Dr. Jeff Masters' WunderBlog

July hurricane outlook

Posted by: Dr. Jeff Masters, 07:26 PM GMT del 02 Luglio 2009

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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1701. Tropicsweatherpr 07:24 PM GMT del 05 Luglio 2009
12z ECMWF also has a closed low South of the Cape Verde islands at day 9. Link Member Since: Aprile 29, 2009 Posts: 64 Comments: 8166

1702. Gumbogator 07:26 PM GMT del 05 Luglio 2009
Am praying for a T.S. because everything is dying. We've had 8 days of 100-104 degree weather and heat indicies as high as 115 degrees for 1 month. Surely a seedling will form. Excessive heat potential in the GOM. Member Since: Agosto 21, 2008 Posts: 0 Comments: 97

1703. presslord 07:31 PM GMT del 05 Luglio 2009
Hi Ikester...and KOGster Member Since: Agosto 13, 2007 Posts: 0 Comments: 10377

1704. IKE 07:32 PM GMT del 05 Luglio 2009
Quoting presslord: Hi Ikester...and KOGster lol Member Since: Giugno 9, 2005 Posts: 23 Comments: 37044

1705. presslord 07:32 PM GMT del 05 Luglio 2009
...and K8ster...and CIster... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10377

1706. stormwatcherCI 07:33 PM GMT del 05 Luglio 2009
Quoting presslord: Hi Ikester...and KOGster What about "Presster" ????? Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8039

1707. presslord 07:35 PM GMT del 05 Luglio 2009
Quoting stormwatcherCI: What about "Presster" ????? ...fair enough... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10377

1708. Dr3w 07:35 PM GMT del 05 Luglio 2009
anybody notice the circulation south of Puerto Rico??? http://www.ssd.noaa.gov/goes/east/carb/loop-rb.html

1709. JRRP 07:38 PM GMT del 05 Luglio 2009
Member Since: Agosto 16, 2007 Posts: 0 Comments: 4308

1710. AllStar17 07:40 PM GMT del 05 Luglio 2009
This wave south of Puerto Rico is getting better organized and certainly has rotation. Any thoughts? Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1711. Dr3w 07:41 PM GMT del 05 Luglio 2009
oops.... i meant The Dominican Republic and Haiti

1713. Skyepony (Mod) 07:42 PM GMT del 05 Luglio 2009
Quoting AllStar17: Skyepony--- What are your thoughts on the wave in the C Caribbean? I think at most we may eventually be watching it as a swirl in the Bay of Campeche that hits Mexico. This week better odds would be something trying to pull together off the SE coast, like GA, Carolinas area. Member Since: Agosto 10, 2005 Posts: 144 Comments: 29327

1714. JRRP 07:42 PM GMT del 05 Luglio 2009
. Member Since: Agosto 16, 2007 Posts: 0 Comments: 4308

1715. Dr3w 07:42 PM GMT del 05 Luglio 2009
Quoting AllStar17: This wave south of Puerto Rico is getting better organized and certainly has rotation. Any thoughts? I say if shear would die down some this thing would probably develop into Ana.

1716. CybrTeddy 07:44 PM GMT del 05 Luglio 2009
Quoting AllStar17: This wave south of Puerto Rico is getting better organized and certainly has rotation. Any thoughts? Looks to me like a typical disorganized wave under heavy shear with convectin. Although we'll have to watch it as it moves into better conditions.. if it can survive that long. Member Since: Luglio 8, 2005 Posts: 253 Comments: 20240

1718. NOWCAST 07:45 PM GMT del 05 Luglio 2009
That convection is from the big wave that tolled off africa a few days ago it looks nice. Member Since: Agosto 11, 2007 Posts: 0 Comments: 10

1719. CaribBoy 07:45 PM GMT del 05 Luglio 2009
Good Afternoon dear wunderblogers. Did someone notice the cluster of storms just off the african coat near 10N 18W. It looks good with cold to very cold tops but I think it wont be able to keep its convective activity as it progresses westward through the central atlantic. Anyway... let's see how this thing evolves in coming days. http://www.ssd.noaa.gov/PS/TROP/DATA/RT/EATL/IR4/20.jpg Member Since: Ottobre 6, 2007 Posts: 0 Comments: 2848

1720. RMM34667 07:46 PM GMT del 05 Luglio 2009
All EL Nino's are not the same? Interesting read. Even if I don't understand 1/2 of it. Member Since: Settembre 7, 2006 Posts: 5 Comments: 910

1721. presslord 07:50 PM GMT del 05 Luglio 2009
Carolinas area. Gggggrrrrr!!!!! Member Since: Agosto 13, 2007 Posts: 0 Comments: 10377

1722. HurricaneSwirl 07:51 PM GMT del 05 Luglio 2009
Quoting CybrTeddy: Looks to me like a typical disorganized wave under heavy shear with convectin. Although we'll have to watch it as it moves into better conditions.. if it can survive that long. Actually it's not as typical as the 93L and the wave before that. its been hanging on ever since it got off of africa I think and has only been able to fire convection again now that its out of the dry environment.. This thing just may become our next invest if shear dies down. Member Since: Luglio 7, 2008 Posts: 2 Comments: 3237

1725. stormwatcherCI 07:53 PM GMT del 05 Luglio 2009
Quoting Dr3w: oops.... i meant The Dominican Republic and Haiti You're forgiven. They are all in the same general area. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8039

1726. fire635 07:57 PM GMT del 05 Luglio 2009
Quoting AllStar17: This wave south of Puerto Rico is getting better organized and certainly has rotation. Any thoughts? Shear is pretty high.. I'd be surprised to see any development any time soon Member Since: Giugno 24, 2006 Posts: 0 Comments: 478

1727. presslord 08:01 PM GMT del 05 Luglio 2009
1724...Pardon moi? Member Since: Agosto 13, 2007 Posts: 0 Comments: 10377

1728. KEEPEROFTHEGATE (Mod) 08:05 PM GMT del 05 Luglio 2009
AOI/XX/XX MARK 15.1N/69.8W Member Since: Luglio 15, 2006 Posts: 143 Comments: 40520

1729. SomeRandomTexan 08:08 PM GMT del 05 Luglio 2009
Dmin and still holding convection Member Since: Agosto 30, 2007 Posts: 0 Comments: 1814

1730. stormwatcherCI 08:08 PM GMT del 05 Luglio 2009
Although shear is high it seems to be backing off from yesterday when it was 40-50 kts around the same area Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8039

1732. KEEPEROFTHEGATE (Mod) 08:11 PM GMT del 05 Luglio 2009
Member Since: Luglio 15, 2006 Posts: 143 Comments: 40520

1733. HurricaneSwirl 08:14 PM GMT del 05 Luglio 2009
the only thing for sure is that the shear is gonna be a major player in this storm... shear tendencies are rising on the northern part of the storm, yet dropping on the southern side of the storm. why do all of the storms this season have to be so headache inducing? why cant they be simple :( Member Since: Luglio 7, 2008 Posts: 2 Comments: 3237

1734. K8eCane 08:18 PM GMT del 05 Luglio 2009
Quoting presslord: Carolinas area. Gggggrrrrr!!!!! HEY PRESLEY! Member Since: Aprile 26, 2006 Posts: 5 Comments: 2371

1736. Rmadillo 08:22 PM GMT del 05 Luglio 2009
Pressy?? Wish I had bought both those monitors so I could watch all the shows here at once. Member Since: Maggio 27, 2009 Posts: 0 Comments: 102

1737. Seastep 08:24 PM GMT del 05 Luglio 2009
hope everyone's 4th was nice.. DOI is what it is all about. Anyway, I'm certainly watching the E carib wave. Shear is high, but certainly in the upper levels the energy blown by that is being blocked. Very interesting area and atmosphere. If that shear drops off it could be significant. It's one of those "nuthin' or blow up." Looking like nothing based on shear... but very interesting atmosphere. Should be watched. Member Since: Settembre 9, 2008 Posts: 6 Comments: 3406

1738. HurricaneSwirl 08:24 PM GMT del 05 Luglio 2009
Quoting WeatherStudent: What are you even considering to be a so-called ''Storm''? Well it is a storm.. It has thunderstorms and rain so doesn't that make it a storm? Now it's not even close to a tropical storm, tropical depression, hurricane, etc. But it can still be a storm without being tropical, right? I never called it more than a simple thunderstorm. Member Since: Luglio 7, 2008 Posts: 2 Comments: 3237

1739. KEEPEROFTHEGATE (Mod) 08:27 PM GMT del 05 Luglio 2009
Member Since: Luglio 15, 2006 Posts: 143 Comments: 40520

1741. stormwatcherCI 08:32 PM GMT del 05 Luglio 2009
Quoting KEEPEROFTHEGATE: Seems to be holding it's own pretty good. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8039

1742. Rmadillo 08:32 PM GMT del 05 Luglio 2009
Hope you brought the big bag of pretzels, Keeper. Member Since: Maggio 27, 2009 Posts: 0 Comments: 102

1743. AllStar17 08:36 PM GMT del 05 Luglio 2009
It could definitely have a chance to develop if shear continues to die down IMO. Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1744. Seastep 08:37 PM GMT del 05 Luglio 2009
To be clearer... a lot of energy seems to be concentrating into a relatively small area. That's all. Interesting and I am watching. Would love some others' thoughts. Member Since: Settembre 9, 2008 Posts: 6 Comments: 3406

1745. HurricaneSwirl 08:40 PM GMT del 05 Luglio 2009
Upper divergence: Lower convergence: nothing really cyclonic going on right now. both upper divergence and lower convergence nearly match Member Since: Luglio 7, 2008 Posts: 2 Comments: 3237

1746. stormwatcherCI 08:40 PM GMT del 05 Luglio 2009
Quoting Seastep: To be clearer... a lot of energy seems to be concentrating into a relatively small area. That's all. Interesting and I am watching. Would love some others' thoughts. Isn't that how it usually starts ? Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8039

1747. futuremet 08:41 PM GMT del 05 Luglio 2009
Increasing moisture over west caribbean. Member Since: Luglio 19, 2008 Posts: 43 Comments: 4049

1748. presslord 08:42 PM GMT del 05 Luglio 2009
Wow! There appears to be something off North and South Carolina.... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10377

1749. JRRP 08:42 PM GMT del 05 Luglio 2009
Member Since: Agosto 16, 2007 Posts: 0 Comments: 4308

1751. Seastep 08:46 PM GMT del 05 Luglio 2009
How goes it WSter? Member Since: Settembre 9, 2008 Posts: 6 Comments: 3406

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