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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1244. IKE
Member Since: June 9, 2005 Posts: 23 Comments: 37858
1243. KEEPEROFTHEGATE (Mod)
Quoting Weather456:
Hot

and it will get hotter still got the hottest part of the season to come july and first have august
Member Since: July 15, 2006 Posts: 170 Comments: 53517
Quoting futuremet:
The area of interest at the GOM has less than 24hrs to get its acts together.


I don't believe less than 24 hrs base on the current motion.

IS EXPECTED TO BE SLOW TO
OCCUR AS THIS SYSTEM MOVES SLOWLY WEST-NORTHWESTWARD. THERE IS A
LOW CHANCE...LESS THAN 30 PERCENT...OF THIS SYSTEM BECOMING A
TROPICAL CYCLONE DURING THE NEXT 48 HOURS. LOCALLY HEAVY RAINFALL
IS POSSIBLE OVER AREAS OF EAST-CENTRAL AND NORTHEASTERN MEXICO
DURING THE NEXT COUPLE OF DAYS.
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1241. KEEPEROFTHEGATE (Mod)
i wonder if levi felt the earth move
Member Since: July 15, 2006 Posts: 170 Comments: 53517
Hot

Member Since: Posts: Comments:
Cape values in florida between 3 and 5

x10^3...

LOL Little steamy today eh?

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Quoting conchygirl:
Alaska jolted by small earthquake (5.3).



thats not small thats big
Member Since: May 21, 2006 Posts: 5091 Comments: 114919
The area of interest at the GOM has less than 24hrs to get its acts together.
Member Since: July 19, 2008 Posts: 43 Comments: 4051
1235. KEEPEROFTHEGATE (Mod)
Quoting hurricane2009:
according to the discussion the NHC says Andres is continuing to get better organized
yep and once the sun goes down it will become a cane by midnight or shortly there after but not for a long period maybe till early thurs morning then it will begin to weaken
Member Since: July 15, 2006 Posts: 170 Comments: 53517
Despite all the heat, I still a thousand times prefer it than the cold!
Member Since: July 19, 2008 Posts: 43 Comments: 4051
Has anybody tried to fry eggs today? Get a black pan and put it on the cement.. add an egg..
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Quoting Levi32:
Well here comes the first thunderstorms down through Georgia. They're thinking the instability under the jetstream on the eastern side of the upper high will be enough to get something going. I have my doubts about south Florida though as they are in a region of strong upper convergence on the SE quadrant of the high.





The amplifying shortwave should be capable of breaking the cap by early tomorrow morning. I expect high chances of rain tomorrow for FL.
Member Since: July 19, 2008 Posts: 43 Comments: 4051
Quoting hurricane23:
The 200 mb. winds west of Central America have been east and NE over there so far.The reason for the 200 mb shear has nothing to do with EL Nino. Hence , more likely to change as time goes by. The 384 hr. 200 prog. has easterlies over much of the Atlantic Basin.

SST's are never a problem from late July until early October; except that if they are cooler than normal, the hurricanes that do form don't have quite the potential for Cat. #5 intensity, as they do when the SST are above normal.

My gut feeling is for a season in the normal range of named storms. But, it is that period from August 20th thru Sept. 2Oth. that is critical. If a ridge just happens to set up in the Western Atlantic at 500 mb during that period, there will likely be landfalls. If a trough sets up on or just off the east coast, any storms will not make landfall, except in the West Gulf. Just plain luck and chance. The long wave position is always changing, and rarely stays in one place more than 10-14 days.

Than again, as ive noted in previous times if the pattern just sets up right for a few days when an Andrew comes along, **** will hit the fan.S. Florida has had a goodly number of major hurricanes in slow years.

Adrian



Agreed Adrian, I have a strange feeling about this year similar to what I had last year for Texas area that it's Florida's turn again. See those SST's off the South Florida coast? Those are right near were Andrew bombed out and imagine how it's going to look come August. Andrew went from a disorganzed mess of a Tropical Storm to a Category 5 in 48 hours. If the wind shear's low enough, and the high's in the right position we could be in serious trouble. This is all though, just a gut feeling. It just doesn't feel good for me, I think Ike last year when Dr. Lyons said that it's very possible that Ike would hit South Florida as a Category 5 might be a foreshadowing of this year. No one freak out that I said this, its all a gut feeling.
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Alaska jolted by small earthquake (5.3).
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The Florida peninsula will be rain cooled tomorrow as multi-cyclogenesis forms along the splitting trough. You can clearly see that upper level counter clockwise spin at the 500mb heights.

NAM 18Z
Member Since: July 19, 2008 Posts: 43 Comments: 4051
Quoting IKE:


Ready for an afternoon run in the sun?


Maybe another day, that 20ft walk to the van to go get milk did me in! Sweating like I jogged a mile or two, phew...
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1225. Levi32
Member Since: Posts: Comments:
The 200 mb. winds west of Central America have been east and NE over there so far.The reason for the 200 mb shear has nothing to do with EL Nino. Hence , more likely to change as time goes by. The 384 hr. 200 prog. has easterlies over much of the Atlantic Basin.

SST's are never a problem from late July until early October; except that if they are cooler than normal, the hurricanes that do form don't have quite the potential for Cat. #5 intensity, as they do when the SST are above normal.

My gut feeling is for a season in the normal range of named storms. But, it is that period from August 20th thru Sept. 2Oth. that is critical. If a ridge just happens to set up in the Western Atlantic at 500 mb during that period, there will likely be landfalls. If a trough sets up on or just off the east coast, any storms will not make landfall, except in the West Gulf. Just plain luck and chance. The long wave position is always changing, and rarely stays in one place more than 10-14 days.

Than again, as ive noted in previous times if the pattern just sets up right for a few days when an Andrew comes along, **** will hit the fan.S. Florida has had a goodly number of major hurricanes in slow years.

Adrian

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1219. IKE
Quoting WPBWeatherBoy:
Coorough Backyard, West Palm Beach, Florida (PWS)
Updated: 3 min 1 sec ago
100.8 °F
Partly Cloudy
Humidity: 45%
Dew Point: 76 °F
Wind: 6.0 mph from the West
Wind Gust: 6.0 mph
Pressure: 29.70 in (Falling)
Heat Index: 116 °F

Are PWS's accurate?, they always display higher temperatures than ones displayed at the airport.


Some are.

I trust the airports. PWS's...pick and choose.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
1218. IKE
Quoting conchygirl:
Hubby calls me a CRAZY woman. It is much warmer today though so we shall see!


Well.....if you never post again after today, we'll know it was a rough run.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
1217. NEwxguy
Quoting conchygirl:
That would be this stupid person here. Going out around 6 pm, but fully hydrated (I hope). I know, I know...LOL


You got me sitting trying to imagine running in that weather down there,but can't too painful.
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Quoting druseljic:
Ok, this is just crazy...

Pensacola 103.6 F
Heat Index 123 F

enough already!!!!


I get overheated just looking outside .... so I don't now :)
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1215. 0741
fort laudale report temp of 100 at 4pm
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Coorough Backyard, West Palm Beach, Florida (PWS)
Updated: 3 min 1 sec ago
100.8 °F
Partly Cloudy
Humidity: 45%
Dew Point: 76 °F
Wind: 6.0 mph from the West
Wind Gust: 6.0 mph
Pressure: 29.70 in (Falling)
Heat Index: 116 °F

Are PWS's accurate?, they always display higher temperatures than ones displayed at the airport.
Member Since: Posts: Comments:
1212. Levi32
Well here comes the first thunderstorms down through Georgia. They're thinking the instability under the jetstream on the eastern side of the upper high will be enough to get something going. I have my doubts about south Florida though as they are in a region of strong upper convergence on the SE quadrant of the high.



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1210. IKE
8-14 day precip....

Member Since: June 9, 2005 Posts: 23 Comments: 37858
Quoting IKE:


R U kidding me? OMG......
Hubby calls me a CRAZY woman. It is much warmer today though so we shall see!
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1208. IKE
Quoting conchygirl:
That would be this stupid person here. Going out around 6 pm, but fully hydrated (I hope). I know, I know...LOL


R U kidding me? OMG......
Member Since: June 9, 2005 Posts: 23 Comments: 37858
1207. IKE
6-10 day outlook shows the heat retreating slightly to the north.....

Member Since: June 9, 2005 Posts: 23 Comments: 37858
1206. Levi32
Quoting futuremet:


Yup, it will definitely take a lot to break this capping inversion, especially at the southern plains. The anticyclone itself is not that potent compared to the perpetual ones at the eastern subtropical Atlantic.


Well the southern plains have like absolutely no chance for convection lol. They're under the center of deep-layer high pressure. The atmosphere is very stable in that area right now.
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Quoting IKE:


Ready for an afternoon run in the sun?
That would be this stupid person here. Going out around 6 pm, but fully hydrated (I hope). I know, I know...LOL
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1204. pottery
1169, Jeffs.
LOL, it took me a while to figure out what it was, this cool damp stuff, falling in a gentle Hissss.
But on closer examination, and on checking with the Specialists here, I am pretty sure it is that elusive form of H2o, commonly referred to as "Rain".
I could be wrong though.....
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1202. IKE
Member Since: June 9, 2005 Posts: 23 Comments: 37858
While the TCHP system does serve a purpose, it is somewhat flawed. An open ocean with an actual thermocline compared to a shallow area, like the gomex, where the thermocline is just the ocean floor with equal TCHP ratings do not affect tropical systems the same. In the open ocean cold water upwhellings caused by storms cool the ocean water and the effective SSTs far faster than in a shallow area where the water at the bottom that is pulled up is almost as warm as the top.

Plus TCHP ratings are overrated. We've all seen numerous storms that have rapidly intensified in very low TCHP water to a major.
Member Since: Posts: Comments:
1199. IKE
Quoting druseljic:
Ok, this is just crazy...

Pensacola 103.6 F
Heat Index 123 F

enough already!!!!


Ready for an afternoon run in the sun?
Member Since: June 9, 2005 Posts: 23 Comments: 37858
Quoting Levi32:


I think they expected the frontal boundary to help break the cap, and it still may as H2009 said we have all evening to wait for it. There is a lot of upper-level convergence though.....lots of subsidence not conducive for rising air. It would take a lot of heat (which we have lol) to break this cap.



Yup, it will definitely take a lot to break this capping inversion, especially at the southern plains. The anticyclone itself is not that potent compared to the perpetual ones at the eastern subtropical Atlantic.
Member Since: July 19, 2008 Posts: 43 Comments: 4051
1197. Levi32
High-resolution visible loop of Andres and the Bay of Campeche. Andres does not appear to have strengthened since this morning, and is having difficulty getting its core going. Severely limited outflow to the north due to an upper-level low over Mexico is probably part of the problem.
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Ok, this is just crazy...

Pensacola 103.6 F
Heat Index 123 F

enough already!!!!
Member Since: Posts: Comments:
Quoting jeffs713:


Think of it this way...
You have a one-meter cube.
If you will it to the brim with warm (85F) water, it will have more heat content than if you only fill the first 30cm. It takes a lot more energy to heat the full cube of water, than it does to heat the partially full cube. Also, the heat given off by the surface (1 square meter) of the full cube will be greater than the heat given off by the surface (1 square meter) of the partially full cube.


lol your right what your saying, but I dont believe in that system. Just dont think hurricanes are going to notice that kind of difference.
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1194. Levi32
Quoting hurricanelover236:
Does anyone else agree with me that the severe storms in florida today were way overexagerated. They said it would be severe sotmrs and the radar is incredibly clear with not even a shower. Can someone explain how their forecasting can be so off?


I think they expected the frontal boundary to help break the cap, and it still may as H2009 said we have all evening to wait for it. There is a lot of upper-level convergence though.....lots of subsidence not conducive for rising air. It would take a lot of heat (which we have lol) to break this cap.

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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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