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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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!
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1760. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 174 Comments: 54434
Quoting atmosweather:


The ECMWF has performed very well with Bonnie...although it never forecasted TC development in the first place. But the tracks and intensity forecast has been remarkably good so far.


That's what I was thinking. :)
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1758. KEEPEROFTHEGATE (Mod)
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

****************************************************
Member Since: July 15, 2006 Posts: 174 Comments: 54434
1757. scott39
Quoting homelesswanderer:


Lol. She does feel like family by now doesn't she. :)
Yea, I like TC Terminator. It saved our butts---this time!
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Quoting homelesswanderer:


I know. Just wondering how close the EURO had it nailed with an open wave? I call a blob headed west. If it's all gone all the better. :)


The ECMWF has performed very well with Bonnie...although it never forecasted TC development in the first place. But the tracks and intensity forecast has been remarkably good so far.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
Call it "mom" ... it's keeping the waves from causing too much trouble.
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Quoting scott39:
We should have named the ULL too! LOL


Lol. She does feel like family by now doesn't she. :)
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Heavy rainfall could be possible due to ULL interaction providing alot of divergence aloft should bonnie's remnants be in the correct region in relation. Of course this is coupled with SE shear and is not conducive to development. It can however cause nice complexes like the one we are seeing now to form and if you happen to be under it then yes heavy rain could be present. Soundings indicate dry air aloft as well so you have to take the satellite presentation with a grain of salt, however. Rainfall rates may not be as impressive as normally experiences in thunderstorm complexes with that sort of height to them.
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Quoting SouthALWX:
I see that homeless, but an LLC wouldnt even survive as a naked swirl under that ULL. it would die in its entirety.


I know. Just wondering how close the EURO had it nailed with an open wave? I call a blob headed west. If it's all gone all the better. :)
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1750. KEEPEROFTHEGATE (Mod)
Quoting SouthALWX:
my mistake hurricat ... I thought that big swirl in the western gomex was the ULL http://www.goes.noaa.gov/HURRLOOPS/huwvloop.html

I suppose I'm wrong. Ill turn off the lights and turn on the generator now, so I wont have to be caught off guard when Bonnie tries to turn them off for me HA! lol j/k.

What data are you using?
the flush model data
Member Since: July 15, 2006 Posts: 174 Comments: 54434
Quoting SouthALWX:
wrong hurricat .... the shear is quite high and the ULL is still in the gomex. the convective blowup appears assoociated with increased upper divergence associated with that portion of the ULL, as evidenced by the SE to NW streaming clouds and the " V " like presentation, the tip of the V being the connection to the updraft region of the complex. Once the shear breaks that connection, the V will flatten and the high clouds will exit stage right. Nothing has changed and as of now Bonnie is doing nothing but surviving, which given the circumstances is impressive in and of itself.


Exactly, there is divergent flow to the NW of the system as the upper winds on the NE side of the ULL interact with the upper level flow from the ridge to the N. All the convection will be tugged NW-ward by the upper flow...and you can also see the dry air on both sides of the elongated circulation begin to squeeze the system even further.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
1748. scott39
We should have named the ULL too! LOL
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my mistake hurricat ... I thought that big swirl in the western gomex was the ULL http://www.goes.noaa.gov/HURRLOOPS/huwvloop.html

I suppose I'm wrong. Ill turn off the lights and turn on the generator now, so I wont have to be caught off guard when Bonnie tries to turn them off for me HA! lol j/k.

What data are you using?
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Either the LLC is moving north-westward or the convection is once again being sheared off.

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1745. scott39
Quoting SouthALWX:
wrong hurricat .... the shear is quite high and the ULL is still in the gomex. the convective blowup appears assoociated with increased upper divergence associated with that portion of the ULL, as evidenced by the SE to NW streaming clouds and the " V " like presentation, the tip of the V being the connection to the updraft region of the complex. Once the shear breaks that connection, the V will flatten and the high clouds will exit stage right. Nothing has changed and as of now Bonnie is doing nothing but surviving, which given the circumstances is impressive in and of itself.
Can heavy rainfall be expected from Bonnie at landfall?
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1744. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 174 Comments: 54434
1743. KEEPEROFTHEGATE (Mod)
Quoting scott39:
Keeper is that good windshear for developement at the moment?
not really
Member Since: July 15, 2006 Posts: 174 Comments: 54434
The crazy ULL has calmed down a bit and slowed it's spin. However, now it is pulling in moisture from 98l remnants into the gulf.
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I see that homeless, but an LLC wouldnt even survive as a naked swirl under that ULL. it would die in its entirety.
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wrong hurricat .... the shear is quite high and the ULL is still in the gomex. the convective blowup appears assoociated with increased upper divergence associated with that portion of the ULL, as evidenced by the SE to NW streaming clouds and the " V " like presentation, the tip of the V being the connection to the updraft region of the complex. Once the shear breaks that connection, the V will flatten and the high clouds will exit stage right. Nothing has changed and as of now Bonnie is doing nothing but surviving, which given the circumstances is impressive in and of itself.
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1737. scott39
Keeper is that good windshear for developement at the moment?
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1736. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 174 Comments: 54434
Quoting SouthALWX:
Um to be clear ... this storm really can't go to Houston ... If it was steered that way it would be shredded long before arrival ... It's only hope to stay a tropical entity is to stay in that hole between the TUTT and the ULL and that gap is closing with time. if the ULL were to hit the afterburners and scoot it may survive as a TD-TS until landfall. Outside chance imo.


Oh I don't expect it be an organized storm. Just looks like the LLC is moving west.
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1734. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 174 Comments: 54434
1733. vince1
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.

The oil spill concern can't be overstated. This has the real potential to churn up 2-Butoxyethanol and whatever else is in the soup and dump it on those where it comes ashore and well inland.
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1732. scott39
Quoting btwntx08:
The energy is going to the N of LLC. Maybe its forming a new one?
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1730. HadesGodWyvern (Mod)
Japan Meteorological Agency
Tropical Disturbance Summary
TROPICAL DEPRESSION
9:00 AM JST July 24 2010
================================

At 0:00 AM UTC, Tropical Depression (1008 hPa) located at 25.0N 122.0E has 10 minute sustained winds of 25 knots with gusts of 40 knots. The depression is reported as moving north northwest at 10 knots.
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1729. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 174 Comments: 54434
Quoting btwntx08:
hmmm


Hmmm? Is right. On the RGB I clicked lower winds to her south seemed to be a bit southwest. Maybe they just like ya this year Rob. :)
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1727. xcool
THIS SO CRAZY. BAD GIRL


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Um to be clear ... this storm really can't go to Houston ... If it was steered that way it would be shredded long before arrival ... It's only hope to stay a tropical entity is to stay in that hole between the TUTT and the ULL and that gap is closing with time. if the ULL were to hit the afterburners and scoot it may survive as a TD-TS until landfall. Outside chance imo.
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Quoting atmosweather:


I don't think you're too far off...looks to me judging by the RECON fixes and RGB imagery that it is around 26.4N...no further N than there.


I have it on the RGB too. Sure looks like it.
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1722. scott39
Quoting Patrap:
290 true

Patrap will you please post the 850 MB Relative Vorticity Analysis Map?
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Quoting btwntx08:
if it were go to houston it would stay the wnw the rest of the way that may not be the case should turn nw soon


Lol, there are some people living around me that are so worried about it that they evacuated already. I don't think it'll be THAT bad here. I mean, it's going for NOLA, not south-central LA like it was.
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1719. xcool
storm is 412.2 statute miles FROM NOLA.
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1717. xcool



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1716. KEEPEROFTHEGATE (Mod)
03L/TD/B
MARK
27.2N/85.4W
Member Since: July 15, 2006 Posts: 174 Comments: 54434
Quoting cirrocumulus:

Now: BOOM! Later: Fizzle, fizzle..

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1714. Patrap
290 true

Member Since: July 3, 2005 Posts: 426 Comments: 128668
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Quoting homelesswanderer:


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.


I don't think you're too far off...looks to me judging by the RECON fixes and RGB imagery that it is around 26.4N...no further N than there.
Member Since: September 24, 2005 Posts: 33 Comments: 9265
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.
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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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