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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pottery
I would have thought the oil would have been to Europe by now
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1060. KYDan
I have a question if you don't mind. I am learning by reading, but have non seen this asked or addressed in any thread here.

What is the difference in a wave and a low pressure system? Is a wave identified as being over water and a lps identified as being over land?

Thanks
Dan
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Quoting MiamiHurricanes09:
Oh, those images update every 6 hours. Sat24 updates every 15 minutes.


ok, cool thanks!
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1058. Levi32
Quoting beell:


I think so. Still a good one to watch the interaction at the very least. Some ULL's are rumored to be a good thing for tropical cyclones. Most of the time-not, imo.

Can anybody name a cane that owed its existence to an ULL?


Yup, Rita 2005 was one of the most beautiful cases of such a setup. Upper-level low backing away over Cuba ventilated the tropical wave to the east and allowed Rita to develop east of the Bahamas.

Upper winds 0z September 18th, 2005, the time of Rita's first advisory as a tropical depression: Notice the upper low to the WSW over Cuba.



24 hours later.....wow just classic: Backing away SW ventilating Rita as upper ridging develops over the cyclone.



Another 24 hours later on the 20th the upper low south of Cuba was beginning to dissipate due to heat being released into the upper atmosphere, and Rita was nearing hurricane status.



Rita was a very classic case of the upper low being very much responsible for her formation, ventilating the tropical wave as it backs away and providing a favorable pocket to develop in. You won't wind a more classic case than this. That's why trough-splits in the tropics are favorable if the position of a disturbance relative them is right. Always watch to the east of upper lows retrograding away westward in the tropics.
Member Since: November 24, 2005 Posts: 635 Comments: 26556
1057. wxhatt
A couple of hurricanes I've been in, when the center passed by I could hear the loud unmistakable drone of the hunters aircraft overhead. Sound ominous, especially when your in the eye...
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Quoting BahaHurican:
I have a feeling they wouldn't be flying this tonight if they didn't have that oil spill in the way tomorrow...

Bingo :) Give the man a cookie :)
Member Since: October 1, 2007 Posts: 81 Comments: 26511
Which reminds me - didn't somebody have a link to the satellite blackout schedule? I was thinking they wouldn't need to do that again this year, but now I'm wondering.
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Yeah, it's reassuring when governing bodies show they give a poop.
Member Since: July 11, 2006 Posts: 14 Comments: 11180
1053. pottery
Quoting Orcasystems:

Especially with the Oil.

Talking about the Oil...
I find it amazing that the oil has remained virtually where it has been all along.
I would have thought that the currents would have long dispersed it further afield. I was not aware that the GOM in that area is so static, for so long.
Strange to me.........
Member Since: October 24, 2005 Posts: 0 Comments: 24060
BOC looks pretty moist.See some kinda of swirl down there.
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I have a feeling they wouldn't be flying this tonight if they didn't have that oil spill in the way tomorrow...
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I agree with Levi as well, im not sure a reconnaissance mission was necessary, with high power NEXRAD systems in range the mission probably wasn't needed.
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Quoting HurricaneGeek:


thanks, but if right now it's 2:30 UTC according to the site, the satellite pic says it was updated at 00:00 UTC...why?

LINK
Oh, those images update every 6 hours. Sat24 updates every 15 minutes.
Member Since: September 2, 2009 Posts: 130 Comments: 21091
SSEC
Things looking better south of Hispaniola.
Member Since: July 11, 2006 Posts: 14 Comments: 11180
Quoting xcool:
HurricaneGeek .30MIN


thanks, but if right now it's 2:30 UTC according to the site, the satellite pic says it was updated at 00:00 UTC...why?

LINK
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Quoting pottery:

Exactly!

Especially with the Oil.
Member Since: October 1, 2007 Posts: 81 Comments: 26511
Quoting Chicklit:
There are probably people still out in the GOM.



Actually I have a few friends working with the oil spill using their boats and today they were told to tie up until notified.
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1043. wxhatt
Quoting MrstormX:


If anything Bonnie should be declassified... lets face it a bare swirl is not a tropical system.


In other words Poof.
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1042. aquak9
Quoting Patrap:
A Big Ol C-5 Galaxy just flew South Over Uptown NOLA..SSe to be exact..Round 2000 ft too.


Awesome.......................right past the 3/4 Moon.


cool...I saw the NOAA plane w/the big plate on top, either this morning or yesterday morning. But those C5's...man I love that deep wind-up rumble.
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1041. pottery
Quoting Orcasystems:


If they had not flown this mission... people (on here also) would be screaming blue murder that they were not flying it.

Exactly!
Member Since: October 24, 2005 Posts: 0 Comments: 24060
Quoting HurricaneGeek:
Hi, can anybody tell me how often these satellite pictures update?

LINK

thanks
Updates are every 30 minutes (e.g XX:15 and XX:45).
Member Since: September 2, 2009 Posts: 130 Comments: 21091
Quoting MrstormX:


If anything Bonnie should be declassified... lets face it a bare swirl is not a tropical system.
The swirl isn't even closed. I agree, declassification at 11 P.M EDT looks very likely.
Member Since: September 2, 2009 Posts: 130 Comments: 21091
Quoting ShenValleyFlyFish:

D@mn. If I could afford one I'm not sure I would own one. They ain't safe. First Tiger Woods, now this . . . Might be time for a recall.


This is one of the funnier things I've heard in a long time :)
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1037. xcool
HurricaneGeek .30MIN
Member Since: September 26, 2009 Posts: 2 Comments: 15625
There are probably people still out in the GOM.
Member Since: July 11, 2006 Posts: 14 Comments: 11180
Hi, can anybody tell me how often these satellite pictures update?

LINK

thanks
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Quoting Chicklit:
Maybe somebody picked up the phone and told them to check it out anyway.


If they had not flown this mission... people (on here also) would be screaming blue murder that they were not flying it.
Member Since: October 1, 2007 Posts: 81 Comments: 26511
Quoting MiamiHurricanes09:
The upper level environment is unfavorable and dry air is is causing thunderstorm activity to collapse. I'll be surprised to see Bonnie regain tropical storm status.


If anything Bonnie should be declassified... lets face it a bare swirl is not a tropical system.
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Quoting Orcasystems:


and... its going west


My interpretation is they are having a hard time finding a good fix.......

seems unlikely a west motion........
Member Since: September 8, 2005 Posts: 0 Comments: 3805
1031. wxhatt
Quoting Chicklit:
It did just spin up a blob. And it is headed toward the biggest ecological disaster in American history...


Wow, Could a Cat5 atomize the oil and disperse it into the atmosphere?
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1030. Patrap
A Big Ol C-5 Galaxy just flew South Over Uptown NOLA..SSe to be exact..Round 2000 ft too.


Awesome.......................right past the 3/4 Moon.
Member Since: July 3, 2005 Posts: 421 Comments: 127633
Quoting MrstormX:


And it was an escalade! I hope they have insurance.

D@mn. If I could afford one I'm not sure I would own one. They ain't safe. First Tiger Woods, now this . . . Might be time for a recall.
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1028. Patrap
Quoting wxhatt:


Would a slow in foward speed allow it to decouple from the ULL?


Doubt it..as itsa Big Ol' ULL still with lotsa Symmetry.






Member Since: July 3, 2005 Posts: 421 Comments: 127633
Quoting nolacane2009:


Well with a possible slowing of forward speed, a possible more movement to the west. Is it a possible senerio for a Cat 1 or is the dry air still not going to allow that? I am no weatherman so I am dumb when it comes to this. I just love to learn from yall's post.
The upper level environment is unfavorable and dry air is is causing thunderstorm activity to collapse. I'll be surprised to see Bonnie regain tropical storm status.
Member Since: September 2, 2009 Posts: 130 Comments: 21091
1026. Levi32
Quoting beell:


Given their responsibility, mission, and xBonnie's proximity to land and the DWH mess-I think they are doing the right thing.


NHC shouldn't really have bothered sending them into something that is obviously weakening towards declassification....I mean if it starts to show signs of reorganization before hitting Louisiana sure send the plane...but why now? We've had radar and can see the demise of the system before our very eyes. We don't even need the plane. Not a big deal but I just think they are wasting their time. This isn't a threat and they all know that.
Member Since: November 24, 2005 Posts: 635 Comments: 26556
Quoting Chicklit:
It did just spin up a blob. And it is headed toward the biggest ecological disaster in American history...


Small addition to your excellent statement.

"headed toward the biggest "MAN MADE" ecological disaster"
Member Since: October 1, 2007 Posts: 81 Comments: 26511
Maybe somebody picked up the phone and told them to check it out anyway.
Member Since: July 11, 2006 Posts: 14 Comments: 11180
Bonnie is all but convection-less save a few scuddy thunderstorms... so im not 100% sure where these winds are coming from.
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Quoting MiamiHurricanes09:
With everything presented to me Bonnie is a TS. HOWEVER there is no proof suggesting that there actually is a closed surface circulation, don't know why the vortex message was released.


Well with a possible slowing of forward speed, a possible more movement to the west. Is it a possible senerio for a Cat 1 or is the dry air still not going to allow that? I am no weatherman so I am dumb when it comes to this. I just love to learn from yall's post.
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Quoting Chicklit:
It did just spin up a blob. And it is headed toward the biggest ecological disaster in American history...
Atmo has been posting the soundings showing how dry the environment Bonnie is in. None of that convection will last, especially since it has no outflow.
Member Since: September 2, 2009 Posts: 130 Comments: 21091
1020. beell
Quoting Levi32:


They're dang bored....they shouldn't even be out there wasting money on a system that is no longer a tropical cyclone.


Given their responsibility, mission, and xBonnie's proximity to land and the DWH mess-I think they are doing the right thing.
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It did just spin up a blob. And it is headed toward the biggest ecological disaster in American history...
Member Since: July 11, 2006 Posts: 14 Comments: 11180
1018. Patrap
Watch the winds lay down on the Profile as she moves away from the Radar site,

NEXRAD Radar
Tampa, Velocity Azimuth Display Wind Profile Range 124 NMI

Member Since: July 3, 2005 Posts: 421 Comments: 127633
1017. wxhatt
Quoting Patrap:
Itsa slowed in forward speed too.


Would a slow in foward speed allow it to decouple from the ULL?
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1016. Halyn
Quoting atmoaggie:
3 speed column shift...of course.
Starter under the accelerator ?? Or .. better yet .. a button on the dash ??
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Quoting Orcasystems:


I am giving them the benefit of the doubt, that they know what they are doing.
We'll find out within the hour.
Member Since: September 2, 2009 Posts: 130 Comments: 21091
Quoting MiamiHurricanes09:
With everything presented to me Bonnie is a TS. HOWEVER there is no proof suggesting that there actually is a closed surface circulation, don't know why the vortex message was released.


I am giving them the benefit of the doubt, that they know what they are doing.
Member Since: October 1, 2007 Posts: 81 Comments: 26511
Quoting nolacane2009:


So what does all that mean?
With everything presented to me Bonnie is a TS. HOWEVER there is no proof suggesting that there actually is a closed surface circulation, don't know why the vortex message was released.
Member Since: September 2, 2009 Posts: 130 Comments: 21091
did somebody wake up Stormw?
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Quoting Patrap:
Itsa slowed in forward speed too.


and... its going west
Member Since: October 1, 2007 Posts: 81 Comments: 26511

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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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