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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951. stormwatcherCI 12:26 AM GMT del 04 Luglio 2009
Quoting StSimonsIslandGAGuy: Well 1992 was an El Nino year and we got Andrew... 2004 was an El Nino year and we got Ivan Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

954. KEEPEROFTHEGATE (Mod) 12:31 AM GMT del 04 Luglio 2009
to all the american bloggers Member Since: Luglio 15, 2006 Posts: 144 Comments: 40670

955. stormwatcherCI 12:32 AM GMT del 04 Luglio 2009
2008 Gustav and Ike, both major hurricanes 8 days apart Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

956. stormwatcherCI 12:32 AM GMT del 04 Luglio 2009
Quoting KEEPEROFTHEGATE: to all the american bloggers same to you Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

957. stormwatcherCI 12:33 AM GMT del 04 Luglio 2009
Quoting stormwatcherCI: 2008 Gustav and Ike, both major hurricanes 8 days apart Sorry, actually 7 days apart. Gustav Aug 25 and Ike Sept 1 Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

958. 147257 12:33 AM GMT del 04 Luglio 2009
well i voted for him ;) a small thing to ask for all the wonderfull blogs you posted jeff masters :) keep the work up i'm enjoying it for 3 to 4 straight years :) Member Since: Agosto 2, 2005 Posts: 7 Comments: 68

960. CybrTeddy 12:37 AM GMT del 04 Luglio 2009
Quoting StSimonsIslandGAGuy: Ike wasn't a major hurricane. He was 'just' a two. Isabel and Fabian were 'just'a two also. Member Since: Luglio 8, 2005 Posts: 253 Comments: 20280

961. Dar9895 12:41 AM GMT del 04 Luglio 2009
Quoting CybrTeddy: Isabel and Fabian were 'just'a two also. Fabian was a Cat 3 with 120 MPH winds in Bermuda.

962. stormwatcherCI 12:41 AM GMT del 04 Luglio 2009
According to the NHC Ike was a major hurricane and there are other countries in the world besides the US and if I am not mistaken Ike made it to category 4 status and caused many deaths in the Caribbean. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

963. stormwatcherCI 12:42 AM GMT del 04 Luglio 2009
Quoting StSimonsIslandGAGuy: Ike wasn't a major hurricane. He was 'just' a two. Check NHC seasons archive on Ike and see if Ike was "just" a two Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

964. JRRP 12:42 AM GMT del 04 Luglio 2009
i do not like to use analogical years that are not of the active period of hurricanes....like 1992.. but i do not know Member Since: Agosto 16, 2007 Posts: 0 Comments: 4315

965. CybrTeddy 12:52 AM GMT del 04 Luglio 2009
I know. we're talking about US Landfalls. Ike and Frances were a 4, Isabel was a 5. (I ment to put Frances, some reason I put down Fabian) Member Since: Luglio 8, 2005 Posts: 253 Comments: 20280

966. SomeRandomTexan 01:06 AM GMT del 04 Luglio 2009
fish are biting here on the bayou Member Since: Agosto 30, 2007 Posts: 0 Comments: 1814

967. hahaguy 01:07 AM GMT del 04 Luglio 2009
Quoting CybrTeddy: I know. we're talking about US Landfalls. Ike and Frances were a 4, Isabel was a 5. (I ment to put Frances, some reason I put down Fabian) Scary thing is about Frances is they were calling her to be a cat 5 at landfall if she didn't get sheared. Member Since: Agosto 12, 2007 Posts: 2 Comments: 2838

968. Patrap 01:08 AM GMT del 04 Luglio 2009
Member Since: Luglio 3, 2005 Posts: 372 Comments: 111635

969. stormwatcherCI 01:19 AM GMT del 04 Luglio 2009
Quoting Patrap: I don't think some people realize the damage a major hurricane can do and like you sday, it only takes one. Ivan devastatedGrand Cayman but we were blessed enough to have have two deaths, both which could have been prevented but the after effects were life changing for a lot of people. Myself included. We need to love one another and thank God for every day we have. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

970. KEEPEROFTHEGATE (Mod) 01:19 AM GMT del 04 Luglio 2009
wave train has commence look beyond west africa to the north of tazman over the arab sea something comes soon Member Since: Luglio 15, 2006 Posts: 144 Comments: 40670

971. AllStar17 01:19 AM GMT del 04 Luglio 2009
There definitely is rotation at 11 N 61 W, look closely, and some convection firing right over it. Link Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

972. stormwatcherCI 01:25 AM GMT del 04 Luglio 2009
Quoting AllStar17: There definitely is rotation at 11 N 61 W, look closely, and some convection firing right over it. Link It is pretty evident. I noticed it a while ago. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

973. Ossqss 01:25 AM GMT del 04 Luglio 2009
Howdy and Happy 4th! What happened on the 4th in weather history? Here is one interesting item that is quite amazing. Cheers ! 1956 - A world record for the most rain in one minute was set at Unionville, MD, with a downpour of 1.23 inches. (The Weather Channel) (The National Severe Storms Forecast Center This day in weather history You can select any day you want :) Member Since: Giugno 12, 2005 Posts: 6 Comments: 8154

974. hahaguy 01:26 AM GMT del 04 Luglio 2009
Quoting Ossqss: Howdy and Happy 4th! What happened on the 4th in weather history? Here is one interesting item that is quite amazing. Cheers ! 1956 - A world record for the most rain in one minute was set at Unionville, MD, with a downpour of 1.23 inches. (The Weather Channel) (The National Severe Storms Forecast Center This day in weather history You can select any day you want :) Cool link Member Since: Agosto 12, 2007 Posts: 2 Comments: 2838

975. AllStar17 01:27 AM GMT del 04 Luglio 2009
If the rotation and convection persists, IMO I think the NHC should at least mention it in the TWO. And many bloggers on this blog have been pointing out that the environment should become more conducive over the next few days. Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

976. AllStar17 01:27 AM GMT del 04 Luglio 2009
Quoting stormwatcherCI: It is pretty evident. I noticed it a while ago. While I just logged on, and noticed it. Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

977. ALCoastGambler 01:28 AM GMT del 04 Luglio 2009
Don't remind me of Camille. Born and raised in Gulfport, MS

978. KEEPEROFTHEGATE (Mod) 01:31 AM GMT del 04 Luglio 2009
Member Since: Luglio 15, 2006 Posts: 144 Comments: 40670

979. Patrap 01:31 AM GMT del 04 Luglio 2009
Well friend,a lot of us did,I grew up every summer in Bay St.Louis offa 90 by Louise's Gift Shop and Waveland Ave.,near the Beach,over the tracks. Was a Long Summer 69 was,the Lunar Landing July 20th,then Camille a Month Later. Member Since: Luglio 3, 2005 Posts: 372 Comments: 111635

980. ALCoastGambler 01:35 AM GMT del 04 Luglio 2009
Quoting Patrap: Well friend,a lot of us did,I grew up every summer in Bay St.Louis offa 90 by Louise's Gift Shop and Waveland Ave.,near the Beach,over the tracks. Was a Long Summer 69 was,the Lunar Landing July 20th,then Camille a Month Later. LOL not that old. barely remember the hurricane....lol

981. tropicfreak 01:35 AM GMT del 04 Luglio 2009
Quoting Acemmett90: nice view of both waves Theres another nice one coming off Africa. Member Since: Settembre 2, 2006 Posts: 110 Comments: 6566

982. presslord 01:36 AM GMT del 04 Luglio 2009
KOG...thanks for getting it... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10377

983. Ossqss 01:38 AM GMT del 04 Luglio 2009
Camille was the benchmark cane for decades. One that many younger folks don't recall. Link Member Since: Giugno 12, 2005 Posts: 6 Comments: 8154

984. KEEPEROFTHEGATE (Mod) 01:39 AM GMT del 04 Luglio 2009
ok Member Since: Luglio 15, 2006 Posts: 144 Comments: 40670

985. presslord 01:40 AM GMT del 04 Luglio 2009
KOG...thanks for getting it... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10377

987. presslord 01:41 AM GMT del 04 Luglio 2009
I dunno why that posted twice...computer must be drunk... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10377

988. ALCoastGambler 01:42 AM GMT del 04 Luglio 2009
Quoting Ossqss: Camille was the benchmark cane for decades. One that many younger folks don't recall. Link My mother bought every book they sold on the hurricane. I did the same for my kids after Katrina.

989. tropicfreak 01:42 AM GMT del 04 Luglio 2009
Quoting btwntx08: Looks loke some of the activity in the EPac is extending over to the carribean. Also I noticed a Blob over cuba. When it gets back over waters, does it have the chance for development? Member Since: Settembre 2, 2006 Posts: 110 Comments: 6566

990. AllStar17 01:45 AM GMT del 04 Luglio 2009
Quoting tropicfreak: Looks loke some of the activity in the EPac is extending over to the carribean. Also I noticed a Blob over cuba. When it gets back over waters, does it have the chance for development? No, it is just typical land thunderstorms Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

991. JRRP 01:56 AM GMT del 04 Luglio 2009
Member Since: Agosto 16, 2007 Posts: 0 Comments: 4315

992. K8eCane 02:14 AM GMT del 04 Luglio 2009
Quoting KEEPEROFTHEGATE: wave train has commence look beyond west africa to the north of tazman over the arab sea something comes soon my grandma used to say "sometimes a bad wind blows" Member Since: Aprile 26, 2006 Posts: 5 Comments: 2375

993. K8eCane 02:15 AM GMT del 04 Luglio 2009
OMG i cant believe i posted that Member Since: Aprile 26, 2006 Posts: 5 Comments: 2375

994. kmanislander 02:18 AM GMT del 04 Luglio 2009
The next Invest is likely to be 32N 40W Quikscat shows a well defined surface low there. Member Since: Agosto 19, 2006 Posts: 0 Comments: 14939

995. fsumet 02:19 AM GMT del 04 Luglio 2009
This year will NOT be like 2004 in terms of El Nino. Just look at the maps below for the same dates. The sst anomalies weren't similar to 2009 till the end of October. This El Nino will not be weak like 2004, it will be at least a moderate El Nino. June 29, 2004 http://www.osdpd.noaa.gov/PSB/EPS/SST/data/anomnight.6.29.2004.gif June 29, 2009 http://www.osdpd.noaa.gov/PSB/EPS/SST/data/anomnight.6.29.2009.gif October 30, 2004 http://www.osdpd.noaa.gov/PSB/EPS/SST/data/anomnight.10.30.2004.gif Member Since: Agosto 23, 2006 Posts: 0 Comments: 159

996. kmanislander 02:20 AM GMT del 04 Luglio 2009
Quikscat for 32N 40W Member Since: Agosto 19, 2006 Posts: 0 Comments: 14939

997. Ossqss 02:32 AM GMT del 04 Luglio 2009
A few hours old (the link ), but lots of busy and odd looking stuff going on up there Kman, for sure. Link Member Since: Giugno 12, 2005 Posts: 6 Comments: 8154

998. homelesswanderer 02:39 AM GMT del 04 Luglio 2009
Quoting SomeRandomTexan: albeit that 1 of the strongest canes to hit the area was in an El Nino year and that was Hurricane Audrey i think it was 1957 If ever there was an argument of pressure vs category this might just be it. I was amazed at the comparisons. Almost mirror images. Member Since: Agosto 15, 2008 Posts: 10 Comments: 3665

999. jeffs713 02:39 AM GMT del 04 Luglio 2009
Quoting StSimonsIslandGAGuy: Ike wasn't a major hurricane. He was 'just' a two. Ike hit the Turks and Caicos Islands as a Cat 4, and also hit Cuba as a Cat 4. Also... Ike put out a heck of a lot of damage for "just a cat 2". Check out my current blog for some examples. Member Since: Agosto 3, 2008 Posts: 15 Comments: 5722

1000. homelesswanderer 02:44 AM GMT del 04 Luglio 2009
Quoting jeffs713: Ike hit the Turks and Caicos Islands as a Cat 4, and also hit Cuba as a Cat 4. Also... Ike put out a heck of a lot of damage for "just a cat 2". Check out my current blog for some examples. Yes he did. That was one terrible Cat 2. Member Since: Agosto 15, 2008 Posts: 10 Comments: 3665

1001. Ossqss 02:45 AM GMT del 04 Luglio 2009
Just helpin to turn the page. L8R Member Since: Giugno 12, 2005 Posts: 6 Comments: 8154

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