Which model do you trust? And, Arctic sea ice reaches a record minimum

By: Dr. Jeff Masters , 7:33 PM GMT on August 17, 2007

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Hurricane Dean, now a major Category 3 hurricane with 125 mph winds, continues to intensify and grow larger in size. Dean pounded Martinique and St. Lucia this morning, and claimed its first victim when a 62-year old man died on St. Lucia while trying to save his cow from raging flood waters.

Dean's eye is now visible on long range radar out of Puerto Rico. Buoy 42059 is in Dean's path, and should be interesting to watch.

We're fairly confident of the 1-2 day forecast, which has Dean headed west to west-northwest over the Central Caribbean, very close to Jamaica and the Cayman Islands, then into the Western Caribbean. After that, things become murkier. The latest 12Z runs of the NOGAPS, UKMET, GFS, and HWRF computer models all show Dean hitting the Yucatan Peninsula, and continuing on into the Gulf of Mexico towards a second landfall near or south of the Texas border. The HWRF run is slower, and does not take Dean to the coast at the end of its forecast period. The big outlier is the GFDL model, which now takes Dean northwest into central Louisiana. Which model is correct? The problem is that each model has a different solution for the behavior of an upper-level low pressure system expected to be over the Gulf of Mexico early next week. Which model should we trust?

In 2006, the official NHC forecast performed better than any of the individual computer forecast models. However, several "consensus" forecasts made using an average of the "big four" computer models (GFDL, GFS, UKMET, and NOGAPS), slightly outperformed the official forecast at some time periods (Figure 1). The Florida State Super-Ensemble (FSSE), for example, combines the "big four" models on the basis of past performance in an attempt to correct for biases in those models. (The FSSE is owned by a private company, which makes it available to NHC but not the general public). The Florida State Super Ensemble slightly out-performed the official NHC forecast at most forecast times.

The "big four" models are plotted on wunderground.com's computer model page for Dean, (along with the inferior BAMM model, which is plotted since it is always available quickly, and has done well at longer range forecasts in the past). We do not get tracking points for the ECMWF or HWRF models at this point, so you'll have to go the raw plots to see those forecasts. Note that three of the "big four" models performed well in 2006, with the GFDL and GFS performing the best. The UKMET had a very poor showing in the Atlantic in 2006. However, the UKMET was the best-performing model in the Eastern Pacific in 2006, along with the GFDL and BAMM models.

The European Center's model (ECMWF) outperformed the "big four" consensus models for 72, 96, and 120 hours forecasts in the Atlantic. However, the ECMWF model was generally not available in time to be used by forecasters. Efforts are being made to make the ECMWF available in a more timely fashion for the 2007 season, which would be a big help. We also have the new HWRF (Hurricane Weather Research Forecast) model this year. In tests done on a number of hurricanes for past years, the HWRF performed about as well as the GFDL (Figure 2).



Figure 1. Track forecast skill in 2006 of the official forecast and the various models, compared to a "zero skill" forecast using NHC's CLIPER5 model. The CLIPER model (short for CLImatology and PERsistence) is a model that makes a forecast based on historical paths hurricane have taken, along with the fact that hurricanes tend to keep moving in the direction they are going (i.e., their current motion persists). Note that many models had a negative skill for their 120 hour (5 day) forecast. The official NHC forecast had about 10% skill at 5 days. Image credit: NHC.

Figure 2. Track errors for 48-hour forecasts from the 2006 version of the GFDL model (black) and the new HWRF model (red). The HWRF model performed better on some hurricane than the GFDL, and worse on others. Overall, the two models had about the same performance on the cases tested. Image credit: Naomi Surgi, NOAA Environmental Modeling Center.

In conclusion, the official NHC forecast outperforms all the individual models, particularly at long ranges. Looking at the individual model plots can be helpful to determine the uncertainty in the forecast, but it's tough to beat the NHC. In the case of Dean, where one model is an outlier from the rest, it is usually better to believe the consensus of the other models.

If you want to look at plots of the individual models, I've written a description of the various models and where to find these plots on our tropical weather page.

Arctic sea ice shrinks to record low
The National Snow and Ice Data Center announced today that Arctic sea ice has just surpassed the previous single-day (absolute minimum) record for the lowest extent ever measured by satellite. Satellite measurements began in 1979. Sea ice extent has fallen below the 2005 record low absolute minimum and is still melting. Sea ice extent is currently tracking at 5.26 million square kilometers (2.02 million square miles), just below the 2005 record absolute minimum of 5.32 million square kilometers (2.05 million square miles). This new record was set a full five weeks before the usual late September minima in ice extent, so truly unprecedented melting is occurring in the Arctic. The most recent images from the North Pole webcam show plenty of melt water and rainy conditions near the Pole.


Figure 2. Current extent of the polar sea ice, compared to the normal for this time in August (pink line). Image credit: National Snow and Ice Data Center.

I'll have an update Saturday morning.
Jeff Masters

Hurricane Dean near the island Puerto Rico (Hector777)
the ciclonic surge hard mind in Salinas,Puerto Rico mines the Community Las Ochenta in the south of Puerto Rico
Hurricane Dean near the island Puerto Rico

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685. bekroweather
10:59 PM GMT on August 17, 2007
Donna in Galveston:

The waves in Galveston currently have nothing to do with Dean. It is like 2000 miles away from Galveston. We just have a little bit of bad weather in SE TX tonight. If it was associated with anything tropical, that would be with the remnant moisture from Erin.
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684. HurricaneParty
11:01 PM GMT on August 17, 2007
The governor of Louisiana has just declared a state of emergency for Hurricane Dean ... just in case!
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683. StPeteBill
10:56 PM GMT on August 17, 2007
Will Dean keep moving as fast or is there anything that would cause it to slow down?
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682. sngalla
6:58 PM EDT on August 17, 2007
Max also mentioned that the ULL may be influencing the GFDL's track.
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681. Crisis57
10:59 PM GMT on August 17, 2007
looks as Dean is going through an eyewall replacement eye doesn't seems so clear anymore
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680. Metallica1990
10:57 PM GMT on August 17, 2007
Posted By: ThePainkiller at 10:58 PM GMT on August 17, 2007.

Recon is on its way out to Dean now. It will be intresting to see what they find.


I say CAT 4 winds of 140<
pressure <945
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679. DonnaGalveston
10:57 PM GMT on August 17, 2007
Here are some live webcams from the area:

http://www.galveston.com/webcams/

As you can see, the water is already rising, and waves are crashing. Roads would probably go under water well in advance of landfall if it comes here.
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678. barbadosjulie
10:57 PM GMT on August 17, 2007
thanks, the talk here in Barbados is that there is forcasted to be another storm in the coming days that will also affect us. Is this tru?
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677. CaymanThriller
10:56 PM GMT on August 17, 2007
Hi,
Does anyone know if the models take in account the effect of landfall? Previously we have seen that there is a "bounce" and shift in track when hurricanes hit the mountainous Jamaica?
Anything to avoid another Ivan..
Cheers
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676. ThePainkiller
10:54 PM GMT on August 17, 2007
Recon is on its way out to Dean now. It will be intresting to see what they find.
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675. extreme236
10:56 PM GMT on August 17, 2007
Posted By: barbadosjulie at 10:55 PM GMT on August 17, 2007.

any thoughts on the wave that left africa so we need to watch this in Barbados or does it look like just another wave


its something to watch but nothing to get concerned about now. its just a wave it may not even develop.
Member Since: August 2, 2007 Posts: 19 Comments: 19234
674. DonnaGalveston
10:54 PM GMT on August 17, 2007
Our news here is saying we would be getting outer rain bands by Wednesday if the current track is valid. I am watching my other neighbors unload their plywood from storage.
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673. barbadosjulie
10:53 PM GMT on August 17, 2007
any thoughts on the wave that left africa so we need to watch this in Barbados or does it look like just another wave
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672. Johnhopefan
10:53 PM GMT on August 17, 2007
Forget Louisiana, last night the same model had a Cat 5 into Houston!

The run into Louisiana keeps Dean a cat 3 until Jamaica, it will be a Cat 4 by the evening. The strength of the storm will lessen the influence of the Upper level low...... to a degree. But if the most accurate models are right then it will still be influenced a bit, maybe enough to mean HOUSTON.... which would big picture.....be 10000000 times worse then Louisiana!

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671. strangesights
10:46 PM GMT on August 17, 2007
Destruction due to flooding resulted south of Mobile were destroyed in Bayou La Batre, Heron Bay, Dauphin Island, ...
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670. StormJunkie
10:52 PM GMT on August 17, 2007
Good points about the models and intensity STL, but there is no way the GFS is accurate with track. Even accounting for the fact that it does not do intensity very well, it way underestimated Dean imho.
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669. WPBHurricane05
6:53 PM EDT on August 17, 2007
Max Mayfield said on Local 10 that the GFDL has Dean moving slower than the other models, which might be the reason why it has Dean going north.
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668. jake436
4:49 PM CST on August 17, 2007
For all of the "experts"...Katrina pushed water INTO Lake P...not out of it. The storm was moving NORTH...and the NORTHEAST quadrant forced water in the Lake.

And Bayoubrother...yes...parts of Mobile were devastated by Katrina. It amazes me how folks think Katrina only affected NOLA.
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667. wederwatcher555
10:53 PM GMT on August 17, 2007
what about the gsf ensemble mean? is it completely useless?
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666. snowboy
10:40 PM GMT on August 17, 2007
Dean is already a strong Cat3, and the tiny eye means further intensification is occurring and it hasn't even hit the very warm waters of the northwestern Caribbean..

This system may well develop into one of the strongest of all time in the Atlantic basin, so please if there is even a tiny chance of it coming by your area get prep's going now. Don't wait for a change in forecast track to get you moving..
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665. Metallica1990
10:52 PM GMT on August 17, 2007
i thought sepat made landfall as a borderline 3/4
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664. StormJunkie
10:46 PM GMT on August 17, 2007
Thanks 456 :~) I am all up for hearing your opinions!

Donna, you get my mail? Again, so sorry!!!

lf, I have spent 8hrs working a java issue because some jerk thought it would be fun to mess with me. Strange this happened after I used a certain java app last night...Arghh. 150$ and hours later I am almost done. Tech folks get paid a fortune! So, no, I have barely looked at Dean, much less that far East. Will get back in the swing here soon.

On that note, Dean is looking very impressive. The next set of islands will not get off as light as the windwards did.

The 18z GFS is nuts, it has no handle on the intensity of this system...
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663. FTmyersZ
6:49 PM EDT on August 17, 2007
Quick question I thought of as I was writing my last post. If a model has the intensity totally wrong, how can it accurately forecast track? The steering currents for a cat 1 and a cat 5 can be quite different right?
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662. hurricane91
6:50 PM EDT on August 17, 2007
everyone stop calling it a she, its a him
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661. marinetech47
10:49 PM GMT on August 17, 2007
South Central La. here. New Iberia. What time do the latest models come out. Got home from work and was very suprised to see where the GFDL was bringing Dean.
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660. H2PV
10:50 PM GMT on August 17, 2007
A landfalling Hurricane is a rapidly changing thing. SEPAT still carried as 150 mph cat 4/5 borderline typhoon is well into Taiwan.

http://www.h2-pv.us/wiki_100mpg/img/wiki_up/20070817.2130.09WSEPAT.jpg
http://www.h2-pv.us/wiki_100mpg/img/wiki_up/20070817.2130.09WSEPAT.jpg

In the image below the bright green represents the most rain, predicted to drop 40 inches in places, as the clouds are scrunched against steep mountains, wringing them out like a sponge. SEPAT has not suffered much change of form with it's interaction with the island of Taiwan.

http://www.h2-pv.us/wiki_100mpg/img/wiki_up/ft-l.jpg
http://www.h2-pv.us/wiki_100mpg/img/wiki_up/ft-l.jpg
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659. nola70119
10:50 PM GMT on August 17, 2007
Surf is already up in Texas....
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658. auminer68
10:48 PM GMT on August 17, 2007
animalrsq wrote: Just a plea to all of you to spread the word. If you or anyone you know plans to evacuate, PLEASE take your pets with you. If it is not safe for you, it's not safe for them. Most people who evacuated for Katrina did not realize we'd be pulling their animals out of the bathroom three and four weeks after the storm, some alive, some not. Thank you.


HEAR HEAR!

I second that. There were some heartbreaking times last year at the Lamar-Dixon Expo Center... believe it!
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657. Crisis57
10:49 PM GMT on August 17, 2007
Posted By: IKE at 10:48 PM GMT on August 17, 2007.

Posted By: Crisis57 at 5:47 PM CDT on August 17, 2007.
whats the status on the ULL


It's just east of south Florida, moving slowly west.


seems to be in the same spot it was this morning but i can be wrong
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656. bekroweather
10:47 PM GMT on August 17, 2007
FTmyersZ

I've just mentioned the result of the UKMET. It's only 18Z model I managed to find.

Global models have also resolution issues. We should not expect a global model to be able to accurately predict a storm's minimum pressure which typically exists in a very small area.
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655. jj292
10:45 PM GMT on August 17, 2007
All the major models except the NOGAPS seem to at least somewhat agree that the center will pass either right on or just north of Jamaica. Even the UKMET has shifted north of Jamaica now.
654. IKE
5:47 PM CDT on August 17, 2007
Posted By: Crisis57 at 5:47 PM CDT on August 17, 2007.
whats the status on the ULL


It's just east of south Florida, moving slowly west.
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652. dean2007
10:45 PM GMT on August 17, 2007
The GFDL would be pretty bad, maybe a WIlma before she hit the Yucatan peninsula in terms of size.
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651. WPBHurricane05
6:47 PM EDT on August 17, 2007
The UKMET is a global model.
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650. WPBHurricane05
6:45 PM EDT on August 17, 2007
Global models do not properly represent storm strength.
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649. Crisis57
10:45 PM GMT on August 17, 2007
whats the status on the ULL
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648. Baybuddy
10:44 PM GMT on August 17, 2007
running, It was a mess alright. I certainly understand we got lucky Parts of Mississippi Got nuked.
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647. Metallica1990
10:44 PM GMT on August 17, 2007
all the conditions are right for rapid deepening I think we have a Category 5 getting ready to show its ugly face
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646. DonnaGalveston
10:42 PM GMT on August 17, 2007
Thank you so much everyone for answering my questions. I just went to the store and it is getting more crowded. The news is on and they are telling everyone to get prepared. Evacuations could start here as early as Monday. If we leave we will try to beat the rush and leave on Sunday maybe. I am very afraid and I know that it may not even come here. This is just nerve wracking.
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645. extreme236
10:45 PM GMT on August 17, 2007
well if dean's strength intensifies even to 115kts, we now have a cat 4 storm, so expect rapid intensification potentially
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644. littlefish
10:42 PM GMT on August 17, 2007
SJ- have you looked at 15N 32W yet? Looks like one of the LBs from last year LOL. Maybe it can traverse the Atlantic w/o getting obliterated? It looks naked w/o convection... See how it is bending the ITCZ a bit?
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643. bekroweather
10:42 PM GMT on August 17, 2007
littlefish: Indeed UKMET seems to be the on fluctuating the most.

Is there access to the 18Z runs for the other models yet?
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642. nolesjeff
10:44 PM GMT on August 17, 2007
Just watching the evening news, Interesting that they have Max Mayfield on. Wonder how that makes Ed Rapport feel, and what it does for his credibilty? Max is awesome, but I think its time for Ed to assume his new role.
Member Since: June 20, 2006 Posts: 9 Comments: 1391
641. dean2007
10:43 PM GMT on August 17, 2007
Dean looks larger and he's stronger then she was before rapid deepening. Sorry about the double post.
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640. miken62
10:40 PM GMT on August 17, 2007
NO one said 150 miles west would worst case; just that it would be very bad ..IF THE STORM WERE AS BIG AS IN AREAL COVERAGE AND STRENGTH AS THE GFDL SAYS .....but it is one model ...NO ONE KNOWS FOR SURE WHERE HE'S GOING ...WE JUST HAVE TO WAIT AND SEE....
all these predictions are just that ..predictions; most of us here are amatuers and not professionals; and some people just want to see who's right and who's wrong....let's just wait and see .....and time will tell

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639. FTmyersZ
6:39 PM EDT on August 17, 2007
bekroweather

I would not buy the UKMET intensity, as it initialized the storm at about 996mb when it was actually more like 970-960. Any interaction with land will obviously weaken the storm, but he is already ALOT stronger than most of the models are showing. This has implications on track as well.


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638. Weather456
10:43 PM GMT on August 17, 2007
StormJunkie,

i hear u......next time
Member Since: July 24, 2005 Posts: 407 Comments: 19076
637. runningfromthestorms
10:37 PM GMT on August 17, 2007
baybuddy - I live on the Fort Morgan Peninsula near Dauphin Island - remember Katrina well - lots of water and sand! But not as much as from Ivan.
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636. animalrsq
6:40 PM EDT on August 17, 2007
Just a plea to all of you to spread the word. If you or anyone you know plans to evacuate, PLEASE take your pets with you. If it is not safe for you, it's not safe for them. Most people who evacuated for Katrina did not realize we'd be pulling their animals out of the bathroom three and four weeks after the storm, some alive, some not. Thank you.
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635. OUFan919
10:42 PM GMT on August 17, 2007
Is it me or is Dean looking more and more like Wilma when she went under rapid intensification but a little larger?
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About

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