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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1751. Seastep 08:46 PM GMT del 05 Luglio 2009
How goes it WSter? Member Since: Settembre 9, 2008 Posts: 6 Comments: 3407

1752. stormwatcherCI 08:47 PM GMT del 05 Luglio 2009
Quoting Seastep: How goes it WSter? LOL Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8058

1754. presslord 08:50 PM GMT del 05 Luglio 2009
Gonna be kinda hard to take care of a baby in a dorm, I'd think... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10405

1755. Seastep 08:50 PM GMT del 05 Luglio 2009
Good, thanks. Where you at w/ the education? Member Since: Settembre 9, 2008 Posts: 6 Comments: 3407

1756. futuremet 08:51 PM GMT del 05 Luglio 2009
Quoting WeatherStudent: Thoughts, FM? The west caribbean will have to be watched during mid to late next week. Member Since: Luglio 19, 2008 Posts: 43 Comments: 4049

1759. Santamaria 08:53 PM GMT del 05 Luglio 2009
@ RMM34667 Thanks for the El Niño link. The article is pretty useless but Google gives more info on El Niño Modoki, a name you'll hear a lot more of. It will affect North Australian rainfall patterns mainly and as that continent is semi-arid anyway, rainfall problems are something they can do without. El Niño Modoki sets up further west in the Pacific, i.e closer to Australia than our normal El Niño which is closer to Ecuador/Peru.

1761. presslord 08:54 PM GMT del 05 Luglio 2009
...sure... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10405

1763. Ossqss 08:56 PM GMT del 05 Luglio 2009
Press,with all do respect, I just can't figure this Carolina thing out. Now this variant :) Member Since: Giugno 12, 2005 Posts: 6 Comments: 8154

1764. Seastep 08:56 PM GMT del 05 Luglio 2009
Quoting WeatherStudent: My senior year for my B.A. in Political Science. No... say it ain't so! ;) You've certainly honed some skills. Member Since: Settembre 9, 2008 Posts: 6 Comments: 3407

1765. presslord 08:56 PM GMT del 05 Luglio 2009
No...no...no... you said "thanks"....I said "sure".... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10405

1766. presslord 08:58 PM GMT del 05 Luglio 2009
Oss....maybe Carolina was a woman... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10405

1768. stormwatcherCI 09:02 PM GMT del 05 Luglio 2009
Quoting WeatherStudent: :), Thanks, I really appreciate that. My bad than, Press. I apologies. I think you are getting too used to being dissed you are unsure of a compliment. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8058

1769. AllStar17 09:02 PM GMT del 05 Luglio 2009
I do not see the wave in the Central Caribbean going into the EPAC, I see it going into the western or northwestern Caribbean, where, if it survives, may have a chance at development. Caribbean Visible Loop Member Since: Giugno 29, 2009 Posts: 13 Comments: 5145

1770. presslord 09:03 PM GMT del 05 Luglio 2009
...all is well... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10405

1771. Seastep 09:05 PM GMT del 05 Luglio 2009
Education is good. Certainly in the pure sense. Press... sincerely hope you had a nice 4th. Not memorial, but still... heartfelt thanks to patrap and others for defending the freedom of this great country. Member Since: Settembre 9, 2008 Posts: 6 Comments: 3407

1772. AllStar17 09:05 PM GMT del 05 Luglio 2009
INVEST 94E Infrared Loop Needs more organization and convection. Earlier it looked much better than it does now. Maybe it will rebound later. Heading into dry/stable environment. Member Since: Giugno 29, 2009 Posts: 13 Comments: 5145

1773. presslord 09:05 PM GMT del 05 Luglio 2009
Sea...it was restful...thanks...hope you're well... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10405

1774. KoritheMan 09:09 PM GMT del 05 Luglio 2009
Quoting WeatherStudent: Don't believe me, I could truly care less. Do you me or my mother, alright then. Been a smartass here lately, eh JFV? Member Since: Marzo 7, 2007 Posts: 421 Comments: 15641

1775. HurricaneSwirl 09:09 PM GMT del 05 Luglio 2009
Quoting AllStar17: INVEST 94E Infrared Loop Needs more organization and convection. Earlier it looked much better than it does now. Maybe it will rebound later. Heading into dry/stable environment. plus its moving into cooler SST's. the best shot of development for it was today. Member Since: Luglio 7, 2008 Posts: 2 Comments: 3237

1776. Ossqss 09:10 PM GMT del 05 Luglio 2009
Quoting presslord: Oss....maybe Carolina was a woman... Well, perhaps, but this Carolina Met, certainly seems to have a different take on usage of all 4 variants. Be well. L8R Carolina Weather Member Since: Giugno 12, 2005 Posts: 6 Comments: 8154

1778. presslord 09:14 PM GMT del 05 Luglio 2009
Quoting Ossqss: Well, perhaps, but this Carolina Met, certainly seems to have a different take on usage of all 4 variants. Be well. L8R Carolina Weather Oss...he's in North Carolina...that explains everything... Member Since: Agosto 13, 2007 Posts: 0 Comments: 10405

1779. KoritheMan 09:14 PM GMT del 05 Luglio 2009
Quoting WeatherStudent: No I wasn't. You should try and word posts like the one directed at press (admittedly, it was a misunderstanding on your part, but still, just take my advice) a bit more politely in the future, so that it doesn't get construed as a smartass comment. Member Since: Marzo 7, 2007 Posts: 421 Comments: 15641

1780. Ossqss 09:15 PM GMT del 05 Luglio 2009
Quoting presslord: Oss...he's in North Carolina...that explains everything... Understood, we have similar issues down here with the folks up there in Florgia :) Member Since: Giugno 12, 2005 Posts: 6 Comments: 8154

1782. cg2916 09:18 PM GMT del 05 Luglio 2009
I'm making this message from an iPod Touch! It really is amazing! Member Since: Dicembre 21, 2007 Posts: 13 Comments: 2954

1784. stormwatcherCI 09:19 PM GMT del 05 Luglio 2009
Quoting KoritheMan: You should try and word posts like the one directed at press (admittedly, it was a misunderstanding on your part, but still, just take my advice) a bit more politely in the future, so that it doesn't get construed as a smartass comment. That's what I was trying to tell him in post 1687. Not trying to be hard on him cause he is young and maybe still needs to learn it is not what you say but how you say it. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8058

1785. AllStar17 09:25 PM GMT del 05 Luglio 2009
Maybe I am seeing things, but can anyone else see rotation in the clouds, moving WNW/NW? Tropical Wave Visible Member Since: Giugno 29, 2009 Posts: 13 Comments: 5145

1786. KoritheMan 09:25 PM GMT del 05 Luglio 2009
Quoting stormwatcherCI: That's what I was trying to tell him in post 1687. Not trying to be hard on him cause he is young and maybe still needs to learn it is not what you say but how you say it. Yeah, exactly. Member Since: Marzo 7, 2007 Posts: 421 Comments: 15641

1787. stormwatcherCI 09:26 PM GMT del 05 Luglio 2009
Quoting AllStar17: Maybe I am seeing things, but can anyone else see rotation in the clouds, moving WNW/NW? Tropical Wave Visible Looks more westerly to me. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8058

1788. AllStar17 09:27 PM GMT del 05 Luglio 2009
Quoting stormwatcherCI: Looks more westerly to me. but do you see any rotation? Member Since: Giugno 29, 2009 Posts: 13 Comments: 5145

1789. KoritheMan 09:28 PM GMT del 05 Luglio 2009
Quoting AllStar17: Maybe I am seeing things, but can anyone else see rotation in the clouds, moving WNW/NW? Tropical Wave Visible It does appear that way. CIMSS data also supports this: 850 mb vorticity: Member Since: Marzo 7, 2007 Posts: 421 Comments: 15641

1790. stormwatcherCI 09:28 PM GMT del 05 Luglio 2009
Quoting AllStar17: but do you see any rotation? Yes. It's been there for a while now. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8058

1791. TexasHurricane 09:29 PM GMT del 05 Luglio 2009
Maybe I am seeing things, but can anyone else see rotation in the clouds, moving WNW/NW? Not really sure about the rotation part but it does seem to be blowing up..... Member Since: Luglio 2, 2009 Posts: 0 Comments: 2811

1792. hahaguy 09:29 PM GMT del 05 Luglio 2009
Decent looking wave off Africa . Member Since: Agosto 12, 2007 Posts: 2 Comments: 2838

1793. stormwatcherCI 09:30 PM GMT del 05 Luglio 2009
Quoting TexasHurricane: Maybe I am seeing things, but can anyone else see rotation in the clouds, moving WNW/NW? Not really sure about the rotation part but it does seem to be blowing up..... You can see it clearly on the RGB satellite picture. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8058

1794. HurricaneSwirl 09:30 PM GMT del 05 Luglio 2009
interesting. its gone to taking a dash-like shape, to taking a shape that looks like a plus, and now i think its trying to condense into a circle like shape. either its attempting to organize in the face of 30 knots of shear or its just the illusion of the convection.. Member Since: Luglio 7, 2008 Posts: 2 Comments: 3237

1795. TexasHurricane 09:32 PM GMT del 05 Luglio 2009
"You can see it clearly on the RGB satellite picture" Have a link to that? Member Since: Luglio 2, 2009 Posts: 0 Comments: 2811

1796. AllStar17 09:33 PM GMT del 05 Luglio 2009
Quoting hahaguy: Decent looking wave off Africa . Yep, we will have to track this too. Member Since: Giugno 29, 2009 Posts: 13 Comments: 5145

1797. stormwatcherCI 09:34 PM GMT del 05 Luglio 2009
Quoting TexasHurricane: "You can see it clearly on the RGB satellite picture" Have a link to that? I don't know how to post links. Maybe someone else can help please. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8058

1798. AllStar17 09:34 PM GMT del 05 Luglio 2009
RGB Caribbean Loop/a> Member Since: Giugno 29, 2009 Posts: 13 Comments: 5145

1799. Tazmanian 09:35 PM GMT del 05 Luglio 2009
ok what did i miss Member Since: Maggio 21, 2006 Posts: 5089 Comments: 111615

1800. AllStar17 09:36 PM GMT del 05 Luglio 2009
I think shear is less than maps are indicating, this looks like it is developing somewhat. Member Since: Giugno 29, 2009 Posts: 13 Comments: 5145

1801. stormwatcherCI 09:36 PM GMT del 05 Luglio 2009
Quoting AllStar17: RGB Caribbean Loop/a> Thanks for your help. I keep trying but so far no good. Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8058

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