why did the winds weaken?

[jrtalon]

hello first post! I've been monitoring this storm for the past several days(just fasinated by hurricanes is all) Anyway I just read the winds have dropped from 175 to 165...yet the pressure has fallen down to 902mb? I was expecting it to get stronger. Can someone explain this to me? Don't get me wrong it good that its weaker (every bit helps I suppose). Also shouldn't the NHC be issuing 2 Intermediate advisories between full updates instead of just the one at 7CDT??

[lurkerinthemidst]

I was wondering the same thing. Hopefully someone can explain this.

[Mac]

Could be normal fluctuations in intensity, or it could be due to interaction with the trough to the north. Regardless, don't expect Katrina to suddenly decreast in intensity. If anything, she will likely increase in intensity between now and landfall. Lest ye forget, we're going to be going into the diurnal period. Overnight, she's going to have even better conditions for intensification, and in the morning--boom--landfall.

[canegrl04]

In their last report,the NHC mentioned there would be fluxuation with intensity.This is not unusual . Katrina may ramp up again thru the course of the evening

[Downdraft]

Major hurricanes go through eyewall replacement cycles. The dynamics of this is not totally understood. Also, it is impossible for a hurricane in the Atlantic basin to sustain such strength for a long period of time. The problem is Katrina replaces her eyewall before landfall she will be going through an intensification phase right at landfall. However, its pretty much academic if you were in it I doubt you could tell the difference between 160 or 170 mile per hour winds. Katrina will remain a Cat 5 storm and make landfall that way.

[txwatcher91]

Could be normal fluctuations in intensity, or it could be due to interaction with the trough to the north. Regardless, don't expect Katrina to suddenly decreast in intensity. If anything, she will likely increase in intensity between now and landfall. Lest ye forget, we're going to be going into the diurnal period. Overnight, she's going to have even better conditions for intensification, and in the morning--boom--landfall.

Excuse me if this is a dumb question, but what exactly is the diurnal period and how does it help hurricanes strengthen when conditions are favorable?

[InimanaChoogamaga]

It sounds like they are saying it was never 175mph. But that it could still strengthen to or past that speed.

DATA FROM THE STEPPED-FREQUENCY MICROWAVE RADIOMETER INSTRUMENT ON BOARD THE AIRCRAFT SUGGEST THAT THE SURFACE TO 700 MB FLIGHT LEVEL WIND RATIO IS NOT QUITE AS LARGE AS WE TYPICALLY USE...AND THE INITIAL INTENSITY IS ADJUSTED SLIGHTLY TO 145 KT.

[Mac]

Personally, I do not believe she is going to go through another ERC prior to landfall. Her eye is 33 nm at last report, and she is almost structurally perfect. Her eye would have to shrink considerably and there would have to be some need for her to undergo an ERC in order for it to happen. Simply put, she's perfect just the way she is, and I do not expect she'll need to undergo another ERC before making landfall.

[cjrciadt]

Major hurricanes go through eyewall replacement cycles. The dynamics of this is not totally understood. Also, it is impossible for a hurricane in the Atlantic basin to sustain such strength for a long period of time. The problem is Katrina replaces her eyewall before landfall she will be going through an intensification phase right at landfall. However, its pretty much academic if you were in it I doubt you could tell the difference between 160 or 170 mile per hour winds. Katrina will remain a Cat 5 storm and make landfall that way.

Agreed, though she went up only to 948 during EWRC last time, and barely weakenend over FL. She does not really do things "by the book".

[LSU2001]

[Scorpion]

Looks to be getting better organized.

[Mac]

Could be normal fluctuations in intensity, or it could be due to interaction with the trough to the north. Regardless, don't expect Katrina to suddenly decreast in intensity. If anything, she will likely increase in intensity between now and landfall. Lest ye forget, we're going to be going into the diurnal period. Overnight, she's going to have even better conditions for intensification, and in the morning--boom--landfall.

Excuse me if this is a dumb question, but what exactly is the diurnal period and how does it help hurricanes strengthen when conditions are favorable?

As I understand it...

The dirunal period occurs at night, after the sun goes down. The sun is no longer heating the atmosphere, and temps rapidly decrease at altitude. Meanwhile, the SSTs remain fairly constant (water retains heat better than air). Because there is a greater variance between SSTs and air temps aloft, the atmosphere becomes more conducive to storm deepening. Kind of like what happens when a warm front and a cold front collide to cause T-Storms.

[WindRunner]

It sounds like they are saying it was never 175mph. But that it could still strengthen to or past that speed.

DATA FROM THE STEPPED-FREQUENCY MICROWAVE RADIOMETER INSTRUMENT ON BOARD THE AIRCRAFT SUGGEST THAT THE SURFACE TO 700 MB FLIGHT LEVEL WIND RATIO IS NOT QUITE AS LARGE AS WE TYPICALLY USE...AND THE INITIAL INTENSITY IS ADJUSTED SLIGHTLY TO 145 KT.

What they were saying there was that their typical 90% of flight level winds conversion does not seem to work at the moment. So, since they upgraded to 150kt/175mph on that, they have not actually found those winds at the surface, and they have had that problem with Katrina before (as they did yesterday afternoon). The winds did not necessarily go down, but they are adjusting them because they are not as sure that they are there anymore.

[ConvergenceZone]

actually, the outflow is looking a bit more ragged on the last couple of frames, check it out. They also mentioned that the pressure has come up just slightly.. I don't think it will strengthen, but don't think it will weaken below 140 mph either.

[txwatcher91]

Could be normal fluctuations in intensity, or it could be due to interaction with the trough to the north. Regardless, don't expect Katrina to suddenly decreast in intensity. If anything, she will likely increase in intensity between now and landfall. Lest ye forget, we're going to be going into the diurnal period. Overnight, she's going to have even better conditions for intensification, and in the morning--boom--landfall.

Excuse me if this is a dumb question, but what exactly is the diurnal period and how does it help hurricanes strengthen when conditions are favorable?

As I understand it...

The dirunal period occurs at night, after the sun goes down. The sun is no longer heating the atmosphere, and temps rapidly decrease at altitude. Meanwhile, the SSTs remain fairly constant (water retains heat better than air). Because there is a greater variance between SSTs and air temps aloft, the atmosphere becomes more conducive to storm deepening. Kind of like what happens when a warm front and a cold front collide to cause T-Storms.

OK, thanks.

[canegrl04]

She gained 30mph overnight last night. She will probably gain alittle more strength over the next 6 hours or so.

[jrtalon]

if this is a EWRC, is this coming at a good time where it won't have a chance to complete the cycle before landfall or at a bad time where it will complete the cycle and then intensify?

[deltadog03]

whats scary is the fact that this could strengthen all the way to the coast

[Mac]

if this is a EWRC, is this coming at a good time where it won't have a chance to complete the cycle before landfall or at a bad time where it will complete the cycle and then intensify?

It's not an ERC. Again, she is almost structurally perfect right now. ERCs happen when an outer eye forms around and inner eye, and the inner eye collapses as it can no longer sustain itself. Before an ERC takes place, you will see the eye shrink dramatically--usually almost to a pinpoint. That is simply not occurring here, and I doubt it will. At 33 nm wide, her eye would have to do a hell of a lot of contracting before it would collapse. I don't think it will happen between now and landfall.

[x-y-no]

Mac is correct. There's no ERC likely before landfall, given the large eye and classic structure.