Irene roars into life; may become the season's first hurricane

By: Dr. Jeff Masters , 12:03 AM GMT on August 21, 2011

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Tropical Storm Irene roared into life this evening, transitioning from a tropical wave to a 50 mph tropical storm in just a few short hours. An Air Force hurricane hunter aircraft in the storm was finishing up its mission when it suddenly came across a region with intense thunderstorms and surface winds of 50 mph. The aircraft found that a center of circulation had barely closed off on the southwest edge of this region, though the plane found almost no winds from the west around the circulation center. The 6:10pm EDT center fix found a central pressure of 1007mb, which is quite high for the observed 50 mph winds. Dry air to the north and west is slowing development, as well as moderate wind shear of 10 - 20 knots, as analyzed by the University of Wisconsin CIMSS group. Infrared satellite loops and radar out of Martinique show the storm is poorly organized, with no evidence of spiral bands. The center of Irene is expected to cross over the Caribbean island of Dominica early Sunday morning, but the heaviest thunderstorms lie to the north of the center, and will affect Guadeloupe, Antigua, and St. Kitts and Nevis.


Figure 1. Evening satellite image of Irene.

The computer models have shifted southwards since yesterday, and now take Irene south of Puerto Rico on Monday, and along the south shore of the Dominican Republic on Tuesday. On Wednesday, Irene should pass near or over southern Haiti, Eastern Cuba, and Jamaica. On Wednesday and Thursday, the models agree that a trough of low pressure will dip down over the Eastern U.S., which is likely to turn Irene to the north. The exact timing and strength of this trough varies considerably from model to model, and will be critical in determining where and when Irene will turn to the north. We can expect that Irene will impact Central Cuba, the Bahamas, and the Florida Keys on Thursday, but it is uncertain whether Irene's turn to the north will take the storm into the Gulf of Mexico or not. Irene most reminds me of Tropical Storm Fay of 2008. Fay formed just off the coast of Puerto Rico, and was never quite able to get organized enough to become a hurricane, due to passage over Hispaniola and Cuba. Fay topped out as a strong tropical storm with 70 mph winds, and did over $500 million in damage in the U.S., mostly due to flooding rains in Florida that accumulated to over 25 inches in a few areas. Fay also dumped heavy rains on Hispaniola, triggering flooding that claimed eight lives.


Figure 2. Track of Tropical Storm Fay of 2008.

Irene will be battling dry air and moderate wind shear of 10 - 20 knots through Sunday, and it will take the storm 1 - 3 days to build up an eyewall and intensify into a hurricane. Irene is more of a threat than Tropical Storm Emily of early August was, since Irene has closed off a center farther east than Emily did and has more time to organize before encountering Hispaniola. I don't think passage over Hispaniola will destroy Irene, since it is a fairly large storm, and is likely to be a hurricane by then. However, if Irene follows the NHC forecast, it will have an extended encounter with Hispaniola and Cuba on Tuesday through Wednesday that will probably weaken the storm below hurricane force. Keep in mind that the average error for an official 5-day forecast from NHC for a developed storm is 200 - 250 miles. Irene could easily miss Florida and move up the East Coast and hit North or South Carolina, or pass through the Florida Keys and into Gulf of Mexico, ending up who knows where. Given the uncertainties, this weekend would be a good time to go over your hurricane preparedness if you live anywhere in the Caribbean, Bahamas, or U.S. coast, since Irene could well be paying you a visit as a tropical storm or hurricane sometime in the next week.

Harvey hits Belize
Tropical Storm Harvey made landfall at 2pm EDT on Saturday near Dangriga Town, Belize, as a tropical storm with 60 mph winds. Harvey continues to dump very heavy rains on northern Guatemala, Belize, and portions of Mexico's Yucatan Peninsula as the storm tracks westwards at 12 mph. Dissipation is expected Sunday as the storm pushes inland. Harvey was a small storm, and the strongest winds were confined to a short stretch of coast near where the center came ashore. Winds at Belize City, Belize on Saturday topped out at 15 mph.


Figure 3. Radar image of Harvey taken at 11:30am EDT on Saturday, August 20, 2011, a few hours before landfall in Belize. A small closed eye is visible just south of the offshore islands of Belize. Image credit: Belize National Meteorological Service.

An exceptionally active early part of hurricane season
It's been a strangely hyperactive season for weak storms in the Atlantic so far this year. Tropical Storm Irene is the 9th named storm this year, and its formation date of August 20 ties 2011 with 1936 as the 2nd earliest date for formation of the season's 9th storm. Only 2005 had an earlier date. The first eight storms this year have stayed below hurricane strength, making 2011 the first hurricane season since record keeping began in 1851 to have more than six consecutive tropical storms that did not reach hurricane strength. As I discussed in Friday's post, a major reason for this is the lack of vertical instability over the tropical Atlantic so far this year. We've had a large amount of dry, sinking air over the tropical Atlantic, and the usual amount of dry, dusty air from the Sahara, both helping to keep the atmosphere stable and stop this year's storms from intensifying into hurricanes. Hurricane activity typically ramps up big-time by August 20, with more than 80% of all the hurricanes and 65% of all the tropical storms occurring after that date. At our current pace, 2011 will become the second busiest Atlantic hurricane season on record, with 24 - 27 named storms. There are only 21 names in the list of names for a hurricane season, so we may have to break out the Greek alphabet again in late October this year, as occurred in 2005. Ironically, this was the last time the current set of names was used in the Atlantic, so 16 of this year's 21 names are repeats of 2005. I'm not too happy about seeing another hurricane season challenge the Hurricane Season of 2005 in any way, and let's hope we don't retire another five names this year, like occurred in 2005! With vertical instability much lower this year than in 2005, and that year having already seen one storm (Dennis) retired by this point in the season, I doubt that will happen, though.


Figure 2. The annual cycle of average hurricane frequency in the Atlantic. Historically, about 35% of all the tropical storms and 15% of all the hurricanes will have occurred by August 20.

Invest 98L near the Cape Verde Islands
A tropical wave near Cape Verde Islands off the coast of Africa, Invest 98L, is spreading heavy rains and strong gusty winds to those islands today. So far this morning, top sustained winds measured in the Cape Verde Islands were 23 mph at Mindelo. 98L has a long stretch of ocean to cross before it could affect any land areas. Approximately 70 - 80% of all tropical cyclones that pass this close to the Cape Verde Islands end up curving out to sea and not affecting any other land areas, according to Dr. Bob Hart's excellent historical probability of landfall charts. The latest set of long-range model runs go along with this idea, and I'd be surprised if 98L threatens any land areas.

Jeff Masters

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Quoting WxLogic:
Good Morning...

Hi WXLogic. Looks like Irene has consolidated overnight. LinkShortWaveLoop

Do you think she's too far north to go through the Windward Passage? Or is that just an illusion?
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2770. WxLogic
HH is on its way by the way:

Product: Air Force High Density (HDOB) Message (URNT15 KNHC)
Transmitted: 21st day of the month at 10:34Z
Date: August 21, 2011
Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 300)
Storm Number: 09
Storm Name: Irene (flight in the North Atlantic basin)
Mission Number: 2
Observation Number: 02
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Located @ Dominica

Well that was weird. Quietest night for weeks. Not much rain and no wind to speak of. The center moved North by 60 miles, but you would still think there would be something here.

Normally one cloud and we get wind and rain ... yet nothing.

I can see the summit of our highest mountain quite clearly.

Strange but who cares? as long as we are safe.
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Question if the models are now taking it away from Florida with the exception of the two western outliers why hasn't the cone shifted more to the right?
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2766. Gearsts
I see and eye forming on radar very small and looks like is a bit north of the NHC forecast point.Link
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westward she goes.....

Quoting CaicosRetiredSailor:


Guadeloupe Radar
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2764. WxLogic
Eye feature trying to develop around 16.9N 61.9W.

Models have trended some to the right due to the strength of Irene being a bit stronger than expected so able to fight of the building Bermuda High from E some but still impacting US.
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2763. Gearsts
Quoting CaicosRetiredSailor:


Guadeloupe Radar
WOW!
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Quoting SavannahStorm:
Good morning all!


Well, I see we can throw out the GFS and ECMWF-

Both show Georgia landfalls, and we all know thats just silly!





Now where is my supply checklist....

That is a steep angle too.
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Guadeloupe Radar
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Quoting WxLogic:
Good Morning...


Morning Logic... just getting on myself... 5A disc... center could be attempting to relocate 50NM north... do you think she now tries to get north of Hispaniola?
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Quoting QMiami:
she looks top heavy - dry air on the south side?


Yes you can see it getting sucked in here...mainly from the N-ern portion of S America where a lot of dry air originates from in the E-ern Carribean. Its not overly significant though, she is mixing it out of her inner core decently.
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2757. GetReal
<
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Quoting Chicklit:


uh, no.
We're watching a tropical storm develop that will impact people and property, livelihood and lives.
This is real.
Sorry Chiklit, I was just joking. I know this is serious stuff. I'm a learned tropical weather person, and the forecaster, if you will, for my local emergency management. That was really about Facebook anyway. I just thought others would get a kick out of it.
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2755. QMiami
Yesterday the models covered florida now they look they are avoiding it lol
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2754. WxLogic
Good Morning...
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Quoting connie1976:
Don't the models tend to go back and forth a lot? ...so today it will be ga and tomorrow it will be la?


There won't be any huge swings with this one, some variation, but it probably will be close to us one way or the other. All I can say is - make sure you are prepared just in case.
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Quoting weatherjr:
To me the big potent cyclone in Fla is out of reality.


We can't give people in Florida and the rest of the SE US a false sense of security, there are a lot of uncertainties still to be worked out but the fact remains that this has the potential to be a very strong storm if things fall into place for her. Land interaction will play the major role in her fate since the large scale environment is very favorable along the entire path of the storm. If she does not cross a significant portion of land before reaching the US coast its very likely she will arrive as a hurricane.
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I was really hoping when I woke up this morning the cone would have shifted or the storm would have gone poof ( unlikely I know, but a girl can still dream). I guess the dream is over. Time to get out if bed eat breakfast, get gas, check the generator, and put up some shutters.
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2749. QMiami
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Quoting HimacaneBrees:


Wow they have a much wider cone.


In that graphic the Navy calculates the area by the radius of TS winds plus the cone radius. NHC only calculates from the forecast point.
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Quoting chrisdscane:
when do the next set of models come out?
I think 8 a.m., but I'm not sure. Sometimes I mix up EDT with the Greenwich Mean Time, whatever that acronym is that I can't think of right now!
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2746. QMiami
she looks top heavy - dry air on the south side?
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Quoting popartpete:
Right up at the crack of daylight, checking on Miss Irene, and worrying. Now, I copied this quote, which I think is hilarious, "I just realized.. We sit & stare at a screen.. We talk to ourselves.. We have imaginary friends, zoos, farms, cities, & fake animals.. We cook imaginary food in imaginary bakeries.. We play bingo that gives no money.. We poke people & think its OK.. We even write on walls.. Think about it.. Facebook (or this blog perhaps) is a mental hospital & we are all it's patients."


uh, no.
We're watching a tropical storm develop that will impact people and property, livelihood and lives.
This is real.
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Quoting Chicklit:
Hi Savannah,
Do you think the track will shift right or is it headed toward GOM?


I think we'll see a windshield-wiper effect in the models for the next day or so as the upper-air pattern presents itself. I don't see Irene making it much farther west than Naples before making landfall and pulling N. The high angle of approach towards Florida and the SE coast, though makes things very uncertain- if Irene's track is off by a degree, the eventual path can be off by hundreds of miles. That's why its important to remember that it is not just a point on a map- it is likely that every state from Florida through New England is going to be affected by Irene.
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Don't the models tend to go back and forth a lot? ...so today it will be ga and tomorrow it will be la?
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Quoting InTheCone:
GA, SC border....



That sucks. Puts me on the ugly side of the storm
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Quoting weatherjr:
To me the big potent cyclone in Fla is out of reality.


well, that is good news!! lol
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Quoting Chicklit:


From Navy site.


Wow they have a much wider cone.
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Right up at the crack of daylight, checking on Miss Irene, and worrying. Now, I copied this quote, which I think is hilarious, "I just realized.. We sit & stare at a screen.. We talk to ourselves.. We have imaginary friends, zoos, farms, cities, & fake animals.. We cook imaginary food in imaginary bakeries.. We play bingo that gives no money.. We poke people & think its OK.. We even write on walls.. Think about it.. Facebook (or this blog perhaps) is a mental hospital & we are all it's patients."
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GA, SC border....

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Hi Savannah,
Do you think the track will shift right or is it headed toward GOM?
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Quoting TampaBayWX:


atmos.. does this mean we could expect to see it possibly strengthen soon?


I think there is a pretty good chance Irene begins to intensify later this morning if the central core continues to consolidate and tries to form an eyewall. There is still some dry air entrainment but she is doing a decent job mixing this out. The upper environment is very favorable for strengthening and I wouldn't be surprised to see Irene become a hurricane in the next 12 hours or so.
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From Navy site. Concensus has taken a rightward shift, instead of entering GOM and crossing Florida diagonally from west to east, Irene is now headed straight for Miami as a weaker storm. This may even change by 8 a.m.!
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Quoting SavannahStorm:
Good morning all!


Well, I see we can throw out the GFS and ECMWF-

Both show Georgia landfalls, and we all know thats just silly!





Now where is my supply checklist....


Was just thinking that this run shoots it all the way up the coast right into Georgia, wild ride for a lot of people if that verifies.
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Quoting atmosweather:
Wind analysis from CIMSS showing the upper level high over Irene starting to expand and slide over the core of the storm, which is helping the storm's outflow improve steadily especially on the N and E sides. The trough to the E of the storm is also acting to improve the outflow channel in that direction as the NHC stated in the last discussion.


atmos.. does this mean we could expect to see it possibly strengthen soon?
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LinkWVLoop

On satellite view Irene looks north of forecast points if it's going through the Windward Passage.
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Good morning all!


Well, I see we can throw out the GFS and ECMWF-

Both show Georgia landfalls, and we all know thats just silly!





Now where is my supply checklist....
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Wind analysis from CIMSS showing the upper level high over Irene starting to expand and slide over the core of the storm, which is helping the storm's outflow improve steadily especially on the N and E sides. The trough to the E of the storm is also acting to improve the outflow channel in that direction as the NHC stated in the last discussion.
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WOW....

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Quoting InTheCone:
06z GFS is running, and looks BAD for SE FL...



I hope that ends up being false... :-(
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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.