CAT 5 Hurricane Dean - Archived threads

[Wthrman13]

Its sortof funny that Sanibel is arguing with a pro met. However, I think his main point has some validity and that he's managed to obscure it.

Please correct my statements if they are incorrect, but I think this is the idea:

Wind is caused by pressure gradients.
Lower pressure means there must either be higher winds or a broader (larger) windfield.
Surge and waves are caused by wind blowing on water over time (duration), and space (fetch).

Therefore, disregarding topography of the area (basically, two storms coming into the same area at the same direction), pressure determines wind impact on water (again must either be higher wind or larger area for any given pressure) and therefore indirectly determines surge.

Bingo! That's absolutely correct. It's an indirect effect due to the pressure gradients, which create winds, which create the storm surge. The low pressure itself does have a small effect on raising the water level, but it's swamped by the direct effect of the winds.

My interpretation of Sanibel's argument was that he was implicating pressure as a main direct cause, and was bringing up things such as "low pressure pan dynamics", which as a meteorologist, I have never heard of. Again, I'm not trying to be a nitpicker, just trying to clear the waters a bit.

[Coredesat]

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----------------

Intense Hurricane Dean (04L)
Tropical Cyclone Warning - Atlantic
Forecast #7 - 2100 UTC 20 August 2007

...Dean approaching the Yucatan Peninsula...

Estimated Position: 18.2°N 83.8°W (confidence good)
Est. Maximum Sustained Winds: 130 knots (150 mph)
Est. Minimum Pressure: 925 hPa
Movement: WNW at 17 knots

DISCUSSION

Dean (04L) has changed little in appearance this afternoon despite passing over warm waters with very high heat content. Recon measured flight-level winds that would translate to a 135 kt system, but SFMR obs do not agree, and neither do Dvorak estimates. System no longer elongated but eye has moved a bit toward the northern side of convective mass. Outflow good in all quadrants, outermost rainbands beginning to affect coast of Yucatan Peninsula. Cold convection wrapping around large eye, no sign of concentric eyewalls according to sat imagery or recon. Dvorak T was T6.5/6.5 from SAB and TAFB, CIMSS ADT has 6.6, intensity held at 130 kt.

Track guidance has shifted southward from previous forecast, so the forecast track is also shifted south a bit. Landfall on the Yucatan Peninsula should occur within 12-24 hr, and final landfall in Mexico should occur by 72 hr. Dean may attain Category 5 strength before the first landfall, but it isn't looking likely at this point. Forecast strength after entering Bay of Campeche depends on how well the circulation survives; opting for a lower forecast intensity than the models are indicating.



FORECAST POSITIONS AND MAX WINDS

Init...18.2°N 83.8°W...130 kt
12 hr...18.7°N 86.3°W...135 kt
24 hr...19.3°N 88.9°W...105 kt...inland
36 hr...19.9°N 91.4°W...75 kt...over water
48 hr...20.6°N 93.8°W...85 kt
72 hr...22.0°N 99.0°W...25 kt...inland dissipating

[dtrain44]

NHC Discussion at 5 EDT:

Same line on potential for intensification - but we've been hearing that for the last day.....

Just so that we fair to the NHC, they've been saying that there has been POTENTIAL for Dean to strengthen into a Cat 5 hurricane. If Dean does not do so that doesn't mean that the NHC forecast is a bust. Indeed, they have correctly and prudently noted the POTENTIAL for intensification.

I know you weren't blasting the NHC, but I just wanted to make sure nobody misinterpreted the "potential for intensification to Cat 5" to mean "Dean will intensify to Cat 5".

That's exactly right. I think Dean is a real success story for the NHC, especially when it must have been very tempting to heavily weigh the early GFDL forecasts and bring Dean straight into the Gulf. The earliest NHC forecasts were clearly well north of where Dean eventually ended up, but they did a good job of sticking to the meteorology and avoiding the emotional tendency to bring it to the worst case for the U.S.

The point that I did hope to make (and poorly phrased) is that Dean hasn't done a lot in the way of strengthening in the last 24 hours, despite objective conditions and forecasts to the contrary. The NHC is definitely right on the mark to be cautious and to note that this terrible storm could get worse. There are certainly no good reasons to expect any weakening and all signs point toward strengthening, if anything.....

[WxGuy1]

Its sortof funny that Sanibel is arguing with a pro met. However, I think his main point has some validity and that he's managed to obscure it.

Please correct my statements if they are incorrect, but I think this is the idea:

Wind is caused by pressure gradients.
Lower pressure means there must either be higher winds or a broader (larger) windfield.
Surge and waves are caused by wind blowing on water over time (duration), and space (fetch).

Therefore, disregarding topography of the area (basically, two storms coming into the same area at the same direction), pressure determines wind impact on water (again must either be higher wind or larger area for any given pressure) and therefore indirectly determines surge.

That's a leap that's only sometimes valid. Minimum central pressure CAN be used as a rough proxy for wind speeds sometimes. But, it's important to remember that it's the pressure gradient we'd need to look at. So, as I'm sure you know, a 910mb large storm may have weaker winds than a 925mb smaller storm. In addition, if the data used to support a particular intensity comes from flight-level data, you need to consider the low-level lapse rates and "internal dynamics" that can play a huge role in the efficiency in which surface winds are represented by flight-level winds.

Finally, size and time are important. The first should be fairly obvious (large storms likely have a larger surge over a larger area), but perhaps the 2nd isn't. Take two storms with different "histories" -- a storm that has rapidly intensified to 910mb and has 120kt winds may very well have a lower surge than a mature 910mb storm with the same winds (particularly if that second storm used to be even stronger). In this case, pressure != surge height.

I'm by no means an expert in storm surge, but it seems that surge height is mostly a function of wind speed, storm size, and storm duration (how long the storm has existed w/o much land interaction). Yes, wind speed and size are also functions of minimum central pressure, but linking minimum central pressure by itself to surge height is dangerous.

* Of course, ocean floor and shore geometry play a massive role in determining surge height. But, we've assumed "two storms coming at the same area from the same direction/track".

EDIT: Ooops, didn't see Wthrman13's reply above first... We say the same thing, though.

[Bluefrog]

cat 5 = catastrophic (period)

[MGC]

As I recall, one inch of Hg yeilds about 10 inches of water. So, if a hurricane has a CP of say 2 inches of Hg below the background pressure than the water level would be sucked up into the eye about 20 inches. The remainder of the surge is caused by wind driven water piling up on the shore.....MGC

[RL3AO]

The NHC has nailed this storm. They were a little slow the first 24 hours or so, but after that they were incredible. My hat goes off to an incredible job well done by the NHC on Dean.

[Scorpion]

Dean is looking great right now... I really wish we could get some recon readings

[miamicanes177]

The NHC has nailed this storm. They were a little slow the first 24 hours or so, but after that they were incredible. My hat goes off to an incredible job well done by the NHC on Dean.

You mean the GFS? The GFS had this thing nailed when it was still in Africa!

[Dionne]

cat 5 = catastrophic (period)

Can the mangroves survive a CAT5?

[fasterdisaster]

Dean is looking great right now... I really wish we could get some recon readings

Same here, is the NHC at least getting them?

[RL3AO]

The NHC has nailed this storm. They were a little slow the first 24 hours or so, but after that they were incredible. My hat goes off to an incredible job well done by the NHC on Dean.

You mean the GFS? The GFS had this thing nailed when it was still in Africa!

Yeah. The GFS took some crap, but it gets the last laugh.

[Wthrman13]

Its sortof funny that Sanibel is arguing with a pro met. However, I think his main point has some validity and that he's managed to obscure it.

Please correct my statements if they are incorrect, but I think this is the idea:

Wind is caused by pressure gradients.
Lower pressure means there must either be higher winds or a broader (larger) windfield.
Surge and waves are caused by wind blowing on water over time (duration), and space (fetch).

Therefore, disregarding topography of the area (basically, two storms coming into the same area at the same direction), pressure determines wind impact on water (again must either be higher wind or larger area for any given pressure) and therefore indirectly determines surge.

That's a leap that's only sometimes valid. Minimum central pressure CAN be used as a rough proxy for wind speeds sometimes. But, it's important to remember that it's the pressure gradient we'd need to look at. So, as I'm sure you know, a 910mb large storm may have weaker winds than a 925mb smaller storm. In addition, if the data used to support a particular intensity comes from flight-level data, you need to consider the low-level lapse rates and "internal dynamics" that can play a huge role in the efficiency in which surface winds are represented by flight-level winds.

Finally, size and time are important. The first should be fairly obvious (large storms likely have a larger surge over a larger area), but perhaps the 2nd isn't. Take two storms with different "histories" -- a storm that has rapidly intensified to 910mb and has 120kt winds may very well have a lower surge than a mature 910mb storm with the same winds (particularly if that second storm used to be even stronger). In this case, pressure != surge height.

I'm by no means an expert in storm surge, but it seems that surge height is mostly a function of wind speed, storm size, and storm duration (how long the storm has existed w/o much land interaction). Yes, wind speed and size are also functions of minimum central pressure, but linking minimum central pressure by itself to surge height is dangerous.

* Of course, ocean floor and shore geometry play a massive role in determining surge height. But, we've assumed "two storms coming at the same area from the same direction/track".

EDIT: Ooops, didn't see Wthrman13's reply above first... We say the same thing, though.

Right, I think quandary recognized it's the pressure gradient. What he was saying, basically, was that for a given minimum central pressure, the wind speeds would be determined by the size of the storm (i.e. the slope of the pressure gradient from the center to the environment), which is correct. So, we are all in agreement and saying basically the same thing

[senorpepr]

Dean is looking great right now... I really wish we could get some recon readings

Same here, is the NHC at least getting them?

Judging from this statement, I say no: COMMUNICATIONS PROBLEMS ARE PREVENTING THE RECEIPT OF MOST DATA FROM THE AIR FORCE RECONNAISSANCE AIRCRAFT THIS AFTERNOON...ALTHOUGH A 1930Z VORTEX FIX WAS RECEIVED.

[fasterdisaster]

Dean is looking great right now... I really wish we could get some recon readings

Same here, is the NHC at least getting them?

Judging from this statement, I say no: COMMUNICATIONS PROBLEMS ARE PREVENTING THE RECEIPT OF MOST DATA FROM THE AIR FORCE RECONNAISSANCE AIRCRAFT THIS AFTERNOON...ALTHOUGH A 1930Z VORTEX FIX WAS RECEIVED.

So, in other words, they are only going off satellite?

[RL3AO]

pojo said when they had the same problems last week that if we don't get the data, the NHC isn't getting the data. You would think in 2007 they could email it to them later or something.

[Scorpion]

This is ridiculous.. it can easily be a Cat 5 right now. 151 kts is only 2 kt away from Cat 5. It certainly has strengthened since then.

[fasterdisaster]

This is ridiculous.. it can easily be a Cat 5 right now. 151 kts is only 2 kt away from Cat 5. It certainly has strengthened since then.

Agreed. I'll be mad if they don't at the VERY least up it to 155 at 8 PM.

[Solaris]

Latest recon:

916 mb, 142 kt NW-Quad

[miamicanes177]

000
URNT12 KNHC 202202
VORTEX DATA MESSAGE AL042007
A. 20/21:37:20Z
B. 18 deg 09 min N
084 deg 11 min W
C. NA mb 2371 m
D. 104 kt
E. 087 deg 009 nm
F. 206 deg 126 kt
G. 098 deg 011 nm
H. 916 mb
I. 10 C/ 3046 m
J. 20 C/ 3041 m
K. 13 C/ NA
L. CLOSED WALL
M. CO16-30
N. 12345/7
O. 0.03 / 1 nm
P. AF303 1404A DEAN OB 20
MAX FL WIND 151 KT NE QUAD 19:18:00 Z
MAX FL TEMP 21 C, 47 / 7NM
MAX FL WIND 142 KT NW QUAD 21:08:50 Z