August hurricane outlook

By: Dr. Jeff Masters , 3:02 PM GMT on July 31, 2009

The Atlantic remains quiet today, with no threat areas to discuss and no models calling for tropical storm formation over the seven days. Not much has changed in the Atlantic since my mid-July Atlantic hurricane outlook posted two weeks ago. However, we are now at the cusp of when hurricane activity begins a steep rise (Figure 1). Early August is typically when wind shear begins a major decline, sea surface temperatures continue to rise, African dust and dry air outbreaks diminish, and the African Monsoon and Intertropical Convergence Zone (ITCZ) become quite active, spawning frequent and powerful tropical waves. These tropical waves serve as the instigators of about 85% of all major hurricanes.

Since the current active hurricane period began in 1995, ten out of fourteen years (71%) have had a named storm form during the first half of August, with an average of 1.4 named storms per year. The last nine years in a row have had a named storm form during the first half of August, but the previous four year stretch (1996 - 1999), did not have any storms form.


Figure 1. The seasonal distribution of Atlantic hurricane activity shows a steep rise at the beginning of August. Image credit: NHC.

Sea Surface Temperatures
Eighty-five percent of all major hurricanes form in the Main Development Region (MDR) of the Atlantic, from the coast of Africa to the coast of Central America, between 10° and 20° latitude. This region also spawns 60% of all weaker hurricanes and tropical storms. Sea Surface Temperature (SST) anomalies in the MDR have slowly but steadily risen during July, and now stand at a respectable 0.5°C (0.9°F) above average (Figure 2). SSTs are well below the record levels observed in 2005 and 2006, when they were up to 2°C above average over large portions of the Main Development Region. Still, there is plenty of heat energy available for strong hurricanes to form this year. The strength of the Azores-Bermuda high has been below average over the past month, driving below average trade winds. Weaker trade winds don't mix up as much cold water from the depths, and cause less evaporative cooling. The latest 2-week run of the GFS model predicts continued slightly below average-strength trade winds through mid-August, so SST anomalies should continue to warm during this period.


Figure 2. Sea Surface Temperature (SST) departure from average for July 30, 2009. SSTs were about 0.5°C (0.9°F) above average over the tropical Atlantic's Main Development region for hurricanes, from Africa to Central America between 10° and 20° North Latitude. Note the large region of above average SSTs along the Equatorial Pacific off the coast of South America, the hallmark of an El Niño episode. Image credit: NOAA/NESDIS

El Niño
El Niño conditions have remained steady over the tropical Eastern Pacific over the past month. Ocean temperatures in the area 5°N - 5°S, 120°W - 170°W, also called the "Niño 3.4 region", remain at 0.8°C above average, which is 0.3°C above the threshold for a weak El Niño, according to the Australian Bureau of Meteorology (Figure 3). An increase of another 0.2°C would push the current El Niño into the "moderate" category. NOAA's Climate Prediction Center issued an El Niño Advisory earlier this month, and predicts that El Niño conditions will intensify over the next few months, and last through the coming winter. The latest set of mid-July runs of the El Niño computer models are almost universally calling for El Niño conditions to remain well-established for the peak months of hurricane season, August - October. It is likely that Atlantic hurricane activity will be suppressed in 2009 due to the strong upper-level winds and resulting wind shear an El Niño event usually brings to the tropical Atlantic. The NOAA CFS model is calling for continued above-average wind shear over most of the tropical Atlantic for the August-September-October peak part of hurricane season.


Figure 3. Sea Surface Temperature (SST) departure from average for the the equatorial Eastern Pacific (the area 5°N - 5°S, 120°W - 170°W, also called the "Niño 3.4 region"). El Niño conditions exist when the SST in this region rises 0.5°C above average. As of July 31, 2009, SSTs in the Niño 3.4 region had risen to 0.83°C above average. To be considered an "El Niño episode", El Niño conditions must occur for five consecutive months, using 3-month averages. Image credit: Australian Bureau of Meteorology.

Wind shear
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart. The jet stream's band of strong high-altitude winds is the main source of wind shear in July over the Atlantic hurricane breeding grounds, but in August the jet stream retreats to the north, and wind shear typically falls.

Wind shear over the past month (Figure 4) has mostly been above average over the tropical Atlantic, particularly over the Caribbean. The presence of El Niño conditions over the tropical Eastern Pacific may be primarily responsible for this enhanced shear. However, wind shear has been slowly falling over the southern portion of the Caribbean and southern MDR over the past week, and is forecast by the GFS model to fall to near-average levels by mid-August. This should present a more favorable environment for hurricanes to form in by mid-month.


Figure 4. Departure of wind shear from average in m/s for the 1-month period ending July 27, 2009. Higher than average wind shear (blue colors) was observed over the Caribbean. The El Niño conditions over the tropical Eastern Pacific may be primarily responsible for this enhanced shear. Image credit: NOAA Climate Prediction Center.

Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa, and the Saharan dust storms have been quite active over the past month. Expect dust from Africa to diminish in the coming month, allowing a greater chance for African tropical waves to develop.

Steering currents
The steering current pattern has remained virtually the same all summer. A persistent trough of low pressure has remained entrenched over the Eastern U.S., bringing cool and relatively moist weather to the northeastern portion of the country. This trough is strong enough to recurve any tropical storms or hurricanes that might penetrate north of the Caribbean Sea. Steering current patterns are predictable only about 3 - 5 days in the future, although we can make very general forecasts about the pattern as much as two weeks in advance. At present, it appears that the coming two weeks will maintain the strong trough over the Eastern U.S., which decreases the hurricane risk to the U.S. Gulf Coast. It is often difficult to break a months-long steering current pattern like the current one, and it's reasonable to forecast that the current steering pattern will continue to dominate into September.

Summary
Recent history suggests a 71% chance of a named storm occurring in the first half of August. However, this is not a typical year. The ITCZ has been remarkably inactive, and there have been an unusually low number of tropical waves coming off the coast of Africa. Although SST anomalies should continue to rise and wind shear should slowly fall over the next few weeks, the computer models suggest no significant changes to the current inactive weather pattern. I'll go with a 30% chance of a named storm forming in the first half of August.

I'll have a new post on Monday.

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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I might be placed on ignore for saying this, but ....., I heard persons lowering their predictions to 5 or 6 storms even someone calling for 0 storms for the year with over 4 months or 16 weeks left in the season, a season by the way that is unique and unpredictable and we are just entering the peak months. Oh does anyone remember 2006 (an EL NINO year) when there were three systems out in the Atlantic all at once?
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Quoting stoormfury:
the tropical wave which is trying to form into an area of disturbed weather in the same location date of Hurricane Allen in 1980. Is that Devaju all over again


I think the wave over Africa is the real player, the GFS may have finally latched on to something realistic

IT forms a low with the wave as it comes off Africa in 42 hours and has held on to it strong through 90 hours
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the tropical wave which is trying to form into an area of disturbed weather in the same location date of Hurricane Allen in 1980. Is that Devaju all over again
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Speaking about sonic booms and the Shuttle, is't thunder a kind of sonic boom also? (lightning traveling at the speed of light displaces the air temporarily and it slaps back into place?)
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58. auburn (Mod)
The federal government has predicted a "near normal" hurricane season for the Atlantic, with a 25% chance of above-normal outbreaks and 25% chance of below-normal outbreaks -- though overall, forecasters expressed a greater degree of uncertainty this year than they have in past years. The National Oceanographic and Atmospheric Administration's predicts a 70% chance of:

* Named storms: 9-14
* Hurricanes: 4-7
* Major hurricanes: 1-3

The other major forecaster in the U.S., Colorado State University, recently revised down its expectations for the 2009 Atlantic hurricane season:

* Named storms: 12
* Hurricanes: 6
* Major hurricanes: 2

Further, the Colorado forecasters predicted the following probabilities that a major hurricane could strike the U.S., all of which are about average for the past century:

* Entire U.S. coastline: 54%
* U.S. East Coast (including peninsula Florida): 32%
* Gulf Coast (from the Florida Panhandle westward to Brownsville, Texas): 31%
* Caribbean: Average

Forecasters warn that the number of storms, and their intensity is only one key determinant of risk of property damage and loss of life: The biggest factor is who lives in harm's way, and how well they prepare. Some 35 million U.S. residents live in hurricane-prone regions, and experts urge them to prepare.

When those storms do come, they will be given names. Tropical cyclones are given names when they achieve tropical storm strength, with sustained winds of at least 39 mph. Hurricanes are tropical storms that have sustained winds that exceed 74 mph, and major hurricanes have sustained winds that exceed 111 mph.
Here are the tropical storm and hurricane names for 2009:
2009 Hurricane and Tropical Storm Names - Atlantic

1. Ana
2. Bill
3. Claudette
4. Danny
5. Erika
6. Fred
7. Grace
8. Henri
9. Ida
10. Joaquin
11. Kate
12. Larry
13. Mindy
14. Nicholas
15. Odette
16. Peter
17. Rose
18. Sam
19. Teresa
20. Victor
21. Wanda

2009 Hurricane and Tropical Storm Names - Eastern North Pacific

In the eastern Pacific, Tropical Storm Andres formed June 21 and reached hurricane strength for a few hours on June 23 without doing any damage.

July has seen three storms form in the Pacific: Tropical Storm Blanca formed July 6 and dissipated by July 8; Hurricane Carlos flared up twice from tropical storm to hurricane strength between July 10 and July 16; and Tropical Storm Dolores, which briefly flared up July 15 and July 16.

1. Tropical Storm Andres - June 21-23
2. Blanca - July 6-8
3. Carlos - July 10-16
4. Dolores - July 15-16
5. Enrique
6. Felicia
7. Guillermo
8. Hilda
9. Ignacio
10. Jimena
11. Kevin
12. Linda
13. Marty
14. Nora
15. Olaf
16. Patricia
17. Rick
18. Sandra
19. Terry
20. Vivian
21. Waldo
22. Xina
23. York
24. Zelda
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;)

that's ww-ette...

;)
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56. IKE
Thanks NRA....ww
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( no offense, IKE....if you post on this blog, you get lumped into the ww catagory...)

;)
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three weather weenies walk into a blog....IKEster, Drakster, and STORMTOPster....
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51. IKE
Plenty of convection over me.....

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I am still wtching an area 11N 37W which has cylonic turning ,but at the moment lacks any significant convection. what i have noticed the last few hrs is that low level clouds are racing to the area indicative of some form of convergence. the area is conducive for development with only the dry air to it's north , nice area of interest
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Quoting jeffs713:

OMG stretching like gumby! ;) (don't mind me, though, my office still uses IE6, and won't let me install FF or IE7/8)

You are definitely learning, though. Thanks for the images, I don't have all the sites bookmarked here @ work.
Thanks - and I will adjust widths to pxs.
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Quoting mobilegirl81:
Why are there sonic booms?

When the shuttle is coming in to land, it is slowing down from approximately Mach 25 (25 times the speed of sound) to 225 knots. Whenever an object reaches the speed of sound, the air in front of it is compressed into a shockwave that is basically a compression wave. This compression wave is basically just a powerful sound wave - it sounds like a boom. The shuttle has a pair of sonic booms, because of the shockwave created by the nose and the wings. (I think that is why there are 2... someone confirm this?)
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Quoting mobilegirl81:
Why are there sonic booms?
That's the sound barrier being broken. The shuttle was going 25 times the speed of sound I think.
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Quoting IKE:
#28..........

There's no convection....look at the entire Atlantic Ocean.......the only thing I see is a wave heading south of Jamaica...it has some.

Thanks Ike. I put convergence as neutral, but did not really address convection - did not add satellite data. Looking at that now : )

NOTE ADDED: My learning experiment was meant to see if I could evaluate a wave - not necessarily to say a wave will develop.
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Quoting KEHCharleston:
Good morning, folks
IF THE GRAPHICS IN THIS POST STRETCH THE BLOG - LET ME KNOW
I have adjusted the width using '%' instead of 'px'. - Using FF, so I cannot see any stretches.

StormW, Congrats on the recognition (per word of mouth) of NHC


NOTE: I decided to pick a wave (35W 'sh), evaluate and follow it. I have purposefully not read the individual weather blogs of StormW, 456, Tspin, etc - so as not to be influenced. This is what I came up with.

The case FOR the Wave

MOIST ENVIRONMENT
SAL has retreated quite a bit from last week.


Total Precipital Water is high


TC Formation Probability


SHEAR
Low shear ahead of the wave




NEUTRAL Findings

SST's
Not really, really cooking, however, I expect the sea temperature to increase now that the SAL is retreating.


Upper level divergence -


Lower level convergence


VORTICITY - not particularly impressive - but getting there :)



The case AGAINST the Wave
Not much to look at


Not picked up by any of the models

Has not been singled out by any of the excellent WU Bloggers as having any more chance than the others


ONE MORE THING - You be the judge


Let me know what you think.. Am I learning or what! Thanking you in advance

OMG stretching like gumby! ;) (don't mind me, though, my office still uses IE6, and won't let me install FF or IE7/8)

You are definitely learning, though. Thanks for the images, I don't have all the sites bookmarked here @ work.
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Thanks Dr. At present, it appears that the coming two weeks will maintain the strong trough over the Eastern U.S., which decreases the hurricane risk to the U.S. Gulf Coast. It is often difficult to break a months-long steering current pattern like the current one, and it's reasonable to forecast that the current steering pattern will continue to dominate into September.

Something can always slip through but that is good news for our friends in the Gulf. Now lets hope that a "big one" does not impact the Caribbean or Eastern Coast of Florida.
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43. IKE
Quoting NRAamy:
...but.......this is soooo quiet.

IKE...I can fix that....

;)


Yeah you can, can't you.:)
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Why are there sonic booms?
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Quoting CybrTeddy:
Anyone hear em sonic booms! loud!

I heard and felt 'em!!
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Quoting leftovers:
not a good time for a hit on s.fl. the hospitals are short staffed already because of swine flu and a hurricane hit this next month could overwelm the system

I think every hospital is short staffed already.
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...but.......this is soooo quiet.

IKE...I can fix that....

;)
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A little surprised that the mass about to hit the Philippines has not been classed as a TD. It seems to have some circulation and a lot of convection.
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36. IKE
#28..........

There's no convection....look at the entire Atlantic Ocean.......the only thing I see is a wave heading south of Jamaica...it has some.

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My Forecast: 9/5/2
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SAL diminishing. Monster wave coming off africa. Right on Q.
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Quoting P451:


But seriously, everyone is pretty jumpy right now down there. Down right expecting another Andrew to pop up.


Totally irresponsible post! I live in SW FL, in the path of Charley - everyone is NOT "pretty jumpy".

How about citing the sources for your assertions.
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Quoting P451:


Do you happen to have the same map of the same area that posted all of Andrew's "records" so to speak? Does anyone? Had hurricane hunters saying "192mph at 12,000 feet" things of that nature.



Link
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What I am seeing out there now is rather favorable conditions starting to appear. SAL has retreated quite a bit, warm SSTs, much lower shear than a couple weeks ago. The ingredients are starting to show up, but there is nothing taking advantage of it.
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Quoting IKE:
No mention of MJO?


i think dr. masters doesn't go too far into expert terms such as MJO, TUTT, etc. not that he cant, but he chooses a shorter and more understandable explanation for weather n00bs like me
Member Since: December 31, 1969 Posts: Comments:
Good morning, folks
IF THE GRAPHICS IN THIS POST STRETCH THE BLOG - LET ME KNOW
I have adjusted the width to '500 px'. - Using FF, so I cannot see any stretches. CHA

StormW, Congrats on the recognition (per word of mouth) of NHC


NOTE: I decided to pick a wave (35W 'sh), evaluate and follow it. I have purposefully not read the individual weather blogs of StormW, 456, Tspin, etc - so as not to be influenced. This is what I came up with.

The case FOR the Wave

MOIST ENVIRONMENT
SAL has retreated quite a bit from last week.


Total Precipital Water is high


TC Formation Probability


SHEAR
Low shear ahead of the wave




NEUTRAL Findings

SST's
Not really, really cooking, however, I expect the sea temperature to increase now that the SAL is retreating.


Upper level divergence -


Lower level convergence


VORTICITY - not particularly impressive - but getting there :)



The case AGAINST the Wave
Not much to look at


Not picked up by any of the models

Has not been singled out by any of the excellent WU Bloggers as having any more chance than the others


ONE MORE THING - You be the judge


Let me know what you think.. Am I learning or what! Thanking you in advance
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27. IKE
No mention of MJO?
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26. IKE
Quoting Patrap:

..No "wearing and tearing for weeks"..


Great kick-a** song by the legends of rock.
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23. IKE
Quoting Patrap:
The slow or non season so far is a good break for many area's in recovery mode from previous years.


Exactly.

I was watching ABC news early this morning and they were talking about Houston only having 2 trauma centers...there was one, I think in Galveston, that hasn't opened back, since IKE...and the stress it's put on the 2 that are open. That's been almost a year since IKE.

Sometimes peacefulness is bliss.
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But living in paradise sometimes has a price...



Checking out now. Have a great day everyone!
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..No "wearing and tearing for weeks"..
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A little tidbit of hurricane info last year.

The most active day in last year's hurricane season was September 2, when Gustav was a Tropical Depression inland over Louisiana, Hanna was a Hurricane north of Hispaniola, Ike was a strengthening Tropical Storm over the Central Atlantic, and Tropical Depression Ten(Josephine) was just forming.
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I'm just a cheeseburger in Paradise...
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The slow or non season so far is a good break for many area's in recovery mode from previous years.
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15. IKE
Quoting SavannahStorm:


"Don't know the reason, stayed here all season..."


Time for me to book a Caribbean vacation.


Amen.
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13. IKE
Dr. Masters...when was the last year the central Pacific had a named storm before the Atlantic?
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Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather

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