Atlantic hurricane outlook for the last half of July

By: Dr. Jeff Masters , 2:41 PM GMT on July 16, 2007

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Atlantic tropical cyclone activity is usually low during the last half of July. Since the current active hurricane period began in 1995, eight of 12 years have had one or two named storms form during the last half of July. Two named storms formed in both 1995 and 2005. In 2005, we were already up to "E" in the alphabet at this point, so 2007 is certainly not going to be a repeat of 2005, thank goodness! As seen in Figure 1, most of the late July activity occurs in the Gulf of Mexico, but there are a few long-track "Cape Verdes" hurricanes beginning to occur. These are spawned by tropical waves that come off the coast of Africa. African tropical waves serve as the instigators of about 85% of all major hurricanes.


Figure 1. Tracks of all tropical storms and hurricanes since 1851 that formed July 16-31. The Gulf of Mexico coast is the preferred strike location.

Sea Surface Temperatures
Sea Surface Temperature (SSTs) remained near average over the tropical Atlantic between Africa and the Lesser Antilles in July, thanks to plenty of African dust keeping sunlight from heating up the ocean. However, SSTs are 0.5-1.0 C above average over much of the Caribbean and Gulf of Mexico, so there is still good reason to expect an above average number of tropical storms and intense hurricanes this season--but not the 15 or 16 more named storms predicted by the Klotzbach/Gray team and TSR. I think it is more likely we will see 10-12 more named storms in the Atlantic this season, for a total of 12-14 when we include Andrea and Barry.


Figure 2. Sea Surface Temperature (SST) departure from average for July 16, 2007. Image credit: NOAA.

Tropical Cyclone Heat Potential
It's not just the SSTs that are important for hurricanes, it's also the total amount of heat in the ocean to a depth of about 150 meters. Hurricanes stir up water from down deep due to their high winds, so a shallow layer of warm water isn't as beneficial to a hurricane as a deep one. The Tropical Cyclone Heat Potential (TCHP, Figure 3) is a measure of this total heat content. A high TCHP over 80 is very beneficial to rapid intensification. There is less heat energy available this year than in 2005, which recorded the highest SSTs and TCHP ever measured in the tropical Atlantic. However, this is not true in the Western Caribbean, where we have very high TCHP this year. The African dust storms have not penetrated all the way to the Western Caribbean, and SSTs and TCHP have stayed above average. In the unlikely event we get an intense hurricane in late July, it would probably be in the Western Caribbean.


Figure 3. Tropical Cyclone Heat Potential (TCHP) for July 14 2005 (top) and July 14 2007 (bottom). TCHP is a measure of the total heat energy available in the ocean. Record high values of TCHP were observed in 2005. Image credit: NOAA/AOML.

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 by tearing a storm apart. Wind shear below 8 knots is very conducive for tropical storm formation.

Wind shear during most of June and July has been above 20 knots along the two branches of the jet stream--the polar jet, which has been positioned along the U.S.-Canadian border, and the subtropical jet, which has been over the Caribbean. This pattern is apparent in this morning's wind shear map (Figure 4, top). However, a major shift the Northern Hemisphere weather pattern is expected over the next two weeks. The GFS model is predicting that the persistent trough of low pressure that has been over the Eastern U.S. will move off and be replaced by a ridge of high pressure about ten days from now. The subtropical jet will weaken, bringing pockets of very low wind shear all across the tropical Atlantic by the end of the month (Figure 4, bottom image). The shear will remain high enough to discourage tropical storm formation over the coming week, but chances for a named storm will increase sharply by the beginning of August.



Figure 4. Top: Wind shear analyzed by the GFS model at 00 GMT Monday, July 16 2007. Bottom: Forecasted wind shear for August 1, 2007. Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots). In most circumstances, wind shear above 20 knots (10 m/s, the light purples in the image) will act to inhibit tropical storm formation. Wind shear below 8 knots (4 m/s, the lightest red color) is very conducive for tropical storm formation. Note the large increase in low wind shear areas expected by August 1 (bottom image, red colors).

Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa. Despite the fact that the Sahel region of Africa has seen two straight years of above-average rains, which should result in soil stabilization and fewer dust outbreaks, 2007 has seen very high levels of dust coming from Africa. This activity continued over the tropical Atlantic during the first half of July, and I expect this activity to continue for the remainder of July. This dry air and dust will act as a major deterrent to any storms that tries to form between Africa and the Lesser Antilles Islands the remainder of July. With the coming Northern Hemisphere weather pattern shift, it is possible that the dry air coming off of the Sahara will fade some at the end of the month, though.

Steering currents
The steering current pattern for June and the first half of July featured a pattern much like we saw in 2006, with an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. As I discussed in my blog on Friday the 13th, this steering current pattern is expected to shift next week, bringing a ridge of high pressure over the Eastern U.S. and an extension of the Bermuda High westwards over the U.S. This pattern will act to block recurvature of any tropical cyclones that might form in the last half of July. Such a pattern puts the Western Caribbean, Gulf of Mexico, and east coast of Florida at highest risk, as we saw in 2004 and 2005. Note that the the northern Lesser Antilles Islands, Puerto Rico, and the U.S. East Coast from the Carolinas northwards can expect reduced risk under this steering pattern. Steering current patterns are not predictable more than about two weeks in advance, and there is no way to tell if this new steering current pattern will remain in place for a few days or a few months.

Summary
Recent history suggests a 75% chance of at least one named storm occurring in the last half of July. This July, I put the odds at 50%, due to the unfavorable conditions for the coming week. Any storms that occur this July will probably be towards the end of the month, due to the lower shear, warmer SSTs, and potential for less African dust and dry air then. The areas of highest risk are the Western Caribbean, Gulf of Mexico, and east coast of Florida.

Jeff Masters

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1462. weatherblog
3:56 PM GMT on July 18, 2007
texascanecaster...earlier you said if a storm would come around the Boston area in July, it wouldn't be tropical in nature. Well, around the same time last year we had Beryl, as a 50 mph *tropical* storm hit Nantucket, Mass.
Member Since: July 10, 2006 Posts: 27 Comments: 1623
1461. NoNamePub
2:47 PM GMT on July 18, 2007
Texascanecaster -
No wishcasting is going on. I simply pointed out that CMC (even though unreliable) showed something heading that way. NGPS shows some (albeit slight) agreement. Then I asked for opnions. Apologies (as I am new) if this is considered wishcasting. Trust me - I live in SE Florida - I would be thrilled if NO canes came this year.

Member Since: July 13, 2007 Posts: 0 Comments: 530
1460. texascanecaster1
2:41 PM GMT on July 18, 2007
no moonlight wishcasting is going on again i moving to new blog.
1459. nolesjeff
2:39 PM GMT on July 18, 2007
Link new blog
Member Since: June 20, 2006 Posts: 9 Comments: 1391
1458. texascanecaster1
2:39 PM GMT on July 18, 2007
let me clairfy some things.First of all can you guys see my comments or are they being blocked? second cmc is dead wrong as it has been all year. third of all whoever said that ngps showed a dsiturbance hitting boston was dead wrong ngps shows absoulotuley nothing. fourth of all niether does gfs. 5th of all it would be extremely unlikley if on the miracle chance something did form in the atlantic like that that it would hit boston ethier it would move west into florida or sharply recurve out to sea it is remotley possible it could move into one of the carolinas but this is not an issue as this disturbance will not form cmc is wrong. no offenses meant to anyone.
1457. moonlightcowboy
2:39 PM GMT on July 18, 2007
http://manati.orbit.nesdis.noaa.gov/dataimages21/cur/zooms/WMBds32.png

Good morning, all.
...wave behind the spinning low. 2nd low about to form here? or just itcz action?

...and why are we talking about Boston. There's no danger there with this high sitting the way it is? Did I miss something in the night?
Member Since: July 9, 2006 Posts: 184 Comments: 29610
1455. hurricane23
2:35 PM GMT on July 18, 2007
Posted By: crownwx at 10:32 AM EDT on July 18, 2007. (hide)
Last hurricane with a major impact on Boston was Hurricane Carol in 1954.

I nailed it.
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1454. crownwx
2:32 PM GMT on July 18, 2007
Last hurricane with a major impact on Boston was Hurricane Carol in 1954.
Member Since: December 27, 2004 Posts: 3 Comments: 207
1453. texascanecaster1
2:32 PM GMT on July 18, 2007
however at a later date in the hurricane season it would be more possible but not until late agust do i think that something tropical in nature could form and hit boston.
1452. NoNamePub
2:30 PM GMT on July 18, 2007
Good Info thanks 23.

LOL - Only thing I rememeber about 1985 (I was 6) is that my Dad took my Grandma instead of me to game seven of the Royals/Cards series....
Member Since: July 13, 2007 Posts: 0 Comments: 530
1451. texascanecaster1
2:29 PM GMT on July 18, 2007
any system that forms in that area as a disturbance will be purley extrattropical in nature. if anything hit boston it would be an sts or extratropical baroclinic storm. It is remotley possible that a hybrid storm could form but i hgighly doubt that we would see a fully flegded tropical system hit boston. chances of that happening: less than 1%
1450. blogger4life
2:29 PM GMT on July 18, 2007
Hey Taz. What do you think is going to happen with that wave over the lesser antillies?
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1449. hurricane23
2:26 PM GMT on July 18, 2007
Here's a page on BOB and Gloria.
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1448. gthsii
2:23 PM GMT on July 18, 2007
I guess Bob and Gloria from the 80s-90s dont count as major storms. i lived in boston at the time and remember sleeping thru both of 'em
1447. hurricane23
2:17 PM GMT on July 18, 2007
Side question - When was the last time a Major storm hit Boston?

From the top of my head besides the 1938 hurricane i remember Carol back in 1954 which had winds of 100 mph and produced a surge of 8-13 feet above mean tide.

Here's a good page on hurricane carol.
Member Since: May 14, 2006 Posts: 8 Comments: 13839
1446. NoNamePub
2:12 PM GMT on July 18, 2007
Posted By: hurricane23 at 2:08 PM GMT on July 18, 2007.

The CMC is just to funny next thing you know it will predict a cat5 in boston.This model folks is no better then the NAM when it comes to forcasting tropical cyclones.Atleast in my opinion.Adrian

NOTE-Its done very poor on genesis this season.



H23 - I see some agreement with NGPS though. Although it shows a much weaker distrubance, it is in the same area around the same time.

Side question - When was the last time a Major storm hit Boston?
Member Since: July 13, 2007 Posts: 0 Comments: 530
1445. weathers4me
2:09 PM GMT on July 18, 2007
Cyclone: Well put. You are so right. Has anyone seen the latest position of the Bermuda high and where the jet stream is now? It would be interesting if the trend Dr. Masters was referring to above has any signs of changing or are we going to be locked into the steering patterns of 2006. Take care.
Member Since: May 24, 2007 Posts: 0 Comments: 118
1444. hurricane23
2:08 PM GMT on July 18, 2007
The CMC is just to funny next thing you know it will predict a cat5 in boston.This model folks is no better then the NAM when it comes to forcasting tropical cyclones.Atleast in my opinion.Adrian

NOTE-Its done very poor on genesis this season.
Member Since: May 14, 2006 Posts: 8 Comments: 13839
1443. Hellsniper223
2:06 PM GMT on July 18, 2007
Cyclone0z... Touching... ... ... ...
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1442. CycloneOz
2:03 PM GMT on July 18, 2007
One last thing before I go:

Always remember this!

No matter how destructive weather may be, no matter how sudden or terrifying earthquakes can be...if we didn't have those things, our planet's core would be dead and our water and atmosphere would be ripped away.

People will tell you that storms and earthquakes are signs of the end-times. Certainly, these things should remind each of us as individuals that our time here is short and in a state of constant flux.

Storms and earthquakes are signs...but not of the "end-times." They're signs that your planet is as alive as you are!

That is why I find these phenomena so interesting, captivating, and compelling.

See ya'll again this weekend.

Go out there and enjoy your planet!
Member Since: August 26, 2006 Posts: 10 Comments: 3887
1441. texascanecaster1
1:55 PM GMT on July 18, 2007
Now the fun begins. There are two areas of interest at this point. both are vigorous well formed tropical waves on in the antillies and one in the atlantic. these are the kinds of things that can instigate tropical cyclone development. if we saw these same things in the antillies a week from now we would have a depression. However althought conditions over the antillies system a favorable now they won't stay that way for more than another 24 hr. and then i think this disturbance will fizzle. the other one is the wave in the entral antlantic which has alot of dry air issues. However if it can maintain present form and convection until it reaches the antillies sometime next week it could have a chance. sometime during the first week of agust i expect to see one if not two tropical depressions form.
1440. Tazmanian
1:49 PM GMT on July 18, 2007
: initforwaves i am not sure but i dont think it will do march of any thing right now
Member Since: May 21, 2006 Posts: 5091 Comments: 115349
1439. CycloneOz
1:47 PM GMT on July 18, 2007
Thanks Mr. Nice and gthsii!

You guys ain't seen nuthin' from me yet! :)

This weekend...I'm releasing the 2005 GOES IR HURRICANE SECTOR time lapse animation FROM JUNE - NOVEMBER 2005!

It's almost finished...I just got November to go and it's done. You folks should see it. It's truly remarkable to see so many storms spinning around.

I'm as excited as I can be. I've got special audio prepared for the show. This one will not be updated. It will be the historical copy.

To save things from youtube.com, simply get to the video you want to save, type "kiss" in front on "youtube.com" (ex: this video here would be "http://www.kissyoutube.com/watch?v=kZajlV0E5V8". Press the "Get It" button and save the file with a filename you'll recognize and the suffix ".flv" **IMPORTANT YOU DO THAT**. Finally, load the .flv file into a video converter that can handle flash videos and then convert it to an mpg. It's really that easy.

See ya'll again this weekend! Till then...have fun!
Member Since: August 26, 2006 Posts: 10 Comments: 3887
1438. initforwaves
1:47 PM GMT on July 18, 2007
Taz what do you think about that low at 15N32W? Could it potentially suck moisture out of the ITCZ (as MrNice was saying) and develop?
1437. texascanecaster1
1:44 PM GMT on July 18, 2007
shaer is low over the wave in the antillies and convection is concentrating near the northern part in a circlular form. if it gets a little better organized some slow development may be possible otherwise it go poof.
1436. Tazmanian
1:40 PM GMT on July 18, 2007


Member Since: May 21, 2006 Posts: 5091 Comments: 115349
1435. gthsii
1:37 PM GMT on July 18, 2007
CycloneOZ: that was awesome. best 5 minutes i've spent killing time at work. thanks for that, just amazing to watch weeks worth of weather...kind of puts it all into perspective, watching the patterns, storms flare up. i live in sofla and its amazing to watch the peninsula just go poof with clouds
1434. texascanecaster1
1:35 PM GMT on July 18, 2007
ben in islemorada in the florida keys for the last week and hallf caught a lot of big fish but i am back now. i heard all about bill proenza being fired the typhoon and earthquake in japan and the blob in the antillies i am currently checking shear and models.
1433. MrNiceville
1:34 PM GMT on July 18, 2007
Late? I work flex hours - I just stay later (much to my wife's chagrin)...
1432. NoNamePub
1:33 PM GMT on July 18, 2007
Nr Nice - you are going to be late for work!
LOL
Member Since: July 13, 2007 Posts: 0 Comments: 530
1431. texascanecaster1
1:32 PM GMT on July 18, 2007
im back
1430. BahaHurican
1:29 PM GMT on July 18, 2007
Morning everybody.
Member Since: October 25, 2005 Posts: 19 Comments: 22561
1429. MrNiceville
1:27 PM GMT on July 18, 2007
Hehehe - if you watch the in the lower right corner - you can see 96L just make it into the frame (toward the end of the loop) and get blown apart...

Appropriate that they use Jurassic Park theme for historical info...
1428. CycloneOz
1:20 PM GMT on July 18, 2007
From Dr. Master's Current Blog:

Steering currents
"The steering current pattern for June and the first half of July featured a pattern much like we saw in 2006, with an active jet stream bringing many troughs of low pressure off the East Coast of the U.S."


Wanna see what he's talking about? Look at this latest time lapse animation of the GOES IR Hurricane Sector imagery from 06/01/2007 - 07/12/2007.

Those troughs, they're literally one after another.

Member Since: August 26, 2006 Posts: 10 Comments: 3887
1427. groundman
1:19 PM GMT on July 18, 2007
Posted By: MrNiceville at 1:17 PM GMT on July 18, 2007.
OK - I lied - had to get one more in.

Just looked at the CATL visible loop. That low around 15N32W appears to acutally be drawing moisture out of the ITCZ. Can it "feed" on the ITCZ like that? If so, that's the one to watch over the next 3 days...

I wondered that too, certainly is looking like we may have something to watch? I know, I know, don't get excited. LOL
1426. MrNiceville
1:17 PM GMT on July 18, 2007
OK - I lied - had to get one more in.

Just looked at the CATL visible loop. That low around 15N32W appears to acutally be drawing moisture out of the ITCZ. Can it "feed" on the ITCZ like that? If so, that's the one to watch over the next 3 days...
1425. MrNiceville
1:11 PM GMT on July 18, 2007
Time for the 3 s's folks - gotta head to the office sometime...

Have a great day - see you all LATE tonight...
1424. NoNamePub
1:10 PM GMT on July 18, 2007
Actually NOGAPS Shows a much lighter disturbance up heading that direction around the same time. Good to hear that Conditions don't favor development.
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1423. MrNiceville
1:08 PM GMT on July 18, 2007
Personally, I think it's just another crack pipe dream that the CMC had overnight...

No other models show it spinning up and I personally think the AB high is too strong to allow something like that to migrate to the north side...
1422. NoNamePub
1:05 PM GMT on July 18, 2007
WOW -

Thank goodness CMC is off its rocker.
Did you guys see the Monster it is predicting to spin up to Boston Area?
Member Since: July 13, 2007 Posts: 0 Comments: 530
1421. MrNiceville
1:05 PM GMT on July 18, 2007
As always, thanks, Pat...

I guess "wind shear" and "vertical wind shear" are the same thing. I am curious as to why the NOAA and CIMSS wind shear maps differ so greatly...
1420. BahaHurican
1:00 PM GMT on July 18, 2007
Morning all.

Some interesting comments about Barbados in the early 70s.

Like others, I'm watching the wave set to enter the Caribbean with interest. All this increased moisture seems a clear indicator of the MJO we've been learning about this season.

I'm also noticing the way the ITCZ has shifted almost 5 degrees in the last 4 weeks or so. With it near 10N all across the ATL, looks like the wave train is set to roll out of the station . . .

Also interesting to note is the plume of dust NHC mentioned in this morning's discussion. While this is rapidly moving across the ATL, it is still further north than previously seen, and there doesn't seem to be much more of it where it came from (no obvious dust signatures across the western Sahara).

All in all, I think the next 10 days or so could get pretty interesting in a hurry.
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1419. Patrap
12:55 PM GMT on July 18, 2007
Wind shear map explained

Link
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1418. weathermanwannabe
12:54 PM GMT on July 18, 2007
Sorry if a double post...Seems to me that shear will be the determining factor on survival of the Antillies wave as the WV shows that is it "pushing" the dry air out in front of it as opposed to sucking it in (perhaps a different story if a low were to develop)...
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1417. MrNiceville
12:53 PM GMT on July 18, 2007
Whoops - you're right, Ike - I need to brush up on my reading skills (LOL)
1416. Patrap
12:53 PM GMT on July 18, 2007
Vertical Shear Link
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1415. MrNiceville
12:51 PM GMT on July 18, 2007
Anybody - what is the difference between a "vertical shear map" and a "wind shear" map? I noticed the "current shear" analysis from SSD has shear at 20kts max over the Caribbean, while the "Atlantic Wind Shear" analysis has the shear at 40+ kts. What am I missing here???
1414. IKE
12:52 PM GMT on July 18, 2007
Posted By: MrNiceville at 7:46 AM CDT on July 18, 2007.
The shear in the Caribbean is forecast to intensify over the next 24 hours, but then it drops significantly:


That shear map on WU....the second frame is 12Z Wednesday..which is now....it shows favorable from tonight on, in the Caribbean.
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1413. Patrap
12:49 PM GMT on July 18, 2007
anytime init
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1412. Bamatracker
12:49 PM GMT on July 18, 2007
oh yea...alright well even though im in the big easy its on business and I have to get going. I talk to yall later!!
Member Since: May 17, 2006 Posts: 6 Comments: 1367

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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