Eye of the Storm

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Friday, August 26, 2005

Hurricane Katrina Update 261500Z

At 11 AM EDT, the center of Hurricane Katrina was at 25.1 North 82.2 West, 45 miles northwest of Key West Florida and moving to the west at 7 mph. Maximum sustained winds are up to 80 mph (now significantly stronger, see update below) and minimum central pressure is down to 981 millibars (28.97") (now lower, see discussion).

Hurricane Katrina Advisory Number 12

Recon just found pressure of 971 millibars. Winds observed by doppler radar suggest a surface wind of 85 mph (the recon plane has only sampled the northwest quadrant thus far, finding flight level winds that suggest 80 mph at the surface).

Katrina continues to move south of west. That is expected to flatten to a due west motion during the next 12 hours. Model guidance is widely split on the timing of the erosion of the high pressure ridge that is currently dictating Katrina's westward movement. The 06Z run of the NOGAPS model shifted its forecast landfall position to Louisana (the GFDN model also shows a LA landfall, but it has been on the western outlier for most of the duration of Katrina). The balance of the models take Katrina over the northeastern Gulf coast. Strengthening to a category three is forecast. A special advisory and forecast is now being drafted to cover the lower pressure found by recon.

Hurricane Katrina Discussion Number 12

Official track forecast

I had been expecting Katrina to react favorably to the Gulf of Mexico, so the stronger than expected winds and lower than expected pressure is not a huge shock, but it is a small surprise nonetheless.

I had been expecting a category three with a chance of a category four for the second landfall. The jump that Katrina has gotten on re-intensification makes me think a strong category three to a minimal category four (130-140 mph winds) will be making landfall.

My unofficial watch area continues to be from Mobile Alabama to St Mark's Florida. My warning area is adjusted westward to run from Destin to Mexico Beach Florida. As mentioned in my last update, coastal areas west of my warning area are more likely to receive effects of Katrina than areas inland, while the opposite is true for areas east of my warning area.

Tropical Cyclone Heat Potential
is greater now than it was in when Dennis made landfall
so weakening before landfall will be less significant than it was for Dennis.

Start preparing Florida panhandle, Katrina is heading your way and she's going to be carrying a strong punch.

SLIGHTLY LATER: Dropsonde from the recon plane in the eyewall recorded 95 knots at 925 millibars, which suggests 100 mph winds at the surface... which caused a Special Advisory to be released showing this and the new pressure. Intensity forecast adjusted to show winds in the vicinity of 125 mph at landfall.

The intensity estimate dervied from the dropsonde may be too high. Dropsondes do not record sustained winds, but rather instanteous winds, so it is quite possible that it recorded a gust. Flight level winds at the time suggest that 80 knots/90 mph is more likely. However, the forecaster may be assuming stronger winds will be found in the northeast quadrant. We shall see.

A BIT MORE LATER: The discussion argues the pressure reading of 971 millibars supports 88 knots, hence 85 knots for this advisory. A valid argument, but not one that all forecasters would agree to. Flight level winds will support 100 mph at some point this afternoon, but it may be a while. (Tied into this is the problem associated with having the official forecasts being in knots (converted to mph for the sake of the general public); because the possible values are in increment values, you can have 80 knots or 85 knots, which amounts to 90 or 100 mph. A value of 95 mph is not possible).


At 12:46 PM, Blogger Brendan said...

Question for you... supposing a more westward track than we're currently anticipating... if a Category 3 or 4 hurricane hits New Orleans from the south (as opposed to from the east-southeast, which I understand to be the worst-case scenario), crossing some land (the Mississippi Delta, I guess?) on its way up, is that likely to cause the doomsday scenario envisioned by disaster planners (flooding the bowl, etc.)?

Also... would crossing the Mississippi Delta really weaken a hurricane much before hitting New Orleans... or would that be sort of like the Everglades?

At 1:24 PM, Blogger Brendan said...

P.S. I don't know Louisiana geography that well, so correct me if any of my underlying assumptions or premises are wrong. I'm talking about, or trying to talk about, the scenario envisioned by NGPS in its 06:00 run.

At 3:52 PM, Blogger Brendan said...

Thanks for the comment over on my blog.

I just re-read the Times-Picayune special report, and it includes this passage:

"The worst case is a hurricane moving in from due south of the city," said [LSU engineer Joseph] Suhayda, who has developed a computer simulation of the flooding from such a storm. On that track, winds on the outer edges of a huge storm system would be pushing water in Breton Sound and west of the Chandeleur Islands into the St. Bernard marshes and then Lake Pontchartrain for two days before landfall.

"Water is literally pumped into Lake Pontchartrain," Suhayda said. "It will try to flow through any gaps, and that means the Inner Harbor Navigation Canal (which is connected to Breton Sound by the Mississippi River-Gulf Outlet) and the Chef Menteur and the Rigolets passes.

"So now the lake is 5 to 8 feet higher than normal, and we're talking about a lake that's only 15 or 20 feet deep, so you're adding a third to a half as much water to the lake," Suhayda said. As the eye of the hurricane moves north, next to New Orleans but just to the east, the winds over the lake switch around to come from the north.

"As the eye impacts the Mississippi coastline, the winds are now blowing south across the lake, maybe at 50, 80, 100 mph, and all that water starts to move south," he said. "It's moving like a big army advancing toward the lake's hurricane-protection system. And then the winds themselves are generating waves, 5 to 10 feet high, on top of all that water. They'll be breaking and crashing along the sea wall."

Soon waves will start breaking over the levee.

"All of a sudden you'll start seeing flowing water. It'll look like a weir, water just pouring over the top," Suhayda said. The water will flood the lakefront, filling up low-lying areas first, and continue its march south toward the river. There would be no stopping or slowing it; pumping systems would be overwhelmed and submerged in a matter of hours.

"Another scenario is that some part of the levee would fail," Suhayda said. "It's not something that's expected. But erosion occurs, and as levees broke, the break will get wider and wider. The water will flow through the city and stop only when it reaches the next higher thing. The most continuous barrier is the south levee, along the river. That's 25 feet high, so you'll see the water pile up on the river levee."


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