July hurricane outlook

By: Dr. Jeff Masters , 7:26 PM GMT on July 02, 2009

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Atlantic tropical cyclone activity typically picks up a bit during the first half of July. Since the current active hurricane period began in 1995, seven of 14 years (50%) 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. Last year's Hurricane Bertha was one such rare early July Cape Verdes hurricane. Bertha's 120 mph winds made it the sixth strongest early-season Atlantic hurricane on record. Bertha also set the record for farthest east formation as a tropical storm, hurricane, and major hurricane, so early in the season.


Figure 1. Tracks of all tropical storms and hurricanes 1851 - 2006 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 (SST) anomalies have warmed slightly over the past two weeks, but are close to average over the tropical Atlantic between Africa and Central America (Figure 2). These are the are the coolest SST anomalies we've seen since 1994. The strength of the Azores-Bermuda high has been near average over the past two weeks, driving near-average trade winds. Stronger-than-average trade winds were observed through most of the period November 2008 - May 2009, which helped cool the tropical Atlantic substantially. Strong winds mix up colder water from the depths and cause greater evaporative cooling. The latest 2-week run of the GFS model predicts continued average-strength trade winds through mid-July, so SSTs should remain near average during this period.

Typically, July tropical storms form over the Gulf of Mexico, Western Caribbean, and Gulf Stream waters just offshore Florida. SSTs are about 1.0°C above average for this time of year in the Gulf of Mexico, but near average elsewhere. July storms typically form when a cold front moves off the U.S. coast and stalls out, with the old frontal boundary serving as a focal point for development of a tropical disturbance. There will be one or two fronts moving off the U.S. coast over the next two weeks, and we will need to watch these for development. Wind shear is too high and SSTs are usually too cold in July to allow African tropical waves to develop into tropical storms. African tropical waves serve as the instigators of about 85% of all major hurricanes,

Figure 2. Sea Surface Temperature (SST) departure from average for July 2, 2009. SSTs were near 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 a developing El Niño episode. Image credit: NOAA/NESDIS

El Niño
El Niño conditions continue to amplify over the tropical Eastern Pacific. Ocean temperatures there rose 0.5°C over the past two weeks, and are now 0.45°C above the threshold for El Niño, according to the Australian Bureau of Meteorology (Figure 3). NOAA's Climate Prediction Center issued an El Niño Watch in early June, saying "that conditions are favorable for a transition from neutral to El Niño conditions during June - August 2009". The pattern of changes in surface winds, upper-level winds, sea surface temperatures, and deeper water heat content are all consistent with what has been observed during previous developing El Niños, and latest set of mid-June runs of the El Niño computer models are almost universally calling for El Niño conditions to become 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.


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 "Niña 3.4 region"). El Niño conditions exist when the SST in this region rises 0.5°C above average. As of June 28, 2009, SSTs in the Niño 3.4 region had risen to 0.95°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, since the jet is very active and located quite far south this time of year.

The jet stream over the past two months has been locked into a pattern where a southern branch (the subtropical jet stream) brings high wind shear over the Caribbean, and a northern branch (the polar jet stream) brings high wind shear offshore of New England. This often leaves a "hole" of low shear between the two branches off the coast of North Carolina, which is where Tropical Depression One formed at the end of May.

The jet stream is forecast (Figure 4) to maintain this two-branch pattern over the coming two weeks. This means that the waters offshore of the Carolinas are the most likely place for a tropical storm to form during this period.


Figure 4. Wind shear in m/s between 200 mb and 850 mb, as forecast by the 06Z July 02, 2009 run of the GFS model. The position and strength of the subtropical jet stream is forecast to change little over the next two weeks, and this jet will bring high wind shear to the Caribbean and Gulf of Mexico into mid-July. There will at times be a region of low shear between the polar jet (northern set of arrows on the plots) and the subtropical jet, allowing for possible tropical development off the coast of North Carolina. Wind speeds are given in m/s; multiply by two to get a rough conversion to knots. Thus, the red regions of low shear range from 0 - 16 knots.

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 be a major deterrent to any storms that try to form between Africa and the Lesser Antilles Islands in July.

Steering currents
The steering current pattern over the past few weeks has not changed much, and is typical for June and July. We have an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. These troughs are frequent enough and 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 typical July pattern, bringing many troughs of low pressure off the East Coast capable of recurving any July storms that might form. There is no telling what might happen during the peak months of August, September, and October--we might be in for a repeat of the favorable 2006 steering current pattern that recurved every storm out to sea--or the unfavorable 2008 pattern, that steered Ike and Gustav into the Gulf of Mexico.

Summary
Recent history suggests a 50% chance of a named storm occurring in the first half of July. Given that none of the computer models are forecasting tropical storm formation in the coming seven days, and SST and wind shear patterns look pretty average, I'll go with a 20% chance of a named storm forming during the first half of July.

Vote for Mike Theiss as an Antarctica blogger
Extreme weather photographer Mike Theiss, who wrote our Ultimate Chase photography blog for two years until a new job took him to South America, wants your help. He's entering a Quark Expeditions competition to receive an expense-paid 2-week trip to Antarctica, where he will do some intensive photography and blogging. In order to go, he needs the votes to show that he's a popular blogger. So, if you liked his posts while he was blogging for wunderground, and want to see him blog for wunderground during this potential Antarctica voyage, go to http://www.blogyourwaytoantarctica.com/blogs/view /220 and cast a vote. It takes about 3 minutes navigate through the registration and voting process. Mike will be back chasing hurricanes this August, and has promised to post his excellent storm photos on wunderground should we help him secure the Antarctica gig.

Have a great holiday weekend, and I'll be back Monday with a new post.

Jeff Masters

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Quoting AllStar17:


Which, if it survives the shear, it may have a chance in the NW Caribbean for development.
Hush.
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TexasHurricane, did you give up already ? It took me a while to figure it out too so keep trying.
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so whats new on El Nino
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Quoting extreme236:


Yeah the shear is high across much of the Caribbean, with the exception being the NW part. It would have to go there to develop.


Which, if it survives the shear, it may have a chance in the NW Caribbean for development.
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Quoting extreme236:


Yeah the shear is high across much of the Caribbean, with the exception being the NW part. It would have to go there to develop.
We don't want it thank you very much.
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It appears that if anything makes it in to the gulf it will go east towards FL.
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Quoting Seastep:
Shear is very high.

To me, it's not about moving into a more favorable environment.... don't see that happening... it's about energy sitting there waiting for a favorable environment to coalesce.

If anthing, that is how it will play out.

bbl... dinner time!


Yeah the shear is high across much of the Caribbean, with the exception being the NW part. It would have to go there to develop.
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Quoting TexasHurricane:
stormwatcherCI

did it work this time?



you need to hit the Qute i got under line in Bold

Action: Quote | Ignore User
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sometimes with IE you have to do it twice...
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1838. Seastep
Shear is very high.

To me, it's not about moving into a more favorable environment.... don't see that happening... it's about energy sitting there waiting for a favorable environment to coalesce.

If anthing, that is how it will play out.

bbl... dinner time!
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Quoting HurricaneSwirl:
when does the CIMSS site update? i wanna see
what the upper divergence/lower convergence is
doing and updated shear tendencies.


reposting
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Quoting AllStar17:


No
Keep playing around with it. You will get it eventually.
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Quoting TexasHurricane:
ok - did it work?


No
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Quoting AllStar17:


It did come off Africa....earlier Weather456 said he could trace it back to when it was at 20 E on June 24th. Speaking of Africa...we should watch the wave that just came off the coast.
A few days ago right before it entered the Caribbean I think it went through about 40-50 kts of shear if I understand the shear maps correctly.
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ok - did it work?
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Quoting stormwatcherCI:
I think it traversed quite a bit of shear already to get where it is now and so far has held together imo. I might be mistaken but I think this was a wave that came off of Africa.


It did come off Africa....earlier Weather456 said he could trace it back to when it was at 20 E on June 24th. Speaking of Africa...we should watch the wave that just came off the coast.
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Quoting AllStar17:


I agree it is being sheared, but I do not think it is undergoing 30 knots of it, I think it is less.


It looks like 30 knots to me.
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1830. Relix
That nasty wave hit me hard here in north PR for like 2 hours. Heavy rains with winds... was a pretty cool event. May be the only thing like this I see the whole year!!

I kid I kid =P
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1829. beell
Interaction with the south side of a developing TUTT and a source of the wind shear over this wave.
Photobucket

SSD WV Loop
Member Since: September 11, 2007 Posts: 137 Comments: 15303
Quoting TexasHurricane:
I don't get it, it is bringing me to the "new comment" area.
Yes, that is where you need to comment on the quote and then hit post comment. Try again.
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Quoting AllStar17:


The wave may end up dissipating, but continue to keep an eye on it, because if it survives the shear it could enter a favorable environment.
I think it traversed quite a bit of shear already to get where it is now and so far has held together imo. I might be mistaken but I think this was a wave that came off of Africa.
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I don't get it, it is bringing me to the "new comment" area.
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Blog is heating up.
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stormwatcherCI

did it work this time?
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Quoting tennisgirl08:


Yes, the wave will probably dissipate. The shear is just too high. I think the weather mets in here would agree!


The wave may end up dissipating, but continue to keep an eye on it, because if it survives the shear it could enter a favorable environment.
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Quoting TexasHurricane:
I'm clicking on it but it isn't working....
Try refresh. Sometimes it doesn't show up right away.
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1821. fire635
Quoting TexasHurricane:
Maybe I am seeing things, but can anyone else see rotation in the clouds, moving WNW/NW?

Not really sure about the rotation part but it does seem to be blowing up.....


I agree... its still not looking too organized, but the convection popping up is beginning to catch my attention.
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1820. Ossqss
Concerning the deliberations earlier with post interpretation.

He who feeds a stray dog, ends up with a new pet. Just my take on the last post I make (׿×) out--
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Quoting TexasHurricane:
stormwatcherCI

not working....
Not yet. Try again right under whichever comment you want to quote.
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1818. Seastep
Quoting AllStar17:
I think shear is less than maps are indicating, this looks like it is developing somewhat.


I agree. And I suspect it is from the energy that is being kept in the carib... not the same effect as normal. JMO.
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Quoting SomeRandomTexan:
looks like its forming new T-storms near the vort as well.... could be interesting.. shear doesn't look like 30 knots though just looking at the cloud tops.. it is getting sheared but thats just my opinion


I agree it is being sheared, but I do not think it is undergoing 30 knots of it, I think it is less.
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when does the CIMSS site update? i wanna see
what the upper divergence/lower convergence is
doing and updated shear tendencies.
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I'm clicking on it but it isn't working....
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Quoting Tazmanian:
ok what did i miss


well theres a decent wave coming off africa
and a huge flare up in the east caribbean.
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stormwatcherCI

not working....
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looks like its forming new T-storms near the vort as well.... could be interesting.. shear doesn't look like 30 knots though just looking at the cloud tops.. it is getting sheared but thats just my opinion
Member Since: August 30, 2007 Posts: 0 Comments: 1837
did it work?
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Quoting KoritheMan:
I don't even have to look at CIMSS shear maps to know that the central Caribbean tropical wave is undergoing some westerly shear, and is about to run into the bulk of it. This is evident by cirrus clouds streaming eastward toward the western end of the wave.


Yup
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Quoting KoritheMan:
I don't even have to look at CIMSS shear maps to know that the central Caribbean tropical wave is undergoing some westerly shear, and is about to run into the bulk of it. This is evident by cirrus clouds streaming eastward toward the western end of the wave.


Yes, the wave will probably dissipate. The shear is just too high. I think the weather mets in here would agree!
Member Since: July 17, 2008 Posts: 0 Comments: 1242
Quoting TexasHurricane:
can someone please tell me how to quote?
On the left below each passage it says action:quote. Click on that and the section to type your comment will come up. When you finish click on post comment.
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Quoting TexasHurricane:
can someone please tell me how to quote?


Click "quote" on whatever post you would like to quote (it is at the bottom of the post)
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1806. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 163 Comments: 52257
Quoting stormwatcherCI:
Thanks for your help. I keep trying but so far no good.


All you do is click "Link" and enter in the image URL (right click on the image) and click "View Image" and then copy the URL and paste it into the URL box on the post.
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can someone please tell me how to quote?
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NHC discussion about the tropical wave in ECaribb:

A TROPICAL WAVE IS ALONG 64W S OF 20N MOVING W 15-20 KT. A WELL
DEFINED INVERTED-V LOW CLOUD BAND IS NOTED AND THE SSMI TOTAL
PRECIPITABLE WATER IMAGERY ALSO SHOWS A LARGE MOISTURE PLUME.
SCATTERED MODERATE CONVECTION IS OVER THE E CARIBBEAN FROM
10N-20N BETWEEN 60W-70W.

Doesn't sound concerning? What is a well-defined inverted V low cloud band?
Member Since: July 17, 2008 Posts: 0 Comments: 1242
I don't even have to look at CIMSS shear maps to know that the central Caribbean tropical wave is undergoing some westerly shear, and is about to run into the bulk of it. This is evident by cirrus clouds streaming eastward toward the western end of the wave.
Member Since: March 7, 2007 Posts: 521 Comments: 19119
Quoting AllStar17:
RGB Caribbean Loop/a>
Thanks for your help. I keep trying but so far no good.
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I think shear is less than maps are indicating, this looks like it is developing somewhat.
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ok what did i miss
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RGB Caribbean Loop/a>
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Quoting TexasHurricane:
"You can see it clearly on the RGB satellite picture"

Have a link to that?
I don't know how to post links. Maybe someone else can help please.
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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.