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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1551. IKE 02:39 PM GMT del 05 Luglio 2009
12Z NAM @ 72 hours....95L? Believable? Member Since: Giugno 9, 2005 Posts: 23 Comments: 37044

1552. AllStar17 02:39 PM GMT del 05 Luglio 2009
Quoting Weather456: Only 1 of 5 models decrease shear across the Western Caribbean - Navy NOGAPs, but the others, more reliable does not develop the feature and keeps wind shear unfavorable. Shear may drop but only to marginal values (20-25 knots). Currently, I'm not anticipating development. OK, but sounds like we will have some other things to track over the next few weeks according to the models. Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1553. KEHCharleston 02:40 PM GMT del 05 Luglio 2009
STUPID QUESTION TIME The usefulness of the Shear Tendency Chart for the last 24 hours, would depend on how fluid shear is - how quickly it changes? Does shear change over a matter of days or hours? What would be a better predictor. Shear Tendency Charts or Shear Models? I hope this makes sense. Member Since: Agosto 19, 2008 Posts: 6 Comments: 2490

1554. AllStar17 02:41 PM GMT del 05 Luglio 2009
Quoting IKE: 12Z NAM @ 72 hours....95L? Believable? Looks like it has it slamming into Belize. I do not know if that is a believable scenario or not. Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1555. futuremet 02:43 PM GMT del 05 Luglio 2009
Quoting IKE: 12Z NAM @ 72 hours....95L? Believable? Yes, shear will marginally favorable across the area. If you look at the 200mb levels, you will notice anticyclogenesis occurring right above the wave. Member Since: Luglio 19, 2008 Posts: 43 Comments: 4049

1556. Cavin Rawlins 02:44 PM GMT del 05 Luglio 2009
Probaly 93L all over again so model consensus is the ticket here. Member Since: Luglio 24, 2005 Posts: 407 Comments: 19076

1557. ajcamsmom2 02:45 PM GMT del 05 Luglio 2009
I sure would like a few nice slow, steady rain showers here...It is hot and dry...So dry we had to refrain from celebrating with fireworks last night... Does anyone have a link for the website that lists SST's for different areas on a month to month basis for the past and present??? Not the one with maps, the one with easy to understand numbers... Member Since: Marzo 15, 2008 Posts: 4 Comments: 2482

1558. IKE 02:45 PM GMT del 05 Luglio 2009
Quoting Weather456: Probaly 93L, all over again so model consensus is the ticket here. Good point. Fireworks on the blog. Probably best if it dissipates...lol. Member Since: Giugno 9, 2005 Posts: 23 Comments: 37044

1559. AllStar17 02:49 PM GMT del 05 Luglio 2009
Quoting futuremet: Yes, shear will marginally favorable across the area. If you look at the 200mb levels, you will notice anticyclogenesis occurring right above the wave. Do you believe it will landfall in Belize? Or do you think it will move more northerly and over the Yucatan and into the BOC or GOM? Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1560. futuremet 02:49 PM GMT del 05 Luglio 2009
Quoting Weather456: Probaly 93L all over again so model consensus is the ticket here. Yes lol Almost the same location, and steering patterns. But there is only one major difference--A ULL should not be too close to it hinder its development. Member Since: Luglio 19, 2008 Posts: 43 Comments: 4049

1561. WxLogic 02:50 PM GMT del 05 Luglio 2009
Future... & 456. I concur... NAM did a pretty good job forecasting 93L last time and we might actually end up with another 93L situation once again assuming subsequent runs do yield the same scenario with hopefully more model support than last time. Member Since: Agosto 14, 2008 Posts: 4 Comments: 4666

1562. futuremet 02:51 PM GMT del 05 Luglio 2009
Quoting AllStar17: Do you believe it will landfall in Belize? Or do you think it will move more northerly and over the Yucatan and into the BOC or GOM? That depends on how far north it goes. If it were to reach near the Yucatan channel, a track toward the northeast is plausible. lol keep in mind that forecast is from the NAM...so I am reluctant to make any quick conclusions. Member Since: Luglio 19, 2008 Posts: 43 Comments: 4049

1563. AllStar17 02:53 PM GMT del 05 Luglio 2009
I should not bother to watch the TWC Tropical Update anymore......it is something different every hour. First, Steve says last hour that they would watch the wave in the Caribbean for possible development....this hour, he says it will not form into a tropical cyclone. Now, more than likely it wont, but at least have some consistency in your updates!!! Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1564. futuremet 02:54 PM GMT del 05 Luglio 2009
Quoting AllStar17: I should not bother to watch the TWC Tropical Update anymore......it is something different every hour. First, Steve says last hour that they would watch the wave in the Caribbean for possible development....this hour, he says it will not form into a tropical cyclone. Now, more than likely it wont, but at least have some consistency in your updates!!! I only watch them when something actually forms. Member Since: Luglio 19, 2008 Posts: 43 Comments: 4049

1565. IKE 02:54 PM GMT del 05 Luglio 2009
Quoting futuremet: That depends on how far north it goes. If it were to reach near the Yucatan channel, a track toward the northeast is plausible. lol keep in mind that forecast is from the NAM...so I am reluctant to make any quick conclusions. Agree. NAM spins up things like 93L, that it had as a cane at one time. I wouldn't jump on the NAM. Member Since: Giugno 9, 2005 Posts: 23 Comments: 37044

1566. AllStar17 02:54 PM GMT del 05 Luglio 2009
Quoting futuremet: That depends on how far north it goes. If it were to reach near the Yucatan channel, a track toward the northeast is plausible. lol keep in mind that forecast is from the NAM...so I am reluctant to make any quick conclusions. Yeah, we need more model consensus. Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1567. futuremet 02:55 PM GMT del 05 Luglio 2009
06z WRF forecasting increasing moisture in the Caribbean. Member Since: Luglio 19, 2008 Posts: 43 Comments: 4049

1568. beell 02:55 PM GMT del 05 Luglio 2009
Quoting Weather456: ...Currently, I'm not anticipating development in the Carib. Shear is expected to be favorable in the GOM by then but unsure if it will amplify enough to enter the central GOM. A developing TUTT with its base near 15N 90W and TUTT axis extending NE to at least 25N 75W should keep higher levels of shear over the Carribean. Interaction with the TUTT should provide good upper support for continued convection. If this TUTT follows the recent pattern, the base will cut off and the TUTT lifts out to the NE and as you say, shear levels may ease. Member Since: Settembre 11, 2007 Posts: 124 Comments: 12807

1569. futuremet 02:56 PM GMT del 05 Luglio 2009
I'm out see ya later Member Since: Luglio 19, 2008 Posts: 43 Comments: 4049

1570. AllStar17 02:57 PM GMT del 05 Luglio 2009
Quoting futuremet: I only watch them when something actually forms. Yeah, I do enjoy their live coverage, and I like watching Storm Alert. Of course, I never wish a 'cane on anybody, but I do like their coverage. It seems like they always downcast developing systems for some reason! Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1571. AllStar17 02:58 PM GMT del 05 Luglio 2009
Quoting beell: A developing TUTT with its base near 15N 90W and TUTT axis extending NE to at least 25N 75W should keep higher levels of shear over the Carribean. Interaction with the TUTT should provide good upper support for continued convection. If this TUTT follows the recent pattern, the base will cut off and the TUTT lifts out to the NE and as you say, shear levels may ease. All we can do is watch and see what happens! Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1572. IKE 02:59 PM GMT del 05 Luglio 2009
I'm convinced that's the wave the Houston met is talking about. Worth keeping an eye on. Member Since: Giugno 9, 2005 Posts: 23 Comments: 37044

1573. AllStar17 03:02 PM GMT del 05 Luglio 2009
Quoting IKE: I'm convinced that's the wave the Houston met is talking about. Worth keeping an eye on. Yes, that would make sense. I think it will go more northerly than the NAM predicts. Houston does not need another tropical cyclone, they dealt with Ike last year! Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1574. beell 03:06 PM GMT del 05 Luglio 2009
Quoting IKE: I'm convinced that's the wave the Houston met is talking about. Worth keeping an eye on. Like we would do anything less LOL! In addition to this wave, there seems to be a pattern change afoot over the GOM. For the first time in a while, the GOM is wide open. Fairly good low level flow from the S and SE. This is where part of the moisture may come from and should start to show up towards the middle of next week. Member Since: Settembre 11, 2007 Posts: 124 Comments: 12807

1575. AussieStorm 03:13 PM GMT del 05 Luglio 2009
EPAC Member Since: Settembre 30, 2007 Posts: 5 Comments: 13249

1576. Ossqss 03:20 PM GMT del 05 Luglio 2009
1505. Weather456, Where is the reference to the hydrothermal activity along the Mid-Atlantic Ridge in your blog ? That is an integrated and required part of the equation :) Member Since: Giugno 12, 2005 Posts: 6 Comments: 8154

1577. AllStar17 03:22 PM GMT del 05 Luglio 2009
94E looking better organized, may become Tropical Depression 3-E later today or tonight IMO. NHC will probably up the development chances to high at 2 PM TWO, and say TD is possible later today or tonight. Getting a nice round shape, with the center on the northern side of the convection. SST's way too cold up towards CA and the N Baja Peninsula to support a tropical cyclone. Interestingly enough, the BAMD continues to recurve the storm back towards land. Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1578. Cavin Rawlins 03:33 PM GMT del 05 Luglio 2009
Ridiculously Hot NOAA-15 July 05 at 1105 UTC Member Since: Luglio 24, 2005 Posts: 407 Comments: 19076

1579. sporteguy03 03:37 PM GMT del 05 Luglio 2009
Hi Wx Logic how are things in SW FL? Member Since: Luglio 7, 2005 Posts: 0 Comments: 4824

1580. sporteguy03 03:38 PM GMT del 05 Luglio 2009
Impressive picture 456. Member Since: Luglio 7, 2005 Posts: 0 Comments: 4824

1581. CybrTeddy 03:41 PM GMT del 05 Luglio 2009
Wow.. I dont want a Strong TS this year with low shear in the GOMEX. That wont be fun. Member Since: Luglio 8, 2005 Posts: 253 Comments: 20178

1582. AussieStorm 03:41 PM GMT del 05 Luglio 2009
Quoting AllStar17: 94E looking better organized, may become Tropical Depression 3-E later today or tonight IMO. NHC will probably up the development chances to high at 2 PM TWO, and say TD is possible later today or tonight. Getting a nice round shape, with the center on the northern side of the convection. SST's way too cold up towards CA and the N Baja Peninsula to support a tropical cyclone. Interestingly enough, the BAMD continues to recurve the storm back towards land. Member Since: Settembre 30, 2007 Posts: 5 Comments: 13249

1583. WxLogic 03:42 PM GMT del 05 Luglio 2009
Quoting sporteguy03: Hi Wx Logic how are things in SW FL? Hehe... Hi Sport... I'm actually in CFL. :P Not so bad yet... but getting hotter. Tracking some showers coming out of the TPA area in hopes these would bring some cooler temps later on today to these parts. Member Since: Agosto 14, 2008 Posts: 4 Comments: 4666

1585. WxLogic 03:45 PM GMT del 05 Luglio 2009
Quoting Weather456: Ridiculously Hot NOAA-15 July 05 at 1105 UTC Definitely any tropical development that do manages to enter or develop in the GOM will have a feast. Member Since: Agosto 14, 2008 Posts: 4 Comments: 4666

1586. sporteguy03 03:46 PM GMT del 05 Luglio 2009
Quoting WxLogic: Hehe... Hi Sport... I'm actually in CFL. :P Not so bad yet... but getting hotter. Tracking some showers coming out of the TPA area in hopes these would bring some cooler temps later on today to these parts. Oh, I thought you were in Sarasota near StormW. I am out in the 4 corners area of CFL. Member Since: Luglio 7, 2005 Posts: 0 Comments: 4824

1587. AussieStorm 03:49 PM GMT del 05 Luglio 2009
Member Since: Settembre 30, 2007 Posts: 5 Comments: 13249

1589. JRRP 03:51 PM GMT del 05 Luglio 2009
Link PR radar Member Since: Agosto 16, 2007 Posts: 0 Comments: 4298

1590. AussieStorm 03:52 PM GMT del 05 Luglio 2009
Quoting WxLogic: Definitely any tropical development that do manages to enter or develop in the GOM will have a feast. Member Since: Settembre 30, 2007 Posts: 5 Comments: 13249

1591. beell 03:56 PM GMT del 05 Luglio 2009
Quoting leftovers: until i see a developing cv system im bearish on any development near in. why? nature can only fool a foolish character so many times. Certainly no shortage of fools here (j/k). You have to be a little foolish to try to see a week ahead. Bears and Bulls are no stranger to foolishness. Member Since: Settembre 11, 2007 Posts: 124 Comments: 12807

1592. IKE 03:57 PM GMT del 05 Luglio 2009
Buoy 42001...180 NM south of SW Pass,LA., yesterday afternoon... water temp...88.9 degrees. Member Since: Giugno 9, 2005 Posts: 23 Comments: 37044

1593. beell 04:00 PM GMT del 05 Luglio 2009
Awareness Test Member Since: Settembre 11, 2007 Posts: 124 Comments: 12807

1594. AussieStorm 04:01 PM GMT del 05 Luglio 2009
Member Since: Settembre 30, 2007 Posts: 5 Comments: 13249

1595. AllStar17 04:02 PM GMT del 05 Luglio 2009
Quoting Weather456: Ridiculously Hot NOAA-15 July 05 at 1105 UTC It would be a terrible scenario. If any TS/minimal hurricane entered the GOM under favorable conditions, rapid intensification would be almost certain. That is why this early season quietness is somewhat bad, it allows the water temperatures to become like a sauna! Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1596. AllStar17 04:03 PM GMT del 05 Luglio 2009
Wait, 456 that says 15-Jul 05 Member Since: Giugno 29, 2009 Posts: 13 Comments: 5135

1597. beell 04:08 PM GMT del 05 Luglio 2009
1577. WU got their alphabet confused again. Member Since: Settembre 11, 2007 Posts: 124 Comments: 12807

1598. AussieStorm 04:11 PM GMT del 05 Luglio 2009
4Day Precipitation Member Since: Settembre 30, 2007 Posts: 5 Comments: 13249

1599. stormwatcherCI 04:14 PM GMT del 05 Luglio 2009
Couple of flare ups in the Caribbean Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8038

1600. stormwatcherCI 04:16 PM GMT del 05 Luglio 2009
http://www.nrlmry.navy.mil/archdat/atlantic/winds/scatt/qscatt/20090705.1400.qscatt.scatt.wind.atl.x. png Member Since: Ottobre 9, 2008 Posts: 0 Comments: 8038

1601. Patrap 04:18 PM GMT del 05 Luglio 2009
GOM 60 Hour Water Surface Temperature Forecast Model Member Since: Luglio 3, 2005 Posts: 370 Comments: 111244

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