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Monthly Archives: August 2008

Gustav’s approach

August 31st, 2008 - 8:10 pm

The 11:00 PM EDT advisory is out. Gustav’s intensity appears to have plateaued, and it’s expected to make landfall right around its current intensity — a borderline Category 2/3 hurricane — in the late morning or around noon, in the marshlands south of Houma. That city of 30,000, where The Weather Channel’s Jim Cantore is stationed, will take the brunt of the storm. Significant deviation from this track now appears unlikely (though small, last-minute “wobbles” are always possible).

New Orleans will most likely dodge a bullet, avoiding catastrophic flooding, unless the levees perform worse than expected. This will not be the “mother of all storms.” There may be significant flooding in the West Bank, but if so, it will be mostly because of the levee system’s fragile and incomplete state, not because of anything extraordinary about Gustav. In any case, we should not see a citywide repeat of Katrina — let alone something worse, as once seemed quite possible — assuming the Army Corps of Engineers has done its job this time.

The media should, at this point, be ramping down the hype about Gustav. Twenty-four hours ago, the hype was justified, and the evacuation of New Orleans was totally appropriate and necessary. But now, anyone who is still treating Gustav like some sort of unprecedented apocalypse is just ignoring the data. There will be death and destruction, yes, but certainly not on a “storm of the century” scale. Gustav is no longerexpected to rival Hurricane Katrina in its destructive power.” And the worst effects will be south and west of the Crescent City — again, assuming the New Orleans levees do their job. The storyline now isn’t, will the monstrous Hurricane Gustav destroy New Orleans? It is, will the levees perform up to snuff this time, and survive a surge that they manifestly should be able to handle? (This is a reasonable question, of course, since they also should have handled Katrina.)

Journalists often fail to grasp how quickly and drastically things can change with hurricanes. Yesterday evening, we were looking at a 150 mph monster in the Caribbean, and imagining intensification in the Gulf today to perhaps 175 mph, with limited opportunity for pre-landfall weakening. Instead, thanks to its totally unexpected post-Cuba weakening and failure to intensify significantly over the Loop Current, Gustav now looks to be a run-of-the-mill, low-end Cat. 3 event, at worst. Top winds right now are 115 mph, which are of course nothing to sneeze at — but this is just a regular old major hurricane, not a world-historical event. It may even weaken to Cat. 2 before coming ashore. If journalists continue to act like Gustav is going to be the end of the world, it will only feed public cynicism about hurricane warnings once it comes ashore and “disappoints.”

Anyway… I’m about to go to bed. I’ll try to post an update early in the morning, but I have plans tomorrow and I may not be able to blog again until midday — at which point Gustav will probably be making, or will have just made, landfall in Louisiana. To keep things fresh, here is a constantly updated, live NWS radar loop from New Orleans:

Also, there’s a wealth of good information at the sites listed in my sidebar at right, so if you want the latest even when I haven’t updated this site in a while, just look there. In particular:

* For news from Louisiana, the Times-Picayune Hurricane Center, the WWL-TV live stream, and Best of New Orleans Blog.

* For the latest official information on the storm, the National Hurricane Center. Specifically, the latest forecast track is always here.

* For live satellite imagery of Gustav, the visible satellite loop, the infrared satellite loop.

* For radar imagery of Gustav, the New Orleans long-range and short-range loops; the Mobile, AL long-range and short-range radar loops; and the Lake Charles, LA long-range and short-range radar loops.

* For weatherbloggers’ perspectives, Dr. Jeff Masters, Eric Berger and Alan Sullivan.

Best of luck to everyone on the Gulf Coast. My thoughts and prayers are with you.

P.S. A reminder: timestamps at the top of each post are in PDT, which is three hours behind EDT.

Times-Picayune reporter Sheila Grissett, “embedded” with the Army Corps of Engineers, reports:

The latest prediction of reduced Hurricane Gustav storm surge should be good news for the Industrial Canal and St. Bernard Parish levees, but may still potentially put water over deficient levees on the west bank of Jefferson Parish, the Army Corps of Engineers’ ranking officer said Sunday. …

Just 24 hours ago…predicted surges at landfall ranged from 20 to 25 feet along the coast. They have since fallen to a range of 12 to 16 feet.

There’s much more local news from Louisiana in the New Orleans Hurricane Center.

Meanwhile, another great resource is the Best of New Orleans Blog. Among other things, they’re reporting on major evacuation problems along I-59 (the contraflow is called “a joke”), giving us an introduction to Houma, the town that may take Gustav’s hardest blow (and the town, as I mentioned earlier, where The Weather Channel’s Jim Cantore is riding out the storm); and posting pretty, eerie photos of “The Big Empty,” post-evacuation New Orleans.

P.S. Speaking of those “pretty, eerie photos,” the Times-Picayune has a video along the same lines:

GOP cancels most Monday convention activities

August 31st, 2008 - 5:31 pm

Hurricane Gustav has forced the cancellation of all but the most basic activities on the first day of the Republican National Convention in Minneapolis-St. Paul tomorrow. “This is a time when we have to do away with our party politics, and we have to act as Americans,” said presumptive nominee John McCain.

According to Politico, “four days of festivities were to open Monday, but now a party business session is all that is scheduled.” There will be no “political rhetoric,” according to McCain’s campaign manager, Rick Davis.

Depending on what happens with Gustav (and, I might add, Hanna, though I don’t know if they’re tracking that one yet), officials hope to return to a more traditional convention schedule later in the week:

Convention planners said McCain hopes to show up later in the week to accept his party’s nomination, but there was no guarantee of it, and he is not required to be in the hall for the nomination to be official.

Officials said they will execute “certain basic minimum requirements” of party rules, including receiving the report of the credentials committee, adopting rules and the party platform and electing officials of the convention. …

Davis said the convention will be decided day to day, with delegates notified by e-mail and text message. Davis said the convention hopes to conduct a roll call, but doesn’t know when that will be. …

McCain said he “can hardly wait” to get to St. Paul. “I hope and pray we’ll be able to resume some of our normal operations as quickly as possible,” he said. “But some of that is, frankly, in the hands of God.”

The delegates will also raise funds for disaster relief efforts, and the companies planning convention parties are being asked to “be respectful of the situation that exists in the Gulf” and link those events with the fund-raising efforts. In addition, there may even be some sort of joint charity effort with the Obama campaign, though that remains to be seen.

Many in McCain’s campaign hope — privately, of course — that Gustav will provide them with an opportunity to repair the damage done to the GOP’s image by Hurricane Katrina three years ago. In addition, the fact that McCain’s convention will no longer include speeches by President Bush and Vice President Cheney is not exactly seen is a political detriment, according to Politico.

According to a recon report at 7:03 PM EDT, Hurricane Gustav’s minimum central pressure has fallen another 4 millibars — from 957 to 953 millibars — since 3:17 PM. That’s in addition to the 5-millibar drop between 9:48 AM and 3:17 PM EDT. Gustav’s intensification isn’t rapid, but it’s steady, and the storm is looking better on the visible satellite loop.

I’ve mentioned before that wind speed increases often take a while to “catch up” with pressure drops, and that appears to be happening here. Dr. Jeff Masters predicts that “the winds should start to increase by late tonight in response to the falling pressure.” It seems unlikely that Gustav will regain Category 4 intensity, but Dr. Masters writes: “Given the recent improvement in Gustav’s organization, I believe that the storm has time to intensify into at least a Category 3 hurricane with 125-130 mph winds by landfall.”

On the other hand, it’s still possible that Gustav’s winds could ramp up to 125-130 mph — and then ramp back down to ~115 mph just before landfall. Precision in the intensity forecast is impossible, even at this late date. Regardless, though, it looks like this will be a Category 2 or 3 at landfall, not a Category 4 or 5.

The million-dollar question remains, landfall where? And what of New Orleans?

With regard to the first question, the official forecast track “line” has Gustav’s center coming ashore just southwest of Houma — where The Weather Channel’s Jim Cantore has been sent — and then proceeding directly over Morgan City and Lafayette.

However, as I mentioned earlier, the National Hurricane Center has hinted at a possible westward shift in the track later tonight. On the other hand, Alan Sullivan, who tends to rely on a more intuitive reading of the satellite map rather than scrutinizing the computer models, still thinks a worst-case scenario for New Orleans — landfall near Grand Isle — is possible.

Even if that happens, it might not be Apocalypse Now for the Big Easy, given that Gustav is unlikely to come ashore as the monster hurricane once anticipated. After I linked to Sullivan’s post stating that Gustav “certainly seems to be heading in [Grand Isle's] direction,” he added this addendum:

Will the major levees hold? Probably. Gustav may not quite be strong enough to cause another wholesale flood of the city.

Eric Berger agrees that New Orleans is likely to be spared the worst, as does Dr. Masters, who has a detailed analysis of the threat to New Orleans:

Gustav’s storm surge is not likely to breach the New Orleans levees–if they perform as designed
Gustav is a very large storm. Like Katrina, Gustav may carry a larger storm surge to the coast than its wind speeds might suggest. Currently, Gustav’s diameter of tropical storm force winds is 340 miles. By landfall, this number is forecast to increase to 360 miles, which would make Gustav 80% as large as Katrina was at landfall. NHC’s current storm surge forecast calls for a storm surge of 10-14 feet to the right of where the center of Gustav comes ashore. The latest computer generated storm surge map shows that highest surge will be along the levee system along the east side of New Orleans. Storm surge levels of this magnitude are characteristic of a Category 3 hurricane. The levee system of New Orleans is designed to withstand a Category 3 storm surge. If Gustav intensifies more than the NHC forecast is calling for, there is a significant threat of multiple levee failures in the New Orleans levee system resulting in flooding of portions of the city. However, the latest 12Z (8 am EDT) model runs have shifted their landfall points a bit further west, reducing the odds of a Category 4 storm surge in New Orleans. My best guess is that New Orleans will suffer a Category 2 or 3-level storm surge. The levees will hold with that level of storm surge, if they perform as designed.

Barring unexpected changes in the forecast, it will soon be time for the media to ramp down the pre-landfall hype about a possible calamity in New Orleans, lest they contribute to the public’s sense of cynicism about storms “never living up to the hype.” The “hype” surrounding Gustav was fully justified last night, before the unforeseen post-Cuba weakening, but now it is growing less so, at least with regard to the Big Easy.

In other news, Tropical Storm Hanna — which I confess I’ve barely been following at all, as Gustav has been so all-consuming — is increasingly looking like a hurricane threat to the Southeast coast late this week or early next weekend. Hanna to Savannah? Maybe. I’ll have more to say about Hanna once we’re finished with Gustav.

For nearly a week now, forecasters have been worried about what would happen when Hurricane Gustav tracked over the warm, deep waters of the Gulf of Mexico’s Loop Current. It’s easy to see why: it was the Loop Current, after all, that strengthened Katrina and Rita into Category Five super-hurricanes in 2005. It seemed almost a fait accompli that something similar would happen with Gustav, particularly once it strengthened to a 150-mph Category Four monster yesterday over the Caribbean, meaning it would presumably have a more advanced “starting point” for its expected intensification in the Gulf.

It was this fear of rapid, massive intensification over the Loop Current that spurred New Orleans Mayor Ray Nagin to declare yesterday that Gustav would be the “mother of all storms” and the “storm of the century.” And indeed, while Nagin’s comments were slightly hyperbolic, there seemed little reason when he made them to hope that Gustav would fail to significantly intensify this morning and afternoon over the southern Gulf. The best we could hope for, we thought, was a gradual, non-rapid intensification, followed by significant weakening over the northern Gulf.

Instead, we’ve been blessed by a totally unexpected positive development: Gustav has essentially failed to strengthen at all over the Loop Current. Now those dangerous waters are in the storm’s rearview mirror. Gustav is moving toward less energetic waters — still capable of supporting intensification, but probably not rapid intensification.

gustav-pastloop-sm.jpg
Gustav’s track (black for its previous course, red for its forecast track, black dot for its current position), roughly drawn by yours truly on the Tropical Cyclone Heat Potential map.

Admittedly, Gustav has shed a small handful of millibars from its central pressure in recent hours — but not enough to qualify as significant deepening. Meanwhile, the storm’s core has remained woefully disorganized for a major hurricane, as you can see on the satellite loop, and the winds are barely Category 3 strength, if that. Officially, the top sustained winds are 115 mph, but that may be generous; a strong case can be made that Gustav has been a glorified Cat. 2 all morning and afternoon today. (The NHC has, I suspect, been reluctant to downgrade it because they don’t want people to prematurely sound the “all clear,” lest Gustav then re-strengthen unexpectedly.)

Why has the Loop Current failed to strengthen Gustav much? Alan Sullivan explained this morning:

In time-lapse water-vapor imagery, a skilled eye can see the plume of mid-level dry air wrapping around the outside of Gustav in an arc from the western Gulf, across Yucatan, curving north over western Cuba, and entering the east side of the cyclonic whirl. This plume is hindering convection there, but the fierce winds in the storm core are lifting energy from the Loop Current and cycling it aloft through the western eyewall. Yes, it’s counterintuitive, but this is how hurricanes work.

Now, as of 5:00 PM EDT, there are signs Gustav is finally beginning to get its act together — too late for the Loop Current to do its worst, but soon enough, perhaps, for some intensification before landfall (followed, hopefully, by some weakening). The National Hurricane Center’s 5pm discussion states: “VISIBLE SATELLITE IMAGERY DURING THE PAST COUPLE OF HOURS SUGGESTS THAT GUSTAV IS GETTING A LITTLE BETTER ORGANIZED…WITH A HINT OF AN EYE RETURNING.” Sullivan sees the same thing:

[I]n the last two hours I have seen a marked increase in convection around the center of the storm. Although the eye is still cirrus-lidded, there is a complete ring of overshooting cloud tops. Gustav has reformed a real eyewall, for the first time since it left Cuba. I expect [the] pressure drop to sharpen at the 8 PM update. In that case we shall have an answer to the question of intensity: some strengthening this evening; some weakening just before the eye reaches the Delta. I’m guessing 120 mph winds at landfall.

But landfall where? The NHC has shifted its official forecast track at 5:00 PM, and suggests in the discussion that it may go further left:

THE NEW GUIDANCE RUNS…MOST NOTABLY THE HWRF…SHOW A SLIGHT LEFT TURN AFTER 12 HR WITH A LEFTWARD SHIFT IN THE LANDFALL POINT ON THE LOUISIANA COAST. THE NEW FORECAST TRACK IS SHIFTED SLIGHTLY TO THE LEFT OF THE PREVIOUS TRACK…AND SINCE IT LIES NEAR THE EASTERN EDGE OF THE GUIDANCE ENVELOPE SOME ADDITIONAL SMALL SHIFTS MAY BE NECESSARY LATER.

This would be excellent news for New Orleans. But Sullivan, watching not the computer models but the satellites, sees things differently:

With forward speed at 18 mph, Gustav will be ashore much sooner than previously expected. This means less time for the core to decay before it passes just SW of New Orleans — the worst track. And it certainly seems to be heading in that direction. Maybe it will waver left or right. Maybe.

We’ll see who is right.

I’ll have more to say about the track in my next update, probably around 8:00 PM EDT. (Please note, the timestamps at the top of these posts are in PDT. Sorry for any confusion.)

I don’t have time to post a full update right now, but I highly recommend Alan Sullivan’s latest post, including updates. He summarizing things excellently.

Also, Eric Berger has promised a comprehensive rundown on the prognosis for New Orleans in the next couple of hours.

More later. In the mean time, keep checking the links in the sidebar at right for the latest.

P.S. The timestamp on this post, and all posts on this blog, is in Pacific time. I apologize for any confusion.

I will try to post an update after the 5:00 PM EDT (2pm PDT) advisory from the National Hurricane Center.

Hurricane Gustav is still a serious threat to New Orleans and the rest of the Louisiana coast — but, after its unexpectedly severe weakening over Cuba, there is now reason to hope that perhaps Gustav won’t be the “mother of all storms.”

Dr. Jeff Masters explains what has happened, in a post written at 8:16 AM EDT:

Gustav roared over Cuba as a Category 4 hurricane with 150 mph winds last night, but Cuba exacted a heavy toll on the storm. Gustav weakened to a Category 3 storm after passage over the island, and is still weakening, according to the latest Hurricane Hunter reports. At 7:03 am EDT, the Hurricane Hunters reported a pressure of 960 mb, up 1 mb from their previous pass through the eye at 5:12 am. Surface and flight level winds measured by the Hurricane Hunters suggest that Gustav may only be at Category 2 strength. Top winds seen by the plane’s SFMR instrument so far this morning have been about 105 mph. The top flight level winds of 103 knots at the flight altitude of 10,000 feet also support a surface wind of 105 mph. NHC has elected to keep Gustav at Category 3 120 mph strength with their 8am advisory, probably because the satellite appearance still supports a Cat 3. The boundary between Category 2 and Category 3 is at 115 mph.

Visible satellite loops show a ragged and lopsided looking hurricane. Upper level winds from the south are creating 10-15 knots of shear over Gustav, and restricting the upper-level outflow on the south side. The 28-mile wide eye is not very distinct, and the Hurricane Hunters reported that the eyewall was missing a chunk on the south side. Long range radar from Key West also shows the missing southern portion of the eyewall.

Like I said, that post was written at 8:16 AM, but Gustav still looked ragged as of 9:45 AM, the most recent satellite image on the NOAA loopthough its eye did become distinct again, around 9:00 AM. [UPDATE: The 11am EDT discussion says the "eye-like feature" is not actually at the center of the storm's circulation. The discussion vividly describes Gustav's lack of organization:

GUSTAV CONTINUES TO LOOK SOMEWHAT RAGGED IN SATELLITE APPEARANCE THIS MORNING. THE DEEP CONVECTION IS VERY ASYMMETRIC...WITH THE COLD TOPS DUE MAINLY TO ONE HOT TOWER IN THE WESTERN EYEWALL. WHILE AN EYE-LIKE FEATURE IS APPARENT...IT IS DISPLACED TO THE NORTHEAST OF THE AIRCRAFT-REPORTED CENTER. THE LATEST REPORT FROM A NOAA HURRICANE HUNTER INDICATES THE CENTRAL PRESSURE HAS RISEN TO 962 MB...ALONG WITH AN ELLIPTICAL 30 BY 20 N MI WIDE EYE OPEN TO THE SOUTHEAST. THE INITIAL INTENSITY IS REDUCED TO [120 MPH]…AND THIS MIGHT BE A LITTLE GENEROUS BASED ON THE AIRCRAFT WINDS.]

Bottom line, Hurricane Gustav is not an especially healthy storm at the moment, certainly nothing like the textbook major hurricane we saw south of Cuba yesterday. This is good for two reasons. One, Gustav has to get itself structurally re-organized before it can rapidly intensify, and if it takes long enough to do that, it may run out of warm Loop Current water before it can turn back into a monster. (As Eric Berger says, now is the time for rapid intensification to occur, if it’s going to happen at all.)

Two, even if Gustav does turn back into a monster a bit later today (say, late this afternoon), that’s better than doing so this morning, because it decreases the number of hours of Category 4+ winds blowing over the Gulf of Mexico and building up a massive storm surge. You’ll recall that Katrina’s surge was enormous because it spent so much time over the Gulf as a huge, intense hurricane. With each hour that passes without Gustav strengthening, the odds of a similar phenomenon occurring decrease. (Not that a borderline Cat. 2/3 hurricane doesn’t build up a significant surge, mind you. But it’s a far cry from a Cat. 4 or 5.)

And another bit of a good news: Dr. Masters says “the upper level wind environment is favorable for intensification, but not as favorable as during yesterday’s rapid intensification.”

So, what will Gustav’s impact on New Orleans be? It’s too soon to say, and it could still be the “storm of the century” that Ray Nagin fears. But that seems less likely than it did 12 hours ago. Again quoting Dr. Masters:

NHC’s current storm surge forecast calls for a storm surge of 18-25 feet to the right of where the center of Gustav comes ashore. The latest computer generated storm surge map shows a bit lower maximum storm surge levels — about 15-18 feet on the east side of New Orleans. Storm surge levels of this magnitude are characteristic of a Category 4 hurricane. The levee system of New Orleans is designed to withstand a storm surge characteristic of a Category 3 storm. If the NHC storm surge forecast is correct, there will likely be multiple levee failures in the New Orleans levee system resulting in flooding of portions of the city.

However, the extent of Gustav’s current weakening was unexpected, and this could substantially reduce the storm surge. Given the current intensity forecasts, I believe there is a 60% chance that a lower storm surge of 12-15 feet, characteristic of a Category 3 hurricane, will affect the city. If the Army Corps of Engineers’ assertion that the levee system can withstand a Category 3 hurricane is correct, the levees will hold.

P.S. In terms of watching whether Gustav intensifies rapidly over the Loop Current, keep in mind that the barometric pressure is the first thing to change when a storm “bombs out.” The wind speed often takes a few hours to “catch up” with a pressure drop. So the pressure is the thing to watch for. If Gustav exits the Loop Current without a significant pressure drop, then we’re probably pretty much in the clear. On the other hand, if there’s a last-minute pressure drop just before it exits the warm-water area, an increase in wind speed will likely follow, even if by that point the storm is over a cooler-water area.

Based on Gustav’s current location and forward speed, I’d say we’re looking at a six-hour window, or thereabouts. If the pressure doesn’t drop significantly by, say, the 5:00 PM advisory, we’ll probably be able to say we’ve dodged a bullet. (That’s an off-the-top-of-my-head estimate; someone please correct me if you think I’m wrong.)

Gustav weakens!

August 31st, 2008 - 4:08 am

Good news from the 5:00 AM EDT discussion: Gustav’s “starting point” for today’s expected intensification will be lower than previously thought.

APPARENTLY…THE INTERACTION WITH WESTERN CUBA TOOK MORE OF A TOLL ON THE HURRICANE THAN EARLIER ESTIMATED. DATA FROM THE AIR FORCE HURRICANE HUNTER SHOW THAT GUSTAV HAS WEAKENED WITH MAXIMUM FLIGHT-LEVEL WINDS OF [129 MPH]…SFMR WINDS OF [113 MPH]…AND A MINIMUM CENTRAL PRESSURE OF 958 MB. SATELLITE IMAGES CONFIRM THIS WEAKENING TREND AS THE EYE NO LONGER VISIBLE. AIRCRAFT AND RADAR OBSERVATIONS ALSO INDICATE THAT THE EYE HAS EXPANDED TO AROUND 25 N MI IN DIAMETER. THE INITIAL INTENSITY IS LOWERED TO [125 MPH]…AND GUSTAV MAY EVEN BE A LITTLE WEAKER FOR THE MOMENT. HOWEVER THIS WEAKENING IS FORECAST TO BE SHORT-LIVED AS THE HURRICANE TRAVERSES THE RELATIVELY HIGH HEAT CONTENT OF THE GULF LOOP CURRENT. BEYOND 24 HOURS…THE HEAT CONTENT DECREASES AND MOST MODELS INCREASE THE VERTICAL WIND SHEAR NEAR GUSTAV DUE TO AN UPPER-LEVEL LOW OVER THE WESTERN GULF. IN ADDITION…NONE OF THE NUMERICAL GUIDANCE SHOW SIGNIFICANT RESTRENGTHENING OF GUSTAV…ALTHOUGH ADMITTEDLY THE SKILL OF THESE MODELS IS RATHER LOW. THE OFFICIAL INTENSITY FORECAST IS REDUCED FROM THE EARLIER ONE…BUT STILL SHOWS GUSTAV AS A MAJOR HURRICANE AT LANDFALL.

[UPDATE: Gustav's top winds are now down to 120 mph, as of 8am EDT. This decrease probably does not represent not further weakening, but rather confirmation from the latest recon mission that Gustav was, and remains, "a little weaker" than the previous advisory estimated.]

Hopefully the weakening is such that Gustav, in its less-organized state, will be unable to fully take advantage of the Loop Current. We’ll see. Certainly, my “175 mph” scenario seems far less likely now.

I’ll have another update later in the morning, probably after the 11:00 AM advisory comes out.

UPDATE, 8:06 AM: Alan Sullivan pours cold water on my hopes that the Loop Current won’t do its worst:

Winds are estimated at 120 mph. Central pressure has risen a bit more to 960 mb. That’s the good news. But Gustav’s eye is beginning to reappear in the satellite imagery, and intense convection is wrapping more completely around the center. I expect we will be seeing a fully developed core again within a few hours. I still anticipate a significant pressure drop and increase in winds later today. Weakening will resume before tomorrow’s landfall, but it may be too late, because the storm is moving so fast.

Threat to New Orleans increases

August 30th, 2008 - 10:35 pm

[NOTE: I've reorganized my blogroll, and added a whole bunch of Gustav-specific and New Orleans-specific links. Of particular interest, perhaps, is this page displaying four New Orleans TV live-streams in one place. Anyway, take a look at the blogroll. There's lots of good stuff there.]

As expected, the National Hurricane Center’s 11:00 PM EDT advisory on Hurricane Gustav shifted the official forecast track ever-so-slightly to the right — closer to the New Orleans doomsday scenario. I’ve illustrated the shift below. The blue line is the 5pm track; the black line is the 11pm track; the red line is, roughly speaking, the worst-case track:

gustav-trackshift.gif

As you can see, the new track is not quite the worst-case scenario for New Orleans. But it’s plenty bad, and as Eric Berger writes, “if it moves much further east New Orleans will really, really be under the gun.”

Specifically, if Gustav’s eye comes ashore around Grand Isle, just west of that peninsular jut of land which is the Mississippi River delta, the storm’s right-front quadrant will track directly over New Orleans, bringing the strongest winds and — more importantly — the strongest storm surge right into the city. As Berger says, “Let us hope that is not the case.”

Indeed. Let us, in fact, pray that it is not the case. In such a scenario, New Orleans could see a storm surge far worse than Katrina, indeed worse than anything it has ever experienced. It would be so bad that, unlike in Katrina, the levees would be unable to protect the city even if they work as designed. The scenario envisioned long before Katrina would come to pass. Depending on how many people refuse to evacuate and foolishly stay in the city, thousands could die, perhaps many thousands. It would be a calamity of epic proportions.

It is for this reason that the slight rightward shift in the track — combined with the recently developed consensus that Gustav will not slow down before landfall, as had previously been anticipated (which means there will be a more powerful surge, and less time for weakening over the northern Gulf) — is so alarming. After a couple of days in which the smart money was on a “dodging the bullet” scenario, things are getting rather scary for New Orleans. You know it’s bad when Alan Sullivan, Mr. Anti-Alarmist, is sounding the alarm: “New Orleans is really at risk now.”

Sullivan was reacting not to the changed NHC track, but to the event that precipitated it: the rightward shift in the computer models at 8:00 PM EDT. I alluded to this earlier, and below, you can see for yourself the 8pm “spaghetti” model track from CSU. The models are now mostly “clustered” on a landfall in southeast, rather than southwest, Louisiana:

gustav-trackshift-models.png

Moreover, as this Weather Underground map shows, the important “global” models are almost unanimous in bringing Gustav ashore dangerously close to that worst-case Grand Isle spot.

I can understand why Mayor Nagin, looking at this data, would throw out terms like “storm of the century” and “mother of all storms.” The threat to New Orleans is grave. Once again, the city’s fate will probably depend on tiny variables: last-minute wobbles, puffs of dry air, etc.

I bear no ill will toward the folks in Houma, Morgan City, or points west, but I hope Gustav comes ashore in those areas rather than near Grand Isle. In the grand scheme of things, anything would be preferable to the New Orleans nightmare scenario.

Of course, the best-case scenario for everyone would be substantial weakening before landfall. The problem is, Gustav is likely to get stronger before he gets weaker — perhaps significantly stronger, thanks to the Loop Current, and that’s problematic when his starting point is 140 mph — and then, thanks to the relatively fast forward speed, the time window for weakening will be limited.

The reality is, we simply do not, and cannot, know what Gustav’s landfall intensity will be. We think he’ll be a major hurricane — i.e., Cat. 3 or higher — but that’s about as precise as we can get. Officially, the forecast calls for a landfall intensity in the 140-150 mph range, making Gustav a mid-to-high-end Category 4. However, the discussion says, “THIS IS A LOW CONFIDENCE INTENSITY FORECAST.” That’s saying something, considering that intensity forecasts are almost never very high-confidence.

This forecast is especially “problematic,” as the NHC puts it, because there are so many variables. We aren’t sure how much wind shear there will be, or how much impact it will have; we can’t be certain exactly how much energy the Loop Current will pump into the storm (as we have very, very little skill at forecasting rapid intensification); we don’t know how rapidly the cooler waters north of the Loop Current will start to take their toll, weakening the storm; we have no way of confidently predicting whether (and if so, how much) dry air will get sucked into the circulation when it’s near land; and, of course, eyewall replacement cycles are totally unpredictable. So anything from 115 mph to 165 mph seems totally reasonable.

Here’s the full intensity analysis from the discussion:

THE INTENSITY FORECAST REMAINS PROBLEMATIC. ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN SUGGEST THAT THE INTENSIFICATION [SATURDAY MORNING] OCCURRED DESPITE ABOUT 20 KT OF SOUTHERLY VERTICAL SHEAR CAUSED BY AN UPPER-LEVEL TROUGH OVER THE GULF OF MEXICO. THE PRESENCE OF THIS SHEAR IS SUPPORTED BY RADAR DATA FROM THE KEY WEST WSR-88D… WHICH CURRENTLY SHOWS A WEAK SOUTHERN EYEWALL AND LITTLE PRECIPITATION SOUTH OF THE EYEWALL [AFTER THE CROSSING OF CUBA]. THE SHIPS MODEL SUGGESTS THE SHEAR SHOULD DIMINISH SOME DURING THE NEXT 18 HR…THEN INCREASE AGAIN AS GUSTAV MOVES BETWEEN THE TROUGH AND AN UPPER-LEVEL ANTICYCLONE TO THE SOUTHEAST. ON THE OCEAN SIDE…GUSTAV IS FORECAST TO PASS OVER THE LOOP CURRENT DURING THE NEXT 24 HOURS… THEN POSSIBLY PASS OVER A COUPLE OF COLD EDDIES NORTH OF 26N. ALL GUIDANCE FORECAST RE-INTENSIFICATION DURING THE NEXT 24 HR…SO THE INTENSITY FORECAST WILL CALL FOR A PEAK INTENSITY OF [155 MPH] IN 24 HR…FOLLOWED BY SLIGHT WEAKENING BEFORE LANDFALL DUE TO SHEAR AND LOWER HEAT CONTENT…THEN FASTER WEAKENING OVER LAND. DUE TO THE VARIOUS FACTORS…THIS IS A LOW CONFIDENCE INTENSITY FORECAST.

It’s entirely possible Gustav could come ashore significantly weaker than the forecasted 140-150 mph. It’s also possible it could come ashore stronger. Category Five starts at 156 mph. I don’t want to be alarmist, but what if Gustav strengthens tomorrow morning/afternoon to 175 mph, or something ridiculous like that, as Katrina did on the Sunday before its landfall? The Loop Current is plenty warm enough to support such intensification, just as it was in 2005. And then, what if there isn’t the same lucky combination of dry air and an eyewall replacement cycle that so drastically weakened Katrina at the last minute? What if, instead, there’s only gradual weakening as the storm encounters cooler water over the northern Gulf? There simply may not be enough time to significantly ramp down the strength from its peak.

Moreover, there’s also the crucial fact that last-minute weakening won’t significantly reduce the storm surge, as Katrina taught us. Surge-wise, we need to hope Gustav never strengthens that much today — perhaps because of unexpectedly strong shear, or what-have-you. That seems unlikely… and even a 140-mph storm is still going to cause plenty of storm surge… but we gotta hope for the best.

Anyway… I need to go to bed. But there will be several things to watch for overnight. First, the 2:00 AM EDT intermediate advisory from the National Hurricane Center should include initial data from the reconnaissance aircraft currently en route to Gustav. This data will give a better read on the storm’s current intensity — the 140 mph is only a satellite-based estimate — which is effectively its “starting point” for the expected intensification later today.

A fuller discussion of the aircraft’s findings will be in the discussion at 5:00 AM, including the matter of what Gustav’s organizational structure looks like after the crossing of Cuba. If the core has been significantly disrupted, intensification could be retarded just enough to keep the wind speed down a tiny bit. That would be good, obviously.

Also worth watching at 5:00 AM will be the new forecast track. Will it stay put? Will it edge further to the right? That, in turn, will be informed by the computer model data, which will be coming out over the next few hours. Charles Fenwick plans a wee-hours update on the recon data and the computer models runs, so I’d highly recommend checking out his blog for that, if you’re awake.

I’ll probably post a brief update around 7:30 AM EDT, and a fuller update sometime after 10:00 AM. In the mean time, keep checking the links in my sidebar for the latest.

The Houston Chronicle‘s Eric Berger is looking at the 8pm EDT computer model runs, and he has some bad news for New Orleans:

I … expect the official track to be nudged slightly eastward toward New Orleans [at 11pm EDT] tonight. This raises the likelihood of the ["worse than Katrina"] New Orleans surge scenario I laid out below, but it’s by no means a certainty.

However, it’s not looking good.

Mayor Nagin must be looking at the same data, given his rhetoric in ordering a mandatory evacuation:

Mayor Ray Nagin late Saturday warned that Gustav is the “mother of all storms” and ordered a mandatory evacuation for the West Bank of New Orleans for 8 a.m. Sunday and noon for the East Bank.

“We want 100 percent evacuation,” said Nagin. “It has the potential to impact every area” . . .

“This is worse than a Betsy, worse than a Katrina,” Nagin said.

The mayor speculated that Gustav is so fierce Baton Rouge likely will experience 100 mph winds.

“You need to be scared and you need to get your butts out of New Orleans right now,” Nagin said.

Nagin said he expects Gustav to “punch holes in the Harvey Canal,” which will cause the West Bank to become a bath tub.

The West Bank has 8-foot-high to 10-foot-high protection, he said. Gustav’s storm surge may be 15-, to 18- to 24-feet high.

Anyone who opts to remain in New Orleans “will be on your own,” Nagin warned, adding that services will not be available.

Needless to say, I agree with the decision to evacuate. However, I think Nagin is actually going a bit overboard with his rhetoric here, perhaps overcompensating for his Katrina failures. More on that a little later.

One thing Nagin’s right about, for sure, is that New Orleans residents need to be scared — at least scared enough to, as the mayor said, get their butts out of New Orleans right now. One reason to be scared is that Gustav is actually accelerating, which is very bad news. As Alan Sullivan writes:

Forward speed has increased to 15 mph, and the steering current has really steadied. As I watch the time-lapse imagery, I can see that Gustav has settled into alignment with the other weather systems in the region, and it will probably not deviate from its present track before Gulf Coast landfall. Because it is moving faster than previous projections suggested, it will bring more of its force to the coast.

While Gustav wavered in the Caribbean, bouncing from island to island, I was hopeful that it would not reach the US as a major hurricane. Worst case scenarios rarely come to pass. First Gustav would have to escape the bind of other weather systems and mountainous terrain; then it would have to spin through a rapid intensification phase south of Cuba; finally it would have to find a strong steering current that would bear it swiftly across the Gulf before shear or cooling waters could weaken it. All these things have happened. Now it is just a question of where the eye goes ashore.

If landfall occurs near Atchafalaya Bay or further west, the eyewall will pass safely SW of New Orleans. In that case conditions at the city should not severe enough to breach major levees, unless they are weaker than they were before Katrina. But if landfall occurs near Grand Isle, then New Orleans will be hammered by the northeast quadrant of the storm — the strongest part.

That would the worst-case scenario, and indeed, the true “mother of all storms.”

But it’s only one scenario, and that’s where I think Nagin’s rhetoric is a bit overheated. Gustav may be worse than Betsy and Katrina, but that remains to be seen; it all depends on the track, how much the storm strengthens tomorrow morning and afternoon, and how much it weakens tomorrow night and Monday before landfall. Residents definitely need to know that it could be worse than Betsy and Katrina, you don’t want to tell people it’s definitely going to be worse than those storms, lest your overconfident forecast fail to verify, and you become the “boy who cried wolf” — unable to motivate people to leave the next time a potential “mother of all storms” threatens.

Moreover, Nagin made one statement that makes no sense whatsoever to me, which I edited out of my earlier blockquote because I think it’s just flat incorrect. According to the Times-Picayune paraphrase, Nagin said “Katrina had a footprint of about 400 miles … [whereas] Gustav is about 900 miles and growing.” As far as I can tell, that’s simply not true. According to the NHC, Gustav’s tropical-storm force winds extend a maximum of 170 miles out from the center, on the northeast side; on other sides, the boundary is about 140 or 85 miles. You could say it has a 300-mile “footprint,” perhaps a bit more if you include peripheral effects outside the sustained tropical-storm-force area — but certainly not 900 miles, which is roughly the width of the entire Gulf of Mexico from Texas to Florida!

And while Gustav is indeed growing, it’s not bigger than Katrina, at least not yet — and there’s no possible way it will ever be more than twice Katrina’s size. I don’t even think that’s physically possible at the Gulf’s latitude.