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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I think the West Caribbean is as due for something as any where else with this HP.
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Quoting 1173. unknowncomic:
October varies either can be very busy, or climate can put an early end to the season. No storms from shear or all recurves.


What do u think could happen this October?
Member Since: July 27, 2013 Posts: 0 Comments: 501
Quoting 1170. mitchelace5:


I'm not too sure about October though
October varies either can be very busy, or climate can put an early end to the season. No storms from shear or all recurves.
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1172. nigel20
Good evening fellow bloggers!

Current Weather Conditions:
Kingston / Norman Manley, Jamaica

Conditions at

2013.08.01 0400 UTC

Wind: from the NNE (030 degrees) at 5 MPH (4 KT)

Visibility: greater than 7 mile(s)
Sky conditions: mostly clear
Temperature: 82 F (28 C)
Dew Point: 73 F (23 C)
Relative Humidity: 74%
Pressure (altimeter): 29.97 in. Hg (1015 hPa)
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144 HRS (6days)or less is supposed to be what is valid on models.
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Quoting 1165. unknowncomic:
Yep, historical plays a big part in this. Early August is usually quiet then about mid month a wave or two moves off. Then like someone turns on the light they all come alive and the blog goes nuts.


I'm not too sure about October though
Member Since: July 27, 2013 Posts: 0 Comments: 501
Quoting 1168. mitchelace5:


Models that far out aren't reliable.
I read it completely wrong lol, it's 192 hours out.
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Quoting 1166. MiamiHurricanes09:
574 hours is nothing.


Models that far out aren't reliable.
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Quoting 1154. mitchelace5:
Are any models showing any activity in the Atlantic next week?
NO
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.
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Quoting 1163. mitchelace5:


Maybe 3 storms for August alone. September could be worse.
Yep, historical plays a big part in this. Early August is usually quiet then about mid month a wave or two moves off. Then like someone turns on the light they all come alive and the blog goes nuts.
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Quoting 1148. HurricaneKing:


I agree to some point but I also disagree.

Most of what we saw today was associated with the tropical wave that came from Dorain opening up. It surged westward and has basically became diffuse near western Cuba.

What we are seeing tonight is........that mid level low that was left behind by Dorain. It's found a sweet spot in the southern Bahamas of fairly low shear but increased upper level divergence from that upper level low. The vorticity has been increasing at the 850mb level all day while the 700mb is becoming more and more stacked with it. The 500mb circulation is still not very colocated though. All in all the leftover mid level circulation actually looks better tonight than it did when it had a yellow circle on it.

If I had to call it anything id say it was Dorain's pal lol.


D's diffuse tropical wave that's at Western Cuba...if it can manipulate the moisture around it...can it close off again? Because I would consider that re-Dorian, not the MLL ditched back in the Bahamas, although the NHC always gives the MLC dibs on whatever name it got kicked out of.

Thank you for your analysis, Sir. Very down-to-earth.
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Quoting 1161. unknowncomic:
No, but any day now it will. Soon thereafter I predict three systems at once in the Atlantic. Eventually the weather people will be over-welmed and thinking about taking that vacation.


Maybe 3 storms for August alone. September could be worse.
Member Since: July 27, 2013 Posts: 0 Comments: 501
Quoting 1153. Bluestorm5:
West Pacific might have a strong typhoon two weeks from now. We'll see.



Probably not lol if that model's predictions for Dorian were accurate I'd be under a Hurricane Warning right now...
Member Since: August 28, 2008 Posts: 79 Comments: 3774
Quoting 1157. KoritheMan:


No.
No, but any day now it will. Soon thereafter I predict three systems at once in the Atlantic. Eventually the weather people will be over-welmed and thinking about taking that vacation.
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Quoting 1158. bigwes6844:
Happy August to people in the central time zones!


It's also August here on the East coast.
Member Since: July 27, 2013 Posts: 0 Comments: 501
Welcome to the meat part of the season! Hurricane season has officially started. Today is the day that we see NHC come out with the 5 day forecast for storms at 8am
Member Since: July 25, 2010 Posts: 0 Comments: 2751
Happy August to people in the central time zones!
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Quoting 1154. mitchelace5:
Are any models showing any activity in the Atlantic next week?


No.
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Getting close to that coveted 1000 comment mark, the sign that I am a bonafide TWU blogger. Let's see want kind of BS I can think of to get there.
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Quoting 1148. HurricaneKing:


I agree to some point but I also disagree.

Most of what we saw today was associated with the tropical wave that came from Dorain opening up. It surged westward and has basically became diffuse near western Cuba.

What we are seeing tonight is........that mid level low that was left behind by Dorain. It's found a sweet spot in the southern Bahamas of fairly low shear but increased upper level divergence from that upper level low. The vorticity has been increasing at the 850mb level all day while the 700mb is becoming more and more stacked with it. The 500mb circulation is still not very colocated though. All in all the leftover mid level circulation actually looks better tonight than it did when it had a yellow circle on it.

If I had to call it anything id say it was Dorain's pal lol.


It's just a blob, bro. I ain't comin' back. lol
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Are any models showing any activity in the Atlantic next week?
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West Pacific might have a strong typhoon two weeks from now. We'll see.

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Gil need to get eyewall going if he want a shot at a major hurricane.
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Keep in mind that ideally, yes, spaced out evenly, this would mean that we get one storm every six days. However, we know that things aren't always spaced out this perfectly and we could get storms spaced out in as little as two or three days time. In fact, I remember that during August 1995, there was one particular day where three storms were named...Humberto, Iris, Jerry. That was insane, and i don't recall any other season in the past 20 or so years where three or more storms became classified in the same day(not even the 2005 season had this happen).
Quoting 1124. sar2401:

So I'm assuming that means 9 tropical storms of some type? We've had 4, so that leaves 5 of which 1 would be a tropical storm and 4 would have to be hurricane, 2 of which would be majors. That means one tropical storm every six days. It's not impossible, but, if I was bookie, I'd be willing to give you some pretty good odds that's not going to happen.
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Excellent microwave pass show that there isn't much of an eye feature though lol.



Quoting 1147. MiamiNative:
MiamiHurricanes09, what area are you in? I live down by US1 & SW 160, Work in Down Town.
You live way far hahaa. I live on Brickell Ave in downtown...probably very near where you work. :P
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I'm interested in any weather that is going to affect me.
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Quoting 1140. AussieStorm:


There is no centre of ex-Dorian.... there is no ex-dorian. It's a T-wave and that is all it will be now and in the future til it drifts either over CONUS or out into the N-ATL and dissipates.


I agree to some point but I also disagree.

Most of what we saw today was associated with the tropical wave that came from Dorain opening up. It surged westward and has basically became diffuse near western Cuba.

What we are seeing tonight is........that mid level low that was left behind by Dorain. It's found a sweet spot in the southern Bahamas of fairly low shear but increased upper level divergence from that upper level low. The vorticity has been increasing at the 850mb level all day while the 700mb is becoming more and more stacked with it. The 500mb circulation is still not very colocated though. All in all the leftover mid level circulation actually looks better tonight than it did when it had a yellow circle on it.

If I had to call it anything id say it was Dorain's pal lol.
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MiamiHurricanes09, what area are you in? I live down by US1 & SW 160, Work in Down Town.
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Since Karen and Jose have been mentioned now and then,years after they went caput, I see no reason why we should not be talking about Ex-Dorian/T wave/Whatever you want to call it.

I personally find it worthwhile and would keep watching it whether it develops or not. Cyclogenesis with real time data is far more interesting than some storm far far away in the Atlantic.
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Quoting 1140. AussieStorm:


There is no centre of ex-Dorian.... there is no ex-dorian. It's a T-wave and that is all it will be now and in the future til it drifts either over CONUS or out into the N-ATL and dissipates.


You missed it earlier; Miami radar with a big black circle ringed by tstorms in the middle of FL and Central Cuba. Ask Pat if you don't believe me.

I was frankly surprised to see it, too.

Then a Cayman resident weighed in with winds starting in the East, then shifting North, then finally strongly West. Guess you had to be there.
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You can see where the eye feature will clear out if you tilt your monitor back a bit lol.

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1009mb low
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Quoting 1110. mitchelace5:

the Bahamas

Specifically Long Island.
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Quoting 1140. AussieStorm:


There is no centre of ex-Dorian.... there is no ex-dorian. It's a T-wave and that is all it will be now and in the future til it drifts either over CONUS or out into the N-ATL and dissipates.


I find it really aggravating that people are still talking about it tbh
Member Since: July 27, 2013 Posts: 0 Comments: 501
Quoting redwagon:


That firing S of Cuba is xDorian's southern.. 'grab' on the moisture. 'Center' of xD is about 250 miles due North.


There is no centre of ex-Dorian.... there is no ex-dorian. It's a T-wave and that is all it will be now and in the future til it drifts either over CONUS or out into the N-ATL and dissipates.
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Quoting 1138. MiamiNative:
Happy August Everyone! How many inches am I going to get this weekend?
10 inches of rain for all of august!
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Happy August Everyone! How many inches am I going to get this weekend?
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Quoting 1133. Kumo:


The date in your photo threw me for a loop, as most folks around here only talk about the TS Allison from 2001. I had to do a search and turned up some uncanny similarities between both Allisons.

- Both made landfall at Freeport, TX
- Both storms had casualties associated with them, numbering in the double digits.
- Both formed and made landfall in June.
- Both dumped an incredible amount of rain.
- Both were very slow movers.
- Both were forced south by a ridge.

http://en.wikipedia.org/wiki/Tropical_Storm_Allis on_(1989)

http://en.wikipedia.org/wiki/Tropical_Storm_Allis on


Going further, the name 'Allison' promptly replaced 'Alicia' from 1983, which was the first 'A' storm to appear on this list of the Atlantic rotation. Look where Alicia ended up:



Cool coincidence.
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1136. HadesGodWyvern (Mod)
India Meteorological Department
Tropical Cyclone Advisory #8
DEPRESSION BOB03-2013
5:30 AM IST August 1 2013
===============================

SUBJECT: Depression over southeast Madhya Pradesh and adjoining areas of Vidarbha & Chhattisgarh.

At 0:00 AM UTC, The depression over east Madhya Pradesh and adjoining Chhattisgarh moved slightly westwards and now lays centered over southeast Madhya Pradesh, adjoining areas of Vidarbha & Chhattisgarh, or about 100 km south southeast of Jabalpur.

The system would move west northwestwards and weaken gradually into a well marked low pressure area during next 24 hours.
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1135. Thrawst
Quoting 1081. Gearsts:
The power of saharan air layer or dust.


I can really tell the difference here in the Bahamas. That map seems about accurate in my area.
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Quoting 1125. GetReal:


IMO it will have to get a lot bigger before a blob alert is called for.




That firing S of Cuba is xDorian's southern.. 'grab' on the moisture. 'Center' of xD is about 250 miles due North.
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1133. Kumo
Quoting 1060. Grothar:
Tropical Storm Allison was just a little blob, but it killed over 50 people. One of the most dangerous Tropical Storms I can remember



The date in your photo threw me for a loop, as most folks around here only talk about the TS Allison from 2001. I had to do a search and turned up some uncanny similarities between both Allisons.

- Both made landfall at Freeport, TX
- Both storms had casualties associated with them, numbering in the double digits.
- Both formed and made landfall in June.
- Both dumped an incredible amount of rain.
- Both were very slow movers.
- Both were forced south by a ridge.

http://en.wikipedia.org/wiki/Tropical_Storm_Allis on_(1989)

http://en.wikipedia.org/wiki/Tropical_Storm_Allis on
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Quoting BaltimoreBrian:


Thanks Aussie!


I got an answer from Levi...

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Quoting 1124. sar2401:

So I'm assuming that means 9 tropical storms of some type? We've had 4, so that leaves 5 of which 1 would be a tropical storm and 4 would have to be hurricane, 2 of which would be majors. That means one tropical storm every six days. It's not impossible, but, if I was bookie, I'd be willing to give you some pretty good odds that's not going to happen.


It's actually a lot more viable than you think.

2010 saw the formation of 12 named storms during the August - September period; 1998 saw the formation of 10 named storms during that same period, and 2004 saw the formation of 12 named storms subsequent to August 1.

It's really not that uncommon during active years. Even if August is not as active as 5 named storms, September is usually the more volatile month anyway.
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Quoting 1126. sar2401:

I was impressed, especially for this season so far. The models don't want to take it above a cat 1 but it's looking pretty healthy right now.

I've actually not been impressed by this season so far. Sure, we've seen five Category 1 hurricanes by this point, but that's all they've been. The storms all look the same developing, they all reach 75-85 mph, and they all die over cooler waters (with the exception of Barbara, which made landfall). The average date for the first Cat. 3 or higher storm was July 19.
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1129. sar2401
Quoting GetReal:


IMO it will have to get a lot bigger before a blob alert is called for.

There's a blob south of Cuba that appears to be more blobish than the wandering remnants to the north.
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1128. TxLisa
Quoting 1060. Grothar:
Tropical Storm Allison was just a little blob, but it killed over 50 people. One of the most dangerous Tropical Storms I can remember

I literrally drove around the outer loop of the City of Houston after Allison trying to find a clear rodeway to get home (death in the family) and I was shocked at the number of 18 wheelers and pickup trucks that were underwater. Amazing the amount of damage and devastation a little tropical storm can cause!
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Quoting 7544:
evening all do i see a blob alert comin in the next hour just east of andros island grooo?

Gro has gone to bed. You missed him by about an hour. Is you read back you'll see gro has been commenting on it. Though most of us think nothing will happen from ex-Dorian
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1126. sar2401
Quoting TropicalAnalystwx13:

A 35 mph increase in winds in 24 hours qualifies as rapid intensification, William sorry. Lol.

From the NHC in the 8pm PDT advisory:

"GIL HAS BEEN RAPIDLY INTENSIFYING FOR THE PAST 24-30 HR."

I was impressed, especially for this season so far. The models don't want to take it above a cat 1 but it's looking pretty healthy right now.
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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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