Time to leave New Orleans

By: Dr. Jeff Masters , 8:24 PM GMT on August 29, 2008

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Gustav is now a hurricane, according to wind data taken by the latest Hurricane Hunter flight. At 3:11 pm EDT, the Hurricane Hunters visually estimated surface winds of 90 mph, and the SFMR instrument also measured a patch of surface winds of hurricane force, 75-90 mph. Gustav's pressure has fallen 8 mb since it exited the western tip of Jamaica this morning, and now stands at 980 mb, its lowest pressure so far.


Figure 1. Current satellite image of Gustav.

Visible satellite loops continue to show a well-organized and intensifying storm that is growing larger in size. Upper-level outflow is established in all quadrants and is growing. Low-level spiral bands are multiplying and intensifying, and the amount and intensity of Gustav's heavy thunderstorms are steadily increasing. A well-defined eye has appeared, and Gustav appears poised to enter a phase of rapid intensification. Radar from Pilon, Cuba shows the developing spiral bands of Gustav quite well. Dry air is not evident anywhere close to Gustav.

It's time to leave New Orleans
Today is the 3rd anniversary of Hurricane Katrina's catastrophic hit on the Louisiana/Mississippi/Alabama coast. Unfortunately, I think that people living in New Orleans should mark the anniversary of Katrina by getting the heck out of the city. You live at the bottom of a bowl, much of it below sea level. While New Orleans must exist where it is, this is not natural. Nature wants to fill up this bowl with huge quantities of Gulf of Mexico sea water. There is a storm capable of doing that bearing down on you. If you live in New Orleans, I suggest you take a little Labor Day holiday--sooner, rather than later, to beat the rush--and get out of town. Gustav is going to come close to you, and there's no sense messing with a major hurricane capable of pushing a Category 3 storm surge to your doorstep. Don't test those Category 3 rated--but untested--levees. Conventional pre-Katrina wisdom suggested that the city needed 72 hours to evacuate. With the population about half of the pre-Katrina population, that lead time is about 60 hours. With Gustav likely to bring tropical storm force winds to the city by Monday afternoon, that means that tonight is a good time to start evacuating--Saturday morning at the latest. Voluntary evacuations have already begun, which is a good idea.

The latest computer models
After analyzing the latest 12Z (8 am EDT) model runs, I don't think Gustav is going to bring hurricane force winds to New Orleans, and I estimate there is 10% chance Gustav will bring a Category 3-level storm surge to the city. However, if I lived in the city, I'd get out tonight, because the storm is going to be plenty mean and is going to come uncomfortably close to you. A 10% chance is a big chance when it's your life.

The models continue to fall into two camps: models that foresee a track over Louisiana, making landfall Monday night or Tuesday morning (the GFS and GFDL models), and the models that bring Gustav very close to the Louisiana coast on Monday, then turn the storm westward towards Texas. The HWRF model, which was in the former camp, has now gone over to the westward-turn-to-Texas camp. The general trend of most of the models has been to build a stronger ridge of high pressure over the southern U.S. on Monday and Tuesday, and I forecast that Gustav will come very close to the Louisiana coast on Monday as a Category 3 hurricane, stall, then move westwards towards Texas. This is the solution of the latest HWRF model.

Specifically, the GFDL model continues to be a day faster than the other models, and brings Gustav over New Orleans on Monday morning as a Category 3 hurricane. It is the furthest east model. The GFS predicts a western Louisiana landfall Tuesday morning, with Gustav stalling inland. The ECMWF predicts a landfall near the TX/LA border Monday night. The NOGAPS targets Galveston on Tuesday afternoon, and the UKMET targets Galveston on Wednesday. The HWRF does not bring Gustav to a landfall by the end of its forecast cycle (5.25 days). It predicts a Category 3 hurricane 100 miles south of the central Louisiana coast on Monday morning, that stalls and wanders west, weakening to a Category 1 hurricane.

The intensity forecast for Gustav
Gustav has low wind shear (less than 10 knots), is under an upper-level anticyclone that aids intensification, is over the highest heat content waters of the Atlantic, and has no land masses or dry air to interfere with it between now and landfall in Cuba. I expect Gustav will intensify into a Category 3 or 4 hurricane before hitting Cuba. These favorable conditions will continue once Gustav enters the Gulf of Mexico. However, on Monday afternoon, when Gustav will be nearing the Gulf Coast, the SHIPS intensity model is predicting wind shear will increase to 15-20 knots. This may act to weaken Gustav, as predicted by the HWRF model. If Gustav stalls near the coast, as I am expecting, this will churn up cold water from the depths that may also act to weaken the storm.

Yesterday, a NOAA Hurricane Hunter research aircraft dropped a network of 60 specialized buoys (Air eXpendable BathyThermographs, or AXBTs) in the Gulf of Mexico to provide precise measurements of ocean temperatures in order to aid intensification forecasts for Gustav. This data has not yet been assimilated into the GFDL and HWRF computer models, I am told, since the procedure has not yet been automated. The data must be entered manually by NHC staff into the models. Presumably, NHC has their hands full with Hanna and Gustav, so it is not certain this data will be used for real forecasts of Gustav. However, I'm told by Morris Bender of the GFDL lab that this year's intensity forecasts for hurricanes in the Gulf should be much better than in years past. To quote:

The detailed structure of the warm core ring and the Loop Current is getting assimilated into the initial ocean structure for BOTH GFDL and HWRF. There was a major advancement in this technique that became operational this season.


How big will Gustav get?
Gustav is currently expanding significantly in size, and will be a medium to large sized hurricane the remainder of its life. But will it rival Katrina in size, bringing an enormous Katrina-like storm surge to the coast? Well, our ability to predict size changes in hurricanes is poor. We do know that as storms move further from the Equator, they grow in size. This is because the Coriolis force increases as you move away from the Equator. An increased Coriolis force provides more spin to the storm, and the hurricane responds by growing in size. Thus, expect Gustav to grow in size as it approaches landfall along the Gulf coast. Hurricanes also tend to grow in size as they intensify. These two factors are taken into account when NHC makes a wind radius forecast (Figure 2). NHC is forecasting that Gustav's diameter of tropical storm force winds will increase from 210 miles (estimated at 11 am today) to 310 miles by 8 am EDT Monday, just before landfall. This is about 70% as wide as the 440 mile-wide region of tropical storm force winds Hurricane Katrina had at landfall (5 am August 29, 2005). We really don't understand why Katrina got so huge, though an interesting theory was provided by Pat Fitzpatrick of Mississippi State University at a recent hurricane conference. Here's the technical gist:

Katrina nearly doubled in size on 27 August, and by the end of that day tropical storm-force winds extended up to about 140 n mi from the center. A cursory examination of satellite imagery shows the possible influence of a trough or confluence zone to the north that may have contributed angular momentum to the intensifying cyclone.

Although the rapid intensification of Katrina was noteworthy, the expansion of the tropical storm-force winds is the key forecast issue. The devastation wrought by this storm upon landfall is attributable more to its size rather than its intensity, as it landed as a Category 3 hurricane. This large hurricane caused a record storm surge and exposed the coastal regions of Louisiana and Mississippi to hurricane-force winds for an extensive period of time.

Observations, as well as a Weather Research Forecast model simulation, suggest that an influx of vorticity associated with a remnant front near north Florida contributed to the wind field expansion.


Basically, Dr. Fitzpatrick is saying that satellite observations and computer modeling studies suggest that Katrina got extra spin that helped it grow in size by ingesting a portion of an old front that had stalled out over northern Florida. As Gustav approaches the U.S., we should be on the lookout for similar clumps of clouds with some extra spin that the hurricane could use to help grow in size.

Our interactive Wundermap is a good way to study the predicted size changes of a hurricane. Click on the "Hurricane" layer, then check the "Wind Radius" box to see the predicted extent of hurricane force (65 kt), storm force (50 kt) and tropical storm force (34 kt) winds, for the four quadrants of the storm.


Figure 2. Extent of hurricane force (65 kt, yellow), storm force (50 kt, yellow-green) and tropical storm force (34 kt, green) winds, for the four quadrants of Gustav, as predicted by the National Hurricane Center at 11 am EDT August 29, 2008. Gustav's tropical storm-force winds are predicted to have a diameter of 270 nm (310 miles) when it approaches the coast of Louisiana. For comparison, Hurricane Katrina had a 380 nm (440 mile) diameter region of tropical storm force winds at landfall. Thus, Gustav's wind field is predicted to be 70% as large as Katrina's. Image is from our interactive Wundermap. Hurricane force winds are not included in the final two forecast points, because NHC did not make a forecast of these winds for those times.

Links to follow
Wundermap for the Cayman Islands
Pilon, Cuba radar
Cayman Brac, Cayman Islands
Grand Cayman

Hanna and the rest of the tropics
Not much has changed in my thinking on Hanna and the rest of the tropics, which I presented in this morning's blog. The new set of model runs for Hanna continue to give a wide range of solutions for where the storm might go, with a track over Cuba into the western Caribbean, through the Keys into the Gulf of Mexico, and into the east coast of Florida all possibilities. I'll discuss Hanna more in my next blog, which will be Saturday morning.

In harm's way, on Cayman Brac Island
Wunderground member mangroveman is on Cayman Brac Island, directly in the projected path of Gustav. Here's his report from noon today:

Hi Dr. Jeff

Well, things are picking up here. Winds from the ENE have been steadily increasing over the past 3 or 4 hours. I'd estimate them at around 30 mph with 45 mph gusts. We live on the north side of the island, so have battened down E, N and S with shutters. W ready to set up so we're not in total darkness. Sea state is angry with white tops at about 6 feet. Rain bands giving short showers, none of them very heavy. No thunder and lightning as of yet, since last night.

Will give you more of an update in a couple of hours.

cheers
MM

And from 4pm:

OK. Wind speeds have increased, now from ENE. I'd say 40 mph and gusts now up over 50 mph. This started about an hour ago, and it's been raining solidly for the same period of time. Sea state (from what we can see) has gotten rougher, maybe 8 feet crests. We may be losing power. Just had a couple a brief power outages...probably tree branches on wires. We're watching the eye and movement on the satellite image and it looks like it's going to come pretty close. T& L just started a few minutes ago, probably have to unplug soon, anyway.

more later, I hope, Cayman Brac Power & Light willing.....

MM


Jeff Masters

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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