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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...Up to 40% for the TWO @ 2am, now...
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TROPICAL WEATHER DISCUSSION
NWS NATIONAL HURRICANE CENTER MIAMI FL
205 AM EDT FRI AUG 02 2013

TROPICAL WEATHER DISCUSSION FOR NORTH AMERICA...CENTRAL
AMERICA...GULF OF MEXICO...CARIBBEAN SEA...NORTHERN SECTIONS OF
SOUTH AMERICA...AND ATLANTIC OCEAN TO THE AFRICAN COAST FROM THE
EQUATOR TO 32N. THE FOLLOWING INFORMATION IS BASED ON SATELLITE
IMAGERY...WEATHER OBSERVATIONS...RADAR...AND METEOROLOGICAL
ANALYSIS.

BASED ON 0000 UTC SURFACE ANALYSIS AND SATELLITE IMAGERY THROUGH
0515 UTC.

...SPECIAL FEATURES...
A SURFACE TROUGH...REMNANTS OF DORIAN...EXTENDS FROM 28N78W
BETWEEN THE BAHAMA ISLANDS AND S FLORIDA TO THE COAST OF CUBA
NEAR 23N81W MOVING NORTHWARD. SCATTERED SHOWERS/THUNDERSTORMS
ARE WITHIN 120 NM ALONG THE N COAST OF CUBA BETWEEN 76W AND THE
SURFACE TROUGH WITH SCATTERED SHOWERS AND POSSIBLE ISOLATED
THUNDERSTORMS WITHIN 150 NM W OF THE REMAINDER OF THE SURFACE
TROUGH. SOME DEVELOPMENT OF THIS SYSTEM IS POSSIBLE BEFORE UPPER-
LEVEL WINDS BECOME UNFAVORABLE IN A DAY OR TWO. CLOUDINESS AND
SHOWERS WILL SPREAD OVER SE FLORIDA ON FRI AND SAT. THIS SYSTEM
HAS A MEDIUM CHANCE OF BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS.
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Quoting 3507. CosmicEvents:
Gro....do you recall a cane, TS, TD, or L that traversed the Atlantic and then pulled a straight 90 degree turn "just" off the coast here?

Hope you dont mind butting in, but I did see your post and wanted to say that when I was young I remember Dennis heading towards the SFl area , then it did a sharp turn to the north...
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3523. Grothar
Quoting 3519. galvestonhurricane:


Thanks for the response! I welcome disagreement and reasonable discussion, and I definitely understand from where you are coming.


It's just that I hate getting wet.
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3522. Grothar
Quoting 3507. CosmicEvents:
Gro....do you recall a cane, TS, TD, or L that traversed the Atlantic and then pulled a straight 90 degree turn "just" off the coast here?


As a matter of fact I do. I also remember when a little disturbance was right on the coast and they told us it might be a small rain event and really didn't need to put our shutters up. ON the coast it turned into a little hurricane called Katrina and cost me about 14,000 in damages.

You can see it didn't become a hurricane until it hit the coast

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Quoting 3520. HurricaneAndre:
a little.


It should be able to fight the shear as well IMO
Member Since: July 27, 2013 Posts: 0 Comments: 501
Quoting 3517. mitchelace5:


Also, any high shear ahead of it?
a little.
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Quoting 3516. Grothar:


You know I don't often disagree with most people, I have to on this one. Even though you live in the Gulf region, the coast of Florida has a unique weather oddity. Since the Gulfstream is on our east and mostly warm waters on our west and obviously totally surrounded by warm waters, I have seen these spin into terrible flooding events many times. I remember one in the early 90's when they said to expect 1-3 inches of rain and we all of South Florida got stuck in rush hour traffic when we got 7 inches of rain in less than 2 hours and a total of of 15 inches. It also just happened with Andrea when most of Fort Lauderdale and Miami were flooded.

I never for a minute, thought this was going to be a hurricane, but I think many of us don't see a Cat 5 and we dismiss it.

As I wrote earlier, even depressions cause us tremendous amount of damage much more than do most states. I remember that Texans didn't take Tropical Storm Allison seriously and more than 50 people died when you got over 40 inches of rain.

It most definitely is worth tracking.


Thanks for the response! I welcome disagreement and reasonable discussion, and I definitely understand from where you are coming.
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Quoting 3514. Thrawst:
OK, I stand a little corrected.

This wave (ex-Dorian), I thought it had gone by with no problem... but NOW, this thing is backbuilding. We had some serious wind gusts over 35mph just now before a massive thunderstorm came and basically wiped out our beach party (hehe, awwww yeah)... so now i'm home.

Flooding is starting to become scattered over many places in Nassau.


Interesting observations.. keep em coming
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Quoting 3515. HurricaneAndre:
it'll make it.


Member Since: July 27, 2013 Posts: 0 Comments: 501
3516. Grothar
Quoting 3503. galvestonhurricane:


No offense, Gro, but this thing is not even worth tracking. I've tracked normal thunderstorms stronger than this blob.


You know I don't often disagree with most people, I have to on this one. Even though you live in the Gulf region, the coast of Florida has a unique weather oddity. Since the Gulfstream is on our east and mostly warm waters on our west and obviously totally surrounded by warm waters, I have seen these spin into terrible flooding events many times. I remember one in the early 90's when they said to expect 1-3 inches of rain and we all of South Florida got stuck in rush hour traffic when we got 7 inches of rain in less than 2 hours and a total of of 15 inches. It also just happened with Andrea when most of Fort Lauderdale and Miami were flooded.

I never for a minute, thought this was going to be a hurricane, but I think many of us don't see a Cat 5 and we dismiss it.

As I wrote earlier, even depressions cause us tremendous amount of damage much more than do most states. I remember that Texans didn't take Tropical Storm Allison seriously and more than 50 people died when you got over 40 inches of rain.

It most definitely is worth tracking.
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Quoting 3513. mitchelace5:


This one looks pretty vigorous to me. What do you think?
it'll make it.
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3514. Thrawst
OK, I stand a little corrected.

This wave (ex-Dorian), I thought it had gone by with no problem... but NOW, this thing is backbuilding. We had some serious wind gusts over 35mph just now before a massive thunderstorm came and basically wiped out our beach party (hehe, awwww yeah)... so now i'm home.

Flooding is starting to become scattered over many places in Nassau.
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Quoting 3512. HurricaneAndre:
Some storms can fight the dry air.


This one looks pretty vigorous to me. What do you think?
Member Since: July 27, 2013 Posts: 0 Comments: 501
Quoting 3511. mitchelace5:


But what about that dry air around it though?
Some storms can fight the dry air.
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Quoting 3510. HurricaneAndre:
maybe,there's a low with it too.


But what about that dry air around it though?
Member Since: July 27, 2013 Posts: 0 Comments: 501
Quoting 3509. mitchelace5:


Could the NHC highlight that area near Africa tomorrow or something? I'm not sure, because although Its a big, vigorous wave, it's fightin' dry air.
maybe,there's a low with it too.
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Quoting 3497. mitchelace5:


What's AOI? Area of Interest?


Could the NHC highlight that area near Africa tomorrow or something? I'm not sure, because although its a big, vigorous wave, it's fightin' dry air.
Member Since: July 27, 2013 Posts: 0 Comments: 501
Quoting 3502. HurricaneAndre:
I'm no troll.
You sure act like one...
Member Since: August 1, 2011 Posts: 28 Comments: 8077
Gro....do you recall a cane, TS, TD, or L that traversed the Atlantic and then pulled a straight 90 degree turn "just" off the coast here?
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Quoting 3504. TomTaylor:
amen.
thanks.
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3505. Grothar
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Quoting 3502. HurricaneAndre:
I'm no troll.
amen.
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Quoting 3500. Grothar:
latest models.




No offense, Gro, but this thing is not even worth tracking. I've tracked normal thunderstorms stronger than this blob.
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Quoting 3501. galvestonhurricane:


Don't quote trolls.
I'm no troll.
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Quoting 3494. Bluestorm5:
Nope... DMAX peak at sunrise. DMIN peak at sunset.


Don't quote trolls.
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3500. Grothar
latest models.


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Quoting 3497. mitchelace5:


What's AOI? Area of Interest?
yes.
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Quoting 3497. mitchelace5:


What's AOI?
AOI'S are Areas of Interest,highlighted by the NHC with three colors,yellow,orange,and red.
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Quoting 3495. HurricaneAndre:

Still a good wave,maybe AOI.


What's AOI? Area of Interest?
Member Since: July 27, 2013 Posts: 0 Comments: 501
Quoting 3494. Bluestorm5:
Nope... DMAX peak at sunrise. DMIN peak at sunset.
okay.
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Still a good wave,maybe AOI.
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Quoting 3493. HurricaneAndre:
DMAX peaks at noon right.
Nope... DMAX peak at sunrise. DMIN peak at sunset.
Member Since: August 1, 2011 Posts: 28 Comments: 8077
Quoting 3492. Grothar:



This current flareup began at 10:13
DMAX peaks at noon right.
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3492. Grothar
Quoting 3213. Grothar:
Beginning to fire up some convection. Starting early tonight.




This current flareup began at 10:13
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Got a new blog,go to my blog,Dorian the fighter.Link
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I wrote a blog on the East Pacific, focusing on Gil and the two invests (though less for the last two). I also mentioned 91L in there and talked about the potential for Caribbean mischief in 10-15 days. Read if you wish.

East Pacific basin lit up with activity; watching the Atlantic basin
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3489. Grothar
Quoting 3416. MeteorologistTV:
I'm really concerned about an area in Ft Lauderdale beach. This area under construction is about 2 miles long and the ocean is the closest i've ever seen to houses and businesses.I'm certain even mid tropical storm can cause significant damage around that area.


What beach? There's nothing left. You can even see sand anymore for the entire stretch where I live. We are all waiting for a disaster.
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3488. Kumo
Quoting 3458. Bluestorm5:
Ex-Dorian been doing this over and over the past week with not much success. Give it up.


Let them have some fun, they are not breaking any rules, besides some of us are learning a lot by watching the discussion.
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Quoting 3471. TampaSpin:


The way that burst is occurring....one would say that it certainly could change it some.....the burst is occurring directly over the LL spin....We shall see. My guess NHC will raise the bar to 50% at 2pm.


AM lol I got you
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3486. crankin
downcaster
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Quoting 3481. sar2401:

From what I can see on radar, and radar precipitation estimates, this is a 1"-2" storm. That's why I said the best thing it can do is get moving west and rain itself out or get moving north and east and avoid south Florida altogether.
I think those 1-2" estimates are based on the hit and run north-east forecast. Let's hope that happens and it giddy-ups out of here. If it sits I think the estimates and final totals would rise.
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3484. sar2401
Everyone who wants to lose sleep watching 91L and DMax again, enjoy yourselves. I've seen enough this thing. See you all tomorrow
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gonna stay up for the 2am update....guess I will play some hearts online for a while....BBL
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Quoting 3469. CosmicEvents:
That's true, but it's also the worst case scenario for this area. The last thing needed is "whatever-it-is" sitting there raining itself out and dumping another 5-10" on lake O.


I remember on the flip side, just a couple years ago, one would only hope that something like this would come along and dump that kinda rain on Lake O. I remember it being some 5' below normal pool. Which is disastrous to South Florida. I remember seeing "beaches" on Lake O. Feast or famine...
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3481. sar2401
Quoting CosmicEvents:
That's true, but it's also the worst case scenario for this area. The last thing needed is "whatever-it-is" sitting there raining itself out and dumping another 5-10" on lake O.

From what I can see on radar, and radar precipitation estimates, this is a 1"-2" storm. That's why I said the best thing it can do is get moving west and rain itself out or get moving north and east and avoid south Florida altogether.
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3480. sar2401
Quoting yankees440:
Let it begin....Again

If you use the Miami longe range radar rather than that rainbow floater, I think you'll get a far better idea of what's really developing. The rainbow exaggerates the actual potential of the storm, but the radar will show you the precipitation and intensity.



Lots of different thing for Florida weather
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Quoting 3473. sar2401:

No, there's no evidence of convergence yet, and it's past 12:30 am. The longer we go with just this small center and almost no convection away from the center, the less likely there will be for any convergence to help build the system. It looked far, far better by this time of night when it was north of Puerto Rico than it does now.
however this is the warmest water and lightest shear it has experienced.
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3477. flsky
Quoting 3468. sar2401:

There still is, and never will be, no RI from 91L. What exactly are we supposed to tip our hats for? They were wrong then and, if they are saying now, they are still wrong.

Power on again just now in PI. Still flickering tho.
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Quoting 3474. HurricaneAndre:
You mean AM right.


ya.....AM sorry
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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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