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 IKE:
The oven's on bake in Florida.


I can't remember it being this hot in June! Thankfully the A/C at the job is working great!
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Thanks Keeper...

Im not sure I like the term "meander" lol.
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Quoting rainraingoaway:
Afternoon all. So what the heck is the blob in the BOC???? Certainly looks like something trying to form there.


Trying....but may not succeed. It has little time over water and the whole thing isn't even over water yet. It's mostly a mid-level feature what we're seeing right now. It's small which makes it something that could wind up fast and something to watch but it's not a significant development threat at this time.
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Member Since: July 15, 2006 Posts: 169 Comments: 53296
...DISCUSSION...

GULF OF MEXICO...
UPPER LEVEL DIFFLUENCE ASSOCIATED WITH AN UPPER LEVEL LOW
CENTERED OVER MEXICO NEAR 25N101W IS SUPPORTING NUMEROUS
SHOWERS/ THUNDERSTORMS OVER THE S GULF OF MEXICO S OF 23N W OF
93W. ACROSS THE N AND CENTRAL GULF...A 1012 MB STATIONARY
SURFACE HIGH NEAR 29N92W DOMINATES BRINGING FAIR WEATHER. THE
SURFACE HIGH HAS UPPER LEVEL SUPPORT FROM AN UPPER LEVEL HIGH
OVER S MISSISSIPPI NEAR 31N90W WHICH IS PROVIDING DEEP LAYER DRY
AIR AND SUBSIDENCE. ANTICYCLONIC FLOW AROUND THE SURFACE HIGH IS
AFFECTING MUCH OF THE GULF...WITH SE WINDS OF 10-15 KTS
AFFECTING THE NW GULF W OF 93W. EXPECT THE SURFACE HIGH TO MOVE
W ACROSS THE N GULF TOMORROW AND THEN MEANDER OVER THE WESTERN
WATERS WED.
Member Since: July 15, 2006 Posts: 169 Comments: 53296
Afternoon all. So what the heck is the blob in the BOC???? Certainly looks like something trying to form there.
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Wow, can this be right? It's from an airport station, not a PWS:

Thomaston, Georgia (Airport)
Updated: 25 min 18 sec ago
Clear
97 °F
Clear
Humidity: 89%
Dew Point: 93 °F

Wind: 9 mph from the North
Pressure: 29.83 in (Steady)
Heat Index: 155 °F


Did they set the station up over the sprinkler system or something?
Member Since: September 22, 2005 Posts: 18 Comments: 2317
934. IKE
BREAKING NEWS!

The sun just went behind something in the sky.....a cloud!
Member Since: June 9, 2005 Posts: 23 Comments: 37858
Melbourne, FL
95°F
Partly Sunny Real Feel: 105°F (41°C)
Relative Humidity: 50%
Barometer: 29.75"Hg (F)
Wind: WNW at 14 mph (23 kph)
Visibility: 10 miles (16 km)

Praying for some nice cooling rain!
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932. IKE
The oven's on bake in Florida.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
Quoting Chicklit:
What's with the 120 Heat Index?
I don't get that...
Daytona Beach (Ponce Inlet), FL,
Updated: 1 sec ago
96.8 °F
Clear
Humidity: 61%
Dew Point: 81 °F
Wind: 7.0 mphfrom the NW
Wind Gust: 11.0 mph
Pressure: 29.74 in (Falling)
Heat Index: 120 °F
Visibility: 10.0 miles
UV: 14 out of 16
Clouds: Clear -
(Above Ground Level)
Elevation: 12 ft




Heat index calculator.


96.8 F X 61% humidity= 119.7F heat index
Member Since: September 22, 2005 Posts: 18 Comments: 2317
930. IKE
Ah....nice....cool day outside....

"
Crestview, Florida (Airport)
Updated: 24 min 5 sec ago
Scattered Clouds
97 °F

Scattered Clouds
Humidity: 49%
Dew Point: 75 °F
Wind: 7 mph Variable
Pressure: 29.85 in (Falling)
Heat Index: 109 °F
Visibility: 9.0 miles
UV: 10 out of 16
Clouds:
Scattered Clouds 4500 ft
(Above Ground Level)
Elevation: 210 ft"
Member Since: June 9, 2005 Posts: 23 Comments: 37858
Tropical Tidbit for June 22nd with video
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Quoting Weather456:
The low level circulation of the BOC area remains over the Tehuantepec Peninsula. Should it continue north into the Bay, it could become something of interest.



exactly what I said earlier,the area should be over water(the strong area of convergence)in 6-10hrs and then we could see a invest in the BOC,IMO
Member Since: October 5, 2007 Posts: 20 Comments: 4970
Deer Run, Casselberry, Florida (PWS)
Updated: 1 sec ago
95.9 °F
Scattered Clouds
Humidity: 49%
Dew Point: 74 °F
Wind: 0.0 mphfrom the NNW
Wind Gust: 5.8 mph
Pressure: 29.71 in (Falling)
Heat Index: 107 °F
Visibility: 10.0 miles
UV: 12 out of 16
Clouds: Scattered Clouds 4600 ft
(Above Ground Level)
Elevation: 60 ft

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BOC area is looking good...i think it is worthy of an invest... if it had more time over water..i think it could really develop...
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Quoting Levi32:


It could get interesting and it's small which means it doesn't need much energy to get going but that doesn't necessarily mean development will be rapid. It's something to watch.


Wind shear is marginal too. Its definatly an area to watch.
Member Since: July 8, 2005 Posts: 259 Comments: 23627
Quoting canesrule1:
what is that with 850mb vorticity in the Gulf? looks like it needs to be monitored for rapid development


It could get interesting and it's small which means it doesn't need much energy to get going but that doesn't necessarily mean development will be rapid. It's something to watch.
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Quoting Levi32:


Bay of Campeche, the southern part of the Gulf of Mexico.
thanks a lot
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Quoting canesrule1:
i need help, what is BOC?
Bay of Campeche
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Quoting IKE:
I've got 100 degrees at my house.
here at my house in miami its 102 the index is over 120, i think you could fry some eggs on the sidewalk.
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Quoting canesrule1:
i need help, what is BOC?


Bay of Campeche, the southern part of the Gulf of Mexico.
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i need help, what is BOC?
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what is that with 850mb vorticity in the Gulf? looks like it needs to be monitored for rapid development
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916. IKE
I've got 100 degrees at my house.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
Member Since: July 8, 2005 Posts: 259 Comments: 23627
What's with the 120 Heat Index?
I don't get that...
Daytona Beach (Ponce Inlet), FL,
Updated: 1 sec ago
96.8 °F
Clear
Humidity: 61%
Dew Point: 81 °F
Wind: 7.0 mphfrom the NW
Wind Gust: 11.0 mph
Pressure: 29.74 in (Falling)
Heat Index: 120 °F
Visibility: 10.0 miles
UV: 14 out of 16
Clouds: Clear -
(Above Ground Level)
Elevation: 12 ft

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Wouldn't surprise me to see the bOC area mentioned in the TWO, it has some 850 MB vorcity already and it's holding its own in terms of convection.
Member Since: July 8, 2005 Posts: 259 Comments: 23627
Quoting Weather456:
Dust was noticeable below normal for June, which is a peak month for SAL, the other being July. Along with sea surface temperatures, the only thing that appears to be holding the season back is wind shear, and that will eventually slacken somewhere. The longer Ana stays to form, probably the stronger she will be. Ana maybe a out first named storm and hurricane.

Another thing I've notice, is that persons expectations for June is/was high, and even higher for July. I'm already sensing that persons have given up on the season already and we are weeks (2 months 1 week) away from the peak. That maybe 2009's weapon of choice. Don't become complacent.


Like Alex was a Category 3.
Member Since: July 8, 2005 Posts: 259 Comments: 23627
Andres Dvorak T-number history:

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Quoting StormW:
"Dust"



Hehe... plenty of it. Also noticed at the coast line... there's more coming out.
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Member Since: September 22, 2005 Posts: 18 Comments: 2317
Andres best opportunity to make a run for hurricane status is while he's east of 105W. Once he gets west of that line SSTs drop off rapidly and dip below 26C south of the Baha Peninsula.

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Quoting Weather456:
Dust was noticeable below normal for June, which is a peak month for SAL, the other being July. Along with sea surface temperatures, the only thing that appears to be holding the season back is wind shear, and that will eventually slacken somewhere. The longer Ana stays to form, probably the stronger she will be. Ana maybe a out first named storm and hurricane.

Another thing I've notice, is that persons expectations for June is/was high, and even higher for July. I'm already sensing that persons have given up on the season already and we are weeks (2 months 1 week) away from the peak. That maybe 2009's weapon of choice. Don't become complacent.


Exactly right, W456. Complacency is what gets people killed. I hope everyone uses the first few months of each season to refresh their imagery/model links, and do some learning about the Tropics. We have plenty of experience and knowledge here - use it.
Member Since: August 3, 2008 Posts: 16 Comments: 5875
Quoting jeffs713:


The GOM and area around the Bahamas certainly are toasty. If I remember correctly, wasn't the GOM below average just a month ago?


yes it was... the waters have been warming rapidly
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Quoting JRRP:
mmmmm


The GOM and area around the Bahamas certainly are toasty. If I remember correctly, wasn't the GOM below average just a month ago?
Member Since: August 3, 2008 Posts: 16 Comments: 5875
Dust was noticeable below normal for June, which is a peak month for SAL, the other being July. Along with sea surface temperatures, the only thing that appears to be holding the season back is wind shear, and that will eventually slacken somewhere. The longer Ana stays to form, probably the stronger she will be. Ana maybe a out first named storm and hurricane.

Another thing I've notice, is that persons expectations for June is/was high, and even higher for July. I'm already sensing that persons have given up on the season already and we are weeks (2 months 1 week) away from the peak. That maybe 2009's weapon of choice. Don't become complacent.
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891.

Indeed... a possible repeat of last weeks SRV WX event.
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901. JRRP
mmmmm

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Quoting futuremet:


Looking at the 12Z models, it seems that they have a hard time initializing this.


It has been for a while... except for the NOGAPS as we all know... but one thing that I've noticed is that GFS has been trending towards a more "clustered" disturb area in the W Carib when before it would have an ample area of moisture surging up and diverging in different directions... but on the last couple runs it has been trying to consolidate this "mess" in a more organized system. I guess we'll see if NOGAPS stills stubborn and if other models concur.
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Quoting AussieStorm:
Current SAL


That is a nice-sized SAL blob coming off of Africa...
Member Since: August 3, 2008 Posts: 16 Comments: 5875
The low level circulation of the BOC area remains over the Tehuantepec Peninsula. Should it continue north into the Bay, it could become something of interest.
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Good afternoon all.
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Quoting hurricane2009:
vorticity is actually better over mainland Mexico than the BOC, that should tell you something right there



That was 200MB vorticity
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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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