Hurricane season 2005--why so active?

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

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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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992. dashwildwood
12:43 AM GMT on August 05, 2005
Hey guys, this is my first post here but I have been reading the blogs for about a week now, so I hope to post more in the future. I really don't want my first post to make me sound like an alarmist or an idiot but I just looked at the 18UTC GFS and saw that it has a storm hitting NC around the 15th I know this is 11 days away and there is plenty of time for there to be change, but do you guys think that will be what is eventually irene or a new storm or just something the GFS has thrown on there for fun like it has before?
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991. drlDerek
8:44 PM EDT on August 04, 2005
up to 6500 strikes in the past hour 55mph winds 1" hail.
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990. Raysfan70
12:41 AM GMT on August 05, 2005
Just had some of them storms over here near tampa one jsut moved by my house about 20 minutes ago.
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989. drlDerek
8:38 PM EDT on August 04, 2005
wicked lightning south of orlando. 5000 strikes an hour wow
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988. Raysfan70
12:32 AM GMT on August 05, 2005
can someone give me one of the pages for the long range models would like to see what they are showing?
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987. turtlehurricane
12:11 AM GMT on August 05, 2005
td 9 looks really good now with the large central convection, might be Irene at 11 pm. Definitely by 11 am.
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986. Randyman
11:59 PM GMT on August 04, 2005

An Exclusive Tropical Update:


Tropical Depression Nine in Far Eastern Atlantic - Harvey Heading Out to Sea


Issued: 7:00 PM CDT Thursday, August 4, 2005


At 7pm CDT, Tropical Depression Nine was estimated near 12.7N/35.0W, or about 700 miles west-southwest of the Cape Verde Islands. Maximum winds were 30-35 mph with higher gusts. Central pressure is estimated to be 1009 millibars or 29.80 inches. Movement is to the west at 11-12 mph. This system will move west to west-northwest over the next few days, strengthening to a hurricane in 2-3 days. For further details, please refer to the Tropical Depression Nine section of your ImpactWeather tropical page.


At 7pm CDT, Tropical Storm Harvey is near 32.7N/61.2W, or about 200 miles east of Bermuda. Movement is to the east-northeast at 12-14 mph. Maximum sustained winds are near 65 mph with higher gusts. Minimum pressure is 994 millibars, or 29.35 inches. There is no change to Harvey's forecast track. Harvey will continue to head out to sea into the open Atlantic, where it will slowly weaken and transition to a non tropical low pressure area over the next several days.


Meteorologist: George Harvey
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985. MDweather
7:38 PM EDT on August 04, 2005
hey im back guys and girls where can i get a link to the new models that will be coming out soon
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984. Toyotaman
11:43 PM GMT on August 04, 2005
Hey everyone, I think that its a little to early to predict where this storm will go. Lets wait a week and then make predictions. You never know what can happen to a storm in 7 days time. By then we should have a decent idea where it is heading.
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983. turtlehurricane
11:43 PM GMT on August 04, 2005
goodbye, dont crash the blog while im gone
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981. pcolaFL
6:40 PM CDT on August 04, 2005
I'm having trouble with this blog-it keeps freezing and scrolling funny. I'll check back later. Need to clean the kitchen and play with my mutts anyway. Bye y'all.
980. outrocket
6:35 PM CDT on August 04, 2005
P'cola..maybe maybe not,just if a ridge does build then it wont bode well for the U.S...right now it's way to early to tell where TD9 may go alot of factors.Things change fast and just one little non seen factor could change track ever so slightly but extropolated over time..could mean difference in 100s of miles..
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979. turtlehurricane
11:35 PM GMT on August 04, 2005
nevermind its working again
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978. turtlehurricane
11:34 PM GMT on August 04, 2005
my comments are dying, this blog is starting to freak
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977. Alec
7:32 PM EDT on August 04, 2005
Just an observation: If you guys go to the wunderground tropical homepage and look at the SST map youll see a kind of a triangle of warm water throughout the oceans. i find it funny there is either a depression or a storm at each of the tips of the triangle. well gotta go for now...............Alec...OUT!!!
976. turtlehurricane
11:28 PM GMT on August 04, 2005
20 degrees north is gonna be a long time from now. It might never even get there for all we know.I think it will meet the 20,50 criteria for risk in the U.S but, its just a guess
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975. pcolaFL
6:30 PM CDT on August 04, 2005
outrocket, if the high builds in after the low moves on, wouldn't that be "protection" for the gulf coast? Or am I confused?
974. Alec
7:28 PM EDT on August 04, 2005
definitely turtle. when charley came through the se gulf the temps were near 90 and with the right conditions it exploded into a very compact(eye was about 5 miles in diameter) storm with ferocious winds.
973. outrocket
6:26 PM CDT on August 04, 2005
Its a rain maker...thats all,but its what it may do in time..like high build in after.....uh oo
Member Since: July 15, 2005 Posts: 104 Comments: 11027
972. txweather
11:13 PM GMT on August 04, 2005
One key point to look for in this storms progress is the 20,50 point. If it stays south of there(and has a westerly motion) it has a chance to hit the US. If it goes north of there the odds hitting become very very very small(Unless its going straight w or wsw at that time). Thats why any n component of the storm right now is excellent. My statements about it going out to see were based on the assumption it will go north of there. If it doesn't we face a different game.
971. turtlehurricane
11:24 PM GMT on August 04, 2005
the sst in that spot is insane even for hurricane season. its over 90 degrees. if a hurricane goes through there it would be scary.
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970. Alec
7:25 PM EDT on August 04, 2005
i think its too close to land to be much of anything. its supposed to weaken with time and drift very slowly westward.
969. turtlehurricane
11:18 PM GMT on August 04, 2005
it looks a little cyclonic but, looks can be very decieving
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968. Alec
7:18 PM EDT on August 04, 2005
Ok. Ill explain. Troughs are associated with low pressure within themselves and with higher pressure around their environment. When troughs move off the coast they displace air behind them. Now this air has to be filled up because higher pressure goes to lower pressure filling in the gap. Since the gap gets filled up higher pressure will develop. Hope that helps.
967. outrocket
6:16 PM CDT on August 04, 2005
sure is alec..persistant lil low..been forecast to fade for days but hasn't.Rain is all I have seen today and heavy storms moving through area this afternoon from NE to SW..and you being correct on high pressure moving to fill in low..well this little system may become one of the abstract players that cause the ridge to build in even more..then that may be a factor in Irene's future as she steams towards the west.
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965. turtlehurricane
11:13 PM GMT on August 04, 2005
hurricanewatcher, its possible especially due to the trough but, due to the trough it will be sheared and be weak. also as the forecast said hybrid cause its on a front. i say the worst case scenario is subtropical depression 10. that is remote by the way.
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964. pcolaFL
6:11 PM CDT on August 04, 2005
Alec-are you saying that the current low pressure just offshore will become a high when the trough moves on? Sorry for the dumb question but I am not as scientifically inclined as the rest of you. That's why I enjoy reading what you have to say.
963. Alec
7:15 PM EDT on August 04, 2005
there is a slight counterclockwise circulation near FL,AL, and MS looking at the local radars.
962. turtlehurricane
11:11 PM GMT on August 04, 2005
my prediction is florida will be hit by Irene because the eye of Irene went over my house in 1999
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960. Alec
7:07 PM EDT on August 04, 2005
I explained a week ago about the tendencies for highs to build after a trough swings by since the displaced air has to be filled with higher pressure(higher pressure goes to lower pressure) thats why last yr, troughs swung off the se and right afterward a high built in making the storms hit FL.
959. pcolaFL
5:57 PM CDT on August 04, 2005
Any ideas on how the Gulf of Mexico will do in the next couple of weeks? Is there any high pressure system that would steer things away if "Irene" keeps going west or hits the peninsula and continues on? I know it would be pure speculation, but I would be interested in anyone's thoughts on the subject. Thanks.
958. turtlehurricane
11:08 PM GMT on August 04, 2005
sorry the blog is acting up
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957. turtlehurricane
11:05 PM GMT on August 04, 2005
were speculating over the top! lets just speculate to the Leewards
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955. Alec
7:01 PM EDT on August 04, 2005
If this storm goes more nw than w it has a better chance of catching a trough in the mid atlantic where harvey is being taken. its looking like it might miss the central trough and will probably get taken more westward through the next 3-5 days. After that THE HIGH WILL EITHER WEAKEN OR BUILD but its too early to tell that now.
954. turtlehurricane
11:01 PM GMT on August 04, 2005
ok we are starting to speculate over the top! lets just speculate up to the leeward islands
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953. Jedkins
11:01 PM GMT on August 04, 2005
Severe weather widespread with very heavy tropical thunderstorms reaching severe limits all acroos south florida today,dont know why I really said that just tellin y'all what is goin on near in my state.
952. Jedkins
10:59 PM GMT on August 04, 2005
Yes but there probably wont be any harvey weakness as the ridge will mostlikely at this time build in to strong for that
951. Alec
6:59 PM EDT on August 04, 2005
Ok hawkeye but dont you think its fair that it can catch onto upper lows also? i think the professionals can distinguish them to where it wont be a problem.
950. turtlehurricane
10:55 PM GMT on August 04, 2005
ya, that is the most important thing at the moment. If it went north it would be forgotten but, if it trends west it will be a serious threat to anything in the west indies. Totally different scenarios that can be caused by how strong a high is to the millibar. And as u said the gfs shows it strong enough during the part of the run that might be feasible to happen.
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949. Jedkins
10:52 PM GMT on August 04, 2005
What storm turn to the west,its to far away but a carolina landfall is very unlikely because for one there will almost certainly be a blocking ridge and there have been many troughs moving off the carolina coast northward but maybe southern southcarolina southward down to southflorida is probably where it will go if it keeps moving west and a weakness does not accur but My thinking is Florida east coast if a weakness in the ridge does not develop.BUT IT IS CERTAINLY WAY TOO EARLY TO TELL UST POSSIBLE IDEAS I AM THINKING OF.Oh and turtle hurricane just explaining why I think the caroinas landfall is less likely,so now you know that factors that MIGHT come into play that would inhibit that from happening
948. Hawkeyewx
5:57 PM CDT on August 04, 2005
If that FSU model was consistently right we'd have about 50 hurricanes this season. I think it needs some major tweaking.
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947. Alec
6:57 PM EDT on August 04, 2005
stupid internet. im having trouble with this technology. keep losing a connection
946. Hawkeyewx
5:45 PM CDT on August 04, 2005
Turtle, I agree the second half of the 16-day GFS run can't be looked at for purposes other than fun speculation. However, the important thing is the last several GFS runs have trended further west and the 18z run now does not turn Irene north into the Harvey weakness but instead keeps it moving further west.
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945. turtlehurricane
10:51 PM GMT on August 04, 2005
pcolafl, the FSU forecast has been trying to develop a storm there for a week. i think they have to tweak that problem.
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944. Jedkins
10:49 PM GMT on August 04, 2005
The FSU model is good at predicting where storms will go but bad at predicting a storm to form or intensity,but dont Forget about the FSU because it preformed the best in terms of where storms where go than any aother model last year and is doing very well this year too.
943. txweather
10:45 PM GMT on August 04, 2005
Maybe we should all move to somebodies blog every day otherwise I suspect we'll crash this one one.
942. turtlehurricane
10:49 PM GMT on August 04, 2005
jedkins, u just said in a whole paragraph its too early to tell
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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