07/05/05 -- TALK: Hurricane Dennis

[George Tincher]

This has the potential to be a MAJOR disaster for central and southern Indiana.

According to the Hydrometeorological Prediction Center:

Oh. Crap.

The only saving grace may be that the state has been relatively dry since the widespread 1-3 inch rains of the remnants of Arlene back on 11-13 June. However, 17 inches falling on the confluence of the White and Wabash Basins would be absolutely unbelievable.

It's going to be a busy couple of weeks for us at IND.

Wow, if that pans out, I think the restrictions on water usage will be lifted soon over WRN and NWRN KY. Looks like I am in the 4-5" rainfall zone here.

Geez, that's a lot of rainfall potential. Seems it's either feast or famine doesn't it?

-George

 

[Owen Shieh]

Some preliminary wind data from NWS Mobile. NHC maximum sustained winds are usually around where the highest gust was observed.



HIGHEST WINDS...
NAVARRE...PEAK GUST FROM NE AT 121 MPH AT 219 PM CDT
NEAR PENSACOLA AIRPORT...GUST FROM NW AT 96 MPH AT 247 PM CDT
MARY ESTHER...PEAK GUST 102 MPH
SEMMES...PEAK GUST FROM N AT 44 MPH AT 400 PM CDT
LILLIAN ALABAMA...PEAK GUST 44 MPH AT 129 PM CDT
VALPARAISO (VPS)...PEAK GUST 83 MPH AT 240 PM CDT
PENSACOLA NAS (NPA)...PEAK GUST 58 MPH AT 254 PM CDT
NICEVILLE FLORIDA...PEAK GUST 70 MPH
PENSACOLA (WEAR)...PEAK GUST FROM W AT 53 MPH
MOBILE (USS ALABAMA)...PEAK GUST FROM N AT 77 MPH AT 248 PM CDT
LOXLEY ALABAMA...PEAK GUST FROM N AT 50 MPH AT 245 PM CDT
CRESTVIEW (CEW)...PEAK GUST FROM SE AT 58 MPH AT 320 PM CDT
MOBILE (MOB)...PEAK GUST FROM N AT 48 MPH AT 100 PM CDT
MOBILE BROOKLEY (BFM)...PEAK GUST FROM N AT 44 MPH AT 1158 AM CDT
CRESTVIEW (CEW)...PEAK GUST FROM SE AT 58 MPH AT 320 PM CDT
PENSACOLA (PNS)...PEAK GUST FROM W AT 93 MPH

 

[Karen Politte]

It appears Dennis was a weakening Cat 3 or even possibly a Cat 2 at landfall.

Does anybody know if there were ever any curfews in effect in any areas for this storm? Did any chasers encounter difficulties with things like this?

KR

 

[Brandon Clement]

There was a curfew for every county from Destin, Fl to Moss Point, Ms. They did not enforce it in most areas. During the worse parts of the storm the law enforcement, emergency management, etc. were all hunkered down and not out to enforce it. Just after the worse of the storm it was a completely different story. There were road blocks going to the islands immediatly and from what I was told they were enforcing the curfew and mandatory evacuations very strictly. In the Pensacola area they were not enforcing it at all, but after most hurricanes if you are in the area and not helping you are in the way. The media was pretty bad and swarmed the areas that had damage and since there were not many areas with significant structure damage it became a log jam around the areas that had been damaged and it was hampering the process of the people trying to clean up. In my experiece it is not the police you have to worry about but the home owners who don't appreciate it at all. In New Orleans during Isadore the street flooding was pretty bad in certain places and if you drive your car through the street it causes a wake adding to the damage in people's houses. There were quite a few reports of people shooting at vehicles driving through the flooded waters when they had no business being in the area. If an area really gets hit hard such as Ivan, Charlie and Andrew the police set up road blocks and will arrest you just for trying to get in unless you live there, part of the press or part of the clean up effort. Gulf Shores actually gives you a hurricane re-entry sticker. They set up a road block and if you don't have the sticker your out of luck.

 

[Brandon Clement]

Dennis was Very odd since the most damage and highest gusts have been reported on the north and NW side of the eye. I was a few miles to the NE of the airport and I know we got some gusts over 100. The damage was very minimal by the airport but the farther west you got the debris got worse. You could certainly see evidence of the winds streaking. IMO it was a weak cat 2 when it made landfall and that was for a very small area (couple mile radius). I don't understand how it was not stronger with the pressure readings and the satellite images. I do understand how it it weakened so quickly. The shallow cool water of the gulf coast that had experienced some upwelling just 5 days earlier, dry air getting wrapped in the center and an eye-wall replacement cycle all at the same time beat Dennis up very badly.

 

[Owen Shieh]

Yeah, if you remember, the strongest convective band was to the NW of the eye the entire time throughout landfall. It just pushed into Alabama after crossing the panhandle. And if I weren't given pressure data, I would say this was a strong cat. 1 or weak cat. 2 at landfall. Notice NHC did not indicate the pressure at landfall like they usually do about 1-2 hours afterwards? Well, I think they are reassessing to see if the obs were accurate.....but then it is rare for pressure data to be wrong. I guess we'll have to see what NHC does after July in their monthly summary.

 

[Brett Adair]

Well...I watched Dennis all the way in and this was definately no Ivan. Portions of Alabama did recieve more damage than the coastline did though. There were a couple of factors that might have screwed the situation up.

1. Dennis was just in a reorganization period prior to landfall. An intense convective blowup occured early in the morning in the SE Quad and this caused somewhat of a more northward turn, thus saving the Alabama coastline from a direct hit.

2. A cool water eddy was located just off shore as mid level dry air advection was taking place from the WRN GoM during the morning hours as well. This allowed for rather rapid weakening just offshore and tore alot of the NE Quad apart as the dry air cycled in.

There were not too many in the way of tornadoes given the hurricane never took the "hook" that they normally do around here. During previous landfalling hurricanes we have usually seen a quick NNE or NE right after landfall and this would maximize shear parameters in the NE Quad. Limited instability was also an inhibitor.

I have noted the trends of hurricanes weakening prior to landfall in Gulf tropical cyclones. The cool water eddys along the coastline of the panhandle has saved them many times. They can thank god for that.

 

[Jim Tang]

Max Mayfield at the NHC says Dennis will probably be retired, no surprise in that, but according to many news sources, damage estimates are $1-5 billion! There is a real problem with too many homes along the coastline. This was no wimpy storm, but its windfield was small, and STILL, it is a billion dollar disaster. :shock:

However, IMO Dennis was a Cat 3 at landfall. Here's why:

AF300 1704A DENNIS OB 20
MAX FL WIND 117 KT E QUAD 19:28:00 Z

From the Hurricane Hunter aircraft as Dennis was making landfall. 117 kt flight level wind means 120 mph sfc wind.

 

[Jeff Snyder]

From the Hurricane Hunter aircraft as Dennis was making landfall. 117 kt flight level wind means 120 mph sfc wind.

IF the reduction formula you used is correct... In my experience, weakening storms don't tend to work with the 90% reduction formula... I'd like to see more of the verification of this 90% reduction idea, particularly with weakening storms. The lack of convection in the southeastern 1/2 of the storm probably resulted in less wind momentum transfer, which is probably why we've heard reports of stronger winds in the western 1/2 of the storm versus the eastern half (as someone above noted).

Regardless, if you're talking about verification of surface winds, then I think they should only really use actual surface wind observations... Even the use of the reduction formula isn't really verification, since it's still based on the premise that 90% reduction is correct -- again, not verification.

 

[Scott Olson]

Based on what I've seen I would rate Dennis a strong category 2 with a ridiclously small area of max winds. Looking at wind speed plot data of Dennis & Ivan the two couldn't be more different. Dennis probably did have 90kt sustained winds but they were confined to a very small area. While hurricane force winds were really only limited to a small area to the NE-ENE of the eyewall. From 1630-1930 Dennis fell 5mb per hour to reach 946mb. I haven't seen any obs of sustained winds to support CAT 2-3 but since it was 946mb at landfall and had such a small swath of 80-90kt winds I'd be willing to give this the benefit of the doubt and say it was a CAT 2.

Between 0430z and 0730z (landfall) Ivan fell 2.3mb/hr. The radius on 90-80kt winds on Ivan must be 5-6 times larger than Dennis.

 

[Scott Olson]

From the Hurricane Hunter aircraft as Dennis was making landfall. 117 kt flight level wind means 120 mph sfc wind.

IF the reduction formula you used is correct... In my experience, weakening storms don't tend to work with the 90% reduction formula... I'd like to see more of the verification of this 90% reduction idea, particularly with weakening storms. The lack of convection in the southeastern 1/2 of the storm probably resulted in less wind momentum transfer, which is probably why we've heard reports of stronger winds in the western 1/2 of the storm versus the eastern half (as someone above noted).

Regardless, if you're talking about verification of surface winds, then I think they should only really use actual surface wind observations... Even the use of the reduction formula isn't really verification, since it's still based on the premise that 90% reduction is correct -- again, not verification.

While I believe this storm had some 90-100kt sustained winds just prior to landfall I think Jeff is absolutely right in the way winds are measured. Essentially all we have to count on prior to landfall is estimated winds. Measuring the winds via how the ocean looks (white caps, foam,etc.) while refined is probably not terribly accurate. The prefered method of having a 10% reduction of flight level winds to come up with surface winds is probably fairly accurate when a Hurricane is not quickly strengthning or quickly weakening (like during landfall). Maybe they could come up with a forumla that would take into account weakening and get more accurate surface estimation.

 

[MClarkson]

what the NHC says are 'max sustained winds" are usually closer to 'max gusts', in my opinion, at least for landfalls in the past few years.

 

[Justin Turcotte]

what the NHC says are 'max sustained winds" are usually closer to 'max gusts', in my opinion, at least for landfalls in the past few years.

I've noticed this as well... but keep in mind that the surface obs are far and few between and the peak wind could be at one point within a large area. We know the sensor placed at Navarre Beach recorded a 120+ mph gust. Who is to say that a mile down the road that there wasn't a 130, 135, etc? Dennis' eye was a tiny critter where there would have been a wide variation in the winds over very short distances. This is likely one reason this was no Ivan. Too small and too fast.

 

[Owen Shieh]

I would assume that in order for the reduction principle to even work, a prerequisite must be symmetry. Recon wind obs for a non-symmetrical storm could be very misleading. In the case of T.S. Bret for example, the recon flew into a mesoscale thunderstorm and found a wind gust to tropical storm force. Reduced to the surface, this was 40 mph. But if you fly a plane into ANY thunderstorm, I'm sure you'll find a gust that high at least! So IMO, surface wind speed estimations should only be used on symmetric storms and, as everyone has pointed out above, storms that are neither rapidly strengthening or weakening.

 

[Jim Tang]

I would assume that in order for the reduction principle to even work, a prerequisite must be symmetry. Recon wind obs for a non-symmetrical storm could be very misleading. In the case of T.S. Bret for example, the recon flew into a mesoscale thunderstorm and found a wind gust to tropical storm force. Reduced to the surface, this was 40 mph. But if you fly a plane into ANY thunderstorm, I'm sure you'll find a gust that high at least! So IMO, surface wind speed estimations should only be used on symmetric storms and, as everyone has pointed out above, storms that are neither rapidly strengthening or weakening.

Yes, you do have a good point there, but if one reviews radar imagery from landfall, one would see that most of the convection was on the west side. I remember from one of TWC's (sorry :lol graphics of lightning strikes in Dennis during the storm that there were tons of CG's inside Dennis's eye wall - on the northern and western side.

Note that the wind speeds on the vortex data message were recorded on the eastern quadrant, and at that time, dry air was infiltrating the eastern and southern quadrants and, as a result, those portions of Dennis were relatively devoid of convection. And I remember reading a post in one of the zillions of pages in this topic that that person thought the strongest winds were in the northern and western portions of the eyewall.

In addition, observations rarely survive hurricane winds and/or record accurate measurements during such conditions. Other times, no obs are available where the hurricane makes landfall, especially in this case, where the strongest winds are confined to a very small point.