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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1051. tennisgirl08 05:02 AM GMT del 04 Luglio 2009
Happy Birthday USA!! Member Since: Luglio 17, 2008 Posts: 0 Comments: 1242

1052. Orcasystems 05:03 AM GMT del 04 Luglio 2009
AOI AOI Member Since: Ottobre 1, 2007 Posts: 77 Comments: 26077

1053. K8eCane 05:04 AM GMT del 04 Luglio 2009
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 Member Since: Aprile 26, 2006 Posts: 5 Comments: 2375

1054. TheWeatherMan504 05:10 AM GMT del 04 Luglio 2009
Happy 4th Of July! Member Since: Maggio 18, 2008 Posts: 2 Comments: 1037

1055. messageinabottle 05:15 AM GMT del 04 Luglio 2009
HAPPY 4TH of July everyone. Goodnight. Member Since: Ottobre 17, 2008 Posts: 0 Comments: 29

1056. TexasHurricane 05:19 AM GMT del 04 Luglio 2009
Happy 4th of July all.... Will the tropics be more active next week? Hmmmm..... Member Since: Luglio 2, 2009 Posts: 0 Comments: 2811

1057. Rodek 05:23 AM GMT del 04 Luglio 2009
Happy 4th everyone! Let's be happy that we're not celebrating the 4th named storm of the season!

1058. Levi32 05:24 AM GMT del 04 Luglio 2009
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: novembre 24, 2005 Posts: 586 Comments: 25459

1059. SomeRandomTexan 05:24 AM GMT del 04 Luglio 2009
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: Agosto 30, 2007 Posts: 0 Comments: 1814

1060. K8eCane 05:24 AM GMT del 04 Luglio 2009
Quoting hurricane23: Out to sea garbage... one can only hope... Member Since: Aprile 26, 2006 Posts: 5 Comments: 2375

1061. TexasHurricane 05:29 AM GMT del 04 Luglio 2009
SomeRandomTexan true.... Member Since: Luglio 2, 2009 Posts: 0 Comments: 2811

1062. jeffs713 05:42 AM GMT del 04 Luglio 2009
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. Member Since: Agosto 3, 2008 Posts: 15 Comments: 5722

1063. pottery 10:27 AM GMT del 04 Luglio 2009
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. Member Since: Ottobre 24, 2005 Posts: 0 Comments: 20712

1064. AussieStorm 10:28 AM GMT del 04 Luglio 2009
hello anyone here? helloooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo what happened to the EPAC monster storm? Member Since: Settembre 30, 2007 Posts: 5 Comments: 13359

1065. Joanie38 10:49 AM GMT del 04 Luglio 2009
Good Morning everyone!!! HAPPY 4TH OF JULY ALL!!! Member Since: Giugno 16, 2008 Posts: 0 Comments: 461

1066. stormwatcherCI 10:52 AM GMT del 04 Luglio 2009
The AOI SW of the Azores is looking very healthy this morning. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1067. Joanie38 10:53 AM GMT del 04 Luglio 2009
Quoting stormwatcherCI: The AOI SW of the Azores is looking very healthy this morning. Good Morning Stormwatcher..:) Member Since: Giugno 16, 2008 Posts: 0 Comments: 461

1068. Joanie38 10:55 AM GMT del 04 Luglio 2009
I just have this STRANGE feeling the tropics are gonna start getting active very sooooon. Just my gut instinct...lol... Member Since: Giugno 16, 2008 Posts: 0 Comments: 461

1069. Cazatormentas 10:56 AM GMT del 04 Luglio 2009
Did you see the new situation of the Atlantic blob? Another controversial system (lol). Member Since: Ottobre 18, 2007 Posts: 0 Comments: 149

1070. AussieStorm 10:57 AM GMT del 04 Luglio 2009
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? Member Since: Settembre 30, 2007 Posts: 5 Comments: 13359

1071. stormwatcherCI 11:06 AM GMT del 04 Luglio 2009
Quoting Joanie38: Good Morning Stormwatcher..:) Good morning and Happy 4th ! Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1072. stormwatcherCI 11:08 AM GMT del 04 Luglio 2009
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. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1073. stormwatcherCI 11:09 AM GMT del 04 Luglio 2009
The Caribbean looks kinda weird to me. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1074. stormpetrol 11:19 AM GMT del 04 Luglio 2009
Member Since: Aprile 29, 2007 Posts: 0 Comments: 6404

1075. stormpetrol 11:21 AM GMT del 04 Luglio 2009
Happy 4th Everyone. That blob looks more tropical to me this morning, might have the first TS of the season. Member Since: Aprile 29, 2007 Posts: 0 Comments: 6404

1076. stormwatcherCI 11:22 AM GMT del 04 Luglio 2009
Quoting stormpetrol: I would rather see that where it is than in the Caribbean. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1077. stormwatcherCI 11:23 AM GMT del 04 Luglio 2009
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. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1078. stormpetrol 11:24 AM GMT del 04 Luglio 2009
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. Member Since: Aprile 29, 2007 Posts: 0 Comments: 6404

1079. stormpetrol 11:26 AM GMT del 04 Luglio 2009
Quoting stormwatcherCI: I would rather see that where it is than in the Caribbean. you got that right! Member Since: Aprile 29, 2007 Posts: 0 Comments: 6404

1080. stormpetrol 11:29 AM GMT del 04 Luglio 2009
If it doesn't get named within the next 6 hours it probably never will. Member Since: Aprile 29, 2007 Posts: 0 Comments: 6404

1081. stormwatcherCI 11:32 AM GMT del 04 Luglio 2009
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. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1082. Cavin Rawlins 11:40 AM GMT del 04 Luglio 2009
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: Luglio 24, 2005 Posts: 407 Comments: 19076

1083. beeleeva 11:42 AM GMT del 04 Luglio 2009
Morning all!!! Happy 4th of July!!! Another hot and dry day in Katy Tx.... Member Since: Luglio 22, 2008 Posts: 0 Comments: 81

1084. cg2916 11:54 AM GMT del 04 Luglio 2009
Happy 4th of July! Let's be Patriotic by only posting sattelitte images with red, white and blue! Member Since: Dicembre 21, 2007 Posts: 13 Comments: 2908

1085. stormwatcherCI 11:57 AM GMT del 04 Luglio 2009
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. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1086. homelesswanderer 12:08 PM GMT del 04 Luglio 2009
Good morning. Happy 4th everyone! Member Since: Agosto 15, 2008 Posts: 10 Comments: 3665

1087. cg2916 12:14 PM GMT del 04 Luglio 2009
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. Member Since: Dicembre 21, 2007 Posts: 13 Comments: 2908

1088. stormpetrol 12:17 PM GMT del 04 Luglio 2009
latest quikscat Member Since: Aprile 29, 2007 Posts: 0 Comments: 6404

1089. stormwatcherCI 12:17 PM GMT del 04 Luglio 2009
North Korea test-fires more missiles, says Seoul Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1090. HIEXPRESS 12:23 PM GMT del 04 Luglio 2009
Living in America Member Since: Ottobre 13, 2005 Posts: 4 Comments: 2153

1091. GeoffreyWPB 12:27 PM GMT del 04 Luglio 2009
I see 94L is up on the Navy site. Member Since: Settembre 10, 2007 Posts: 0 Comments: 9127

1092. stormwatcherCI 12:29 PM GMT del 04 Luglio 2009
Quoting GeoffreyWPB: I see 94L is up on the Navy site. 31.9w 38.6n 1009 mb 35kts Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1093. stormwatcherCI 12:30 PM GMT del 04 Luglio 2009
Are the Azores a very populated country ? Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1094. nrtiwlnvragn 12:32 PM GMT del 04 Luglio 2009
AL 94 2009070412 BEST 0 319N 386W 35 1009 EX Extratropical Member Since: Settembre 23, 2005 Posts: 11 Comments: 8943

1095. Greyelf 12:41 PM GMT del 04 Luglio 2009
Member Since: Giugno 5, 2007 Posts: 18 Comments: 838

1096. Cavin Rawlins 12:44 PM GMT del 04 Luglio 2009
Tropics Relatively Quiet: Extratropical Invest 94L Member Since: Luglio 24, 2005 Posts: 407 Comments: 19076

1097. cg2916 12:49 PM GMT del 04 Luglio 2009
Quoting Weather456: Tropics Relatively Quiet: Extratropical Invest 94L Why didn't you mention the EPAC area? Member Since: Dicembre 21, 2007 Posts: 13 Comments: 2908

1098. cg2916 12:53 PM GMT del 04 Luglio 2009
Quoting stormpetrol: latest quikscat Where is that area? Member Since: Dicembre 21, 2007 Posts: 13 Comments: 2908

1099. stormwatcherCI 12:54 PM GMT del 04 Luglio 2009
Quoting cg2916: Where is that area? 94L Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8043

1100. kmanislander 12:54 PM GMT del 04 Luglio 2009
Quoting Weather456: Tropics Relatively Quiet: Extratropical Invest 94L Good morning We have seen a few of these so far this year. I noted this area last night as the next likely Invest ( post 994 ) so no surprise there but as far as the Caribbean and MDR in the Atl. it would appear that shear will continue to hold down development for some time to come. Member Since: Agosto 19, 2006 Posts: 0 Comments: 14939

1101. cg2916 12:55 PM GMT del 04 Luglio 2009
Quoting stormwatcherCI: 94L That's not the Quikscat images I'm using. Member Since: Dicembre 21, 2007 Posts: 13 Comments: 2908

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