot range.

Lots and lots more tidal and lake/river gauges here. Watch the storm surge in real time.

And — again — for the very latest, follow me on Twitter.

[NOTE: check the blog homepage and follow me on Twitter for the very latest.]
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Tropical Storm Isaac remains at the very cusp of hurricane status, declared “nearly a hurricane” as of 7:00 AM. Its barometric pressure remains more typical of a Category 2 hurricane, and the most recent NHC discussion, at 5:00 AM, stated that “THE INNER CORE CIRCULATION IS BECOMING BETTER DEFINED…AND STRENGTHENING MIGHT BE IMMINENT.” (Stop me if you’ve heard that one before. What a confounding storm to forecast!) Yet for now, on the morning of the day it will hit the U.S. mainland, Isaac remains stuck at 70 mph. I confess I’m a bit surprised the NHC hasn’t just gone ahead and declared it a hurricane — it’s so close, either decision would probably be justifiable — and then let the year-end Tropical Cyclone Report retroactively downgrade it if necessary. But they’re playing this one by the book.

Whether it hits as a 70 mph tropical storm or a 75 mph hurricane, Isaac’s effects will largely be the same. I still think inland flooding and perhaps inland wind will be the biggest impacts; storm surge is also a threat. If Isaac does start to (finally) rapidly intensify, and end up approaching Cat. 2 status, its immediate landfall effects will be a bit more severe. But either way, this seems likely to be a serious, but non-catastrophic, hit. (The inland flooding is the one thing that still has some potential to be catastrophic, or nearly so; we’ll see.) Folks in the storm’s path should remain hunkered down, if that’s what they’ve chosen to do, and not let down their guard. But the media should also dial back the hype, as I wrote last night. This isn’t going to morph into an apocalyptic hellstorm in the final hours before the landfall. It genuinely did have that potential, but thanks to some lucky dry air entrainment, it didn’t realize its potential, despite generally near-perfect environmental conditions. So now let’s not pretend a gas station roof blowing in the wind, or some predictable storm surge in an extremely flood-prone coastal area, or inevitable widespread power outages, constitutes armageddon, mmkay?

Anyway… as I did during Irene, I thought it would be helpful to post some live tidal-gauge data here, as a sort of “one-stop shop” for tracking Isaac’s storm surge. But then I realized: why stop there? How about a one-stop shop for landfall-watching data generally? So, before we get to the tidal gauges, how about some satellite and radar maps?

Lots of images after the jump…

[NOTE: check the blog homepage and follow me on Twitter for the very latest.]
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For the third consecutive advisory, the belief that Isaac might imminently become a hurricane was not vindicated at 11:00 PM EDT, as the storm’s winds were held at 70 mph, just shy of hurricane strength — even as the pressure dropped to 979 millibars, which is usually more typical of a borderline Category 1/Category 2 hurricane.

In this sense, Isaac is reminiscent of a poor man’s Hurricane Ike, which at one point had a pressure of 944 mb, typical of a borderline Category 3/Category 4 hurricane, yet its winds refused to budge from 100 mph (borderline Category 1/Category 2). People kept insisting that Ike’s winds would inevitably “catch up” to the pressure, in a burst of rapid strengthening that was always just around the corner. There was also fear that the pressure, plus the storm’s immense size, would lead to an epic storm surge well out of proportion to the winds and category. Similar fears are being expressed with Isaac. Yet Ike’s winds never did “catch up” with its pressure, and the surge underwhelmed — it was a hive, not a pimple.

Likewise, another “I” storm — last year’s Hurricane Irene — at one point had a pressure of 942 mb, typical of a Category 4, but winds of 110 mph that never “caught up” with the pressure. Indeed, Irene eventually weakened to a minimal hurricane, 75 mph, with a pressure of 958 mb (typical of a mid-range Category 3), and then to a tropical storm, 60 mph, with a pressure of 966 mb (typical of a high-end Category 2). Irene, too, produced fears of a storm surge vastly out-of-proportion to its wind and category — yet the reality again underwhelmed.

I’m not saying that Isaac is Ike redux or Irene redux — every storm is different — and I certainly think folks in surge-prone areas should prepare for the worst (or, more precisely, should already have prepared for the worst), including by evacuating if appropriate. But the parallels do give me some pause about the current storm-surge predictions. We shall see.

Meanwhile, I still suspect Isaac will become a hurricane overnight — but I can’t help but wonder, what if my Thursday-morning post, “Might Isaac Never Become A Hurricane?,” turns out to be accurate? I had given up on the notion days ago, but with less than 24 hours until landfall, the storm is running out of time!

In any case, it makes little difference — a 70 mph tropical storm and a 75 mph hurricane are basically the same thing. The question isn’t whether Isaac will become a minimal hurricane, but whether it will have the type of overnight/morning rapid intensification phase, just before landfall, foreseen by the GFS earlier today. It still may. And intensification at landfall would make the storm feel like much more of a beast, as I explained earlier. But every time you think this storm is about to go “rapid,” dry air keeps interfering. That’s very good news for New Orleans & environs, and although I feel a certain dread about the inevitable cries of “OVERHYPE!!!1!” if that good luck continues, it’s obviously best for everyone if it does.

I’ll try to post an update in the morning sometime around 8:00 AM Eastern. Maybe Isaac will finally be a hurricane by then. Stay tuned, as they say.

P.S. One final note. Up until now, talk of various worst-case scenarios, or at least of very-bad scenarios, has been justified and proper, because of the forecast uncertainty and the very real possibilities of calamity that existed. Several people have asserted to me that there was “no evidence” Isaac would become a major hurricane, but that’s just categorically false: multiple reliable computer models had multiple consecutive runs showing a Category 3 or stronger hurricane making landfall on the Gulf coast. Those models weren’t invented by TV pundits or weatherbloggers. They were real, and they were scary. Moreover, although this storm has proven to lack intensity, it’s taking a horrific track from NOLA’s perspective, and if it weren’t for several very persistent dry-air entrainments — something that just can’t be predicted with any degree of certainty — we really could be looking an extremely dire situation right now. This was a close-run thing. The large-scale conditions for strengthening to a major hurricane (good outflow, low shear, warm waters) were nearly perfect; only hard-to-predict small-scale features (dry air, core structure) prevented Isaac from reaching its potential. Meteorologically speaking, particularly in light of the track, this was closer to a disaster than the overhype-troll crowd will ever understand.

However.

Barring significant overnight intensification, the media must dial back the “hype” in the morning. Unless we all wake up to an exploding Isaac that’s finally worked out its dry-air and inner-core issues and is seriously ramping up just before landfall, the worst-case scenarios in terms of intensity will no longer be valid or relevant. So it’ll be time to stop talking about them, and indeed, to explicitly acknowledge that they’re now off the table. Yes, there will still be a significant (though, let’s not kid ourselves, probably not catastrophic) storm surge, plus major threats in terms of inland flooding (that one COULD be catastrophic) and perhaps inland winds, power outages, etc., etc. But those concerns are of a different order from the apocalyptic fears many of us justly felt when we saw model maps like this. It insults readers’ and viewers’ intelligence to treat “ordinary” storm impacts, like typical coastal flooding, downed trees, widespread power outages, gas station roofs blowing in the wind, etc., as epic events of grave significance on par with the sort of impacts we originally feared. This is where media gets hurricanes wrong: not by discussing worst-case scenarios early on, when they’re still plausible and in fact the public needs to be aware of them, but by failing to back off the “hype” once it’s clear that the worst isn’t happening. There’s a reason you’ve seen this blog steadily backing off the worst-case talk today: I always try my best to “keep it real,” and call it like I see it — and it’s become increasingly clear to me that this storm, while very serious (all hurricanes, and near-hurricanes, are!), isn’t going to be an epic catastrophe, after all. It could have been, but now it’s very likely not going to be. I try to make sure my coverage reflects that once it becomes clear. I hope the MSM will do so too.

If I can be permitted a brief personal aside… today was a momentous day in the Loy household. Our eldest daughter (of 3), age 4 1/2, started kindergarten. Then, because she’s taken an interest in Isaac — she’s a huge Daddy’s Girl; she pretty much takes an interest in everything I do — I asked if she wanted to make a hurricane tracking chart. She was thrilled with the idea. So, without further ado, I give you the new kindergartner’s first ever tracking chart:

I wrote the dots; she drew the lines. You’ll note she decided to spare Haiti a direct hit. I don’t think she realized that the hurricane’s track actually is allowed to go over land. Too bad it doesn’t work that way in real life!

She was also curious about how storm surge works, so I gave her & her little sisters (ages 3 and 1) a demonstration using a hose, their back-yard water table, and some grapes (playing the role of houses on the waterfront):

Hopefully the folks in the lowest grape got the Hell out!

As I wrote on Twitter, “Out: SLOSH model. In: LFSSWT (Loy Family Storm Surge Water Table). The latest in grape-based surge modeling technology.”

So, there you go. I don’t mean to make light of Isaac; I just wanted to give y’all a little window into my world. Many of my Isaac updates and tweets have been written amid the chaos of the under-5 set. Good times.

I’ll have more on the storm shortly. Reader’s Digest version: it’s still not quite a hurricane; almost, maybe in an hour, but not yet. Those prayers to the Patron Saint of Dry Air are working, NOLA. Keep it up.

[NOTE: check the blog homepage and follow me on Twitter for the very latest.]
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Just a quick additional note, as an addendum to my post below. There still remains a remarkable amount of uncertainty regarding the details of Isaac’s track, given that we’re only 24-48 hours from landfall. Check out this “4-panel plot” showing the 48-hour positions of the Euro (top left), GFS (top right), HWRF (bottom left) and GFDL (bottom right) models, courtesy of Dr. Ryan Maue and Weather Bell Models:

This particular forecast is, as Maue says, a nightmare. There’s just been much more uncertainty than with most hurricanes, and that remains true, even at this late date.

As an aside, it looks like Isaac may be upgraded to a hurricane — or else very, very nearly one — at 5:00 PM Eastern. Stay tuned.

P.S. Dr. Jeff Masters has some excellent analysis on the potential storm surge from Isaac:

Storm surge is the primary damage threat from Isaac. Isaac is a huge storm, with tropical storm-force winds that extend out 205 miles from the center. For comparison, Hurricane Katrina at landfall had tropical storm-force winds that extended out 230 miles from its center. Isaac’s large size will enable it to set a large area of the ocean into motion, which will generate a large storm surge once the storm approaches land on the Gulf Coast. Water levels at Shell Beach, Louisiana, just east of New Orleans, were already elevated by 1′ this morning. Conversely, water levels have fallen by 2′ this morning at St. Petersburg, Florida, where strong offshore winds due to Isaac’s counter-clockwise circulation have carried water away from the coast. The latest 6:30 am EDT Integrated Kinetic Energy analysis from NOAA’s Hurricane Research Division put the destructive potential of Isaac’s winds near 0.6 on a scale of 0 to 6, but the destructive potential of Isaacs’s storm surge was 2.1 on a scale of 0 to 6. I expect this destructive potential will rise above 3 by time Isaac makes landfall, making Isaac’s storm surge similar to that generated by Category 2 Hurricane Gustav of 2008, which followed a path very similar to Isaac’s predicted path. Gustav brought a storm surge characteristic of a Category 1 hurricane to New Orleans: 9.5′ to Lake Borgne on the east side of the city. A higher Category 2-scale surge occurred along the south-central coast of Louisiana, and was 12.5′ high in Black Bay, forty miles southeast of New Orleans. Recent model runs indicate Isaac may slow down to a forward speed under 5 mph on Tuesday evening and Wednesday morning, close to the coast. If Isaac is just offshore at this time, the coasts of Southeast Louisiana, Mississippi, Alabama, and the Florida Panhandle will be exposed to a large storm surge with battering waves for two high tide cycles. This sort of extended pounding will be capable of delivering more damage than the storm surge of Hurricane Gustav of 2008.

I’m a bit wary of this sort of forecast — likely Category 1 hurricane to produce uncommonly devastating surge due to its size! — because similar predictions regarding Irene in 2011 and Ike in 2008 were not fully borne out by the reality of what occurred. However, the analogy to Gustav seems apt in this case. We shall see. Certainly, if you live in a storm surge zone, take no chances. Get the Hell out; go to higher ground.

Dr. Masters also has a good overview of the surge’s likely impact on New Orleans, including a handy map of the newly upgraded levees. Read the whole thing, as they say.

[NOTE: check the blog homepage and follow me on Twitter for the very latest.]
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Isaac remains a 65 mph tropical storm as of 11:00 AM EDT. It continues to try to get better organized, but has yet to form an eye or take a more “classic” appearance on satellite.

If New Orleans and the Gulf Coast survive this storm relatively intact, they ought to designate a Patron Saint of Dry Air, and thank that saint. If it weren’t for that single atmospheric feature — dry air getting sucked into the circulation — this storm could well be a monster right now, bearing down on New Orleans along a worst-case track. (It’d be worse than Katrina, in terms of the track.)

Instead, even after last night’s re-centering, Isaac is still struggling to get its act together with respect to its internal core structure, which is the only missing prerequisite to significant intensification right now. The overall wind shear is light, the outflow is good, and the water is warm (probably not deeply warm enough for Cat 4/5, but certainly warm enough for Cat 3). But, as I explained in my post four days ago wondering whether Isaac might never become a hurricane (still technically a live question), a storm can’t intensify significantly without a well-structured inner core, no matter how “perfect” the conditions otherwise are. And it’s dry air that’s consistently prevented Isaac from developing a healthy core.

Unfortunately, the possibility of shedding that dry air at the last minute, and intensifying rapidly just before landfall, remains very much alive. It might not happen, but it might, and that’s the chief remaining cause for concern. (Well — the size of the storm, its somewhat slow speed, and the resulting increased & broad-based storm surge and heavy flooding rains are also big concerns.) As the NHC explained this morning:

UNTIL THE DRY AIR MIXES OUT…EROSION OF THE INNER-CORE CONVECTION WILL LIKELY CONTINUE UNTIL ISAAC APPROACHES THE NORTH-CENTRAL GULF COAST IN ABOUT 24 HOURS. AT THAT TIME…INCREASED FRICTIONAL EFFECTS FROM THE GULF COAST LAND MASS IS EXPECTED TO CONTRIBUTE TO AN INCREASE IN CONVECTION IN THE NORTHEAST QUADRANT…WHICH COULD ENABLE THE STRENGTHENING PROCESS.

To illustrate how this might transpire, it’d helpful to look at the most recent run of the most reliable American computer model, the GFS. The model intensifies Isaac more than the NHC is predicting — the NHC forecast intensity tops off at 90 mph; the GFS gets up to 112 mph 99 mph, as you’ll see below — and the GFS’s landfall point is a bit west of the NHC’s. But the general intensification trend is similar. Because Dr. Ryan Maue’s excellent Weather Bell Models site gives us a printout of the GFS projection for every 3 hours between now and landfall, we can go through, step by step, and get a sense of how the next 36 hours might play out. Please don’t focus on the precise details — this is only a single computer model run, not an official forecast, and it is certainly subject to change. I just want to give everyone a general overview of the sort of storm evolution that might occur, according to this trusted model. Don’t make life-or-death decisions based on this model; that’s what the NHC forecast is for. This is just for illustrative purposes.

[UPDATE: As indicated by the crossed-out "112 mph" above, I made an embarrassing and critical error here. I forgot that the wind totals in these graphics, which I converted from knots to mph, also have to be converted from flight level to surface level, which entails a reduction of 10 to 15%. I know that's true of flight-level winds from recon flights and the like, but I totally forgot about it in this context. That helps explain much of the apparent discrepancy between the NHC and GFS forecasts (though the GFS was still a bit more bullish) and forces me to modify some of the text below. I apologize profusely for the error, which I have corrected by reducing each affected figure by 12%. These corrections are made without strikethroughs, for ease of reading.]

First, here’s how the GFS portrayed Isaac as of its “initial” position, at 8:00 AM Eastern Time, with an estimated 55 mph winds and 991.2 mb pressure.

Now, let’s see how the GFS predicted Isaac would evolve from there. (Link goes to the image.)

Monday, 11:00 AM: 56 mph, 989 mb
Monday, 2:00 PM: 58 mph, 987 mb
Monday, 5:00 PM: 60 mph, 985 mb
Monday, 8:00 PM: 65 mph, 983 mb.

The 8:00 PM snapshot shows the first sizable three-hour jump in wind speed — so don’t be surprised if any changes remain incremental all day today. Here’s what it looks like:

Then, according to the GFS, Isaac largely stands pat for about six hours, before beginning to strengthen more rapidly early Tuesday morning:

Monday, 11:00 PM: 65 mph, 982 mb
Tuesday, 2:00 AM: 68 mph, 980 mb
Tuesday, 5:00 AM: 75 mph, 978 mb

So Isaac finally becomes a hurricane in the wee hours of the morning, per the GFS, looking like this:

Tuesday, 8:00 AM: 77 mph, 977 mb

Tuesday, 11:00 AM: 80 mph, 976 mb

Tuesday, 2:00 PM: 82 mph, 973 mb

At this point, it’s tomorrow afternoon, and Isaac is making landfall in Plaquemines Parish, near the mouth of the Mississippi River. According to the GFS (and the official NHC forecast), it will be intensifying as it does so, which is bad news, as intensifying hurricanes are more dynamic, bring heavier wind gusts and generally better organized than steady-state or decaying hurricanes of the same stated intensity. (On the bright side, late intensification — as opposed to earlier intensification and then steady state for a long time before landfall — means Isaac will have less time to build up a massive storm surge ahead of it, a la Katrina and Ike. Given the storm’s size, that’s a very good thing.)

Anyway, GFS has Isaac reaches the borderline of Category 2 status between 2pm and 5pm tomorrow, as it passes over the marshes of Plaquemines:

Tuesday, 5:00 PM: 94 mph, 969 mb

Note the big jump between those last two frames, from 82 mph to 94 mph in just three hours tomorrow afternoon, during landfall in Plaquemines Parish. Not good. But again, the point isn’t the detail of exactly when or where the GFS thinks this will happen, which is subject to change, and is not reflected by the official forecast at this time. The point, rather, is that this sort of thing can happen, and can happen quite close to land, with little or no warning. That’s why the exact track and timing are so important. A storm that might intensify that quickly is a storm that might turn into a mid-range Category 3 a few hours later, if it had just a little bit more time over water to get there. (Conversely, it’s a storm that might remain a low Category 1 or Tropical Storm if it had just a little bit less time.) It looks like Isaac will probably take too long to start strengthening, and thus run out of water before catastrophic last-minute rapid deepening can happen — indeed, contra the GFS, it might even run out of water before it can strengthen much at all — but these model maps illustrate why the difference between a fearsome monster and “overhyped” “dud” of a storm can be a matter of a few hours and/or a few dozen miles in either direction.

Anyway, the next GFS projection takes Isaac up to solid Category 2 status:

Tuesday, 8:00 PM: 99 mph, 967 mb

After that, in the GFS’s forecast, land interaction begins to weaken the storm.

Tuesday, 11:00 PM: 97 mph, 970 mb
Wednesday, 2:00 AM: 99 mph, 974 mb
Wednesday, 5:00 AM: 90 mph, 977 mb
Wednesday, 8:00 AM: 84 mph, 978 mb

The GFS proceeds to show Issac almost stalling over the bayous, and actually gaining a tiny bit of strength midday Wednesday while sitting over that marshy land mass, which would be bad news in several respects. (Primarily due to flooding rains, but also because of a prolonged wind and surge battering.) In the GFS scenario, Isaac retains hurricane or high-end tropical-storm force winds for more than 24 hours over land. I’m not sure how realistic that scenario is, so I won’t dwell on it right now, but it’s certainly something to watch. [UPDATE: I will say this: intensification during landfall can definitely lend itself to slower-than-usual weakening over land. And so can passage over wet, marshy areas. Louisiana ain't Cuba. I do think the inland wind aspect of this storm is something to watch, along with the huge threat of inland flooding rains, of course.]

By Thursday afternoon, the GFS picture of Isaac looks like this:

This would be a significant wind and rain event for all of Louisiana, if it came to pass. But again, my point here is not to “predict” these details, and please remember that this is just one “run” by one computer model, not the official forecast. My point is to illustrate the general parameters of the storm evolution that might occur, and to point out how small differences can have a huge impact on the outcome.

P.S. New Orleans Mayor Landrieu is sticking with the plan of “sheltering in place.” Disregarding the notion of preparing for a storm one category worse than the forecast, he is telling residents to prepare for a “strong Category 1″ (which is precisely what the forecast says), adding, “The good news is it’s not a Category 3.” He’s probably right, of course — and if so, he’ll be hailed for handling the storm well, and nobody will ever read my post bashing him again. But in the less likely but still plausible (20% chance? 10%? 5%?) event that Isaac strengthens dramatically before landfall and brings truly devastating conditions to New Orleans — rather than just a rough, windy, rainy day or two, with minor-to-moderate flooding — history will judge him harshly.

The fact is, Landrieu chose to prepare for the most likely case, and NOT to prepare for the worst. So, hopefully the worst won’t happen. It probably won’t. It usually doesn’t. I still disagree with his decision, and always will, based on the contemporaneous information available yesterday (not 20/20 hindsight) about what the realistic worst-case scenario was. But I do understand the reluctance to evacuate under those maddeningly uncertain circumstances, and I hope it works out for him (because I hope NOLA doesn’t get crushed by Isaac, obviously).

[NOTE: check the blog homepage and follow me on Twitter for the very latest.]
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Isaac remains a tropical storm this morning, still suffering some of the inner core disorganization that has plagued it for days. And while the odds of a direct hit, or near-direct hit, on the New Orleans area are increasing, the odds of a “worst-case scenario” appear to be decreasing, as both the computer models and the official NHC forecast have downgraded their intensity forecasts a bit. Officially, the NHC is now calling for a Category 1 strike, although Category 2 remains very much in play. Category 3+? Still definitely possible, but probably a bit less likely than yesterday.

It’s still way too early for New Orleanians or others in Isaac’s path to let down their guard, though. Intensity is unpredictable, and at least one of the better models, the HWRF, is still calling for major hurricane at landfall:

But the most accurate (on average) American model, the GFS — which has been driving the grave concern for New Orleans since Saturday afternoon, when it began predicting a Category 3 or 4 landfall there — is now calling for a somewhat less fearsome-looking Isaac, albeit still a low-end Cat. 3, coming in just west of the Mouth of the Mississippi River:

That scenario would still drive a ton of water into the New Orleans area, especially with the amount of time Isaac will have over the Gulf to push the Gulf waters toward shore ahead of landfall. Perhaps just as important, the possibility of rapid deepening remains frighteningly present, if Isaac could ever manage to get its inner core issues worked out. If that were to happen, say, 12 or 18 hours before landfall, it’d be really, really bad news. That’s why folks in harm’s way should continue to prepare for the worst.

Still, there is increasingly reason to hope that Isaac will be something less than worst-case. Indeed, most of the intensity models don’t foresee it getting stronger than Category 1:

If the Cat. 1 scenario verifies, that’d be great news (though the surge would still potentially be a big problem, due to the storm’s size and duration). But it would also bring out the cries of “OVERHYPE!!!1!” in force, from folks contending myopically that the entire threat of something worse than Cat. 1 was dreamed up in the fevered imaginations of TV pundits and weatherbloggers. People who are tempted to make this argument would do well to remember what the computer model forecasts looked like Saturday afternoon, and Saturday night, and much of Sunday. I would suggest that those individuals take their unimpeachable 20-20 hindsight and go place a bet on the winner of last year’s Kentucky Derby. Weather forecasting is a difficult and inherently uncertain endeavor, and just because a scenario doesn’t occur, doesn’t mean it wasn’t a realistic concern at the time. If the phrase “prepare for the worst” means anything at all, it makes no sense whatsoever to argue that every worst-case scenario which doesn’t verify was therefore some sort of fraud. Most worst-case scenarios don’t happen. That’s in their nature. But since we don’t know in advance which ones will happen and which won’t, we still have to prepare for each one, in order to ready for the small percentage of them that do happen.

But I’m getting ahead of myself. Isaac remains a real, live threat to life and property, in the here and now. The odds of a total disaster may have dropped somewhat. But there is still plenty of reason to be concerned, especially about the biggest killer, storm surge — and folks in the storm’s path, particularly in New Orleans, need to continue to take it very, very seriously, and prepare accordingly.

[NOTE: check the blog homepage and follow me on Twitter for the very latest.]
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In an overnight development with major implications for both the track and intensity of Tropical Storm Isaac, the storm’s center of circulation has “relocated” underneath the ball of convection (i.e., thunderstorms) that has been blowing up all evening, previously to the north of the center.

This happens sometimes, and it means Isaac’s strengthening will likely begin in earnest now. I wouldn’t be surprised if he’s a hurricane by morning.

Meanwhile, by spontaneously “moving” 55 miles or so to the northwest of its previous position, Isaac’s center may now be on a different track trajectory, and/or may react slightly differently to the steering currents that drive the storm’s movement. The most recent set of computer model runs are less useful now, since they were based on an incorrect initial storm position, so we’ll have to see a new set of model runs to begin to understand what impact this relocation of the center will have on the track. It seems plausible that it could result in a more easterly track — perhaps increasing the risk to the Mobile area or the Panhandle, or maybe increasing the risk to New Orleans as opposed to the TX/LA border region — but we’ll need to see some model runs to be sure.

Speaking of model runs, there was also another significant development moments ago: the 00Z European model, i.e. the ECWMF or “Euro,” has shifted its landfall point from the AL/FL border region west to the LA/MS border region. It has also decreased its landfall intensity to Category 1. So this brings the Euro and the GFS more in line with one another, specifically by way of the European model joining the American model further west (U-S-A! U-S-A!). This implies the possibility of a dangerous consensus for New Orleans (ignoring the intensity issues for the moment, since that’s much harder to predict). But course, as I just said, the center relocation could throw off any such consensus, so we’ll just have to wait and see what the various 06Z models (and eventually, the 12Z Euro) show.

I’d love to stay up for the 5:00 AM EDT advisory and discussion from the NHC, but I’ve got to get to bed now — it’s 12:30 AM local (Mountain) time, I’ve gotten limited sleep all weekend due to Isaac-tracking, and I’ve got both work and my 4 1/2 year old daughter’s first day of kindergarten tomorrow. So I need to Occupy My Bed, as it were.

I’ll do my best to keep this blog updated tomorrow, and I’ll also inevitably be tweeting extensively throughout the day about Isaac.

[NOTE: check the blog homepage and follow me on Twitter for the very latest.]

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Via Nate Silver, I found this excellent blog post by Weather Channel hurricane specialist Bryan Norcross. It crystallizes a couple of things I’ve been fretting about, but says them more clearly and with more authority that I could (since Norcross is, y’know, an actual meteorologist and all), so I will now quote it extensively:

What a horrendous confluence of events. Tropical storms never get on people’s radar, and the fact that Isaac passed Key West with less effect than a gusty afternoon thunderstorm made the situation worse. Even with a Cat 1 hurricane, will people take action tomorrow that may save their life Tuesday night?

Isaac’s extremely large circulation is one of the factors that should make it intensify relatively slowly, and a track that misses the high heat-content pools in the Gulf should help in that direction as well. Plus it has less than two days over the water before landfall. But, the water ahead is still plenty warm – over 85 degrees – and the atmospheric pattern looks very favorable for strengthening. Taken together, the NHC’s Cat. 2 forecast looks reasonable, though preparations should be made for a Cat. 3, given the lack of skill inherent in intensity forecasting.

Apparently, according to Silver, in an earlier iteration of his post, Norcross articulated this as a more general statement: “Always prepare for one category higher than forecast, knowing that intensity-forecasting skill is not high.”

Anyway, Norcross’s point that “tropical storms never get on people’s radar” is precisely why I hope Isaac becomes a hurricane overnight. That’s going to happen eventually anyway, and it would be helpful for preparedness if it goes ahead and happens now. Of course, stubborn locals and hype-trolls will still foolishly say “C’mon! It’s just a Category 1!” — ignoring NHC director Rick Knabb’s admonition, “Don’t make preparations based on current intensity, prepare for the stronger #Isaac we’re forecasting. Evacuate if instructed.” — but at least calling it a hurricane would help a little bit.

Back to Norcorss:

In any case, that same large circulation will move a lot more water toward the coast than an average hurricane. The entire Hurricane Warning area – from Louisiana to the western Panhandle – is extremely vulnerable to storm surge flooding. The NHC is forecasting 6 to 12 feet of water ABOVE THE GROUND in spots along that stretch of coast.

Normally, the exact amount of surge will be dependent on the exact track and where in the tide cycle the storm comes in. In Isaac’s case, however, there will be such a long duration of onshore winds, due to the storm’s size and slow forward speed, the water may stay high for more than one tide cycle.

In fact, that size and slow forward speed will make this a hurricane experience like none in memory, if it comes together as forecast. The weather will deteriorate tomorrow (Monday) in the Florida Panhandle, and tomorrow night across the rest of the northern Gulf coast. Then Tuesday the storm approaches the coast and a whole day later it is just inland on the current timetable. Even Thursday there is still onshore wind over part of the coast.

People will experience strong, howling winds and torrential rain for 24 to 36 hours or more in this scenario. They will be trapped at home, many without power.

And then there’s New Orleans. The storm as currently forecast should NOT exceed the capabilities of the new super-strong levees, but what if the storm comes in stronger? What a decision to have to make! Should they call an evacuation or not? In addition, there are populated areas outside of the levee-protection system.

Tomorrow (Monday) will be a day for big decisions from Louisiana to the Florida Panhandle. With life-threatening storm surge expected well away from the landfall point, widespread evacuation orders are likely.

And then there’s the inland flood threat from the torrential rain.

Unless something unexpected happens, it’s truly a nightmare scenario.