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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1097. cg2916
Quoting Weather456:
Tropics Relatively Quiet: Extratropical Invest 94L


Why didn't you mention the EPAC area?
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Tropics Relatively Quiet: Extratropical Invest 94L

Member Since: July 24, 2005 Posts: 407 Comments: 19076
1095. Greyelf
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AL 94 2009070412 BEST 0 319N 386W 35 1009 EX

Extratropical
Member Since: September 23, 2005 Posts: 14 Comments: 11154
Are the Azores a very populated country ?
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Quoting GeoffreyWPB:
I see 94L is up on the Navy site.
31.9w 38.6n 1009 mb 35kts
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I see 94L is up on the Navy site.
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Living in America
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North Korea test-fires more missiles, says Seoul
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latest quikscat
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1087. cg2916
Quoting Weather456:
Good morning all, the area of near the Azores has fire some deep convection for something that north. I'll have an update soon.

That's not the only thing firing something. Check CNN.
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Good morning. Happy 4th everyone!
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Quoting Weather456:
Good morning all, the area of near the Azores has fire some deep convection for something that north. I'll have an update soon.
Good morning. Looking forward to your update.
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1084. cg2916
Happy 4th of July! Let's be Patriotic by only posting sattelitte images with red, white and blue!
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Morning all!!! Happy 4th of July!!! Another hot and dry day in Katy Tx....
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Good morning all, the area of near the Azores has fire some deep convection for something that north. I'll have an update soon.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
Quoting stormpetrol:
If it doesn't get named within the next 6 hours it probably never will.
The NHC still has it code yellow. Wonder why when it looks pretty organized to me.
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If it doesn't get named within the next 6 hours it probably never will.
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Quoting stormwatcherCI:
I would rather see that where it is than in the Caribbean.


you got that right!
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That quikscat is showing if I'm reading correctly some 45 knot winds on the east side of the circulation, i've seen named TSs look worse that that.
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Quoting stormpetrol:
Happy 4th Everyone. That blob looks more tropical to me this morning, might have the first TS of the season.
Same to you.
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Quoting stormpetrol:
I would rather see that where it is than in the Caribbean.
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Happy 4th Everyone. That blob looks more tropical to me this morning, might have the first TS of the season.
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The Caribbean looks kinda weird to me.
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Quoting Joanie38:
I just have this STRANGE feeling the tropics are gonna start getting active very sooooon. Just my gut instinct...lol...
Sure hope not but unfortunately we don't have any control over it. Mother Nature tends to do whatever she wants.
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Quoting Joanie38:


Good Morning Stormwatcher..:)
Good morning and Happy 4th !
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Quoting Joanie38:
I just have this STRANGE feeling the tropics are gonna start getting active very sooooon. Just my gut instinct...lol...


How true are your gut feelings?
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Did you see the new situation of the Atlantic blob? Another controversial system (lol).

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I just have this STRANGE feeling the tropics are gonna start getting active very sooooon. Just my gut instinct...lol...
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Quoting stormwatcherCI:
The AOI SW of the Azores is looking very healthy this morning.


Good Morning Stormwatcher..:)
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The AOI SW of the Azores is looking very healthy this morning.
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Good Morning everyone!!! HAPPY 4TH OF JULY ALL!!!
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hello anyone here?
helloooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo

what happened to the EPAC monster storm?
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1063. pottery
Happy Independence Day, friends. From Trinidad.

I see that the SAL is in retreat, and that things are still quiet in the Trop. Atl. The wave that passed through has left the Island wet and cool. First time this season it has felt like rainy season.
Not much to anticipate from the weather in the Atlantic. Looks to remain tranquil for a while yet.
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Quoting Levi32:


The tropical wave will be almost nothing by tomorrow and you guys will have forgotten about it lol.
Its already poofing. Fading quickly, and its coming up on dmax, too.
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SomeRandomTexan

true....
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1060. K8eCane
Quoting hurricane23:


Out to sea garbage...



one can only hope...
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Quoting TexasHurricane:
Happy 4th of July all....

Will the tropics be more active next week? Hmmmm.....


it wouldn't take much to beat this past week..haha
Member Since: August 30, 2007 Posts: 0 Comments: 1844
1058. Levi32
Quoting tennisgirl08:


Levi - any thoughts on TampaSpin's view of the ULL?? Just curious. Also, I see that the tropical wave in the Eastern caribbean is still firing convection. Is this still expected to get torn apart by shear?


While I can see why Tampa would think the ULL looks like that, it's not in the right position to form a surface low under it. It will be dissipating and moving SW over the next few days.

The tropical wave will be almost nothing by tomorrow and you guys will have forgotten about it lol.
Member Since: November 24, 2005 Posts: 635 Comments: 26647
1057. Rodek
Happy 4th everyone!

Let's be happy that we're not celebrating the 4th named storm of the season!

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Happy 4th of July all....

Will the tropics be more active next week? Hmmmm.....
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HAPPY 4TH of July everyone. Goodnight.
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Happy 4th Of July!
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1053. K8eCane
Quoting presslord:
K8....You didn't say "the Carolinas", did you?


i did by mistake...it slipped out...i dont know why i did it but i did...LOLOLOLOL
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AOI

AOI
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Happy Birthday USA!!
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Happy Birthday... or Independence Day... or is it both? :)
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Quoting TampaSpin:


Its currently an Upper Level Low....i believe this might try to work its way down to a surface LOw in time.....It just appears to be the type that would do such a thing.....Look at this loop South of the Western Tip of Cuba......Link


Levi - any thoughts on TampaSpin's view of the ULL?? Just curious. Also, I see that the tropical wave in the Eastern caribbean is still firing convection. Is this still expected to get torn apart by shear?
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KEH thanks for the history lesson! I love history especially war stuff
Member Since: August 30, 2007 Posts: 0 Comments: 1844
1047. Levi32
Quoting K8eCane:


ok thanks Levi
he just used to be here a lot
are you still thinking some sort of low off Carolinas next week?


I think we'll see a low try to develop on the tail of that front, but whether it becomes warm-core or not is something we won't know until it happens.
Member Since: November 24, 2005 Posts: 635 Comments: 26647

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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.