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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501. SomeRandomTexan 05:15 AM GMT del 03 Luglio 2009
Quoting Chicklit: care about yourself first for you are the only one who sustains you. now i must say this is true... im all about caring for yourself and not letting someone have to take care of ya... :) Member Since: Agosto 30, 2007 Posts: 0 Comments: 1814

504. Chicklit 05:17 AM GMT del 03 Luglio 2009
Quoting hunkerdown: \ Cuotos Cave VA Souvineer Ceramic Chicken Cuotos cave VA souvineer chicken. Great little chicken with gold trim. Says Cuotos cave renn(a couple of the letters are missing here) VA. Nice older ceramic chicken, probably from the 1950 era. Gold has a little wear, but otherwise mint. $22.00 @ GoAntiques.com Okay, my idea is... How many ways can you cook chicken! Then substitute alligator. Member Since: Luglio 11, 2006 Posts: 14 Comments: 10254

506. hunkerdown 05:20 AM GMT del 03 Luglio 2009
Quoting hurricane2009: I understood him, just figure it out its not that hard geez now we got people attacking Taz for his language? Maybe there are two full moons tonight We have been teasing Taz about his posts for a long time. There is no harm meant by it. I think he knows it. Member Since: Agosto 15, 2008 Posts: 0 Comments: 2514

508. Levi32 05:22 AM GMT del 03 Luglio 2009
Quoting hurricane2009: I understood him, just figure it out its not that hard geez now we got people attacking Taz for his language? Maybe there are two full moons tonight Being transported to Jupiter would explain that.....oh and guess what? There's a permanent hurricane there so we're never lacking for something to track =) --------------------------------------------------------- 0z GFS still showing a stalling tail-end of a front that has the potential to cause trouble next week: Member Since: novembre 24, 2005 Posts: 586 Comments: 25459

509. Chicklit 05:22 AM GMT del 03 Luglio 2009
Whenever Portlight wants me to donate a box of my novel, ending with the Hurricane Season of 2004, just let me know and I will send them a complimentary 60 copies of the book. Member Since: Luglio 11, 2006 Posts: 14 Comments: 10254

510. Chicklit 05:23 AM GMT del 03 Luglio 2009
Levi, I cannot read those maps. Please translate. By the way, I am getting an 'A' or at the very least a 'B' in my 'Grant Writing and Contract Administration' class at UCF...just thought y'all should know that I'm not a complete slacker. Expect an 'A' of course. Member Since: Luglio 11, 2006 Posts: 14 Comments: 10254

511. hunkerdown 05:24 AM GMT del 03 Luglio 2009
Quoting hurricane2009: sorry I am from the world or reality and that didnt look like teasing to me. Usually the smile face is not indicative of seriousness. Member Since: Agosto 15, 2008 Posts: 0 Comments: 2514

515. Chicklit 05:26 AM GMT del 03 Luglio 2009
Hurricane09 are you a reincarnation of JFVHurricane? Member Since: Luglio 11, 2006 Posts: 14 Comments: 10254

516. Levi32 05:27 AM GMT del 03 Luglio 2009
Quoting Chicklit: Levi, I cannot read those maps. Please translate. It's showing an area of enhanced precipitation and lower pressure on the tail-end of a cold front off the North Carolina coast. It was showing a closed low on other runs but not this one. The ECMWF also shows a low. Depending on if the low gets rapidly carried NE and out to sea or not, this could sit and try to become subtropical or tropical in 4-8 days. Member Since: novembre 24, 2005 Posts: 586 Comments: 25459

518. hunkerdown 05:28 AM GMT del 03 Luglio 2009
Quoting hurricane2009: Do hurricanes on Jupiter have pinhole eyes? At 144 hours out, the GFS is predicting a Cat 7 on Jupiter, with a pinhole eye. Member Since: Agosto 15, 2008 Posts: 0 Comments: 2514

520. Chicklit 05:29 AM GMT del 03 Luglio 2009
Oh...that's funny, Hurricane. (please don't ban me...thinking of weather topic gimme a break!) Member Since: Luglio 11, 2006 Posts: 14 Comments: 10254

521. KoritheMan 05:29 AM GMT del 03 Luglio 2009
Quoting Chicklit: Hurricane09 are you a reincarnation of JFVHurricane? He isn't JFV. Member Since: Marzo 7, 2007 Posts: 409 Comments: 15449

522. hunkerdown 05:30 AM GMT del 03 Luglio 2009
Quoting hurricane2009: really read the last post then lol But you quoted me... Member Since: Agosto 15, 2008 Posts: 0 Comments: 2514

524. jeffs713 05:30 AM GMT del 03 Luglio 2009
Quoting pearlandaggie: 505. not attacking Taz, just enlightening him to a problem that's been around for several years. his previous response has been to whine about it until other posters affirm his poor communications skills. all i'm saying is that he brush up on his ability to communicate on an ENGLISH board before posting on it--that's all. i'm sure this will be misinterpreted as a "racist" thing as most things of this sort are....... Yeah, but it is still pretty darn harsh to call someone out like that on the blog. A WUMail would have been more appropriate. Member Since: Agosto 3, 2008 Posts: 15 Comments: 5722

525. beell 05:32 AM GMT del 03 Luglio 2009
short people, jeffs-there everywhere i guess Member Since: Settembre 11, 2007 Posts: 125 Comments: 12884

527. Chicklit 05:33 AM GMT del 03 Luglio 2009
Quoting hurricane2009: huh? You're right, Hurricane. Lost my handle there for a minute. Member Since: Luglio 11, 2006 Posts: 14 Comments: 10254

529. Chicklit 05:36 AM GMT del 03 Luglio 2009
Quoting hurricane2009: It happens to the best of us lol Thanks. At least we can relate. And I still have a productive job! Back to the weather... Member Since: Luglio 11, 2006 Posts: 14 Comments: 10254

531. KEEPEROFTHEGATE (Mod) 05:40 AM GMT del 03 Luglio 2009
well good evening or i should say good morning i am really seeing some true colours on here been lurking for a bit now no fate but that which we make for ourselves good night all time to catch some zzzzzzz Member Since: Luglio 15, 2006 Posts: 143 Comments: 40619

533. jeffs713 05:40 AM GMT del 03 Luglio 2009
Quoting beell: short people, jeffs-there everywhere i guess Sadly, you are very right. I know most of the time on the blog, it is just someone losing their cool a bit, or just typing before thinking. (and that may be the case here) It would just be nice that everyone remembers that when you type here, it is just like you said it. People will remember it, and people will judge you for what you say here. Everyone has their own personality, and everyone has their own merits, too. Some people are more blunt than others, some are painfully PC. All I ask anyone to do is maintain some civil decorum, and respect others opinions. I obviously can't enforce it here (although my poof list is growing), but I definitely enforce it on my own blog. Member Since: Agosto 3, 2008 Posts: 15 Comments: 5722

534. jeffs713 05:42 AM GMT del 03 Luglio 2009
Quoting StSimonsIslandGAGuy: The Great Red Spot is actually an anticylone--high pressure center ;) Well, when your atmosphere is several thousands of miles thick... I'm pretty sure that high pressure can have a pretty dramatic hurricane-like appearance, since that would imply upwelling from lower levels. Member Since: Agosto 3, 2008 Posts: 15 Comments: 5722

535. Levi32 05:42 AM GMT del 03 Luglio 2009
Quoting StSimonsIslandGAGuy: The Great Red Spot is actually an anticylone--high pressure center ;) Huge pressure gradient though means it's basically a hurricane lol. Maybe it's not even a pressure gradient....the whole planet is gas after all. I'm not sure how it works. Member Since: novembre 24, 2005 Posts: 586 Comments: 25459

537. beell 05:45 AM GMT del 03 Luglio 2009
Cheers, and Goodnight! Member Since: Settembre 11, 2007 Posts: 125 Comments: 12884

538. Chicklit 05:45 AM GMT del 03 Luglio 2009
There are 70 people on my 'poof'list. I put them back in based on others' recommendations, quotes. Member Since: Luglio 11, 2006 Posts: 14 Comments: 10254

541. Chicklit 05:49 AM GMT del 03 Luglio 2009
'night all...it's a holiday weekend. Enjoy. Member Since: Luglio 11, 2006 Posts: 14 Comments: 10254

542. pearlandaggie 05:49 AM GMT del 03 Luglio 2009
524. true........i'm sure it's been done many, many times before. the guy refuses to use standard English and people on this board just dismiss it.......well, i've called it out for the benefit of ALL readers! Member Since: Settembre 14, 2007 Posts: 3 Comments: 3963

543. tennisgirl08 05:54 AM GMT del 03 Luglio 2009
pearlandaggie - you are not very nice! What's brewing in the tropics tonight, or in other words, the blog tonight?? LOL!! Member Since: Luglio 17, 2008 Posts: 0 Comments: 1242

545. Levi32 05:56 AM GMT del 03 Luglio 2009
Quoting tennisgirl08: pearlandaggie - you are not very nice! What's brewing in the tropics tonight, or in other words, the blog tonight?? LOL!! LOL good one! Nothing in the tropics. We may have to keep an eye on the Carolina coasts next week, and might also get an east Pacific storm, but for now it's all quiet. I hope it stays that way tomorrow so I can post a blog I've prepared for something else, and I can only post such things during quiet periods lol. Member Since: novembre 24, 2005 Posts: 586 Comments: 25459

546. pearlandaggie 05:57 AM GMT del 03 Luglio 2009
543. actually, i'm a very NICE person so long as the discussion adheres to a simple baseline...that being the TRUTH. i get a bit cranky when the discussion strays from the truth OR THE ENGLISH LANGUAGE....... Member Since: Settembre 14, 2007 Posts: 3 Comments: 3963

547. Houstonweathergrl 06:01 AM GMT del 03 Luglio 2009
Quoting pearlandaggie: 543. actually, i'm a very NICE person so long as the discussion adheres to a simple baseline...that being the TRUTH. i get a bit cranky when the discussion strays from the truth OR THE ENGLISH LANGUAGE....... Member Since: Agosto 8, 2008 Posts: 0 Comments: 135

548. tennisgirl08 06:02 AM GMT del 03 Luglio 2009
Quoting pearlandaggie: i know this is in direct conflict with some of you liberal do-gooders. just send me a PM if you want to continue the discussion. otherwise, support me in the fact that Taz needs to speak better English before posting on this board. and how would you describe yourself? as a conservative a who is an intellectual know-it-all. likes to hear himself talk and speaks excellent English. At least Taz talks about the weather on a "weather" blog. I have been lurking and all you have done is berade others and talk politics. Go carry on your intellectual nonsense in some other blog! POOF!!! Member Since: Luglio 17, 2008 Posts: 0 Comments:** 1242

550. tennisgirl08 06:05 AM GMT del 03 Luglio 2009
Quoting Levi32: LOL good one! Nothing in the tropics. We may have to keep an eye on the Carolina coasts next week, and might also get an east Pacific storm, but for now it's all quiet. I hope it stays that way tomorrow so I can post a blog I've prepared for something else, and I can only post such things during quiet periods lol. all's quiet for now. good to know. Member Since: Luglio 17, 2008 Posts: 0 Comments: 1242

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