Massive African Dust Storm Cooling Atlantic Hurricane Odds for Early August

By: Dr. Jeff Masters , 3:10 PM GMT on July 31, 2013

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A massive dust storm that formed over the Sahara Desert early this week has now pushed out over the tropical Atlantic, and will sharply reduce the odds of tropical storm formation during the first week of August. The dust is accompanied by a large amount of dry air, which is making the Saharan Air Layer (SAL) much drier than usual this week. June and July are the peak months for dust storms in the Southwest Sahara, and this week's dust storm is a typical one for this time of year. Due in large part to all the dry and dusty air predicted to dominate the tropical Atlantic over the next seven days, none of the reliable computer models is predicting Atlantic tropical cyclone formation during the first week of August.


Figure 1. A massive dust storm moves off the coast of Africa in this MODIS image taken at 1:40 UTC July 30, 2013. Image credit: NASA.


Video 1. The predicted movement through August 3 of this week's Africam dust storm, using the NOAA NGAC aerosol model. Image credit: NOAA Visualization Laboratory.

How dust affects hurricanes
Saharan dust can affect hurricane activity in several ways:

1) Dust acts as a shield which keeps sunlight from reaching the surface. Thus, large amounts of dust can keep the sea surface temperatures up to 1°C cooler than average in the hurricane Main Development Region (MDR) from the coast of Africa to the Caribbean, providing hurricanes with less energy to form and grow. Ocean temperatures in the MDR are currently 0.7°F above average, and this anomaly should cool this week as the dust blocks sunlight.

2) The Saharan Air Layer (SAL) is a layer of dry, dusty Saharan air that rides up over the low-level moist air over the tropical Atlantic. At the boundary between the SAL and low-level moist air where the trade winds blow is the trade wind inversion--a region of the atmosphere where the temperature increases with height. Since atmospheric temperature normally decreases with height, this "inversion" acts to but the brakes on any thunderstorms that try to punch through it. This happens because the air in a thunderstorm's updraft suddenly encounters a region where the updraft air is cooler and less buoyant than the surrounding air, and thus will not be able to keep moving upward. The dust in the SAL absorbs solar radiation, which heats the air in the trade wind inversion. This makes the inversion stronger, which inhibits the thunderstorms that power a hurricane.

3) Dust may also act to produce more clouds, but this effect needs much more study. If the dust particles are of the right size to serve as "condensation nuclei"--centers around which raindrops can form and grow--the dust can act to make more clouds. Thus, dust could potentially aid in the formation and intensification of hurricanes. However, if the dust acts to make more low-level clouds over the tropical Atlantic, this will reduce the amount of sunlight reaching the ocean, cooling the sea surface temperatures and discouraging hurricane formation (Kaufman et al., 2005.)


Figure 2. Map of the mean summer dust optical thickness derived from satellite measurements between 1979 and 2000. Maximum dust amounts originate in the northern Sahel (15° to 18° N) and the Sahara (18° to 22° N). The Bodele depression in Chad is also an active dust source. Image credit: Evidence of the control of summer atmospheric transport of African dust over the Atlantic by Sahel sources from TOMS satellites (1979-2000), by C. Moulin and I. Chiapello, published in January 2004 in Geophysical Research Letters.

Dust in Africa's Sahel region and Atlantic hurricane activity
The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18° - 22° N) and the northwestern Sahel (15° - 18° N) (Figure 3.) The dust from the Southwest Sahara stays relatively constant from year to year, but the dust from the Northwest Sahel varies significantly, so understanding this variation may be a key factor in improving our forecasts of seasonal hurricane activity in the Atlantic. The amount of dust that gets transported over the Atlantic depends on a mix of three main factors: the large scale and local scale weather patterns (windy weather transports more dust), how wet the current rainy season is (wet weather will wash out dust before it gets transported over the Atlantic), and how dry and drought-damaged the soil is. The level of drought experienced in the northwestern Sahel during the previous year is the key factor of the three in determining how much dust gets transported over the Atlantic during hurricane season, according to a January 2004 study published in Geophysical Research Letters published by C. Moulin and I. Chiapello. In 2012 (Figure 3), precipitation across the northwestern Sahel was much above average, which should result in less dust than usual over the Atlantic this fall, increasing the odds of a busy 2013 hurricane season.


Figure 3. Rainfall over the Northwest Sahel region of Africa was about 200% of average during the 2012 rainy season. The heavy rains promoted vigorous vegetation growth in 2013, resulting in less bare ground capable of generating dust. Image credit: NOAA/Climate Prediction Center.

The future of African dust: highly uncertain
A September 2013 paper in the Bulletin of the American Meteorological Society by Joseph Prospero and Olga Mayol-Bracero, "Understanding the Transport and Impact of African Dust on the Caribbean Basin," discusses the large uncertainties on how African dust may change due to climate change. Over the past decade, there has been no clear relationship between African dust and climate indices such as rainfall in the Sahel or the El Niño/La Niña cycle, which "makes it difficult to predict how dust emissions and transport might change over the coming decades as climate changes. The problem is exacerbated by the inability of models (IPCC 2007) to agree on future rainfall trends over large areas of North Africa (including the Sahel) that are known to be major dust sources today and in the recent past."

Links
Saharan Air Layer Analysis from the University of Wisconsin

Atlantic dust forecast from the Tel-Aviv University Weather Research Center

The Saharan Air Layer (SAL) was first described in 1972, in this classic paper: Carlson, T. N., and J. M. Prospero (1972), The Large-Scale Movement of Saharan Air Outbreaks over the Northern Equatorial Atlantic, Journal of Applied Meteorology, 11(2), 283-297

Dr. Amato Evan published a study in Science magazine March 2009 showing that 69% of the increase in Atlantic sea surface temperatures over the past 26 years could be attributed to decreases in the amount of dust in the atmosphere.

Kaufman, Y. J., I. Koren, L. A. Remer, D. Rosenfeld, and Y. Rudich, 2005a: The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean. Proc. Natl. Acad. Sci. USA, 102, 11 207–11 212.

Wang, Chunzai, Shenfu Dong, Amato T. Evan, Gregory R. Foltz, Sang-Ki Lee, 2012, Multidecadal Covariability of North Atlantic Sea Surface Temperature, African Dust, Sahel Rainfall, and Atlantic Hurricanes, J. Climate, 25, 5404–5415.
doi: http://dx.doi.org/10.1175/JCLI-D-11-00413.1

Jeff Masters

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Quoting 1607. DocMurphy:
Been watching the blog for several days and have been quite entertained about the events concerning the blob. With gratitude to Monty Python for the creative writing, I'd like to share how the blob conversations have looked...

Bring out your dead!

NHC: Here's one -- nine pence.
xDorianBlob: I'm not dead!
WU Blogger: What?
NHC: Nothing -- here's your nine pence.
xDorianBlob: I'm not dead!
WU Blogger: Here -- he says he's not dead!
NHC: Yes, he is.
xDorianBlob: I'm not!
WU Blogger: He isn't.
NHC: Well, he will be soon, he's very ill.
xDorianBlob: I'm getting better!
NHC: No, you're not -- you'll be stone dead in a moment.
WU Blogger: Oh, I can't take him like that -- it's against regulations.
xDorianBlob: I don't want to go in the cart!
NHC: Oh, don't be such a baby.

I look forward to engaging in future discussions.


Absolutely brilliant!
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Quoting 1660. 954FtLCane:

It's a red state, it's Texas, they won't agree with anything that remotely even mentions GW....LOL


How biased and pathetic of you.
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1674. Patrap
Member Since: July 3, 2005 Posts: 433 Comments: 132803
Quoting 1662. SouthernIllinois:

What a blanket statement to make.

I knew someone would comment. I'm just KIDDING...let me go back and erase before I start WW3.
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Satellite images and surface observations indicate that a LLC appears to be forming just Se of Andros island. remember this is an area that has had history of tropical systems spinning up quickly
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The remnants of Dorian will not reform. We have all seen swirls of low clouds with a few thunderstorms, this is no different.
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1670. LafLA
Quoting 1662. SouthernIllinois:

What a blanket statement to make.


Consider where the source is from...
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Quoting 1664. Camille33:
here we go big wave here with low.
Yep, once SAL moves out of the way and ITCZ lifts north, then the big ones will follow. Also the position of the A-B High is more towards the Azores Islands which would allow for better development, Erin would have the best shot of becoming this seasons first hurricane.
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Quoting 1654. rmbjoe1954:
Hello everyone!
Just checking in and I see a donut over the Bahamas.

Oh well, have a great day everyone.

It's just a donut - I mean it can't kill you, right? Oh, wait, never mind:-)
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I just had to BANN JFV from my FB page....still and idiot on Tropics....LOL
Member Since: September 2, 2007 Posts: 179 Comments: 20450
1666. Patrap
New Orleans queens, Sure know how to schmooze it
Maybe for some that seems alright
When I step out, strut down with my sugar
She'd best not talk like Barry White!



Member Since: July 3, 2005 Posts: 433 Comments: 132803
According to the loops posted, clouds from the blob appear to be approaching Bimini and the Florida coastline, causing thousands of tourists to realize that Florida shouldn't really be called the Sunshine state and instead be called "Sunshine in Winter, Rain and mosquitoes in Summer State"
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Quoting 1663. GTstormChaserCaleb:
here we go big wave here with low.
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Quoting 1640. RitaEvac:
Most folks aren't buying into this.

Report Claims Galveston Could be Submerged

Texas it's a state
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Quoting 1641. FIUStormChaser:


The NHC has even been wrong with the 48 hour predictions, sometimes going from 30% to 80% red. Imagine the ridicule if they forecast no development and a system forms during a holiday.
I agree. Tropcial cyclones can and do make radical changes in their development from hour to hour. The NHC is really taking a great risk with their forcasting credibility with this new five-day outlook.
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That same line of storms just rolled through Pensacola. It went from just one little dot on the radar to a huge red cell in the matter of about 30 min. Tons of lightning and super heavy rain for about 45 min!


Quoting 1642. 69Viking:
Well so much for trying to go 4 days without rain. Heavy thunderstorms just blew up over Fort Walton Beach dropping some very heavy rainfall. I'm never going to get to mow the pond formally called my front yard!

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Quoting 1644. TheDawnAwakening:
Surface low developing in the Bahamas, underneath the deepest convection. Low level cumulus are showing signs of cyclonic curvature.


Your posts are well throughout and knowledgable. And reasonable, this definitely won't blow up into a strong or even moderate system.
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Link
Do I see Katrina had a ull too when it form look at this loop!
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Hello everyone!
Just checking in and I see a donut over the Bahamas.

Oh well, have a great day everyone.
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I got to give this Blog credit...this is where I get my information good, bad, accurate, not so accurate, many, many points of views. If something just looks suspicious on sat pics I go to this site for everybody points of views. I'm usually about 12 hours ahead of the local, national weather services, with information of tropical development..thanks to this blog. Again thanks everyone who contributes to this blog.

Daniel
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Quoting 1637. superpete:
Of note on the latest SFC map is the direction shown for the T wave analyzed as 1011 MB Low off Africa ( SW into the ITCZ )

NHC surface map

Link
Thanks for the link Super Pete. When I see a surface low on this with our blob, I will get excited. Till then I moving around so I don't get a fat A** waiting forit.
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Quoting 1635. mrsalagranny:
http://www.myfoxhurricane.com/custom/models/gfs.h tm l


Is that something in the GOM? Just watch as the Bermuda high pushes into the GOM and it seems as though something is in there. Maybe I am seeing things but was wanting some input on it please.TIA


There *is* something in the GOM, you just can't 'see' it without radar.
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Quoting 1639. CitikatzSouthFL:
FIUStormChaser

Please don't quote eddye. He just likes to post outrageous comments just to stir up trouble. I poofed him a long time ago.

Thanks.




plzs dont tell us oh we can and can not quote well quote oh evere we want re gard less on oh we have on ignore
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Quoting 1607. DocMurphy:
Been watching the blog for several days and have been quite entertained about the events concerning the blob. With gratitude to Monty Python for the creative writing, I'd like to share how the blob conversations have looked...

Bring out your dead!

NHC: Here's one -- nine pence.
xDorianBlob: I'm not dead!
WU Blogger: What?
NHC: Nothing -- here's your nine pence.
xDorianBlob: I'm not dead!
WU Blogger: Here -- he says he's not dead!
NHC: Yes, he is.
xDorianBlob: I'm not!
WU Blogger: He isn't.
NHC: Well, he will be soon, he's very ill.
xDorianBlob: I'm getting better!
NHC: No, you're not -- you'll be stone dead in a moment.
WU Blogger: Oh, I can't take him like that -- it's against regulations.
xDorianBlob: I don't want to go in the cart!
NHC: Oh, don't be such a baby.

I look forward to engaging in future discussions.

So funny! So true!
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Quoting 1631. nrtiwlnvragn:
Loop


If it does form, they should call it "Grothars Donut"

(Ex: "Grothars Dount" made landfall today... )
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Quoting 1641. FIUStormChaser:


The NHC has even been wrong with the 48 hour predictions, sometimes going from 30% to 80% red. Imagine the ridicule if they forecast no development and a system forms during a holiday.


Honestly....for NHC not to at least have a yellow circle up is puzzling.
Member Since: September 2, 2007 Posts: 179 Comments: 20450
Surface low developing in the Bahamas, underneath the deepest convection. Low level cumulus are showing signs of cyclonic curvature.
Member Since: October 21, 2008 Posts: 248 Comments: 3970
img src=""> !!
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Well so much for trying to go 4 days without rain. Heavy thunderstorms just blew up over Fort Walton Beach dropping some very heavy rainfall. I'm never going to get to mow the pond formally called my front yard!

Member Since: August 25, 2006 Posts: 1 Comments: 3253
Quoting 1638. Hurrihistory:
Forcasting tropical development is hard enough for two to three days out but now trying to predict what will happen five days out seems a little too much in my book. I think the NHC should really rethink this concept.


The NHC has even been wrong with the 48 hour predictions, sometimes going from 30% to 80% red. Imagine the ridicule if they forecast no development and a system forms during a holiday.
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Most folks aren't buying into this.

Report Claims Galveston Could be Submerged
Member Since: Posts: Comments:
FIUStormChaser

Please don't quote eddye. He just likes to post outrageous comments just to stir up trouble. I poofed him a long time ago.

Thanks.
Member Since: Posts: Comments:
Forcasting tropical development is hard enough for two to three days out but now trying to predict what will happen five days out seems a little too much in my book. I think the NHC should really rethink this concept.
Member Since: Posts: Comments:
Of note on the latest SFC map is the direction shown for the T wave analyzed as 1011 MB Low off Africa ( SW into the ITCZ )

NHC surface map

Link
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Quoting 1582. Grothar:


Not too many are old enough to remember that one!!!!


Raises his hand
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http://www.myfoxhurricane.com/custom/models/gfs.htm l


Is that something in the GOM? Just watch as the Bermuda high pushes into the GOM and it seems as though something is in there. Maybe I am seeing things but was wanting some input on it please.TIA
Member Since: June 6, 2010 Posts: 0 Comments: 853
1634. whitewabit (Mod)
TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
800 AM EDT THU AUG 1 2013

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE NEXT FIVE
DAYS.

&&

FIVE-DAY FORMATION PROBABILITIES ARE EXPERIMENTAL IN 2013. COMMENTS
ON THE EXPERIMENTAL FORECASTS CAN BE PROVIDED AT...USE LOWER
CASE...

HTTP://WWW.NWS.NOAA.GOV/SURVEY/NWS-SURVEY.PHP?COD E=ETWO

$$
FORECASTER FRANKLIN/ZELINSKY
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Quoting 1620. eddye:
wow guys look like we got katrina part 2 forming off fl and it will be stalled their for 18 hrs so we can be looking at a strong ts mini cat 1 hurricane


-_- At the very most it will only become an invest/tropical depression.

This is early August... Not late August... And conditions are not conducive for that type of scenario.

Just aced my Evolution test.
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Quoting 1619. prcane4you:
Boring people around here just talking about blobs here & there.Why people spend their time doing this?


You know you can always find another website/blog to visit if you don't like it. I'm sorry to tell you but chances are we're probably going to keep on boring you. It's what we do... we talk about blobs and such.
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Loop
Member Since: September 23, 2005 Posts: 16 Comments: 11633
Quoting 1622. unknowncomic:
There's a blowuo of convection, but we need ASCAT or something to confirm a LLC.


no you don't.... just radar at this point and vis sat. This is still very disorganized with a midelevel twist east of Nassau. It's getting better defined but not a Depression or Storm....if convection stays through the day and pressures fall 2-3 mbs over the next 12hrs...then we might have a TD by later tonight. I give it a 30% chance
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1629. VR46L
Quoting 1624. washingtonian115:
Lol.You know the blog has nothing better else to do when they talk about a blob in the bahamas that won't amount to much besides more rain chances for that area.The real stars are about to come off in another two weeks that will be much more worthy of discussion.I'm waiting on those to track until then toodles.I have to help my sister move today.


Ssshh Wash LOL!

Blob watching is better than AGW fights.....
Member Since: March 1, 2012 Posts: 0 Comments: 7093
The AOI over the Bahamas is trying to develop a weak surface low at this hour based on satellite and bouy data. Stations south of the area is reporting SSW to SW winds in Canagua, Cuba. Another station reporting winds from the south on the east part of the Blob and finally a Easterly wind on the northern side of the AOI. If convection continues to fire the NHC may tag an invest with our system



Link

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1619. prcane4you

Then why are you lurking? Perhaps you need to go to another blog to be entertained.
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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