Early August hurricane outlook

By: Dr. Jeff Masters , 2:12 PM GMT on July 29, 2007

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A tropical disturbance (98L) a few hundred miles north of the central Bahamas has become only a little better organized since yesterday. Heavy thunderstorm activity has increased, and the beginnings of some upper-level outflow to the northeast is apparent on visible satellite loops. A QuikSCAT satellite pass from 6:38am this morning revealed that 98L is attempting to form a closed circulation at the surface, and had top winds of at least 25 mph.

Water vapor satellite loops show an upper-level low pressure system to 98L's northeast, and this upper low is bringing about 15 knots of wind shear over the disturbance. The GFS and GFDL models predict that the upper low will move north-northeast in tandem with the disturbance, keeping low enough shear over it that a tropical depression could form. The other reliable models do not develop 98L. At present, it appears that Bermuda is the only place that needs to concern itself with 98L. SSTs are warm enough to support tropical storm formation until 98L reaches a point 500 miles or so north of Bermuda. The Air Force Hurricane Hunter aircraft scheduled to investigate 98L this afternoon was canceled, and no new flights are planned.


Figure 1. Preliminary model tracks for 98L.

Early August hurricane outlook
In the first half of August, Atlantic tropical cyclone activity starts to pick up. Since the current active hurricane period began in 1995, eight of 12 years have had one or more named storms form during the first half of August, including the last seven years in row. The fact that we've had a quiet July does not mean we can expect a slower than average hurricane season. To illustrate, consider 2004--the first storm, Alex, did not get named until August 1, yet that season had 15 named storms, 9 hurricanes, and 6 intense hurricanes. Five named storms formed in first half of August of 2004. One item of comfort, though, is the fact that 2007 is definitely not a repeat of 2005--we were already up to "G" in the alphabet at this point in 2005.


Figure 2. Historical Atlantic hurricane and tropical storm activity, 1851-present. Figure modified from NOAA's original.

As seen in Figure 2, hurricane activity does take a bit of an upward jump around August 1, but the real action doesn't start until August 18. It should not surprise us, then if we go a few more weeks without a named storm. I am still expecting an above-average hurricane season with 12-14 named storms (10-12 more, since we've already had two), with at least one major hurricane hitting the U.S. However, the decline in SSTs relative to normal over the past two months means we should have a less active season than originally thought. I'm guessing that the Dr. Bill Gray/Phil Klotzbach August forecast, due to be released Friday August 3, will have two fewer named storms compared to their May 31 forecast.


Figure 3. Tracks of all tropical storms and hurricanes since 1851 that formed August 1-15.

Sea Surface Temperatures
Early August storms can occur anywhere, and strike anywhere (Figure 3), since the oceans have finally heated up to the point where the entire tropical Atlantic can support hurricane formation. Sea Surface Temperature (SSTs) remained near average over the tropical Atlantic between Africa and the Lesser Antilles in July (Figure 4), thanks to plenty of African dust keeping sunlight from heating up the ocean. A stronger than average Bermuda High has also helped cool the ocean more than normal, thanks to the faster trade winds it brought over the ocean in June and July. 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 hurricane season. The total amount of heat energy in the upper layer of the ocean (the Tropical Cyclone Heat Potential, TCHP) remains high in the Western Caribbean--near the record levels observed in 2005. However, TCHP is much lower over the rest of the tropical Atlantic, including the Gulf of Mexico.


Figure 4. Sea Surface Temperature (SST) departure from average for July 26, 2007. Image credit: NOAA.

Wind shear
Wind shear has been near normal, averaged over the tropical Atlantic during July. Wind shear is predicted to be near or below normal for the first half of August.

Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa. By early August, dust activity typically diminishes, but it is not well-understood how to forecast these dust outbreaks. We have seen a decline in the amount of dust over the past week, as one might expect from climatology. Since the long-range GFS forecast does not show any major changes to the weather pattern over Africa the next two weeks, I am expecting African dust activity to remain near normal through mid-August.

Steering currents
The hurricane steering pattern for the next two weeks should be near normal, with no areas at above-average risk for a hurricane strike. I discussed this in detail in Friday's blog. Steering current patterns are not predictable more than about two weeks in advance, and there is no way to tell if this steering current pattern will remain in place past mid-August.

Summary
Recent history suggests a 75% chance of at least one named storm occurring in the first half of August. I'll go with climatology and forecast a 75% chance this year, as well, since SSTs, wind shear, and African dust/dry air should all be near normal. With the steering current patterns expected to be near normal, no areas can be singled out as being at higher risk than average. We'll have to keep a careful eye out late this week, when a cold front is expected to sweep off the East Coast of the U.S. Several of the models are indicating the possibility that a tropical depression could form at the tail end of the cold front by Thursday or Friday. This would most likely happen off the Carolina coast, but could also occur in the Northeastern Gulf of Mexico, near the Florida Panhandle. I think it is still too early to get a tropical storm forming between Africa and the Lesser Antilles Islands this week, but next week we could see something.

Last blog for a week
This will be my final blog until Monday, August 5, as I am taking a week's vacation to do some camping and paddling along Lake Huron's gorgeous Georgian Bay. I've arranged for an able substitute blogger to make daily posts here this week, but I will be able to do some blogging beginning Friday if something nasty pops up.

Jeff Masters

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702. gthsii
2:06 PM GMT on July 30, 2007
Hey Drak: have you got a current..err...steering currents map? :-) little bit of austin powers there
701. gthsii
2:03 PM GMT on July 30, 2007
gfs long range

last frame of long range GFS model of 99L
699. dewfree
2:02 PM GMT on July 30, 2007
a few things of intrest as far as im concerned . two waves one at 22N63W and the other around 10 Nand 46W both features have intrest .the weather channelsaid that the air force reserve i thinking about looking into the one at 10N 46W
Member Since: August 27, 2006 Posts: 7 Comments: 589
698. dewfree
2:00 PM GMT on July 30, 2007
or is it a ULL humm i cant see it all on this map
Member Since: August 27, 2006 Posts: 7 Comments: 589
697. gthsii
2:00 PM GMT on July 30, 2007
anything of interest today

could you read a little of the blog first?
696. dewfree
1:59 PM GMT on July 30, 2007
Check out the ULL comming oof the coast of africa. looks like it is pulling a tropical wave back onto the coast of africa. lol
Member Since: August 27, 2006 Posts: 7 Comments: 589
695. dewfree
1:57 PM GMT on July 30, 2007
Looks like the wave at 22 N 63W is firing up.Appears it may have circulations with it as well. still no niticable circulation on what was 98l
Member Since: August 27, 2006 Posts: 7 Comments: 589
694. dewfree
1:53 PM GMT on July 30, 2007
the wave at 9 N 46W appears to have circulation with it >It also appears to be traveling at n nw at momment.
weather channel says they are monitoring a wave off the coast of south america.I wonder if it is the same one.yes it is.They are mentioning the air force reserve going into it or making the decision wether they will go into it tommarow. hummm interesting. this one looks like it may have its stuff together.
dew
Member Since: August 27, 2006 Posts: 7 Comments: 589
693. WPBHurricane05
1:49 PM GMT on July 30, 2007
new blog
Member Since: July 31, 2006 Posts: 56 Comments: 8112
691. dewfree
1:45 PM GMT on July 30, 2007
What no storm no depression not anything in atlantic .just waves waves waves.
Member Since: August 27, 2006 Posts: 7 Comments: 589
689. Hawkeyewx
1:38 PM GMT on July 30, 2007
Posted By: Tazmanian at 8:32 AM CDT on July 30, 2007.

looks like its geting stronger fast


It is trying to organize, but it will be a slow process.
Member Since: July 5, 2005 Posts: 2 Comments: 1923
688. seminolesfan
1:36 PM GMT on July 30, 2007
Drak-I already told you my evidence; the hole on the W edge of 99L. You didn't answer my question thou!
Member Since: June 14, 2006 Posts: 0 Comments: 2106
687. Tazmanian
1:36 PM GMT on July 30, 2007
her you go : drusierDMD

Link
Member Since: May 21, 2006 Posts: 5089 Comments: 114041
686. eaglesrock
1:36 PM GMT on July 30, 2007
new blog
684. FMTXWMAN
1:33 PM GMT on July 30, 2007
Atlantic floater 2 is now on the system between 45 and 50. Labeled as invest.

The visible image appears to support mid and high level circulation. Hard to see if it has low.


Is there a link to a higher resolution loops?
Member Since: May 1, 2007 Posts: 2 Comments: 68
683. WPBHurricane05
1:32 PM GMT on July 30, 2007
Hurricane Katrina had to fight off dry air before hitting Florida. That is why it had most of the nasty stuff to the south of the storm.
Member Since: July 31, 2006 Posts: 56 Comments: 8112
682. drusierDMD
1:32 PM GMT on July 30, 2007
Taz can you post a the main page for those spaghetti models? so i can get to them in the future. i tried and couldnt find my way there from the main page. thx
681. Drakoen
1:32 PM GMT on July 30, 2007
lol Kman yea...
Member Since: October 28, 2006 Posts: 57 Comments: 29714
680. Drakoen
1:32 PM GMT on July 30, 2007
i would say that conditions are marginal-favorable for development.
Member Since: October 28, 2006 Posts: 57 Comments: 29714
679. Tazmanian
1:32 PM GMT on July 30, 2007
looks like its geting stronger fast

Member Since: May 21, 2006 Posts: 5089 Comments: 114041
678. kmanislander
1:31 PM GMT on July 30, 2007
Wow

earlier this morning I called for 99L to be an invest by this afternoon and I come back from my morning coffee to see it up already !
Member Since: August 19, 2006 Posts: 0 Comments: 15671
677. stormybil
1:31 PM GMT on July 30, 2007
99l has a better shot than 98l of developing and just might fight the dry air . its happen many times before
676. bobw999
1:31 PM GMT on July 30, 2007
lets wait to see what the 11:30 two says
675. Drakoen
1:30 PM GMT on July 30, 2007
Posted By: WizardApprentice at 1:29 PM GMT on July 30, 2007.

yea but unlike 96L there is weaker shear and much less SAL


exactly.
Member Since: October 28, 2006 Posts: 57 Comments: 29714
674. bobw999
1:30 PM GMT on July 30, 2007
come to my blog for the truth of 99l
673. Drakoen
1:30 PM GMT on July 30, 2007
seminloesfan do you see evidence of a SFC low pulling dry air in?
Dry air is gonna kill everything. Are you aware that dry air moves?
Member Since: October 28, 2006 Posts: 57 Comments: 29714
670. seminolesfan
1:28 PM GMT on July 30, 2007
Drak you don't know what your talkin about!!!

How can a surface low help it fight off dry air?

Just the opposite is true. The low pulls the dry air into the system. You understand pressure differentials, right?
Member Since: June 14, 2006 Posts: 0 Comments: 2106
669. Drakoen
1:27 PM GMT on July 30, 2007
Posted By: bobw999 at 1:25 PM GMT on July 30, 2007.

bobw9 lol. It can fight of the dry air easier because it already has a SFC low.

remember 96l?


96L formed a SFC low when there was dry air aorund most of the system.

Member Since: October 28, 2006 Posts: 57 Comments: 29714
668. Tazmanian
1:27 PM GMT on July 30, 2007
dr m come back you this took off at a bad time
Member Since: May 21, 2006 Posts: 5089 Comments: 114041
667. SWFLdrob
1:27 PM GMT on July 30, 2007
ya...it has some things going for it...but that dry air looks like a fairly big hurdle to overcome.

Gives us something to obsess over for the time being and the very least.
666. Tazmanian
1:26 PM GMT on July 30, 2007
yes but 96L was in a lot of dry aire and SAL 99L is not it has moist aire a round it
Member Since: May 21, 2006 Posts: 5089 Comments: 114041
664. bobw999
1:26 PM GMT on July 30, 2007
One word: [b]ERNESTO!!!!!![/b]

even if it does develop, that is exactly what i see
662. seminolesfan
1:25 PM GMT on July 30, 2007
What are you talkin' about Drak?

Moist upper levels are one of the requirements for cyclogenesis. You can already see the dry air entrainment on the W side of the convection and how its punching a hole in 99L.

I guess time will tell.
My guess: Dry air-1; 99L-0
Member Since: June 14, 2006 Posts: 0 Comments: 2106
661. bobw999
1:25 PM GMT on July 30, 2007
bobw9 lol. It can fight of the dry air easier because it already has a SFC low.

remember 96l?
659. Hawkeyewx
1:25 PM GMT on July 30, 2007
There's no need to go nuts with a possible future track, yet. This thing still has work to do to become a tropical cyclone and there's no guarantee it will make it.
Member Since: July 5, 2005 Posts: 2 Comments: 1923
658. DDR
1:24 PM GMT on July 30, 2007
trinidad & the southern windard islands could use some rain, its been unusually dry :(
Member Since: April 27, 2007 Posts: 14 Comments: 1625
657. Tazmanian
1:24 PM GMT on July 30, 2007
her are the mode runs for any one that miss it


Member Since: May 21, 2006 Posts: 5089 Comments: 114041
656. Drakoen
1:23 PM GMT on July 30, 2007
bobw9 lol. It can fight of the dry air easier because it already has a SFC low.
The corilos effect can steer it north like the spagetti models suggest.
oh its on the SSD site now.
Member Since: October 28, 2006 Posts: 57 Comments: 29714
655. NoNamePub
1:22 PM GMT on July 30, 2007
Where can I get a link for the Spaghetti models for 99L?

Thanks!
Member Since: July 13, 2007 Posts: 0 Comments: 518
654. bobw999
1:21 PM GMT on July 30, 2007
Some dry air on its path to development



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About JeffMasters

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