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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Quoting weathersp:
For all you Floridians... hows the water temp?

Are TCHP indices like through the roof in coastal waters?

TCHP won't be that high in coastal water, since the thermocline = the bottom. If the water is only 50-75m deep, it doesn't have a huge depth of warmth to work with.
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1143. Levi32
Be back later.
Member Since: November 24, 2005 Posts: 635 Comments: 26601
We are now entering Neutral ENSO conditions (per ONI values) with the MEI now +0.344.
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1141. KEEPEROFTHEGATE (Mod)


a little puffer is poofing
Member Since: July 15, 2006 Posts: 170 Comments: 53553
1139. Levi32
Quoting jeffs713:


Remind me about this day in about 6 or 7 months.


In 6 or 7 months I'll tell you it's -10 degrees and you'll say you're so glad you live there and not here! Lol
Member Since: November 24, 2005 Posts: 635 Comments: 26601
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1135. NEwxguy
Quoting sporteguy03:
A heat wave is considered 3 consecutive days 90 or above correct? I know when I was living in Southern New England that was a big deal, does Florida not consider it a heat wave? Since the forecast goes down to 90-92 the rest of the week that still fits heat wave criteria right?


This year,I consider it a heatwave if we get 3 consecutive days hitting 80
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Quoting weathersp:
For all you Floridians... hows the water temp?

Are TCHP indices like through the roof in coastal waters?


85 degrees off the Cape and Volusia County that is mighty toasty.
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Quoting Levi32:


It's a reactive El Nino. This is how I explained it to someone a week ago:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The ENSO is an atmospheric-oceanic relationship. A reactive El Nino means that the ocean is reacting to what the atmosphere is doing. A perfect demonstration of this is happening right now. The MJO is starting to bring another round of downward motion into the West Pacific. All that sinking air spreads outward at the ocean surface, creating high pressure.

Similarly, the MJO upward motion pulse is now over the east Pacific, and all that rising air is creating low pressure at the surface. Since air naturally flows from high to low pressure, you get increased westerlies across the equatorial Pacific. This also lowers the SOI.

This is the classic atmospheric signature of El Nino, and the ocean reacts because the westerlies push warm water eastward usually in the form of Kelvin Waves and pile it up in the central/eastern Equatorial Pacific.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As for how strong it will get, that's the unpredictable part, but it will probably be a moderate event. I hope I made at least some sense lol.

Well said, thanks Levi. Yes, I understand better now.
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Quoting Levi32:
LOL it's 48 degrees outside here.

68 inside..


Remind me about this day in about 6 or 7 months.
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I think the BOC system has a good chance of developing.

I think it will be an invest tonight or tommorow.

Also, I give it a 40-60% chance of becoming a TD in the next 48 hours.
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thanks Levi :)
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For all you Floridians... hows the water temp?

Are TCHP indices like through the roof in coastal waters?
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Link

Wow 108 degrees near me, I checked all the PWS in my area can't find any below 96. This is like 1998 heat in FL.
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1125. Levi32
Quoting SpicyAngel1072:
Hey StormW can you explain to me what the Indian ocean has to do with out hurricane season?

(I'm clueless and trying to learn)


I hope StormW doesn't mind me trying to answer this....I know he kind of pops in and out on a busy schedule.

Our hurricane season is affected by El Nino, and one of the signs that we are entering an El Nino episode is as StormW mentioned, the lack of convection in the Indian Ocean. That's due to all MJO downward motion pulse that has been sitting over there for weeks now shutting down the monsoon that usually creates lots of rain and convection in the north Indian Ocean.

Basically the Indian Ocean is just one of the indicators when looking at El Nino, and in turn has an impact on our hurricane season.
Member Since: November 24, 2005 Posts: 635 Comments: 26601
1122. IKE
Quoting Acemmett90:

florida is in a continuous heat wave
this, this is insanity


LOL.....maybe you need a cool glass of water.
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I'd watch out this season if your in the southeast.This map from jim williams (hurricanecity.com) is a rough draft showing what areas are hit in slow versus busy seasons. If it is a slow season and your area does not get hit if you are in the circled or pink areas consider yourself lucky.
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Quoting hurricane2009:
102 degrees on my thermometer outside


I knew my thermometer was right it said 102 as well in the shade!
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Thanks for the response Levi!
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A heat wave is considered 3 consecutive days 90 or above correct? I know when I was living in Southern New England that was a big deal, does Florida not consider it a heat wave? Since the forecast goes down to 90-92 the rest of the week that still fits heat wave criteria right?
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1115. Levi32
The blob in the Bay of Campeche is showing itself to be very unorganized. The mid-level center is displaced to the north with low-level vorticity max to the south. This system probably won't have enough time over water to get organized.

Member Since: November 24, 2005 Posts: 635 Comments: 26601
1114. IKE
Andres is popping an eye....look for an upgrade soon.
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im jealous Levi
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1112. IKE
Quoting Levi32:
LOL it's 48 degrees outside here.

68 inside..


LOL...I hate you.....
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1111. Levi32
LOL it's 48 degrees outside here.

68 inside..
Member Since: November 24, 2005 Posts: 635 Comments: 26601
BOC disturbance might have a small chance to organize if it stays over water.Andres not to far away is not helping its cause.
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Hey StormW can you explain to me what the Indian ocean has to do with out hurricane season?

(I'm clueless and trying to learn)
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1108. Levi32
Quoting druseljic:
Maybe a silly question, but I come here to learn so here goes... everyone is posting about the extreme heat in the SE. Is there any corrolation to the forming El Nino? I know it can drastically change weather patterns but obviously don't know as well as some of those here. Any enlightenment is appreciated!


In a sense yes because of the MJO. The current phase that it's in supports a strong ridge over the southern US, and obviously we have a monster ridge there right now. Generally though the ENSO impacts southeastern US weather more in the winter time and not so much in the summer, with sometimes the exception of hurricane landfalls.
Member Since: November 24, 2005 Posts: 635 Comments: 26601
I've seen it this hot before in Houston (September 2000 was actually hotter... stayed above 100 for 11 days), but I've never seen the heat last nearly a month, like this heat wave is. The Houston area hasn't had a high below 95 since the last week of May.
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Heatwave wont be around for very long.Strong Instability will lead to some severe storms at times this week.
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Yumm... tasty heat...

Grogan's Mill, The Woodlands, Texas (PWS)
Updated: 1 sec ago
98.1 °F
Scattered Clouds
Humidity: 45%
Dew Point: 76 °F
Wind: 3.0 mphfrom the NNW
Wind Gust: 4.0 mph
Pressure: 29.84 in (Falling)
Heat Index: 113 °F
Visibility: 10.0 miles
UV: 2.9 out of 16
Clouds: Scattered Clouds 5000 ft
(Above Ground Level)
Elevation: 139 ft

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1103. IKE
Quoting jeffs713:


4/70 air conditioning is good, along with a lot of water. 4 windows down, 70 mph.


You are darn right. I can take it, unless I'm stopped.

I'm 51...I've lived here about all my life. I've never seen it this hot, this long.
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Strong looking Tropical wave approaching Barbados and The Windward Islands.
Member Since: April 29, 2007 Posts: 0 Comments: 7833
Point Forecast: 5 Miles S Crawfordville FL
A Few Clouds
101 °F(38 °C) Humidity: 32 %
Wind Speed: N 14 G 23 MPH
Barometer: 29.77" (1007.9 mb)
Dewpoint: 66 °F (19 °C)
Heat Index: 105 °F (41 °C)
Visibility: 10.00 mi.
More Local Wx: 3 Day Histo

Just stepped outside to get something out of the car and throught of the Wicked Witch on the Wizard of Oz...."I'm melting...I'm melting...."
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Quoting Levi32:
I feel so sorry for you guys....



It reached a record temperature of 100 degrees today inside my house lol =]
Member Since: July 19, 2008 Posts: 43 Comments: 4051
Maybe a silly question, but I come here to learn so here goes... everyone is posting about the extreme heat in the SE. Is there any corrolation to the forming El Nino? I know it can drastically change weather patterns but obviously don't know as well as some of those here. Any enlightenment is appreciated!
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1098. IKE
"Milton, Florida (Airport)
Updated: 33 min 51 sec ago
Clear
101 °F

Clear
Humidity: 34%
Dew Point: 68 °F
Wind: 7 mph from the NNW
Pressure: 29.76 in (Falling)
Heat Index: 107 °F
Visibility: 10.0 miles
UV: 13 out of 16
Clouds:
Clear -
(Above Ground Level)
Elevation: 177 ft"
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Quoting IKE:


Try going to town in a car with no AC working.

What's real fun is getting in the car AFTER stopping somewhere and the car has been on that hot asphalt.....it must be 120-130 in my car....



4/70 air conditioning is good, along with a lot of water. 4 windows down, 70 mph.
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1096. Levi32
Quoting StormW:


Nice job Levi.

Also, one of the tell tale signs of this oncoming El Nino is the lack of storms, or basically a shutting down of the Indian Ocean...not too much convection this season.


Yeah the shut-down of the Indian Monsoon really kick-started the whole process. It was interesting to watch how little convection there was in the Indian basin.
Member Since: November 24, 2005 Posts: 635 Comments: 26601
1094. IKE
That Crestview,FL. temp I posted of 101 is a record for the date.
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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.