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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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1113. flsky
9:25 PM GMT on August 01, 2009
Incredible pic of the tornado. Do you know if it's been retouched or not?
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1112. IKE
9:24 PM GMT on August 01, 2009
Last Update: 1-AUG-2009 4:43pm EDT

No tropical development is expected across this basin into early next week.

There are a few tropical waves across the Atlantic Basin, although the overall pattern remains very inactive. Waves are located along 59 west, south of 25 north and along 88 west, south of 20 north. These waves are moving westward at about 4-6 degrees longitude per day. This dry air is suppressing the development of thunderstorms north of 15 north, which continues to impact the wave at 59 west. Meanwhile, the other tropical wave near 88 west is encountering moderate upper-level wind shear. This will prevent thunderstorms from becoming organized over the next few days.


By AccuWeather.com Meteorologists Justin Povick and Andy Mussoline
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1110. BurnedAfterPosting
9:23 PM GMT on August 01, 2009
well maybe around 7-8W actually, either way it has about a day or so before it emerges off the coast and it may or may not be associated with the convection

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1109. Drakoen
9:23 PM GMT on August 01, 2009
Quoting btwntx08:

its closer to 10w look back drakoen's post 1036


Correct.
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1108. mobilegirl81
9:22 PM GMT on August 01, 2009
If enough moisture(Tropical waves)can keep moving off the african coast, and being the time of year it is, that dust will not be an inhibiting factor at all.
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1107. BurnedAfterPosting
9:22 PM GMT on August 01, 2009
yup looking at his pic, the "Low" is right around 6W
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1106. BurnedAfterPosting
9:20 PM GMT on August 01, 2009
Quoting btwntx08:

its closer to 10w look back drakoen's post 1036


actually in his pic I see the low right around 5-6W
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1104. mikatnight
9:18 PM GMT on August 01, 2009
Howdy Folks!


Oldest known photograph of a tornado




Image ID: wea00206, NOAA's National Weather Service (NWS) Collection
Location: 22 miles southwest of Howard, South Dakota
Photo Date: 1884 August 28


• High Resolution Photo Available

For those who are interested, NOAA has a large collection of photos - many depicting the different types of clouds.
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1103. mobilegirl81
9:18 PM GMT on August 01, 2009
Yea P, it must move out from under that dust and the air in the carribean might subside a little while it heads that way.
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1102. BurnedAfterPosting
9:18 PM GMT on August 01, 2009
Quoting gordydunnot:
And then it becomes a sheering experience. If not sooner. I say we got to wait another 2 weeks, maybe upper level winds will calm down by then.


lol uppper winds are favorable for development in the Atlantic

The downcasting continues.....
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1101. gordydunnot
9:17 PM GMT on August 01, 2009
And then it becomes a sheering experience. If not sooner. I say we got to wait another 2 weeks, maybe upper level winds will calm down by then.
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1100. Patrap
.."Sure as the Dust,that blows High in June..,when Moving thru Kashmirrrrrrrrr"...

Current SAL Split window
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The dry air does not seem to be a problem until you get to the carribean.
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1097. Patrap
Ack,coff,dust..spit..
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1096. IKE
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1095. Patrap
Minor earthquake rattles southeastern Tenn., N.C.
by The Associated Press
Saturday August 01, 2009, 12:54 PM



DUCKTOWN, Tenn. -- A minor earthquake rattled the mountainous area of southeastern Tennessee, western North Carolina and northern Georgia on Saturday morning, the U.S. Geological Survey reported.

The 3.2-magnitude temblor happened about 11 a.m. Saturday. Its epicenter was located about 55 miles east of Chattanooga, near the town of Ducktown.
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As was said earlier, the wave axis looks to be further back along 5W, the convection that is coming off the coast may be associated with that wave.

Very nice mid-level rotation
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They are definitly getting stronger in frequency...may plow enough moisture to allow something to form out there.
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It must hold what its got to become something.
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Link
'The Eagle has Landed'
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Quoting Skyepony:
Weather456~ Chris was a similiar end..

Savannah~ Ya'll enjoy the rain. Hope it has a good chance to soak in. Ya'll need it.


Actually, we're near or above average in this part of the state. We just need a way to pump the excess uphill to Atlanta...
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1088. Skyepony (Mod)
Weather456~ Chris was a similiar end..

Savannah~ Ya'll enjoy the rain. Hope it has a good chance to soak in. Ya'll need it.
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It's that time of the year in this part of the world. This has been the story here in South Georgia every day for the last 2 weeks.
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Quoting Skyepony:
RGB loop of Lana is very revealing.. All of the mid & upper layer of the storm are just ripped off with a strong naked swirl left marching on to the west. This maybe painful for some to watch..just letting ya'll know, pretty graphic shredding.

If you watch only the rainbow it looks like it is desperately trying to reform to the North, but the rgb is the true story of what is going on here. Enjoy the waves HI..


reminds me of Chris.
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1084. Skyepony (Mod)
RGB loop of Lana is very revealing.. All of the mid & upper layer of the storm are just ripped off with a strong naked swirl left marching on to the west. This maybe painful for some to watch..just letting ya'll know, pretty graphic shredding.

If you watch only the rainbow it looks like it is desperately trying to reform to the North, but the rgb is the true story of what is going on here. Enjoy the waves HI..
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Model support is not something common in 2009, but when it occurs, most likely a system does form.

90L (model support) (verified)
91L (model support) (verified)
92L (model support) (verified)
93L (no model support) (verified)
94L (model support but for a non tropical low) (verified)
97L (no model support) (verified)
98L (model support for a hybrid or non tropical system) (verified)
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Afternoon....That wave coming off the coast is the most impressive looking one so far of the season.....
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NOGAPS briefly shows a system on the 4th of August in the same area as the other 4 models
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1077. Skyepony (Mod)
Lana needs lifesupport..


Surfs up..

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yea then we will get 1000 fish storm comments instead of 1000 comments of the season is a bust lol

a bit of an improvement but not much
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1075. Skyepony (Mod)
93W
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Quoting WeatherStudent:


Let's hope so, and why would this be? No land threater, then?


put it this way, it's too early to say. We are just going on the models which will change back and forth; and nothing has develop as yet.
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1073. Skyepony (Mod)
Not sure if we will see Hurricane or Typhoon till the MJO quits meandering around the middle.

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Quoting wunderkidcayman:
hi guys what's up when is ana coming and it is pouring in west bay
Lots of clouds and thunder in East End but hasn't started raining yet.
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at the moment I'm not impress with the wave, but it seems it will interact with that area of convection near the coast, we'll see how that goes.
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Quoting WeatherStudent:
456, if this thing were to develop, it would more likely then not, head on out to sea, wouldn't it?


Based on what the models are showing, yes.
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Quoting tennisgirl08:


take it one day at a time, wunderkid.

ok I'm sorry I am so bloody bored of looking at nothing in the tropics
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WS read post 1066
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Quoting wunderkidcayman:
ok so we could see Invest 99L then T.D.2 then T.S.ANA is there a possiblity that it could become Hurricane ANA knowing that some late-season storms are powerful?


take it one day at a time, wunderkid.
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ok so we could see Invest 99L then T.D.2 then T.S.ANA is there a possiblity that it could become Hurricane ANA knowing that some late-season storms are powerful?
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Quoting GinnyinSC:
I'm DEFINITELY a novice when it comes to weather information. So can anyone tell me why an el Nino in the Pacific drastically changes the conditions in the Atlantic? Thanks to anyone who responds.


Its explained in this presentation
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