Dust forecast for the 2009 Atlantic hurricane season

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

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

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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Experiencing the calm before the
storm here on the mid-Pacific coast this morning. Andres is still at tropical storm level, slowly churning up the coast toward us. We're at approx. 19N/105W so watching very closely. Nerve-wracking, to say the least!
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Tampa is under a severe t-storm watch. I'm just not sure where the storms will come from right now. The storms already out there will go well to our south.
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Quoting StormFreakyisher:
Man this is awfully sad.The City of Lake Worth lost power from this storm.FPL always said they are ready for this hurricane season and they "strengthened" power lines.Ha yeah right!
i remember they said the titanic was unsinkable and we know how that turned out
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Man this is awfully sad.The City of Lake Worth lost power from this storm.FPL always said they are ready for this hurricane season and they "strengthened" power lines.Ha yeah right!
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with shear forecast to relax in the western caibbean then the tropical wave in the vicinity could be the trigger for something to evolve the next few days
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Thank you Storm for the information.

Quoting StormW:
This is for Chicklit from yesterday evening, or whoever wants the info:

This may help put into perspective what Levi is telling you:

This first site, you'll see orange and green...this represents the downward and upward motion of the MJO. Orange being downward motion or "sinking air", and green showing upward motion or "rising air" As Levi said, wind flows from higher to lower pressure. Where you have sinking air, pressures tend to be higher, and when the air hits the surface, it spreads out and heads toward lower pressure. So, with the orange over India and Australia, etc, air is sinking and spreading out, and heads east toward the green over South America and GOMEX. As this air sinks BTW, it compresses, and warms. This has a tendency to dry out the atmosphere, not much unlike the ridge we've been dealing with in our area. So, that's kinda what's affected the Indian Monsoon. This creates a westerly wind that helps push the warm water in the WPAC over toward the EPAC. When the MJO pulse is strong enough downward, it pushes a wave of warm water in the subsurface, known as a Kelvin Wave, eastward. This wave sloshes back and up from the coast of South America, and that's where we see the warm tongue that extends from South America, westward. It is the same westerly wind that helps to increase wind shear in the Atlantic Basin.

Where you see green, that is upward motion, and generally where you will see increased convective activity, as this aids in providing "lift" to the atmosphere. Think of it generally as no lift, no clouds.


The next tool is the SOI (Southern Oscillation Index. By looking at the graph, you can basicaly tell which direction the winds in the PAC are are coming from...when this graph is in the negative, like it is now, this indicates some hefty westerly (direction the wind is coming from) winds coming from the WPAC. When this is up in the positive, the trades begin to blow in the opposite direction (from South America and across toward the WPAC.

As it goes from the positive toward zero, this is an indication that the easterly tardes are weakening, and vice versa.

The graph is derived from the difference in the MSLP of Darwin Australia, and Tahiti. When pressures are higher over Darwin, and lower over Tahiti, the SOI heads for the negative, and the opposite occurs when the pressure differences are reversed.


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1635. IKE
I see why we have a chance today...trough is digging down into central Alabama and heading towards the south....
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1633. lavinia

Thank you very much StormW. That helped me immensely.
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UW-CIMSS Experimental Vertical Shear and TC Intensity Trend Estimates

Current Conditions (from TPC) :
Latitude : 16:50:48 N
Longitude : 103:08:46 W
Intensity (MSLP) : 990.5 hPa

Max Pot Int (MPI,from Emanuel) : 905.1 hPa
MPI differential (MSLP-MPI) : 85.4 hPa

CIMSS Vertical Shear Magnitude : 13.1 m/s
Direction : 66.8 deg

Outlook for TC Intensification Based on Current
Env. Shear Values and MPI Differential
Forecast Interval : 6hr 12hr 18hr 24hr

Legend : VF-Very Favorable F-Favorable N-Neutral
U-Unfavorable VU-Very Unfavorable

-- Mean Intensity Trend (negative indicates TC deepening) --
6hr 12hr 18hr 24hr
VF <-3.0mb/ 6hr <-6.0mb/12hr <-9.0mb/18hr <-12.0mb/24hr
F -3.0 - -1.0 -6.0 - -2.0 -9.0 - -3.0 -12.0 - -4.0
N -1.0 - +1.0 -2.0 - +2.0 -3.0 - +3.0 -4.0 - +4.0
U +1.0 - +3.0 +2.0 - +6.0 +3.0 - +9.0 +4.0 -+12.0
VU >+3.0 >+6.0 >+9.0 >+12.0
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1631. IKE
Quoting GeoffreyWPB:
Finally a break in the rain..What a morning! On a sad note...Ed McMahon passed away this morning.

Sorry to hear that.....Carson and McMahon both gone...
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Good morning from Deerfield Beach. Was awakened by a lightning strike that must have only been yards away from my condo. Nasty area of thunderstorms just came thruough here---wind really picked up and the lightning could only be described as excessive.

Stay safe everyone!
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1629. IKE
Quoting gulfcoastdweller:

oh pretty please with a cherry on top?

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1627. WxLogic
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Finally a break in the rain..What a morning! On a sad note...Ed McMahon passed away this morning.
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Thus far, in layman's terms there is/isn't enough dust in Africa which means we are/aren't going to have more/less dust in the atmosphere months before hand to affect/have no affect on the amount or intensity of tropical systems we see this year because sea surface temperatures are/are not being suppressed/repressed/without estrogen, which will then be aided/unaided/laughed at by an assisting full/partial/neutral/non El Nino/La Nina/La Vida Loca event.

LOL That certainly clears things up for me! Thanks
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1622. IKE
NAM puts a lot of moisture in the western Caribbean on the end(84 hours), of it's last run....

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1621. IKE
800 AM EDT TUE JUN 23 2009




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good morn all

ADT-Version 7.2.3
Tropical Cyclone Intensity Algorithm

----- Current Analysis -----
Date : 23 JUN 2009 Time : 104500 UTC
Lat : 17:33:46 N Lon : 103:34:30 W

CI# /Pressure/ Vmax
4.4 / 982.6mb/ 74.6kt

Final T# Adj T# Raw T#
(3hr avg)
3.9 3.7 3.7

Latitude bias adjustment to MSLP : +2.0mb

Center Temp : -72.9C Cloud Region Temp : -67.8C



Ocean Basin : EAST PACIFIC
Dvorak CI > MSLP Conversion Used : ATLANTIC

Tno/CI Rules : Constraint Limits : NO LIMIT
Weakening Flag : ON
Rapid Dissipation Flag : FLAG


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Avon Park, Highlands County, FL
CoCoRaHS Stn FL-HL-2
T-Storms raced through between 6:20 - 7:30 dumping .80 inch of rain, frequent lightning, one strike across the street at the high school sounded like a bomb went off.
Temperature dropped ten degrees, bringing much needed relief from the overnight low temp of sticky 81. Area under severe T-Storm watch until 3PM.
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Update:I am still in the full brunt of this storm in Boca Raton. I still have power and we had gusty winds but lightning is not that bad.There is more storms to the north. Oh!and good luck Broward and Miami dade.
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1617. IKE
Oh yeah....642 more GFS runs and the tropical season is over:)
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1616. IKE
Quoting sporteguy03:

I had a strong T-storm at 5am! 1500 lightning strikes!

Glad ya'll are getting rain....

Panhandle has a chance...30% at my house....today.
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Quoting IKE:
Starting to see a few clouds in central Alabama, heading south.

Our heat wave is close to being over...for now...starting tomorrow....hopefully...please....in a nice way.

I had a strong T-storm at 5am! 1500 lightning strikes!
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1557. StormW 11:32 PM CDT on June 22, 2009

Thanks Storm, but the one I am looking for is just numbers in columns, no maps, it is real easy to understand...just a few columns running down the page, with areas at the top, SST's and anomolies...I believe that's how it went anyway, I haven't seen it for a year now...

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1613. IKE
Starting to see a few clouds in central Alabama, heading south.

Our heat wave is close to being over...for now...starting tomorrow....hopefully...please....in a nice way.
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Yeah, it is raining - ever so lightly now but more to come. Cooler temps on the way!
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Very interesting Storm. Thanks (as always!) for taking the time to teach.
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Morning all. Stay safe Floridians!
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Quoting Weather456:
Tropical Update

Thanks 456, I was wondering about that trough hanging off the low that caused delays at The U.S. Open this past weekend. The tail of the trough looked like a classic set-up for late season trough/tropical development. We'll see. Maybe it will create a decent swell up-coast.
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1606. IKE
Quoting TheCaneWhisperer:
A day late but, we'll take it. I don't think they could have been more wrong about yesterday, lol.

Same thing with the heat-wave letting up. It took longer then they thought.

I think the models they use(we look at), are showing changes too fast.
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Quoting TheCaneWhisperer:
A day late but, we'll take it. I don't think they could have been more wrong about yesterday, lol.

Yeah, I agree. During the day yesterday it was very dry for a June day. Kind of felt like Las Vegas, relatively speaking, for Florida at least. We'll take it today though. I just woke up to the complete opposite weather of yesterday. Nie T-storm just ripped through my house at Deerfield Beach, the lightning and thunder just sent my dog into her closet bunker. LOL This T-storm isn't even from the line heading south from the Palm Beaches, we're about to get slammed.
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A day late but, we'll take it. I don't think they could have been more wrong about yesterday, lol.

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1602. IKE
Looks like Andres is paralleling the coast.

Think the NHC is about right on the track. Looks to stay mainly offshore.
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Tropical Update

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1600. IKE
Today: A 30 percent chance of showers and thunderstorms, mainly after 1pm. Sunny and hot, with a high near 99. Heat index values as high as 106. Calm wind becoming northwest between 5 and 10 mph............

Under 100 for the high....YEEHAW~~~~~~~
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1599. sjm45
Hades - no worries I followed your info to the Japanese Meteo. website. Which serves its purpose. Thanks.
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TS 4 is Tropical Storm Nangka, by the way sjm
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West Pacific ocean there are so many sites

Digital Typhoon
Typhoon 2000
Japan Meteorological Agency (RSMC)
China Meteorological Administration
Hong Kong Observatory
Korea Meteorological Administration
Philippines Atmospherical Geophysical Astronomical Services and Administration

For the Indian Ocean

India Meteorological Department (RSMC)
Thailand Meteorological Department
Pakistan Meteorological Department

The pressure reading for the advisories are found under the RSMC Tropical Cyclone Outlook page.

Arabian Sea usually get at least 1 cyclone a year.
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1596. sjm45
Hades - is there a good site giving info about tropical storms around West pacific and Bay of Bengal/Arabian Sea? Where do you get your info? I can never find pressure readings!

Is it quite rare to have a Cyclone in Arabian sea? Don't seem to see them much.

TS 4 - must be nearly a cyclone by now. It looks well formed to me.
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Japan Meteorological Agency
Tropical Cyclone Advisory #5
15:00 PM JST June 23 2009

At 6:00 AM UTC, Tropical Storm Nangka (998 hPa) located at 12.1N 125.6E has 10 minute sustained winds of 35 knots with gusts of 50 knots. The storm is reported as moving northwest at 10 knots.

Gale-Force Winds
150 NM from the center

Dvorak Intensity: T2.0

Forecast and Intensity
24 HRS: 14.3N 121.4E - 35 knots (Tropical Storm/CAT 1)
48 HRS: 16.7N 120.2E - 35 knots (Tropical Storm/CAT 1)
72 HRS: 19.5N 119.6E - 35 knots (Tropical Storm/CAT 1)
Member Since: December 31, 1969 Posts: Comments:
India Meteorological Department
Tropical Cyclone Warning Number TWO
8:30 AM IST June 23 2009

Subject: Depression over east central Arabian Sea

At 3:00 AM UTC, Depression ARB01-2009 located at 19.0N 71.5E, or about 150 km west of Mumbai and 300 km south-southwest of Surat.

3 minute sustained winds near the center is 25 knots with a central pressure of 998 hPa. The state of the sea is rough to very rough around the system's center.

Satellite imagery indicates gradual organization of convection during the past 12 hours. The intensity of the system is T1.5. Associated broken low /medium clouds with embedded intense to very intense convection seen over Arabian Sea between 16.5N and 21.0N and to the east of 67.0E. The lowest cloud top temperature due to convection is around -70C

Vertical wind shear of horizontal wind over the region is low to moderate (around 10-20 knots). Sea surface temperature is favorable for intensification and is 0.50 to 1.00C above normal. The system lies embedded in the southwesterly flow in lower and middle levels. The upper tropospheric ridge roughly runs along 21.0N.

Considering all the above, the system is likely to intensify into a deep depression and move in a north-northeasterly direction and cross south Gujarat coast near Surat by this evening/night.
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