Wind-Pressure out of Whack

[quandary]

Is there any explanation for why our recent storms have been completely unable to keep up high windspeeds but have maintained extraordinarily low pressures? I'll give a few examples. First, Ivan. Ivan's windspeeds were hovering at 135mph before landfall (not after), but its pressure was sitting down around 929-930mb. It looked to have a huge burst of convection and organization before it weakened significantly, but this didn't seem to bump up the windspeeds at all. In most storms of the past, 930 correlated with 150-155 mph, the most recent examples being Gert and Lenny in 1998, and Isabel in 2003. Yes, Ivan was a large storm, so I will disregard the examples of Keith and Iris, but the others were relatively large.

Then Katrina, probably the best example of all, which made landfall with a pressure of 918-920, which normally always corresponded with winds of 160mph. Examples of this exist throughout the record, almost all the Cat 5s had pressures in this range (H. Hugo, H. David, H. Isabel again, H. Floyd at 155, H. Easy, Dog, Beulah, Donna, etc.). Yet Katrina's winds were barely at 145mph.

Emily earlier this year was also an example. It was true that she had 3 widely spread wind maxima, but a pressure of 945 and winds of 110-115, which she had at some point, don't stack up.

And now Rita. Winds of 120, pressure of 937. Lily's pressure at 140mph was 937. Isabel's at 160mph was 938. What's going on here?

[Scorpion]

Generally weakening storms have lower pressures than typical.

[cjrciadt]

Pressure rises far slower than wind speeds weaken. Katrina was still sub 930 into MS as a mid Cat3!

[isobar]

Scorpion's right on ... big difference between a system weakening vs. intensifying. But the bottom line is pressure gradient dictates wind speed, not central pressure reading.

[vacanechaser]

Scorpion's right on ... big difference between a system weakening vs. intensifying. But the bottom line is pressure gradient dictates wind speed, not central pressure reading.

The fact is though, you dont have a gradient if the pressure is not up or down.. The lower, the bigger the gradient from the surrounding areas.. The higher the pressure, the weaker or smaller the gradient..

I think part of this with Rita, watching the sat loops, I noticed that the upper or mid level center appeared to be pushed off to the east and northeast of the low level center at landfall.. this would not allow the center to be vertically stacked and this would alos not allow the winds to reach the surface. No real organization to the system, no real transport of the higher winds in the upper levels to the surface..

Unsure about Katrina.. I thinl the recon just didnt find them with Katrina.. All you have to do is look at the damage with her!! Although alot of what we are seeing is suge damage. Still inland areas saw a lot of heavy wind damage..

Jesse V. Bass III
http://www.vastormphoto.com
Hurricane Intercept Research Team

[Sanibel]

Exactly correct.

What you are seeing is high SST's. Like I was saying, nature adjusts - so when there are record warm SST's nature adjusts the margins. A sign of this is extremely low pressures with moderate windspeeds.

Gradient is probably correct. The spread of high SST's throughout the entire enclosed Gulf fails to support a steep gradient, that in turn keeps the windspeed down. It also probably pushed the CONUS pattern further north creating less synoptic gradient or forcing. When the entire storm has hot SST's under it it creates very low pressure, but the gradient fails to ram the winds towards the center.

Katrina was there momentarily with that WPAC eye. These storms were stronger in the tropical latitudes because the synoptic there is horizontal with extremely hot SST's being forced to the cold upper atmosphere...

[HurricaneBill]

Scorpion's right on ... big difference between a system weakening vs. intensifying. But the bottom line is pressure gradient dictates wind speed, not central pressure reading.

Which also explains why midget hurricanes have higher pressures in regards to wind speeds.

[SamSagnella]

Well in Rita, wouldn't this be explained by the fact that she had multiple eyewalls during the several hours prior to landfall? If she has two or three wind maxima of 90kts, that would expend a similar amount of energy to, say, one wind maximum of 110kts. Rita clearly was going through some funky ERC-like transition as she came ashore, so this, IMO, likely explains the pressure-windspeed discrepancy.

[amawea]

Excellent analysis vacanechaser!
Amawea

[Derek Ortt]

there also was not a strong ridge of high-pressure nearby to cause the tight pressure gradient that we have seen with storms such as Andrew or even Jeanne

[Aslkahuna]

Other factors as well. There's a degree of subjectivity built into the wind/pressure relationship due to scatter in the data sample used to derive it. Secondly, the pressure gradient will be tighter for a smaller storm than for a larger one all other factors being equal. Finally, the ambient pressure field has something to do with it (which is why WPAC typhoons typically have lower pressures for given intensities most of the year than ATL storms since the ambient pressures are lower). Since one of the factors cited for this (and last year)being an active ATL season is that pressures in the Tropical Atlantic are lower than normal, then we would have lower pressures for given intensities than we would normally have. Bottom line is that wind/pressure relationships are a guide only and good for first guesses but should not be bought into a gospel and the final word. Something to remember the next time JB, who is obviously not a hurricane expert, goes off on a rant about pressures and winds and calls the people at NHC liars because they don't agree with his assessment based solely on pressure.

Steve

[wxmann_91]

Wind is a horrible indicator of a hurricane's strength, as it is dependent on the pressure gradient and the winds aloft mixing down to the surface. Large hurricanes (such as Katrina and Rita) tend to have problems with both, thus they usually have lower windspeeds, and small hurricanes (such as Emily) the opposite. Pressure is best as indicating a hurricane's strength.

[MGC]

Wind is a horrible measure of a hurricanes strength. I'd much rather see pressure used as the measure. I'd rather have gone through Andrew than Katrina. The current system makes these little midget hurricanes like Andrew, Charley and Camille look more dangerous than Katrina, Floyd or even Rita......MGC

[Aslkahuna]

Ah, but it's the winds that do the damage, generate the waves over open water and push the storm surge inland. This is why an understanding of the morphology and dynamics of hurricanes (or typhoons in my case) is necessary to be able to forecast the effects of them at the surface which is related to intensity and size of the windfield. Another thing about pressure reports-the recon reports are often extrapolated from upper air data and errors can result if the wrong assumptions are made about the characteristics of the air column below the level from the extrapolation is made.

Steve

[Tampa Bay Hurricane]

Wow this post has a lot of excellent explanations! Thanks I am learning
a lot!

[no advance]

Thank you everyone.

[gratefulnole]

I thinked I learned alot form this thread.
Is this right:
Pressure is a better indicator of overall strength than windspeed because a large hurricane like Katrina is spreading its energy over a greater area than a smaller hurricane like Andrew that had stronger winds but in a concentrated area.
A large hurricane has trouble maintaining cat 5 winds because it has to pull in the wind from alot further away.
Charlie was relatively compact so it was able to maintain hurricane status even while crossing Florida diagonally.