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 vortfix:
Baroclinic low pressure centers are extremely difficult to classify either way "before" they actually develop Levi.
None of the models are even close to accurate in that regard.

After all my years dealing with the tropics I have found that possible baroclinic features need to be monitored closely as it is never a sure bet either way.

That BOC Blob should be history by 36 hours from what I see.





See?!?!?!?!?! Straight forward, easy to follow, uses common English words in a simple enough fashion that even I can follow it....

That's what I'm talkin' 'bout!!!!!!!!!!
Member Since: August 13, 2007 Posts: 0 Comments: 10479

If you can't read the writing the advisory says winds are now 60mph and minimum central pressure is 995mb
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Quoting CybrTeddy:


Do you have some sort of magic crystal ball to say the BOC blob should be gone in 36 hours? Rofl. The NHC doesn't seem to take notice of the low off the Carolinas.

Seems to me, a crystal ball isnt needed. The whole point of the blog is to state your case, your ideas, your thoughts. Nothing is a given.
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What I'd like to see is that high pressure get west of me and then have a blob (like we have now in the BOC) in the central Gulf get plowed WNW right into TX dumping copious amounts of rain to stop the heat and drought.
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It looks like Andres' outflow is being severely limited to the north by an upper low over Mexico.

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My crystal ball says not a damn thing gonna happen anywhere this week, so how about that
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Do you have some sort of magic crystal ball to say the BOC blob should be gone in 36 hours? Rofl. The NHC doesn't seem to take notice of the low off the Carolinas.


My Magic 8 Ball is what I rely on this time of year!
LOL

The NHC and I rarely agree on these potential baroclinic spin-ups.

Member Since: October 29, 2007 Posts: 135 Comments: 46068
Quoting CybrTeddy:


Do you have some sort of magic crystal ball to say the BOC blob should be gone in 36 hours? Rofl. The NHC doesn't seem to take notice of the low off the Carolinas.


Because it's cold-core lol =)
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Quoting RitaEvac:
Drak what is the link to that Sat loop you posted?

Link
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The BOC blob is the best looking blob so far this season but a little too much sheer right now and it is being squeezed between Mexico to the left and the high pressure area to it's right with not enough room to "breathe" so to speak......Kind of reminds me of the Dr. John tune; "I was in the Right place, but, It must have the wrong time"...
Member Since: August 8, 2006 Posts: 0 Comments: 8782
Quoting vortfix:
Baroclinic low pressure centers are extremely difficult to classify either way "before" they actually develop Levi.
None of the models are even close to accurate in that regard.

After all my years dealing with the tropics I have found that possible baroclinic features need to be monitored closely as it is never a sure bet either way.

That BOC Blob should be history by 36 hours from what I see.




Do you have some sort of magic crystal ball to say the BOC blob should be gone in 36 hours? Rofl. The NHC doesn't seem to take notice of the low off the Carolinas.
Member Since: Posts: Comments:
Quoting vortfix:
Baroclinic low pressure centers are extremely difficult to classify either way "before" they actually develop Levi.
None of the models are even close to accurate in that regard.

After all my years dealing with the tropics I have found that possible baroclinic features need to be monitored closely as it is never a sure bet either way.

That BOC Blob should be history by 36 hours from what I see.




Vortfix i agree with you strongly
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Quoting Levi32:


Sure lots of home-brew systems start off baroclinicly, but IMHO this isn't one of them. I'm sitting here looking at the upper trough in a couple days and it's really sticking it's foot down into the Bahamas and if you were gonna get development it would need to leave the low behind while the baroclinic zone moves out. That's not going to happen here the trough is too deep and the low will be picked up. The piece that gets left behind is the trough split in the Gulf of Mexico where a surface low may form along the north gulf coast and drift SW.

Anyway....that's just my take.


i have to strongly agree with Vortfix
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Drak what is the link to that Sat loop you posted?
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Quoting vortfix:
Baroclinic low pressure centers are extremely difficult to classify either way "before" they actually develop Levi.
None of the models are even close to accurate in that regard.

After all my years dealing with the tropics I have found that possible baroclinic features need to be monitored closely as it is never a sure bet either way.



Sure lots of home-brew systems start off baroclinicly, but IMHO this isn't one of them. I'm sitting here looking at the upper trough in a couple days and it's really sticking it's foot down into the Bahamas and if you were gonna get development it would need to leave the low behind while the baroclinic zone moves out. That's not going to happen here the trough is too deep and the low will be picked up. The piece that gets left behind is the trough split in the Gulf of Mexico where a surface low may form along the north gulf coast and drift SW.

Anyway....that's just my take.
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Think we will see any development out of it Drak?
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There is some 850mb rotation, which shows up nicely on the cimss 850mb vorticity product and the surface observations support broad low level cyclonic flow in the BOC. Shear is around 10-15 knots and is forecasted to lower to 5-10 knots as the deep layered ridge axis sets up. Low level steering currents indicate a movement to the WNW or NW to be expected around the surface anticyclonic flow of a high pressure center over Louisiana.

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Which means we will continue to cook and burn the soil moisture content.
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Baroclinic low pressure centers are extremely difficult to classify either way "before" they actually develop Levi.
None of the models are even close to accurate in that regard.

After all my years dealing with the tropics I have found that possible baroclinic features need to be monitored closely as it is never a sure bet either way.

That BOC Blob should be history by 36 hours from what I see.


Member Since: October 29, 2007 Posts: 135 Comments: 46068
High pressure over the Gulf states is going to steam roll that sucker into Mexico, I don't see any rain chances coming from this for TX, not until the high gets west of us this coming weekend which will begin to pump more tropical moisture into the state.
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Quoting hurricane2009:
I think we are seeing the starts of some rotation with that area in the BOC


Vort 850 MB confirms that, We need a quicksat though to see if there is any type of circulation. I would though watch this area, it could be 93L later tonight
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Quoting hurricane2009:
I think we are seeing the starts of some rotation with that area in the BOC


a nice mid level rotation
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969. IKE
Member Since: June 9, 2005 Posts: 23 Comments: 37858
Heat index here at 12:30 was 125. I'm a mile from the Gulf. If the coastal areas are cooler I don't want to be inland today!
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the AOI in the BOC has TD force winds as seen from the QuikSCAT but they are rain contaminated.
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It may be gone by tommm
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Quoting reedzone:
I think the NHC made a smart decision. Should be 93L later on tonight.


If it were to be an invest it would be tomorrow.
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Quoting vortfix:
The low pressure near the Bahamas is going to be cold-core.


Oh...I see...and you have an infallable tropical crystal ball that guarantees that??

LMAO.



Lol no but I explain this in my video and blog. It's in the middle of a baroclinic zone. This transports heat instead of consolidating it. That's the basis of a cold-core system.
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The low pressure near the Bahamas is going to be cold-core.


Oh...I see...and you have an infallable tropical crystal ball that guarantees that??

LMAO.

Member Since: October 29, 2007 Posts: 135 Comments: 46068
I think the NHC made a smart decision. Should be 93L later on tonight.
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Quoting vortfix:
I just wish we could get some of that rain from the BOC area up into south/central Texas.
Bad drought situation there.

As far as any short term development is concerned...I will be watching around the Northern Bahamas the next couple days.



The low pressure near the Bahamas is going to be cold-core.
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Quoting IKE:


There's a 10...occasionally 15 mph breeze blowing outside today. Scattered clouds......

I've got 99.5 degrees.



98 with a heat index of 108 down here along the coast! So much for the GOM waters keep us cooler than places inland!
Member Since: August 25, 2006 Posts: 1 Comments: 3010
I just wish we could get some of that rain from the BOC area up into south/central Texas.
Bad drought situation there.

As far as any short term development is concerned...I will be watching around the Northern Bahamas the next couple days.

Member Since: October 29, 2007 Posts: 135 Comments: 46068
Interesting.

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

SHOWERS AND THUNDERSTORMS OVER THE SOUTHWESTERN GULF OF MEXICO AND
BAY OF CAMPECHE ARE ASSOCIATED WITH A WEAK SURFACE TROUGH.
DEVELOPMENT OF THIS SYSTEM...IF ANY...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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I just looked at the heat index here in Jesen Beach, Fl, 123 dgrees. I cannot think of the last time I've seen that.
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Now the TWO mentions it, its something to watch throughout the day. I give it a 40% chance of development in the next 48 hours.
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953. IKE
Airport about 90 miles from the COC....

"Ixtapa Zihuatanejo, MX (Airport)
Updated: 54 min 12 sec ago
Overcast
81 °F
Overcast
Humidity: 84%
Dew Point: 75 °F
Wind: 9 mph from the ESE
Pressure: 29.64 in (Falling)
Heat Index: 86 °F
Visibility: 8.0 miles
UV: 3 out of 16
Clouds:
Mostly Cloudy 2000 ft
Mostly Cloudy 8000 ft
Overcast 25000 ft
(Above Ground Level)
Elevation: 20 ft"
Member Since: June 9, 2005 Posts: 23 Comments: 37858
Quoting KEEPEROFTHEGATE:
...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.


Yes, it says the surface high is to move West across the GOM! That should open the door for a better chance of showers and bring some cooler air to the Florida Panhandle. The extended forecast is starting to show this change!!!
Member Since: August 25, 2006 Posts: 1 Comments: 3010
949. IKE
Quoting Levi32:
I still think the BOC should be mentioned in the tropical discussion....small systems like this can wind up fast like Marco in 2005 on short notice. It's not that hard to give it a less than 30% chance for development, which is what I give it.


Ask and ye shall receive.

There is something to it.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
Quoting vortfix:
So what the heck is the blob in the BOC???? Certainly looks like something trying to form there.


It's a Blob in the BOC...that's it.



Not really, its energy being transfered from the EPAC to the Atlantic, plus it has 850 MB vort and its maintaining convection throughout the day.
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947. IKE
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
200 PM EDT MON JUN 22 2009

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

SHOWERS AND THUNDERSTORMS OVER THE SOUTHWESTERN GULF OF MEXICO AND
BAY OF CAMPECHE ARE ASSOCIATED WITH A WEAK SURFACE TROUGH.
DEVELOPMENT OF THIS SYSTEM...IF ANY...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.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.

$$
FORECASTER BLAKE/BRENNAN
Member Since: June 9, 2005 Posts: 23 Comments: 37858
946. IKE
Quoting 69Viking:


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


There's a 10...occasionally 15 mph breeze blowing outside today. Scattered clouds......

I've got 99.5 degrees.

Member Since: June 9, 2005 Posts: 23 Comments: 37858
I still think the BOC should be mentioned in the tropical discussion....small systems like this can wind up fast like Marco in 2005 on short notice. It's not that hard to give it a less than 30% chance for development, which is what I give it.
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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!
Member Since: August 25, 2006 Posts: 1 Comments: 3010

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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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