Weather Nerd

Threat to New Orleans increases

[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.