July Atlantic hurricane outlook

By: Dr. Jeff Masters , 5:31 PM GMT on June 30, 2008

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The first half of July is usually a quiet period in the Atlantic for tropical cyclone formation. Since 1995, six of 13 years (46%) have had a named storm form during the first half of July. The busiest first half of July occurred in 2005, when three hurricanes formed. These included Hurricane Dennis and Hurricane Emily--the strongest hurricanes ever observed so early in the season. As seen in Figure 1, most of the early July activity occurs in the Gulf of Mexico, Western Caribbean, and Carolina waters. However, a few long-track "Cape Verdes" hurricanes begin to occur. These are spawned by tropical waves that come off the coast of Africa. Tropical waves serve as the instigators of about 85% of all major hurricanes.


Figure 1. Tracks of all tropical storms and hurricanes since 1851 that formed July 1-15. North Carolina and the Gulf of Mexico coast from the Florida Panhandle to Texas are the preferred strike locations. Oddly, the Florida Peninsula has been struck by only two storms that formed in the first half of July.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) have cooled about 0.5°C in the past month over the region we care about the most--the hurricane Main Development Region that extends from the coast of Africa to the coast of Central America, between 10° and 20° latitude (Figure 2). SSTs are now about about 0.5°C below average in this region (compare the May 29 SST anomaly image. One notable exception is the region closest to the African coast, which is about 2°C above average. The reason for the cooling over most of the tropical Atlantic is that the strength of the Bermuda-Azores High has increased since May, driving stronger trade winds. These stronger winds cause more evaporative cooling of the sea surface (just like blowing on your wet skin cools it off). In addition, levels of Saharan dust coming off the coast of Africa in June increased dramatically compared to May, which had dust levels about 30% below average. In particular, a major dust storm that began about June 20 off the coast of Africa crossed all the way to Florida by Friday last week, bringing hazy skies across the entire tropical Atlantic (Figure 3). All that dust blocked sunlight, preventing the water from heating up as much as usual. Another large dust storm was observed leaving the African coast around June 14.


Figure 2. Sea Surface Temperature (SST) departure from average for June 30, 2008. Image credit: NOAA.


Figure 3. Satellite image of the West African coast from June 21, 2008, at 14:50 GMT. A huge dust storm moved off the coast of Africa June 20, and arrived over South Florida on June 27. Image credit: NASA.

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.

Wind shear during almost all of June was above average over the tropical Atlantic, making it a very quiet month. This shear was predominately caused by the two branches of the jet stream--the polar jet, which runs along the U.S.-Canadian border, and the subtropical jet, which runs through the Caribbean to North Africa. This is very typical for June, when the jet stream is still very active and quite far south. The jet stream will gradually weaken as summer progresses, bringing lower wind shear and greater chances for tropical storm formation. Indeed, the GFS model is predicting that very low levels of wind shear will develop in the region between Africa and the Lesser Antilles Islands late this week (Figure 4). This is echoed by all the other major global forecast models, which unanimously predict that a tropical depression may form between Africa and the Lesser Antilles Islands by late this week or early next week. The latest 2-week GFS forecast keeps wind shear below average levels through mid-July.


Figure 4. GFS model wind shear forecast for Friday, July 4, 2008. Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots). In most circumstances, wind shear below 12 knots (6 m/s, the lighter red colors) is conducive for tropical storm formation.

Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa. Despite the fact that the Sahel region of Africa has seen three straight years of average to above-average rains, which should result in soil stabilization and fewer dust outbreaks, June 2008 had high levels of dust coming from Africa. Expect dust from Africa to be a major deterrent to any storms that try to form between Africa and the Lesser Antilles Islands in July.

Steering currents
During June 2008, the Bermuda-Azores High (Figure 5) extended farther west than usual. Its strength was about 1 mb below average, which normally would drive slower trade winds than average. This did not occur, as the trade winds were about 2 m/s (4 knots) above average over most of the tropical Atlantic in June. For the first half of July, both the GFS and ECMWF long-range models are predicting that the Bermuda-Azores High will not extend so far west, as the jet stream will bring a persistent trough of low pressure over the East Coast. This pattern will tend to recurve most tropical storms that penetrate north of the Caribbean Sea. Steering current patterns are not predictable more than about two weeks in advance, and there is no telling if this favorable steering current pattern will persist into the peak of hurricane season.


Figure 5. Sea level pressure for June 2008 (left), and average sea level pressure from climatology (the years 1979-1996). The Bermuda-Azores High extended farther west than usual in June 2008, keeping low pressure entrenched in the U.S. Midwest, leading to major flooding. Image credit: NOAA/ESRL.

Summary
Recent history suggests a 46% chance of at least one named storm occurring in the first half of July. All of the major computer models predict the possible formation of a tropical depression in the region between Africa and the Lesser Antilles late this week or early next week. Given the high degree of model unanimity, marginally favorable sea surface temperatures, favorable wind shear, and unfavorable dust levels expected in this region, I put the chances of a tropical storm forming in the region at 30% over the next two weeks. I put the odds of a tropical storm forming elsewhere--in the Caribbean, Gulf of Mexico, or off the U.S. East Coast--at 30% as well, for a combined roughly 50% chance of a first half of July named storm.

I'll have an update Wednesday morning.
Jeff Masters

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1501. sporteguy03
2:02 PM GMT on November 22, 2008
Pre Ike
Member Since: July 7, 2005 Posts: 0 Comments: 5414
1500. sporteguy03
2:02 PM GMT on November 22, 2008
Hi Ike!!!!!!!!!!!!!!!!!!!
Member Since: July 7, 2005 Posts: 0 Comments: 5414
1499. TexasGulf
8:29 PM GMT on July 01, 2008
OK, OK... y'all made your point! Enough of the 'silent treatment' already. An hour is just cruel.

I'll concede that;
Jim Cantore might be a marginal weather reporter who doesn't need to wear an official "TWC hairpiece".

Furthermore, The Weather Channel doesn't necessarily shill their merchandise during news reports. I'm sure all the logos are shown by accident.

Now you've broken me... will somebody speak?

Whimper!
Member Since: April 28, 2008 Posts: 0 Comments: 354
1498. TexasGulf
8:00 PM GMT on July 01, 2008
Man... saying that you like Jim Cantore's weather reporting is a real blog killer.

Everyone suddenly just wen't silent... no crickets... it's kind of spooky!
Member Since: April 28, 2008 Posts: 0 Comments: 354
1497. TexasGulf
7:21 PM GMT on July 01, 2008
I think we need another report from Jim Cantore from the CV Islands. what do you think guys? lol

Personally, I like Jim Cantore's reporting. He looked better when he had more hair, so he may want to ask The Weather Channel to sponsor an official "TWC hairpiece". That way, he can wear the logo without having a ball-cap.

The Weather Channel gets me irritated with their shill for official TWC merchandise. Especially junior reporters... they all seem to wear the "official" TWC windbreaker, with logo ball-cap, a Weather Channel shirt and carry a microphone with a TWC tag on it. Even if it's 100 degrees and sunny, they still have to display the promotional merchandise. I'm just glad TWC doesn't sell "official" boxers and halter-tops. The reporters would have a hard time working those product placements into their storm report. :>)
Member Since: April 28, 2008 Posts: 0 Comments: 354
1496. 69Viking
7:13 PM GMT on July 01, 2008
1460. mda91

Bold prediction!
Member Since: August 25, 2006 Posts: 1 Comments: 3077
1495. Skyepony (Mod)
7:05 PM GMT on July 01, 2008
Come to the new blog...
Member Since: August 10, 2005 Posts: 192 Comments: 38650
1494. IKE
7:04 PM GMT on July 01, 2008
NEW BLOG!
Member Since: June 9, 2005 Posts: 23 Comments: 37858
1493. IKE
7:02 PM GMT on July 01, 2008
1489....

GaryGnu
Member Since: June 9, 2005 Posts: 23 Comments: 37858
1492. TheWeatherMan504
7:01 PM GMT on July 01, 2008

1477. TheCaneWhisperer 6:56 PM GMT on July 01, 2008
Surface Low developing around homestead?


Where is floodman when you need him.
Member Since: May 18, 2008 Posts: 2 Comments: 1042
1490. NEwxguy
7:00 PM GMT on July 01, 2008
speculation,its what drives the stock market and tropical weather blogs
Member Since: September 6, 2007 Posts: 886 Comments: 15948
1489. pearlandaggie
7:00 PM GMT on July 01, 2008
Ike, you're not talking about Forecaster//Stromtop, are you? LOL
Member Since: September 14, 2007 Posts: 3 Comments: 3963
1488. Nolehead
7:00 PM GMT on July 01, 2008
i'm still holding the "Z" card for $1,000 if anyone remembers the other day...still think it's going to hit somewhere...
Member Since: June 3, 2008 Posts: 0 Comments: 1934
1487. IKE
6:59 PM GMT on July 01, 2008
12Z ECMWF puts it near 31N and 55W as a full-blown storm on it's latest run....

Link


Where's that guy that said it won't get to a TD? lol.......
Member Since: June 9, 2005 Posts: 23 Comments: 37858
1486. KEEPEROFTHEGATE (Mod)
6:59 PM GMT on July 01, 2008
hmmm
Member Since: July 15, 2006 Posts: 175 Comments: 54847
1483. tornadofan
6:57 PM GMT on July 01, 2008
Who was the blogger that used to come on all the time and put a curse on a system when it is declared?
Member Since: April 5, 2007 Posts: 83 Comments: 12345
1480. IKE
6:56 PM GMT on July 01, 2008
12Z HWRF puts it near 19N and 45W at the end of it's latest run....

Link
Member Since: June 9, 2005 Posts: 23 Comments: 37858
1478. TheWeatherMan504
6:56 PM GMT on July 01, 2008
LOL IKE!
Member Since: May 18, 2008 Posts: 2 Comments: 1042
1474. pearlandaggie
6:55 PM GMT on July 01, 2008
1465. LOL!

1463. i don't think it was you. the question was more centered around the current strength of the ABH rather than model outputs. i was under the impression that the ABH was fairly strong and extensive and wondered why the models thought any storm that may form would track so far north so quickly....that's all.
Member Since: September 14, 2007 Posts: 3 Comments: 3963
1473. NEwxguy
6:54 PM GMT on July 01, 2008
jp,I agree,its a matter of watching how the atlantic environment evolves,so fish or westward,its all speculation,but thats the fun of it.
Member Since: September 6, 2007 Posts: 886 Comments: 15948
1471. TheWeatherMan504
6:54 PM GMT on July 01, 2008
whats the link to that florida website with all the tropical cyclone tracking models.
Member Since: May 18, 2008 Posts: 2 Comments: 1042
1466. JLPR
6:52 PM GMT on July 01, 2008
so how is 92L doing?
I have been out enjoying summer and I have missed 92L's birth =P
Is the wave holding or breaking apart?
Member Since: September 4, 2007 Posts: 36 Comments: 5223
1465. IKE
6:52 PM GMT on July 01, 2008
1459. pearlandaggie 1:49 PM CDT on July 01, 2008
1453. LMAO...xenophobe, shill, denialist, and now flamed beef....did i miss any? :)


Simmering crow.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
1464. tornadofan
6:52 PM GMT on July 01, 2008
Best to listen to the guy with the flaming skull.
Member Since: April 5, 2007 Posts: 83 Comments: 12345
1463. 69Viking
6:51 PM GMT on July 01, 2008
1450. pearlandaggie

I just think it's too early to look at models in my opinion. The system is still a wave so less likely to be affected by the bumps that could turn it northwest. No attack on you I promise!
Member Since: August 25, 2006 Posts: 1 Comments: 3077
1461. KEEPEROFTHEGATE (Mod)
6:50 PM GMT on July 01, 2008
this is going in what ever direction it wants no possible way to determine its final desination at this point the game is only just beginning
relax lets finish the game
Member Since: July 15, 2006 Posts: 175 Comments: 54847
1459. pearlandaggie
6:49 PM GMT on July 01, 2008
1453. LMAO...xenophobe, shill, denialist, and now flamed beef....did i miss any? :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
1458. NEwxguy
6:44 PM GMT on July 01, 2008
water vapor loop,shows all the bumps it has to overcome,but those bumps aren't going to stay there either,they are moving westward also,interesting to see things develope
Member Since: September 6, 2007 Posts: 886 Comments: 15948
1457. weathersp
6:44 PM GMT on July 01, 2008
Hey all.. been busy at the workplace today.. Can someone fill me in?
Member Since: January 14, 2007 Posts: 17 Comments: 4140
1455. WeatherfanPR
6:44 PM GMT on July 01, 2008
JFV, a predicting ball LOL...anyway, I agree with you in some way.
Member Since: August 23, 2006 Posts: 0 Comments: 1592
1454. Levi32
6:43 PM GMT on July 01, 2008
Back in a bit
Member Since: November 24, 2005 Posts: 635 Comments: 26684
1453. thelmores
6:43 PM GMT on July 01, 2008
NP pearl..... as long as your are not a xenophobe! LOL
Member Since: September 8, 2005 Posts: 0 Comments: 3805
1452. DocBen
6:42 PM GMT on July 01, 2008
Interesting how one model takes it almost to South America and the others send it fishing. I'm placing my bets on westward. That ridge seems awful strong.
Member Since: May 16, 2006 Posts: 0 Comments: 98
1451. thelmores
6:41 PM GMT on July 01, 2008
1441

I think at 6pm.... LOL
Member Since: September 8, 2005 Posts: 0 Comments: 3805

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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.