Bonnie weakens to a tropical depression

By: Dr. Jeff Masters , 8:40 PM GMT on July 23, 2010

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Tropical Depression Bonnie is nearing the end of its traverse of South Florida, and passage over land has significantly disrupted the small storm. Satellite images show almost no heavy thunderstorms near Bonnie's center of circulation, and the center is now exposed to view. Radar-estimated rainfall from the Key West radar shows that Bonnie dumped very little rain on South Florida--maximum rainfall amounts from the storm were about four inches over a small region southwest of Miami. Water vapor satellite loops show that Bonnie is embedded in a large area of dry air, thanks to an upper level low to the west over the Gulf of Mexico. This low has brought an increasing amount of wind shear to Bonnie today, and shear has increased from 20 knots this morning to 25 knots this afternoon. Surface observations in South Florida currently don't show any tropical storm force winds. Bonnie's top winds today were at Fowey Rocks, which had sustained winds of 46 mph, gusting to 53 mph, at 10:45 am EDT.


Figure 1. Satellite image of Bonnie from NASA's MODIS instrument, taken at 17:10 UTC July 23, 2010. Image credit: NASA/.

Track Forecast for Bonnie
The latest set of model runs from 8am EDT (12Z) are very similar to the three previous sets of runs, and this degree of consistency gives me confidence that Bonnie will stay within the cone of uncertainty depicted on the track forecast images. The projected track will take Bonnie over the oil spill region, and the storm's strong east to southeasterly winds will begin to affect the oil slick on Saturday morning. Assuming Bonnie doesn't dissipate over the next day, the storm's winds, coupled with a likely storm surge of 2 - 4 feet, will drive oil into a substantial area of the Louisiana marshlands. However, the current NHC forecast has Bonnie making landfall in Louisiana near 9pm CDT Saturday night. According to the latest tide information, this will be near the time of low tide. This will result in much less oil entering the Louisiana marshlands than occurred during Hurricane Alex in June. That storm brought a storm surge of 2 - 4 feet and sustained winds of 20 - 30 mph that lasted for several days, including several high tide cycles. The latest oil trajectory forecasts from NOAA (Figure 2) predicts potential oil impacts along a 150-mile stretch of Louisiana coast on Sunday.


Figure 2. Oil Trajectory forecast for Sunday for the Deepwater Horizon oil spill. Image credit: NOAA.

Intensity Forecast for Bonnie
Bonnie has been disrupted by its passage over land, and it will take at least six hours for the storm to reorganize once it moves over the warm waters of the Gulf of Mexico tonight. This process will be hindered by the large upper-level low to its west. If the low remains in its present location, relative to Bonnie, it will bring high wind shear of about 20 - 30 knots to the storm. This will allow for only slow intensification, or may even destroy Bonnie. Bonnie is unlikely to intensify to more than a 50 mph tropical storm, and I give a 30% chance it will dissipate over the Gulf of Mexico before making landfall. The GFDL model predicts Bonnie could hit the Gulf Coast as a 50 mph tropical storm, but the other major models such as the HWRF, GFS, ECMWF, and NOGAPS show a much weaker storm. I don't give Bonnie any chance of becoming a hurricane. NHC is putting the odds of Bonnie being a hurricane at 2 pm Saturday at 4% (5pm advisory.)

If you are wondering about the specific probabilities of receiving tropical storm force winds at your location, I recommend the wind probability product from NHC. The latest probabilities of various locations getting tropical storm force winds, 39 mph or higher, from the 5pm EDT advisory:

Buras, LA 30%
New Orleans 28%
Mobile, AL 37%
Pensacola, FL 30%

Resources for the BP oil disaster
Map of oil spill location from the NOAA Satellite Services Division
My post, What a hurricane would do the Deepwater Horizon oil spill
My post on the Southwest Florida "Forbidden Zone" where surface oil will rarely go
My post on what oil might do to a hurricane
NOAA's interactive mapping tool to overlay wind and ocean current forecasts, oil locations, etc.
Gulf Oil Blog from the UGA Department of Marine Sciences
Oil Spill Academic Task Force
University of South Florida Ocean Circulation Group oil spill forecasts
ROFFS Deepwater Horizon page
Synthetic Aperture Radar (SAR) imagery from the University of Miami

Next update
I'll have an update Saturday morning.

Jeff Masters

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Quoting aspectre:
TropicalDepression Bonnie was^heading for a Houston,Texas landfall in ~29hours.
(Straightline^projection using its last 2 positions. Take with HUGE grain of salt)
Copy&paste 26.2N81.9W, 26.4N82.5W, 26.4N83.4W-26.7N84.4W, 26.7N84.4W-29.4N94.7W, BIX, 22.3N74.0W, 18.5N87W, TAM, 28.7N88.4W into the GreatCircleMapper
Shortest red line denotes the movement between last two positions. Below the map shows:
TD.Bonnie had a heading of 288.7degrees (3.8degrees west of WestNorthWest) while traveling
a distance of 65miles(~105kilometres) over 3hours at a speed of ~22mph(~35km/h),
and was 721miles(~1,381kilometres) away from the coast in the direction of its heading.

^ Straightline projections are not forecasts of what will happen in the future,
especially not for TropicalCyclones. They aftcast what has already happened.
* DeepwaterHorizon is marked at 28.7N88.4W


While ago it was Pt Arthur at 55hrs. Might actually be right. For the LLC anyway.
Member Since: August 15, 2008 Posts: 10 Comments: 3665
1710. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 174 Comments: 54407
1709. Patrap
ummmmmm,cakeeeeeeeee
Member Since: July 3, 2005 Posts: 426 Comments: 128668
1708. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 128668
1707. xcool
HMM
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With the hot waters of the GOM one should know/learn that anything can happen, happen quickly. We've just seen it happen too often - storms cane intensify quickly. Numerous examples, but one really only has to look briefly backwards to Humberto.

It's the GOM - it's hot, deep fuel, akin to gasoline (right now, really is) that can pop before you realize what's happening!
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nite levi. i'm out too. i've got a strawberry shortcake swimming party to throw tomorrow afternoon.
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Quoting Levi32:


I would agree, though it doesn't seem impossible for the low-level center to sneak under that circular blob. I would be very surprised to see this get back up to a tropical storm, but the slight possibility is there, surprisingly. If it does slip under there and intensify, I will need to learn a lesson....


To me it looks like the LLC has been moving due west on the 26 north line. I know sats can be deceiving so I guess I could be wrong.
Member Since: August 15, 2008 Posts: 10 Comments: 3665
Quoting Levi32:
Alright, time to call it a night so I can survive school tomorrow. I still don't expect any strengthening of Bonnie before 2nd landfall. No matter what happens, it's a rainmaker, nothing more. Only real concern is for the oil spill area with the slick maybe getting pushed towards the coast. Have a good night everyone.


Good night Levi well done again today! Get some good rest!
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TropicalDepression Bonnie was^heading for a Houston,Texas landfall in ~29hours.
(Straightline^projection using its last 2 positions. Take with HUGE grain of salt)
Copy&paste 26.2N81.9W, 26.4N82.5W, 26.4N83.4W-26.7N84.4W, 26.7N84.4W-29.4N94.7W, BIX, 22.3N74.0W, 18.5N87W, TAM, 28.7N88.4W into the GreatCircleMapper
Shortest red line denotes the movement between last two positions. Below the map shows:
TD.Bonnie had a heading of 288.7degrees (3.8degrees west of WestNorthWest) while traveling
a distance of 65miles(~105kilometres) over 3hours at a speed of ~22mph(~35km/h),
and was 721miles(~1,381kilometres) away from the coast in the direction of its heading.

^ Straightline projections are not forecasts of what will happen in the future,
especially not for TropicalCyclones. They aftcast what has already happened.
* DeepwaterHorizon is marked at 28.7N88.4W
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1700. Levi32
Alright, time to call it a night so I can survive school tomorrow. I still don't expect any strengthening of Bonnie before 2nd landfall. No matter what happens, it's a rainmaker, nothing more. Only real concern is for the oil spill area with the slick maybe getting pushed towards the coast. Have a good night everyone.
Member Since: November 24, 2005 Posts: 635 Comments: 26654
the spooky dvorack looks like little red riding hood. LOL! st. simon would "discredit" our seeing a face in the imagery. perfect example of what we were talking about the other night. none of us really think it's a real face - duh
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1697. scott39
Quoting atmosweather:


The low level flow is so strong and in a slightly different direction to the mid/upper flow...so when the system is decoupled it makes it hard for it to ever vertically stack. It's not impossible but I would say it's only got a 10% chance to intensify...too many negative environmental factors besides what I talked about.
What is the NHC saying before landfall. Thanks for the explanations.
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1696. Patrap
Mobile Radar will reach come Dawn to the coc.

Were losing Heights from Tampa and Tally isnt Far nuff to see the CoC.

So Itsa gonna degrade some on radar for a few Hours..Unless it Keeps building that semi CDO..
Member Since: July 3, 2005 Posts: 426 Comments: 128668
1695. Patrap
Tally Long Range Showing some Tstorms developing in Northern Band/Bands

NEXRAD Radar
Tallahassee, Base Reflectivity 0.50 Degree Elevation Range 248 NMI

Member Since: July 3, 2005 Posts: 426 Comments: 128668
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1693. KEEPEROFTHEGATE (Mod)
from nothing it will come
Member Since: July 15, 2006 Posts: 174 Comments: 54407
Quoting KEEPEROFTHEGATE:
03L/TD/B
MARK
27.1N/85.3W


you can see her building up there
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Quoting scott39:
So strong the convection cant pull the LLC towards it?


The low level flow is so strong and in a slightly different direction to the mid/upper flow...so when the system is decoupled it makes it hard for it to ever vertically stack. It's not impossible but I would say it's only got a 10% chance to intensify...too many negative environmental factors besides what I talked about.
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Quoting Patrap:


She's still got the spin
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1689. Levi32
This is likely another 92L moment....it had 2 different MCCs with the low-level center under the edge of the convection which was sustained for over 12 hours at one point, but it never intensified. The MCC eventually died out. This is what I think the case will be here.
Member Since: November 24, 2005 Posts: 635 Comments: 26654
Quoting Patrap:


Ya Noticed that eh..?

LOL

creepy
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1687. Patrap
Member Since: July 3, 2005 Posts: 426 Comments: 128668
1685. Levi32
Quoting atmosweather:
The reason why there is still convection blowing up NW of the LLC is because that's where the mid level energy is getting pulled by the strong upper SE-erly winds. The LLC is traveling WNW or W and will never be able to realign itself under that energy unless it reforms (unlikely due to the strong low level flow) or slows dramatically (again unlikely for the same reasons). She looks better than she is, the 1013 mb min. central pressure is extraordinarily high for a tropical cyclone. She is still weakening and on the edge of degeneration.


I would agree, though it doesn't seem impossible for the low-level center to sneak under that circular blob. I would be very surprised to see this get back up to a tropical storm, but the slight possibility is there, surprisingly. If it does slip under there and intensify, I will need to learn a lesson....
Member Since: November 24, 2005 Posts: 635 Comments: 26654
1684. xcool
IMO TS
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1683. scott39
Quoting atmosweather:
The reason why there is still convection blowing up NW of the LLC is because that's where the mid level energy is getting pulled by the strong upper SE-erly winds. The LLC is traveling WNW or W and will never be able to realign itself under that energy unless it reforms (unlikely due to the strong low level flow) or slows dramatically (again unlikely for the same reasons). She looks better than she is, the 1013 mb min. central pressure is extraordinarily high for a tropical cyclone. She is still weakening and on the edge of degeneration.
So strong the convection cant pull the LLC towards it?
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1682. Patrap
Quoting atmoaggie:
Uh oh, she looks angry...
lol.


Ya Noticed that eh..?

LOL
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1681. KEEPEROFTHEGATE (Mod)
03L/TD/B
MARK
27.1N/85.3W
Member Since: July 15, 2006 Posts: 174 Comments: 54407
1680. xcool
STORMS LOVE GOM .
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Quoting AussieStorm:

omg,,, is that her face???
Uh oh, she looks angry...
lol.
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1677. xcool
drink of HOTHOTwater --SCOTT39-
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1675. Patrap
A few White Pixels Eddy?

www.ghcc.msfc.nasa.gov
Member Since: July 3, 2005 Posts: 426 Comments: 128668
You never know for sure with these small systems, especially in the Gulf where 29-30C water temps are available. Bonnie looks to have found a small window of lower shear and is organizing slightly. A small system can strengthen suddenly and die off just as quickly as it strengthened. The convective blowup needs to persist for any strengthening to occur, which is a tough feat to accomplish with so much shear and dry air around. Nonetheless, Bonnie doesn't want to die.
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The reason why there is still convection blowing up NW of the LLC is because that's where the mid level energy is getting pulled by the strong upper SE-erly winds. The LLC is traveling WNW or W and will never be able to realign itself under that energy unless it reforms (unlikely due to the strong low level flow) or slows dramatically (again unlikely for the same reasons). She looks better than she is, the 1013 mb min. central pressure is extraordinarily high for a tropical cyclone. She is still weakening and on the edge of degeneration.
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1672. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 174 Comments: 54407
1670. xcool
scott39 LOL
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Quoting Patrap:
Its showtime..


Spookie Dvorak of Bonnie





omg,,, is that her face???
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1668. scott39
Quoting Patrap:
Radar tells no lies.
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1667. Patrap
4 Ringed Dvorak is..well,

interesting
Member Since: July 3, 2005 Posts: 426 Comments: 128668
Quoting Patrap:
Its showtime..

We'll see if it sticks or fizzles...

That easterly shear isn't actually all bad for her development, given her speed. The dry layers have been killing the convection.
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1665. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 174 Comments: 54407
1664. scott39
Shes gettin a big drink of water!
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1663. Patrap
Its showtime..


Spookie Dvorak of Bonnie




Member Since: July 3, 2005 Posts: 426 Comments: 128668
Nite all - check back in tomorrow.
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1661. scott39
Quoting Patrap:
La..La..La



Round 9 Ding Ding-- Bonnie tightened her gloves up!
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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