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August tropical outlook

By: Dr. Jeff Masters, 2:28 PM GMT on August 16, 2006

A well defined surface circulation has developed about 200 miles southeast of the North Carolina/South Carolina border this morning. Heavy thunderstorm activity is limited to the south side of this system, due to 10 knots of wind shear from northerly upper level winds. However, long range radar out of Wilmington, NC shows some impressive echoes and low level rain bands forming, and I imagine NHC will send out a Hurricane Hunter airplane this afternoon to see if a tropical depression has formed.

The computer models are forecasting that this system will not recurve out to sea, as the trough of low pressure that had been pulling this system to the north is now exiting the East Coast. High pressure is building in, which should force the system towards the coast over the next few days. Steering currents are weak, and the track of this system is very uncertain. The storm may go ashore over South Carolina (as favored by the GFDL model), northern Florida (as favored by the NOGAPS model), or perhaps North Carolina or Georgia. Wind shear is expected to increase significantly on Thursday from the north to the south, so this system lilkely does not have long to live.

Figure 1. Current long range radar out of Wilmington, NC.

Figure 2. Preliminary models tracks for the East Coast disturbance.

Eleswhere in the tropics
The big burst of thunderstorms that developed in the Gulf of Mexico last night is gone, and no development is likely there today. A rotating area of clouds a few hundred miles northeast of the northernmost Lesser Antilles is associated with an upper level low pressure system. This is not expected to develop.

August hurricane outlook
What a difference a year makes! By this date in 2005, we were already up to Hurricane Irene, the 9th named storm of the season. Of those nine, four were hurricanes, and two (Dennis and Emily) were record-breaking Category 4 and 5 hurricanes. However, before we congratulate ourselves too much on a safe start to hurricane season, it is instructive to look at the plot of typical hurricane activity for the Atlantic (Figure 3). Peak hurricane season starts about August 18 and runs through October 18. The worst part of hurricane season is in front of us, and I do anticipate that conditions will get active. Witness 1998, when only one named storm occurred prior to August 19, and 10 named storms and 7 hurricanes formed by the end of September. A similar pattern of activity occurred in 2000, with only two named storm by this date, and a season total of 15 named storms. So, those of you who doubt NOAA and Dr. Gray's predictions of 15 named storms this season need to put your skepticism on hold.

Figure 3. Climatological Atlantic hurricane and tropical storm activity.

A major shift in the atmospheric pattern over the Atlantic began at the end of July, and portends an active hurricane season. June and July were characterized by a much stronger than normal Bermuda High, with surface pressures up to 7 mb higher than normal over the Atlantic. Taking a look at the surface pressures the past ten days (Figure 4), we see that surface pressures are now up to 7 mb lower than normal over much of the Atlantic, a complete reversal of the situation in June and July. Lower surface pressures are more conducive for hurricane formation, and drive weaker trade winds. Weaker trade winds mean less evaporative cooling of the ocean, allowing the ocean to heat up more than usual.

Figure 4. Sea level pressure (top) and departure from normal (bottom) for the 10 days ending August 12, 2006. Image credit: NOAA Climate Prediction Center.

We can see that the tropical Atlantic has warmed considerably, relative to normal, between July 28 (Figure 5) and August 12 (Figure 6). The Caribbean has warmed about 1/2 a degree C, relative to normal, and the blue pool of cooler than normal waters near the Bahamas has shrunk. The 2-week forecast from the GFS model predicts a continuation of these conditions, and I expect that the ocean will continue to warm to much above normal levels through September (although not as warm as 2005).

Figure 5. Sea surface temperature departure from normal for July 28, 2006.

Figure 6. Sea surface temperature departure from normal for August 12, 2006.

Wind shear
Wind shear was higher than normal in June and July, and has decreased to near normal levels since August 1. The exception is the region from the Bahamas north along the U.S. East Coast, which has still seen higher than average wind shear, due to the presence of strong upper-level low pressure systems. The 2-week GFS forecast continues to call for strong upper-level low pressure systems to roam the waters of the Atlantic, bringing hostile wind shear to any budding tropical systems that venture near. However, the wind shear averaged over the entire tropical Atlantic is expected to be near normal, and I expect that some systems will begin finding "holes" in the shear and manage to develop during the remainder of August.

Dry air
Outbreaks of Saharan dust and associated dry air have been common this year. The dust acts to reduce the amount of sunlight reaching the ocean, keeping SSTs cool. The dust and dry air also interfere with the formation process of tropical storms. The Saharan dust season peaks in June and July, and should begin a slow decline the remainder of hurricane season. Plots of relative humidity from the latest 2-week GFS forecast support this idea.

Vertical instability
Another important ingredient for tropical storm formation which I haven't talked much about is vertical instability. Simply put, if the air near the surface is very warm and the air at high altitudes is very cold, this is an unstable atmosphere. When air is unstable, thunderstorm and tropical storm activity is enhanced. This occurs because in an instable atmosphere, air from the surface can rise further and faster than air in a stable atmosphere. Rising air pulls up the moist air from the surface to colder regions aloft, where the moisture can condense and fall as rain. Since warm air is less dense than cold air, rising air in an unstable atmosphere finds itself less dense than its surroundings, since it started out very warm to begin with. Thus, the air will continue to rise, until it reaches a region of the atmosphere where the stability is high. In tropical cyclones, this often happens at about 50,000 feet--the beginning of the Stratosphere, a very stable layer of air where temperature increases with height.

Instability over the tropics during the 2006 hurricane season (Figure 7) has been below average. The ocean temperatures have been close to normal, which keeps the atmosphere more stable. Also, the general atmospheric ciculation has brought more stable air into the tropical Atlantic than we saw in 2005. However, with the weak trade winds we've been seeing this August allowing the oceans to heat up to much above normal, I expect that instability will increase to near normal levels by the beginning of September, enhancing hurricane formation.

Figure 7. Vertical instability in 2006 (blue lines) compared to normal (black lines) for the Eastern Caribbean (left) and tropical Atlantic (right). Image credit: CIRA (Cooperative Institute for Research in the Atmosphere).

Steering pattern
The large scale jet stream pattern and associated positioning of the Bermuda High has remained unchanged since early June, and is forecast to remain the same into early September. This pattern puts a trough of low pressure over the U.S. East Coast, which will act to recurve storms approaching the U.S. I expect that many of the strongest hurricanes we will see this year will recurve harmlessly out to sea, perhaps threatening only Bermuda. However, some systems may not recurve in time, which puts the U.S. East Coast at higher risk than average for a hurricane strike. North and South Carolina have the highest risk of any region of the U.S., since they stick out farthest into the ocean. This steering pattern also favors higher than normal hurricane activity in the northernmost Lesser Antilles Islands, such as the Virgin Islands, Anguilla, and St. Maarten. The U.S. Gulf Coast has a lower than average chance of a hurricane strike with this steering pattern. Given that this steering pattern has held for so long, the odds are that it will remain in place through at least mid-September.

The relatively quiet hurricane season we've been enjoying is not going to last. A very active period will start, as soon as the atmosphere destabilizes a bit more. If one believes the long-range 2-week outlook from the GFS model, the current quiet period should last another 4-12 days. Around August 21, I expect it will appear that a switch has been thrown, and the Atlantic will be very active indeed. Expect our first hurricane in the Atlantic by August 26, and a very active September. However, I do expect we will get many recurving storms that will miss land, and that this hurricane season will be similar to the ones we experienced in 1995-2003.

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

i agree wabit and thats the reason that it should dissipate
So if those Highs stay in generally the same place then the sheer should remain low

What are the chances of it moving across and staying alive on Upper Florida
Thunder: In order for any circulation to stay alive it really needs to be a strong storm, especially if it moves slowly over the FL peninsula. Chances of that are not very good.
Above,the models have it going west then SW except for LBAR
If that upper level high continues to sit near 93L, we can forget shear will ever tear it apart.

Fl-There are some nice year to year SST comparison maps for the Gulf and Gulf Stream in my blog.

The convection is dying down a little with 93, if this trend continues then we will get to see what is going on down there at the surface :)
What is that ULH supposed to do
Was just going to say that JP, also in the loop I posted you can see the deep convection tapering off for now, have to see if it comes back tonight.
mcgreen91, that is copyrighted information, do not steal that.
Afternoon all,

Here in Charleston it is sunny and hot.. DO any of you all think i should break out the foul weather gear?? Or do you think it will stay off shore???
It's the same here in GA, seafarer, although I have noticed a cool, gusty breeze in the past hour or so from the NE. I doubt you'll get anything from this except for a passing shower. Most indicators show this thing moving down towards GA or FL.

I would like to point out that it is weak, and I dont how long it is suppose to stick around. Its been there for about 24hrs now.
Could call nash, I was just going to look and see if I had seen that analyisis somewher else.

How you doing 23?

93L looking good
519. SLU
Posted By: SLU at 8:07 PM GMT on August 16, 2006.
This system looks too sick on satellite imagery to be a TD. Any upgrading now in my opinion would be premature. They should monitor the system for another 3 - 6 hours to see it gets better organised before upgrading to a TD even if the plane finds one because the system could easily lose organisation.


The NHC pretty much reflected my earlier comment
520. MahFL
Thanks SavanahStorm..
Too bad though,I could use a day off
Posted By: jphurricane2006 at 4:59 PM EDT on August 16, 2006.
ah ok; so what situation would have to happen to decrease the shear

Nothing will. Its gonna happen. High pressure would have to stop moving down from the Great Lakes. That will not happen.
Afternoon 459

Don't think you need the foul weather gear 459. Looks like it will move WSW then more SW, so maybe a few rain bands, but that is about it here IMHO. I would still keep a close eye on it just to be safe though.
mgreen91-that is very lame,on this blog, we don't use others people's efforts or info from articles without giving credit where it is due.
93L does not look very good, compare it to 91C:

Both are listed at 25 kts/30 mph but 91C is at 1009 mb.
They have to please their Vacation Sponsors and say that everything will weaken even if they know that is not the case or could not be the case

LOL. I guess the NHC and Dr Master are also under their sway. Or could it be...that the thing really is going to weaken?
Oh, 91C is in the Central Pacific, where storms rarely develop... if it develops, that may be telling us something about the weather patterns (storms usually develop in the Central Pacific during El Nino years).
Afternoon to you too SJ,
Would have liked to see some more rain around here. Anyway I'll take what i can get :)
Here's a shear loop showin the impending shear
Nice picture comparision Michael. You can see the difference between great outflow and terrible outflow. Good Job!

but last year Tropical Depression 01C formed.
RWDOBSEN: It's not that TWC might get it right. It's that they only go by ONE solution and report it as such. It's almost as if ST broke into TWC studios, performed a little "Face Off" with Stevie Surfer Lyons and reported the Tropical Update "In Stone".
skye ...can't get that link to open for me can you post the URL???
C'mon, the center at 277 meters is 2C warmer than the edge, meaning it is clearly warm core, there is a closed circulation, and there is an abundance of thunderstorms over the center. This is the very definition of a tropical depression, and it shouldn't matter that winds are only 40 to 45 km/hr.

I am dissapointed with the NHC.
Stormjunkie, what do you think about our prospects for seeing interesting weather from 'the blob' down our way?
I suspect they expect either land or increasing shear to kill it, but if it is a warm core system over water near 28, with thunderstorms over the center, and a closed circulation, then whether or not it only has 12 to 24 hours to live or not, it should be TD #4.
New Blog Up!
Someone please show me where there is a minimum required windspeed of 30 mph to be a depression.
Poll--how strong will the southeastern blob be when it makes landfall?

30 kts 1007 mb is my guess
I have seen them have advisories for a 25 mph depression, but never just 20 mph.
Afternoon SSIG, good to see ya

Depends on what kind of interesting you mean...A heavy rain and 20mph sustained winds is intersting to me..lol

As of right now, it looks like it should head down that way, but if it does not move fairly quickly then it will get sheared apart and won;t be much of anything left of it. How tonight treats it will be a major factor in how organized it gets. If it looses all of the convection around this time as it has done the past few days then I don't think it will be able to make a comeback. Could bring some nice T storms though.

But, as with the tropics, it is spinning so I am going to keep an eye on it.

StormJunkie.com-Forecast models, imagery, marine data, wind data, preparedness info, and much more. Also check out the Quick Links for fast and easy navigation of some to the most used tracking sites.
29 knots at 15oo feet would be darn close to 30 mph at the surface.
Looking at the buoy reports in the area, I see no signs of major pressure falls anywhere.
Link wants to bring it to Texas
Here ya go wabit...

Dr. Lyons said that the "window for development may have passed already"....and the jet stream "may annihilate" the storm .....it is "mostly high cloauds and mid clouds we are looking at"...... the storm may "die in place, or move slowly sw"

ok....critique anybody?????
boy.....93L is fizzling right before our eyes!!!

oh well.....the elusive Debbie is still out there somewhere! :)
The atmosphere tends to stabilize in the hours just after sunset. I am not writing anything off until I see what the diurnal max brings late tonight. Agree with others that the LLC will have to move with the upper steering towards the SW or WSW to avoid getting completely sheared.