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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Thanks KEEPER, That is a nice looking Sat Pic, I tryed to go to it myself but you have to pay for it, I already have a account with Accuweather, So I will have to pass on the subscription for now..
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1810. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 165 Comments: 52263
Quoting atmosweather:


And because of the lack of outflow and low level organization...you can see the convection starting to wane as it cannot sustain it's energy and draw up any more heat.

did you zoom in on the nasa site? on the SE side of the blob you can see the last gasp of the updraft ... it's no longer connected to the coc and is dying PDQ ... interesting that the area providing the updraft for the whole complex was so small.
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Quoting SouthALWX:
looks like it was outflow as the thunderstorms are now collapsing. Amazing how fast it happened after that V notch disconnected. wish it wouldhave been in the day time with visible so I could see it better.

Looks like a perfect halo on RGB.
Member Since: August 15, 2008 Posts: 10 Comments: 3665
1807. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 165 Comments: 52263
Quoting xcool:


And because of the lack of outflow and low level organization...you can see the convection starting to wane as it cannot sustain it's energy and draw up any more heat.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
looks like it was outflow as the thunderstorms are now collapsing. Amazing how fast it happened after that V notch disconnected. wish it wouldhave been in the day time with visible so I could see it better.
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Quoting SouthALWX:
for those good with satellite ... looking at latest IR imagery, neat little event. One of two things: a dry layer has induced an annular outflow, or subsidence around the convective blowup is occurring in a distinct ring ... either way it is pleasing to the eyes... if you cant see it zoom in on the NASA site at 100% quality ...


Oh wow. Neat. I'm glad I got to see it. Thanks for pointing it out. :)
Member Since: August 15, 2008 Posts: 10 Comments: 3665
1802. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 165 Comments: 52263
1801. xcool
Member Since: September 26, 2009 Posts: 2 Comments: 15603
Quoting SouthALWX:
for those good with satellite ... looking at latest IR imagery, neat little event. One of two things: a dry layer has induced an annular outflow, or subsidence around the convective blowup is occurring in a distinct ring ... either way it is pleasing to the eyes... if you cant see it zoom in on the NASA site at 100% quality ...


Yep it always looks pretty when you get one of these MCC's situated between a dry atmosphere on both sides and no outflow to work with. Gives the appearance of a strengthening storm but it is far from the case.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
1799. KEEPEROFTHEGATE (Mod)
its waning now short lived be gone again so may have been last gasp
Member Since: July 15, 2006 Posts: 165 Comments: 52263
Quoting NOLA2005:


Shhhhhh! Don't invoke it!


Oh my! You're right. Yikes! I 'm usually more careful than that. Dang IT! ;)
Member Since: August 15, 2008 Posts: 10 Comments: 3665
for those good with satellite ... looking at latest IR imagery, neat little event. One of two things: a dry layer has induced an annular outflow, or subsidence around the convective blowup is occurring in a distinct ring ... either way it is pleasing to the eyes... if you cant see it zoom in on the NASA site at 100% quality ...
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KEEPER, Do you mind posting a updated sat map like you posted on 1744..Thanks
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Quoting homelesswanderer:


Ok my yearly ant post. When we were walking out the door to evac from Gus we had a trail of ants we had to kill first. Gus missed us. But Ike didn't ten days later. Well we've had another ant problem today. And if Bonnie goes to our east um that would be a little creepy if we get one right after. Lol.


Shhhhhh! Don't invoke it!
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1794. xcool
yayy
Member Since: September 26, 2009 Posts: 2 Comments: 15603
Quoting homelesswanderer:


Oh. Well still think you're more right. :)


LOL thanks :) RECON is en route so we will find out in about an hour.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
Quoting SouthALWX:

yeah but thats not all .. Globals screamed shear and then the GFDL and HWRF would place this nice little upper high just big enough to accommodate a storm right where it would need to be... of course that never happened and we had screaming westerlies instead ... I dont know that the hurricane models could grasp the breadth of the train of westerlies resulting from EPAC outflow. I really think it was the El Nino driving stronger westerlies than the models predicted.


It's very true that the models (particularly the hurricane-specific models like the GFDL and HRWF) struggle more during moderate to strong El Nino/La Nina seasons.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
1791. xcool
she far way from rader .//
Member Since: September 26, 2009 Posts: 2 Comments: 15603
Quoting SouthALWX:
I wouldnt say any model has done well with Bonnie ... I think it's due to the size of the TC. Globals are having a hard time seeing it as a TC. The ECMWF has done the best I think, but Im not sure that says much. At the same time Id like to know where these conservative hurricane models were last year when they were screaming for majors when their own source globals called for 30 kt+ shear .... Im curious as to why the hurricane models behaved differently ... I assume it was an El Nino influence but thats research for another day


I agree. With Bonnie it was the globals vs the hurrcane models for a while. And GFS completely missed Alex til it was already here.
Member Since: August 15, 2008 Posts: 10 Comments: 3665
The last blowup lasted four hours and this has the benefit of being at night so who knows.. may last a long time but doesnt help Bonnie do much regardless other than survive.
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Quoting atmosweather:


Most of the models last year had problems with detecting dry air in the atmosphere inhibiting development. This year a few of the global models have done well in that department.

yeah but thats not all .. Globals screamed shear and then the GFDL and HWRF would place this nice little upper high just big enough to accommodate a storm right where it would need to be... of course that never happened and we had screaming westerlies instead ... I dont know that the hurricane models could grasp the breadth of the train of westerlies resulting from EPAC outflow. I really think it was the El Nino driving stronger westerlies than the models predicted.
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1786. scott39
If this is just a blow up and soon to fade, when should we start seein it fade out, because its been going steady for about 2 hours now.
Member Since: June 13, 2009 Posts: 0 Comments: 6706
Indications of convection about to develop on the east side.
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Quoting atmosweather:


Actually that estimate was in degrees/minutes/seconds...26 degrees 42 minutes is 26.7N...ADT puts the center a little N of where I think it is and where RECON has been finding it. But ADT is very unreliable in terms of precise estimates especially with storms as unorganized as this.


Oh. Well still think you're more right. :)
Member Since: August 15, 2008 Posts: 10 Comments: 3665
1783. xcool
btwntx08 .1770
Member Since: September 26, 2009 Posts: 2 Comments: 15603
It is well past time to be looking at models right now for Bonnie now that she is in radar view.
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Quoting NOLA2005:
Everyone the other night was talking about crazy ant behavior, but my cat has suddenly been driving me nuts since about 2:30 or 3:00 AM this morning and is continuing to do so. Maybe a hurricatscan? ;-)


Ok my yearly ant post. When we were walking out the door to evac from Gus we had a trail of ants we had to kill first. Gus missed us. But Ike didn't ten days later. Well we've had another ant problem today. And if Bonnie goes to our east um that would be a little creepy if we get one right after. Lol.
Member Since: August 15, 2008 Posts: 10 Comments: 3665
that little classic V shape is really showing up now .. fun to watch.
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Quoting homelesswanderer:
Wow! Atmos you nailed her latitude 26.4. Good call. Good eye too. :)


Actually that estimate was in degrees/minutes/seconds...26 degrees 42 minutes is 26.7N...ADT puts the center a little N of where I think it is and where RECON has been finding it. But ADT is very unreliable in terms of precise estimates especially with storms as unorganized as this.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
1778. hcubed
Quoting Levi32:


Well I've never seen a C-130 in Alaska lol.


Was stationed at Galena.

All our supplies were delivered by C130s.
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1777. KEEPEROFTHEGATE (Mod)
Quoting scott39:
Keeper-looks like Neutral for intensification and the pressure has dropped to 1006.6, if im reading this right?
yes your right recon is on the way lets se
Member Since: July 15, 2006 Posts: 165 Comments: 52263
Quoting SouthALWX:
I wouldnt say any model has done well with Bonnie ... I think it's due to the size of the TC. Globals are having a hard time seeing it as a TC. The ECMWF has done the best I think, but Im not sure that says much. At the same time Id like to know where these conservative hurricane models were last year when they were screaming for majors when their own source globals called for 30 kt+ shear .... Im curious as to why the hurricane models behaved differently ... I assume it was an El Nino influence but thats research for another day


Most of the models last year had problems with detecting dry air in the atmosphere inhibiting development. This year a few of the global models have done well in that department.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
Goodnight y'all. Anxious to see what Bonnie becomes tomorrow.

Latest GHCC imagery animation:

Link
(updates every 10 minutes)
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1773. scott39
Keeper-looks like Neutral for intensification and the pressure has dropped to 1006.6, if im reading this right?
Member Since: June 13, 2009 Posts: 0 Comments: 6706
ADT is especially bad for weak storms.
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1771. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 165 Comments: 52263
1770. xcool
Member Since: September 26, 2009 Posts: 2 Comments: 15603
Quoting scott39:
The pressure has dropped!


ADT is an extremely unreliable tool for central pressure estimates. RECON is flying out from the Gulf Coast as we speak so we will soon find out.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
Wow! Atmos you nailed her latitude 26.4. Good call. Good eye too. :)
Member Since: August 15, 2008 Posts: 10 Comments: 3665
1767. xcool
models go west
Member Since: September 26, 2009 Posts: 2 Comments: 15603
I wouldnt say any model has done well with Bonnie ... I think it's due to the size of the TC. Globals are having a hard time seeing it as a TC. The ECMWF has done the best I think, but Im not sure that says much. At the same time Id like to know where these conservative hurricane models were last year when they were screaming for majors when their own source globals called for 30 kt+ shear .... Im curious as to why the hurricane models behaved differently ... I assume it was an El Nino influence but thats research for another day
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Quoting scott39:
Yea, I like TC Terminator. It saved our butts---this time!


Yep. We should take her out to dinner or something as thanks. Or maybe I'm hungry? Lol
Member Since: August 15, 2008 Posts: 10 Comments: 3665
Everyone the other night was talking about crazy ant behavior, but my cat has suddenly been driving me nuts since about 2:30 or 3:00 AM yesterday morning and is continuing to do so. Maybe a hurricatscan? ;-)
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I think the ULL wishes it were a hurricane.
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1762. KEEPEROFTHEGATE (Mod)
03L
TROPICAL DEPRESSION BONNIE 0:00UTC 24July2010
UW-CIMSS Experimental Vertical Shear and TC Intensity Trend Estimates

Current Conditions (from TPC) :
Latitude : 26:42:38 N
Longitude : 82:55:13 W
Intensity (MSLP) : 1009.0 hPa

Max Pot Int (MPI,from Emanuel) : 928.9 hPa
MPI differential (MSLP-MPI) : 80.1 hPa

CIMSS Vertical Shear Magnitude : 13.0 m/s
Direction : 127.4 deg

Outlook for TC Intensification Based on Current
Env. Shear Values and MPI Differential
Forecast Interval : 6hr 12hr 18hr 24hr
N N N N

Legend : VF-Very Favorable F-Favorable N-Neutral
U-Unfavorable VU-Very Unfavorable

-- Mean Intensity Trend (negative indicates TC deepening) --
6hr 12hr 18hr 24hr
VF <-3.0mb/ 6hr <-6.0mb/12hr <-9.0mb/18hr <-12.0mb/24hr
F -3.0 - -1.0 -6.0 - -2.0 -9.0 - -3.0 -12.0 - -4.0
N -1.0 - +1.0 -2.0 - +2.0 -3.0 - +3.0 -4.0 - +4.0
U +1.0 - +3.0 +2.0 - +6.0 +3.0 - +9.0 +4.0 -+12.0
VU >+3.0 >+6.0 >+9.0 >+12.0

Member Since: July 15, 2006 Posts: 165 Comments: 52263
1761. scott39
Quoting KEEPEROFTHEGATE:
UW - CIMSS
ADVANCED DVORAK TECHNIQUE
ADT-Version 8.1.1
Tropical Cyclone Intensity Algorithm

----- Current Analysis -----
Date : 24 JUL 2010 Time : 061500 UTC
Lat : 26:46:14 N Lon : 84:14:44 W


CI# /Pressure/ Vmax
2.3 /1006.6mb/ 33.0kt


Final T# Adj T# Raw T#
1.6 2.0 3.1

Latitude bias adjustment to MSLP : +0.0mb

Center Temp : +15.5C Cloud Region Temp : -12.1C

Scene Type : SHEAR (0.20^ TO DG)

Positioning Method : FORECAST INTERPOLATION

Ocean Basin : ATLANTIC
Dvorak CI > MSLP Conversion Used : ATLANTIC

Tno/CI Rules : Constraint Limits : 0.5T/hour
Weakening Flag : ON
Rapid Dissipation Flag : OFF

****************************************************
The pressure has dropped!
Member Since: June 13, 2009 Posts: 0 Comments: 6706

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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.