Hurricane season 2005--why so active?

By: Dr. Jeff Masters , 7:31 PM GMT on August 01, 2005

This will be my last blog entry until August 12; I'm vacationing far from the tropics (Yellowstone!) to appreciate some mountain weather.

Today's monthly summary of hurricane activity for July issued by the National Hurricane Center (NHC) states:

"The month of July saw unprecedented tropical cyclone activity in the Atlantic Basin...with the development of five named storms...Tropical Storm Cindy...Hurricane Dennis...Hurricane Emily... Tropical Storm Franklin...and Tropical Storm Gert. The previous record for named storms that formed in July was four. The two major hurricanes that developed during the month tied a record set in 1916. The July activity follows an unusually active month of June ...And the seven named storms that have formed thus far in 2005 represent a record level of activity for the first two months of the season."

Why has this hurricane season been so active? Part of the reason lies in a decades-long natural cycle in hurricane activity that in 1995 switched to a high-hurricane activity mode. Hurricane activity has been above normal since 1995, and will likely continue to be for the rest of this decade and the next.

Additionally, there are six key ingredients are necessary for tropical cyclone formation (you can read about these in full detail in the Tropical Cyclone FAQ. We'll focus on three of them in particular that have been highly conducive to tropical cyclone formation during this remarkable hurricane season of 2005.

Vertical Wind Shear
Hurricanes need low values of vertical wind shear between the surface and the upper atmosphere (the jet stream level, typically 35,000 - 40,000 feet high in the tropics). Vertical wind shear is the magnitude of wind change with height. High vertical wind shear can disrupt a tropical cyclone trying to form by literally tearing it apart. High wind shear also can weaken or destroy a healthy tropical cyclone by interfering with the organization of deep convection around the cyclone center. Typically, 20 knots (23 mph or 10 m/s) or less difference in wind speed between the surface and upper atmosphere is considered favorable for hurricanes. In June and July of 2005, wind shear values were 20 - 40% below normal for the Gulf of Mexico and Caribbean Sea, the primary genesis locations for the seven tropical cyclones that formed. Wind shear values this low are highly favorable for tropical cyclone formation (see plots below).

Figure 1. Average amount of vertical wind shear (in black) and observed wind shear (in blue) for 2005 for the western Caribbean. Credit: Colorado State University (NOAA/CIRA)

Figure 2. Average amount of vertical wind shear (in black) and observed wind shear (in blue) for 2005 for the eastern Caribbean. Credit: Colorado State University (NOAA/CIRA)

Sea Surface Temperatures
Hurricanes need ocean waters of at least 26.5C (80 F) through a depth of about 50 meters to form or maintain their strength. The warmer the water, the better, since a hurricane is a huge heat engine. Sea surface temperatures (SSTs) are at the highest levels ever observed in the Atlantic, for the 50 years we have records. As of July 31, typical tropical Atlantic SSTs were about 2F (1.1C) above normal.

Figure 3. The Sea Surface Temperature departure from normal (in degrees C) for July 31, 2005. A large area of above normal SSTs (yellows and light greens) covers virtually the entire North Atlantic Ocean. The cold wake of Hurricane Emily is still apparent between the Yucatan Peninsula and southern Texas. Credit: U.S. Navy.

Moist Air
Hurricanes need moist air in the mid-troposphere (5 km or 3 mi altitude). Dry air interferes with the development of the large thunderstorm complexes needed to get a tropical storm going. Until the last week of July, the air over the tropical Atlantic and Caribbean Sea has been very moist. Since then, several large dust storms have moved off of the coast of Africa, accompanied by copious amounts of dry air that has interfered with tropical storm formation. TOMS aerosol data shows a large area of dust covering the entire tropical eastern Atlantic today.

Is Global Warming to Blame?
How much, if any, of this year's activity is due to global warming? That's a difficult question to answer. The research published so far shows that global warming cannot be linked to an increase in the number of hurricanes. So, this season's exceptional number of storms is probably unrelated to global warming. However, there is considerable debate whether or not sea surface temperatures and hurricane intensity have been affected by global warming. It is possible that the remarkable intensity of the hurricanes seen so far this season can be partially blamed on global warming. However, much more research needs to be done on this subject before we can link global warming with hurricane intensity. I plan to write a detailed article on the subject later this season, after I've had time to read the new research linking hurricane intensity to global warming, due to be published in Nature magazine on Sunday, August 7.

Dr. Jeff Masters

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792. Undertaker
1:06 PM EST on August 04, 2005
147257 hey man what's wrong I just read where you said that we better prepared. whats new with the system, could it hit jamaica?
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791. Randyman
3:39 PM GMT on August 04, 2005
A Randyman Exclusive Tropical Outlook...

Tropical Depression Forming in Eastern Atlantic - Harvey Moving Out to Sea

Issued: 10:00 AM CDT Thursday, August 4, 2005

Tropical Wave 27 appears to have developed into a tropical depression near 13N/32.5W this morning. Currently, thunderstorms are displaced a little to the west of the center of circulation. But the system continues to get better organized, and we think that it will be classified as Tropical Depression Nine later today. Conditions aloft appear to be very favorable for development, and we see no reason why this system won't become a tropical storm within the next 24 hours and a hurricane in 2-3 days. All model data are currently forecasting this developing storm to move generally to the west-northwest for the next 2-3 days, followed by a northwesterly turn. Early long-range projections turn the storm northward along 50W longitude and take it safely out to sea. Although we do not think it is likely that this system will enter the Caribbean Sea and threaten the Gulf of Mexico, we cannot rule out a track that might eventually threaten the east U.S. Coast in 8-10 days.

At 10AM CDT, Tropical Storm Harvey is near 32.0N/63.1W, or about 100 miles east of Bermuda. Movement continues to the east-northeast at 14 mph with a gradual decrease in forward speed expected today. On this track, Harvey will be moving away from Bermuda this morning. Maximum sustained winds are near 55-65 mph with higher gusts. Minimum pressure is 994 millibars, or 29.35 inches. There is no change to Harvey's forecast track. Harvey is passing southeast of Bermuda this morning and will continue to head out to sea into the open Atlantic, where he will slowly weaken and transition to a non tropical low pressure area over the next several days.
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790. 147257
06:02 PM GMT op 04 Augustus, 2005
In the eastern Atlantic, A developing tropical wave along 34 west and south of 17 north is tracking west at 10-15 mph. A 1009 mb low along wave was near 12 north and it is looking better organized as well. Upper level conditions are favorable for development and we could have a tropical depression within the next 12 hours. It already looks like a depression.

this is what theyre saying on accuweather
Member Since: August 2, 2005 Posts: 7 Comments: 68
789. 147257
06:00 PM GMT op 04 Augustus, 2005
new update about harvey i dont expect that he becomes a hurricane
Member Since: August 2, 2005 Posts: 7 Comments: 68
788. 147257
05:58 PM GMT op 04 Augustus, 2005
l00k maybe the odds ar against it but the TD is still moving west
Member Since: August 2, 2005 Posts: 7 Comments: 68
787. txweather
5:39 PM GMT on August 04, 2005
K8 here's my definition of rare. 35 storms were near 21N, 51W. at some point in their life. Of those 5 hit land. 1nC,2SC, 1 nips cape cod and 1 hit nova scotia. ONLY isabel hit NC. 1/35 is a rare event. Even if you use 5% as rare it qualifies. The fact is that it is very unlikely for a storm their to make landfall. If this storm hits 20 further east the odds plummet further and further. I'm using this point based on the comparisons to isabel. Also this point seems to be about a mean position. If the storm stays further south than expected then it might stay further south and hit land.

Now history doesn't tell the whole story, but the conditions that allow the outliers to make it west, simply don't exist. They all had a storng high centered around bermuda or west. This time the hight is cenetred further east. Thus there is good confidence in a north turn. One rule of forecasting I learned(other than to hedge) is that if you are making a prediction that is a rare event, you better have rare conditions. Those conditions do not exist as far a I can tell. It seems to me and most people that there isa weakness left behind after Harvey so that shouuld be where the storm recurves. If it misses that weakness or that weakness fills in then it could take a more westerly path, but still even then my instinct is that the hight doesn't quite extend to the east coast.

Still there might be some high surf coming in from this storm as it churns the Atlantic. Also please understand that this isn't an exact science and the rare can happen, so never let down your guard.
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786. MDweather
1:53 PM EDT on August 04, 2005
the water off the east is really warm. I was in Ocean City a few weeks ago and the water was nearing 80! Pretty warm all the way in Ocean City,MD
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785. 147257
05:49 PM GMT op 04 Augustus, 2005
damn this picture

Jamaica better prepare for a big hurricane happily the water by me is warm :)
Member Since: August 2, 2005 Posts: 7 Comments: 68
784. 147257
05:49 PM GMT op 04 Augustus, 2005
mdweather the importants factors are the current high winds
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783. outrocket
12:46 PM CDT on August 04, 2005
any number..right now the high pressure its going around,but hey all could change so no way will I comment..Atmoshere is complex right now so any one of the systems there may play in...
Member Since: July 15, 2005 Posts: 104 Comments: 11038
782. 147257
05:44 PM GMT op 04 Augustus, 2005
i odnt like the idea that there so much systems i'm going on vacation over 5 days and i'm rather home then stuck on land because the airport is destroyd
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781. MDweather
1:44 PM EDT on August 04, 2005
what factors do you think might effect this strom outrocket?
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780. outrocket
12:36 PM CDT on August 04, 2005
What i see at this moment is the whole hurricane basin is active with alot if different type systems..very complex pattern,so we really dont know since atmosphere is fluid, what will effect what yet and to what degree.. It will be a good system to watch if it develops into a storm, to see which factors do what.
Member Since: July 15, 2005 Posts: 104 Comments: 11038
779. MDweather
1:37 PM EDT on August 04, 2005
goldenhine i tihnk this a depression but by 5 p.m. or 11 p.m. it will be a TS
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778. goldenhine
1:31 PM EDT on August 04, 2005
its at lat 12n lon34w movement west, it looks like a TS
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777. pirateotobx
5:28 PM GMT on August 04, 2005
we're going to Disney world for a week this saturday....not worried about down there as much as here in NC while I'm gone...
776. hootiethebooty
5:24 PM GMT on August 04, 2005
I sure hope you are right txweather. Me and my family are due in the Outerbanks, NC for a two week vacation beginning this Sat.
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775. MDweather
1:25 PM EDT on August 04, 2005
thanks txweather here in MD we only get the left overs of storms but isabel was a wake up call because in MD alot of people that lived on or near the bay DID NOT have flood insurance.
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774. txweather
5:24 PM GMT on August 04, 2005
Also MD here is you NWS office and the Forecast discussion
this is good all the time for understanding reasoning for local forecast and will give you hints ahead o time oten i they see a danger to your area.
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773. pirateotobx
5:23 PM GMT on August 04, 2005
all the models i've looked at i don't see anything that will keep it from turning north out to sea...maybe wishful thinking but eh.....
772. txweather
5:17 PM GMT on August 04, 2005
MDweather, you should always have a general plan for when a storm hits. However don't put any plan into action until a Hurricane Watch is issued, then when the Hurricane warnign is issued, go into high gear. I've seem more people get ready then stand down over and over when they were nveer in danger. Wait till the weather service issues a watch then activate the preexisting plan.

That being said in my opinion up in Maryland there is little danger from this storm. Actually the same is true for the whole east coast on this one. But I'm human and do make mistake and sometimes storms do things we don't foresee, so always be aware, just don't go into real worry until there a reason to worry.
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771. Hawkeyewx
12:20 PM CDT on August 04, 2005
The NHC is always slow to call systems in the eastern tropical Atlantic depressions. Basically, they obviously can't get recon out there, ship reports are few, and they just don't have to be in any hurry to upgrade systems that far out in the middle of nowhere.
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770. pirateotobx
5:17 PM GMT on August 04, 2005
boy all the excitement... i bet my local news anchor, skip waters out of new bern nc is about to have an orgasm watching this develop....he's bored if it's not a he's one of those that seems upset if it doesn't make landfall...such a jerk...
769. K8e1
5:18 PM GMT on August 04, 2005
i stick by my forecast it is not RARE by any means for a storm to do what isabel did....where in the world did you get that idea?
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768. Hawkeyewx
11:55 AM CDT on August 04, 2005
For the last few days most if not all models that develop TD9 take it wnw for a few days before turning it nw and then north into the open Atlantic following the weakness left behind by Harvey. The 12z Canadian model has shifted west a bit and the 12z GFS still turns it nw toward Bermuda but then(beyond 7 days, very unreliable) turns it back due west to near the southeast coast of the US. It is fun to speculate about scenarios like this, but beyond about 4 days out it is really just wild guessing.

Also, I wouldn't really count on this system to be able to maintain itself as a powerful hurricane if it follows the current projected track. If you check out the following link you can see that the real heat-filled water will be staying to the southwest of the track. Of course, all of this can change if the system moves more westerly.

With regard to Harvey, it sure did quite a transformation. Yesterday it looked pretty non-tropical, but it looks 100% tropical now with real nice convection and outflow. It has been a very interesting storm.
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767. MDweather
1:14 PM EDT on August 04, 2005
thanks txweather so does that mean this storm could take a path like isabel or just become a problem for fish in the sea? (or is it wayyyy to early to tell)
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766. 147257
05:14 PM GMT op 04 Augustus, 2005
i think the model xtrp is the best model
Member Since: August 2, 2005 Posts: 7 Comments: 68
765. txweather
5:14 PM GMT on August 04, 2005
I do rememeber one year where the first few storms went right to storm bypassing depression, but I forget when. I think it was a record. that being said this looks like your typical 35mph depression. Broad roation , but not yet a tight core.
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764. txweather
4:59 PM GMT on August 04, 2005
MDweather, isabel actually was slightly north of this system and was one of the rare ones that did move wnw and make it, but during Isabel it was clear that a strong high was going to make it turn toward the US instead of going out to sea. This time the models don't hint at such a ridge(it is far further to the East). BTW the GFDL brings this close to Major Hurricane status.
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763. MDweather
1:11 PM EDT on August 04, 2005
K8e1 i live in Maryland should i start to look at diaster plans and prepare for a possible hurricane/tropical storm event?
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762. K8e1
4:56 PM GMT on August 04, 2005
this storm is going wnw for a while then west then nw in NC i would be worried if i were you but you have plenty of time to get prepared this wont be a fish storm it will be a southeast mid atlantic storm or get perilously close
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761. MDweather
1:04 PM EDT on August 04, 2005
im not a pro so what are those models telling us?
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760. punkasshans
12:01 PM EST on August 04, 2005
All of the models on top of each other:
759. punkasshans
11:59 AM EST on August 04, 2005
72 hours from now:

Looks like it is going to take a big jog north at about 60-72 hours from now.
758. Alec
1:00 PM EDT on August 04, 2005
im gonna back later...bye...Alec....OUT!!!
757. punkasshans
11:58 AM EST on August 04, 2005
Nicole and Otto were both sub-tropical storms. . i believe. So, that is a different situation.

Mathew was so close to land, they probably sent a recon plane in and found out the true strength before they issued an advisory.
756. Alec
12:58 PM EDT on August 04, 2005
that was a joke weatherboyfsu? right?
755. Alec
12:55 PM EDT on August 04, 2005 is the site showing some models as having a slight weakness in the ridge.
754. MDweather
12:54 PM EDT on August 04, 2005
does anyone know if this storm is close to where Isabel started?
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753. weatherboyfsu
4:50 PM GMT on August 04, 2005
Whats ALEC....hey undertaker look at last years hurricane season....the last 3 storms started as tropical storms...matthew nicole and otto....all three, first advisory was tropical storm ....just ask old farts....Alec is an wouldnt surprise me if it is introduce as a tropical storm........going to lunch.....bye......
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752. 147257
04:51 PM GMT op 04 Augustus, 2005
ok punkasshans maybe i see it getting bigger and thats what i see as wsw but we will see ;)

i''m going now cya guys over 3 hours
Member Since: August 2, 2005 Posts: 7 Comments: 68
751. Alec
12:49 PM EDT on August 04, 2005
im currently experiencing computer problems just a second guys......
750. 147257
04:50 PM GMT op 04 Augustus, 2005
do you know lenny i thought 1998 just turned over SSS islands and turned south-west track
Member Since: August 2, 2005 Posts: 7 Comments: 68
749. punkasshans
11:49 AM EST on August 04, 2005
The storm is definately not moving any bit south. I would say it has a definately more westerly movement than anything else, and if anything wnw. I dont expect the NHC to change their direction in the next update (or when they declare it a depression)

748. 147257
04:47 PM GMT op 04 Augustus, 2005
mdweather i think it will go wnw again in less then 3 hours
Member Since: August 2, 2005 Posts: 7 Comments: 68
747. txweather
4:43 PM GMT on August 04, 2005
When storms have that wnw trajectory so far out, they almost never make it west. This should do the same thing. Notice I put words like almost and should just to cover the rare events:). But generally this looks like a fish storm ands its size will work against it and help it get turned north.
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746. Alec
12:46 PM EDT on August 04, 2005
i made a lot of little errors a few posts ago. sorry.
745. LSUHurricaneHunter
4:44 PM GMT on August 04, 2005
alec, which model on which site are you referring to? All of the ones that are currently on fsu's site still have the system moving west-northwestward.
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744. punkasshans
11:44 AM EST on August 04, 2005
I can see everyone on here seems to think this TD9 might already be a tropical storm. . i doubt it.

The storm actually looks less impressive than it did earlier in the day. Yes, it will be a depression soon, but I definately dont see a jump to a storm name just yet. Its a huge circulation, but that doesnt mean it is strong.

REMEMBER: size does NOT equal strength
743. 147257
04:46 PM GMT op 04 Augustus, 2005
is it classified? so its finally TD 9
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742. MDweather
12:44 PM EDT on August 04, 2005
does the wsw wobble or movement cut down on the chance of a northward turn?
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Dr. Masters (r) co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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