Dust forecast for the 2009 Atlantic hurricane season

By: Dr. Jeff Masters , 6:58 PM GMT on June 20, 2009

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There will be less African dust than usual over the tropical Atlantic during this year's hurricane season, according to a new experimental dust forecast issued by Dr. Amato Evan of the University of Wisconsin. Dr. Evan used a statistical model that correlated levels of dust activity in past years with rainfall over the Sahel region of Africa and a natural regional wind pattern known as the North Atlantic Oscillation (NAO). He forecasts that dust levels over the Main Development Region (MDR, 8 - 20°N & 15 - 65°W) for Atlantic hurricanes during this year's hurricane season will be similar to last year's below-average levels, thanks in large part to plentiful rains over the Sahel region of Africa during the 2008 rainy season (Figure 1). However, the dust levels expected this year do not approach the record lows seen in 1994 and 2005. Dust forecasts made in May or June are skillful going out five months, with a skill 11 - 16% better than a "no-skill" forecast using climatology.


Figure 1. Rainfall over the Sahel region of Africa was generally 50 - 100 mm (2 - 4 inches) above average during the 2008 rainy season (about 20 - 80% above average). The heavy rains promoted vigorous vegetation growth in 2009, resulting in less bare ground capable of generating dust. Image credit: NOAA/Climate Prediction Center.

The Sahara and the Sahel: significant sources of dust
The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18° - 22° N) and the northwestern Sahel (15° - 18° N) (Figure 2). The dust that originates in the Southwest Sahara stays relatively constant from year to year. However, the dust from the northwestern Sahel varies significantly from year to year, and 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's rainy season (June - October) 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. A dry rainy season the previous year will make an expanded area of loose soil which can create dust. It is also possible that the corresponding changes in vegetation can alter the regional weather patterns, causing more dust production.


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.

How dust suppresses hurricanes
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) off the coast of Africa, providing hurricanes with less energy to form and grow. Dust also affects the Saharan Air Layer (SAL), an 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. The dust may also act to interfere with the formation of cloud drops and rain drops that these thunderstorms need to grow, but little is known about such effects. It is possible that dust may act to help hurricanes by serving as "condensation nuclei"--centers around which raindrops can form and grow.

For additional reading
Dr. Evan published a study in Science magazine this March 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.

Jeff Masters

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Well thanks for the update Jeff Masters. I agree there will be less African outbreaks but they will be strong when they occur, but my reasoning was slight off.

Good Afternoon All, I'm currently awaiting to return home and it looks the Atlantic is very quiet with a massive area of convection in the EPAC. I will have an update Sunday Morning.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
Ike,
If you are lurking, how hot did it get at your house today? Mine said 99 Heat Index 110. lol.
Member Since: July 7, 2005 Posts: 0 Comments: 5417
Quoting Dr3w:


I dont want to ask a dumb question but
why did Bertha form in that area in early july last year?

I thought that never happens


Bertha was an anomaly. In 2008, dust levels were considerably below normal, especially in comparison to 2007 and 2006, both years of which saw immense levels of dust activity.

I would like to add that typically, if we see a hurricane develop in the deep tropics in July like we did with Bertha, that it is usually a precursor to a very active season.
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Link

ok let's try this.
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Quoting stormpetrol:
I would watch that little " spin" ENE of Roatan, Honduras also.
Wind in the Central Caribbean from ENE Western Caribbean from the E and the Yucatan channel from the ESE
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133. Dr3w

Quoting hurricane2009:
just happened drew, last year was an odd year, and sometimes you get storms that form that far out that early

oddly the last big storm that formed that far out that early in the season was also named Bertha in 1996, that Bertha hit NC as a cat 2 in mid-July


Wow
interesting
im goin to go look that up right now

thanks hurricane2009
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132. Dr3w
Quoting stormpetrol:
I would watch that little " spin" ENE of Roatan, Honduras also.


yeah

it looks fairly impressive on stellite

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Lotsa when & how many, not much where & how strong.

Anybody want to try?


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I would watch that little " spin" ENE of Roatan, Honduras also.
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128. Dr3w
Quoting hurricane2009:


Generally yes


I dont want to ask a dumb question but
why did Bertha form in that area in early july last year?

I thought that never happens
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I personally think 93E might be a TD already, JMO.
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125. Dr3w
Hurricane2009

the Cape Verde season begins in August right?
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Hello all...thought i'd throw in my thoughts..I am thinking around the July 1-7 time frame we will see a brief ramp up in activity mostly in invests and tropical disturbances popping up and maybe the first Tropical Storm of the season...just my opinion.

Out for now..may be back in later on.
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122. Dr3w
Quoting plywoodstatenative:
DR, the thing that worries me the most however is the continued heating of the Caribbean in the area of the Yucatan Channel. Past couple years we have not had that much form down in that region and pass through the channel. The lack of dust this year makes me rethink the idea that the CV season may start up a little late and with a bite.


I see what you mean

Id worry more about how much a storm will intensify if it moves into that area
rather than having a storm form in that area
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Lets enjoy the heat and the calm.
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Once one gets in and taps that Gulf, shear wont have a chance because storm will create its own weather and the hell with the experts, never fails
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Gonna be a helluva season folks, buckle up
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Quoting hurricane2009:
lol gator you have to work it into your posts

like a subliminal message, like this:

I really feel that 93E will be a (July 10th) tropical depression within the next (July 10th) 24 hours.


This is an excellent (July 10th) suggestion and one which i will (July 10th) be using. I will be on later (July 10th) im going to play Wii (July 10th) Punch Out!!
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Man its quiet in the tropics.....OH yea its still June thats why, ok that makes sense!
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DR, the thing that worries me the most however is the continued heating of the Caribbean in the area of the Yucatan Channel. Past couple years we have not had that much form down in that region and pass through the channel. The lack of dust this year makes me rethink the idea that the CV season may start up a little late and with a bite.
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111. Dr3w
Quoting hurricane2009:
also another point about this year, yes we know about El Nino, but we are getting conflicting reports of how strong it will be, so until we see what happens as the season goes on, there is no way of knowing what effect El Nino will have, if any at all.

Say El Nino does have some sort of an effect and we get say 9 named storms, but 7 of them form close to home and 3 make landfall in the US. Would that be a bust? I think not.


your exactly correct
we will just see what mother nature haas in store for us
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Quoting hahaguy:


Just because nothing has formed yet does not mean the season is a bust. May I remind you of 2004.


I would have hated to see this blog from June 1st to August 1st in 2004.
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Quoting Dr3w:


I'll be sure to hold you up to that lol

July 10th haha!
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107. Dr3w
Quoting gator23:
July 10th. All my posts from here till then will just say July 10th and if im wrong I will build a corn field to capture the crows and eat 10 of them for breakfast on July 11th


I'll be sure to hold you up to that lol
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July 10th. All my posts from here till then will just say July 10th and if im wrong I will build a corn field to capture the crows and eat 10 of them for breakfast on July 11th
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I dont know much about forecasting or anything like that but I enjoy reading all your predictions and comments.
All I want to say is this:

Snow is SE La in the winter + Extreme heat in June = nothing but trouble in the height of hurricane season.

Our friends and family are already preparing for what we feel will be a hit or close call this yr.
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102. Dr3w
Quoting gator23:


I will stick with the experts and say July 10th


as you should

im just saying that the Atlantic hurricane season is nothing but a guessing game
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Due to all this talk about the factors that go into a season. I just did a blog about the factors and what they look like in this season. Link
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Quoting Dr3w:


I think that feeling may be right
but its too early to tell


I will stick with the experts and say July 10th
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98. Dr3w
Quoting plywoodstatenative:
Something is bugging me about this season, its almost like the first two almost tropical systems were a teaser for us. I think the season is building up for a shock come August. My prediction will never be on the amount of storms or their seriousness. However it is on when we could see a ramp up of activity: Gut feeling on this is around the first week of August for a ramp up of activity.


I think that feeling may be right
but its too early to tell
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from the NWS
[In fact, the average first named storm in the Atlantic Basin (which includes the Gulf of Mexico and Caribbean) does not occur until July 10th]

CALM DOWN EVERYONE SHEESH!!!! You guys have just been spoiled in recent years!
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Japan Meteorological Agency
Tropical Cyclone Advisory #21
SEVERE TROPICAL STORM LINFA (T0903)
3:00 AM JST June 21 2009
====================================

Subject: Category Two Typhoon in South China Sea

At 18:00 PM UTC, Severe Tropical Storm Linfa (980 hPa) located at 21.8N 118.1E has 10 minute sustained winds of 55 knots with gusts of 80 knots. The storm is reported as moving north-northeast at 8 knots.

Dvorak Intensity: T4.0

Storm Force Winds
=================
40 NM from the center

Gale Force Winds
==================
160 NM from the center in southeastern quadrant
100 NM from the center in northwestern quadrant

Forecast and Intensity
=======================
24 HRS: 24.5N 119.1E - 45 knots (Tropical Storm/CAT 1)
48 HRS: 27.7N 120.8E - Tropical Depression

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Quoting Vortex95:
I'm begining to wonder if nothing forms by mid July if even 9 storms is generous.
I think a lot of it is a guessing game. 2005 had 2 TS form in June lasting a total of 7 days and the rest of the season had another 26 form. Remember they had to use the Greek alphabet that year ?
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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAKE CHARLES LA
328 PM CDT SAT JUN 20 2009

...HOT DRY CONDITIONS TO PERSIST FOR THE NEXT 7 DAYS...

.DISCUSSION...ONE WORD...PERSISTENCE. LATEST SFC ANALYSIS SHOWS
HIGH PRESSURE CENTERED OVER THE LOWER MISSISSIPPI VALLEY PROVIDING
THE FORECAST AREA WITH A LIGHT SRLY LOW-LEVEL FLOW. MEANWHILE
WATER VAPOR IMAGERY SHOWS BLOCKING RIDGE IN PLACE FROM THE SERN
CONUS UP THE PLAINS STATES TO THE CANADIAN PRAIRIES. RESULT IS YET
ANOTHER IN WHAT`S BECOME A LONG LINE OF HOT DRY DAYS OVER THE
REGION WITH JUST SCT HIGH-BASED CU SHOWING UP ON VISIBLE IMAGERY
AND SFC OBSERVATIONS.

NO BIG...IF ANY...CHANGES TO INHERITED FORECASTS. MID/UPPER-LEVEL
RIDGE REMAINS THE PRIMARY PLAYER IN THE REGIONAL WEATHER GAME
THROUGH THE PERIOD. RIDGE AXIS IS PROGGED TO DRIFT WWD WHILE
FLATTENING SOMEWHAT...HOWEVER SUFFICIENT WARM AIR ALOFT IS PROGGED
TO LINGER ENOUGH TO SQUASH ANY CONVECTIVE DEVELOPMENT THROUGH THE
COMING WEEK. AT THE SFC A WEAK FRONTAL BOUNDARY IS SHOWN DRIFTING
SWD INTO THE FORECAST AREA BY MID-WEEK BUT THIS FEATURE CERTAINLY
DOES NOT LOOK STRONG ENOUGH TO OVERCOME THE CAPPING. TEMPERATURES
WILL REMAIN WELL ABOVE NORMAL WITH AFTERNOON MAXES PUSHING THE
CENTURY MARK BY EARLY IN THE WEEK. HEAT INDEX VALUES ARE FORECAST
TO BE IN THE MID 100S...

ETC. ETC. ETC. Sigh. :)
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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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