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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3335. stormhank
2:38 PM GMT on August 18, 2007
Dean isnt moving due west anymore? wonder if that will change path projections on models later today?
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3333. Drakoen
2:41 PM GMT on August 18, 2007
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3332. youngw3ath3rman
2:40 PM GMT on August 18, 2007
could someone send me the website to the models of the gfs, gfdl, nogas, and the rest of em
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3331. StormJunkie
2:38 PM GMT on August 18, 2007
Welcome Pedi ☺

I think due to the angle it is very hard to interpret track forecast from the 3d GFDL. Just my guess.
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3330. noleman
9:37 AM CDT on August 18, 2007
I wasnt too concerned last night. Now with this new movement and the fact the ULL looks to have some pull on Dean I think its time for gas and water.
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3329. KennerLA70065
2:39 PM GMT on August 18, 2007
We're all anxiously watching the storm here in New Orleans. For the sake of the Flood Control system, we're all hoping that if it makes landfall in the area, that the eye is at least to the west of Grand Isle and Port Fouchon. This keeps the peak surge area on the west side of the mississippi and limits the amount of water that will get pushed into the lake to more manageable levels.

Having relatives on Grand Cayman that weathered Ivan (they're new house lost its roof and they had to run, through the storm, to the old house on their property that extended family was living in) I can only hope that the storm either passes them on the north side, or, very far to the south.

Whatever happens, I wish luck to those in Dean's pass and safety to the flight crews that are drilling holes in Dean's clouds right now to help us be safe.
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3328. Drakoen
2:37 PM GMT on August 18, 2007
Posted By: extreme236 at 2:35 PM GMT on August 18, 2007.

Posted By: Drakoen at 2:33 PM GMT on August 18, 2007.

Just looks like convection associated with a tropical wave and the ITCZ...

Yep, but it is something to watch because the gfs and cmc develop it


If more computer models develop the wave then i will be interested.
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3327. weatherboykris
2:38 PM GMT on August 18, 2007
I don't use my filter SJ.
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3326. raindancer
2:36 PM GMT on August 18, 2007
welsh - yup / concur...

I suspect Jamaica could be in much worse shape, though an eye passing just to the north of the island would be much better news.

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3325. weatherboykris
2:37 PM GMT on August 18, 2007
leftovers....you need to consider that by the time the radar beam gets to the eye,it's many miles up in the air.
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3324. StormJunkie
2:32 PM GMT on August 18, 2007
No problem labs, I run FireFox, but I do remember how frustrating it used to be when I ran IE.

Thanks again ☺

And one more request for all...

I think that when new folks sign up they automatically show up hidden if you have your filter set to "show below average". If you set your filter this way then you know who those folks are and then it is easier to give them +'s to get them out of the "below average" setting. I have seen some great posts that are showing up as below average just because the folks are new.

Anywho, back to your regularly scheduled Dean...
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3322. nola70119
2:34 PM GMT on August 18, 2007
Dean's been moving almost NW for eight hours....three hours prior to that it was moving due W.
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3321. weatherboykris
2:36 PM GMT on August 18, 2007
We'll see if it's a Cat 5 when the plane gets to the eye...
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3318. Pedibrainpa
2:31 PM GMT on August 18, 2007
Why is there such a discrepancy in the forecasted path between the 3D GFDL model and the GFDL model on the "computer models" page on wunderground?
Thanks!
Amy
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3317. TerraNova
10:33 AM EDT on August 18, 2007
Im sorry to say this but right now the truth is that Dean is at a borderline Category 5 (it'll likely be classified as a 5 soon) and Jamaica and the Caymans will get hammered whether it hits as a 5 or 4. People are going to get hurt.
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3316. extreme236
2:34 PM GMT on August 18, 2007
Posted By: Drakoen at 2:33 PM GMT on August 18, 2007.

Just looks like convection associated with a tropical wave and the ITCZ...


Yep, but it is something to watch because the gfs and cmc develop it
Member Since: August 2, 2007 Posts: 19 Comments: 19234
3315. weatherboykris
2:34 PM GMT on August 18, 2007
They've made the NE turn...heading towards the eye.
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3314. weatherwonderer
2:29 PM GMT on August 18, 2007
Neponset, correct it would just zoom straight up North if weren't for the blocking high pressure ridge. Sorry if my over simplification caused you problems. I would be surprise given what I have been seeing that Dean passes South of Jamaica but since waves of high pressure or weakness thereof could put it N or S of the island. It is unlikely that huge deviations would occur but If I were in Eastern Cuba I would be paying very close attention right now.
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3313. ScoobaSteve
9:34 AM CDT on August 18, 2007
ScoobaSteve - The Cayman people tend to fare hurricanes quite well. There are series of natural caves on the islands, which have in times past been used for shelter...


raindancer: Thanks for your answer. Where are the caves you speak of? I've been there 9 times on dive trips and was unaware of them.

Good to know they have a place to take shelter.

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3312. noleman
2:34 PM GMT on August 18, 2007
Looks like the track of Dean needs to be tweaked this morning . Im in PC, FL and I hope this just means bigger waves and not a trend.
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3311. Stormchaser2007
10:33 AM EDT on August 18, 2007
All i have to say is WOW!!


none
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3310. welshcayman
2:29 PM GMT on August 18, 2007
Posted By: raindancer at 2:28 PM GMT on August 18, 2007.

ScoobaSteve - The Cayman people tend to fare hurricanes quite well. There are series of natural caves on the islands, which have in times past been used for shelter...


Actually the caves are only on the sister island Cayman Brac and although they were once used as shelter the Brac now has modern shelters.

Most of us just stay in our homes. All new construction on the island (and there was a lot after Ivan) is built to Dade County standards. Most of the condo developments are built from solid concrete, have hurricane rated roofs, and plenty of inside rooms (bathrooms, walk in closets) that have no windows so they can be used as 'safe' rooms if things really hit the fan.

For those in lesser quality housing there are excellent shelters here and the infrastructure is simply amazing here. During the peak of Hurricane Ivan we were still able to talk to others using our cell phones. Not many countries around the world have phone systems that would continue to operate during and after a direct Cat 5 hit I am guessing.
3309. bajelayman2
2:29 PM GMT on August 18, 2007
lowerbamagirl:

Not an Invest yet. Way it looks, I agree, probably will be soon. Here is the Navy site. Keep checking every few hours and you will know when an Invest:

http://www.nrlmry.navy.mil/tc_pages/tc_home.html
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3308. Drakoen
2:33 PM GMT on August 18, 2007
Just looks like convection associated with a tropical wave and the ITCZ...
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3307. CJ5
9:23 AM CDT on August 18, 2007
Posted By: StormJunkie at 9:18 AM CDT on August 18, 2007.
I know CJ! Dean is a mystery for know as far as Mex and the Conus go.

SW told me yesterday that he was looking to 2 pints to help give his guidance. Jamaica and the Yucatan...I think those are good points to watch as well.


I agree. Contrasted against a storm like 04Jeanne or even Katrina, the dynamics with Dean seem almost as simple as they could be with any storm but even that is difficult. It makes you appreciate the complexity of nature.
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3305. youngw3ath3rman
2:32 PM GMT on August 18, 2007
Is there anything coming off of Africa that looks interesting?
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3303. labsr4me
2:30 PM GMT on August 18, 2007
Posted By: StormJunkie at 2:29 PM GMT on August 18, 2007.

labs, I think that image may be stretching the blog for anyone in IE, any chance the width could be reduced? 640 width is always a good number for all browsers and screen resolutions.



sorry
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3302. KYhomeboy
2:29 PM GMT on August 18, 2007
Yea we do have caves...but sadly not enough for 54,000 people! lol
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3301. sporteguy03
2:29 PM GMT on August 18, 2007
Drak,
Coordinates 11N 30W
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3300. Wecouldoit
2:25 PM GMT on August 18, 2007
Last Recon (-2Z) puts Dean pretty much on the NHC track
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3299. StormJunkie
2:27 PM GMT on August 18, 2007
labs, I think that image may be stretching the blog for anyone in IE, any chance the width could be reduced? 640 width is always a good number for all browsers and screen resolutions.
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3298. BuffaloSnow77
2:25 PM GMT on August 18, 2007
CCHS, i seen that ULL start to move more SW to Florida, wouldn't this bring an earlier shift NW than expected?
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3297. Neponset
2:28 PM GMT on August 18, 2007
weatherwonder
In the absence of outside forces, the storm should follow the rules of the coriolis effect, not the straight edge effect.
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3296. raindancer
2:26 PM GMT on August 18, 2007
ScoobaSteve - The Cayman people tend to fare hurricanes quite well. There are series of natural caves on the islands, which have in times past been used for shelter...
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3295. labsr4me
2:27 PM GMT on August 18, 2007
Posted By: sporteguy03 at 2:25 PM GMT on August 18, 2007.

Drak,
Whats behind Dean?


His wake...sorry couldn't resist
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3294. nrtiwlnvragn
2:23 PM GMT on August 18, 2007
None og the models have this nw movement. none

The 06Z GFDL is pretty close, 2:00 AM 15.0 66.6, 8:00 AM 15.5 68.1

NHC advisories, 2:00 AM 15.0 66.7, 8:00 AM 15.4 67.9
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3293. seminolesfan
2:25 PM GMT on August 18, 2007
The NHC is not just one person emailing computer output to TV stations

ROFL!! That's a funny mental pic there.
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3292. StormJunkie
2:26 PM GMT on August 18, 2007
Preparedness info
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3291. weatherwonderer
2:22 PM GMT on August 18, 2007
Right now Dean is pretty much doing what is expected. It is supposed(according to models and the HNC) to get abit of a Northerly influence. Now if it does a couple more jogs to the North as it did earlier today we may have different story.
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3290. labsr4me
2:27 PM GMT on August 18, 2007
Interesting view of last three Consensus Model Runs..
0000 UTC 8-18 (Notice consensus into middle YP)
Photo Sharing and Video Hosting at Photobucket


0600 UTC 8-18 (Notice N shift to nearly all models)Photo Sharing and Video Hosting at Photobucket


1200 UTC 8-18 (Notice S shift again, however models seemed to have missed most recent NW movement of Dean)Photo Sharing and Video Hosting at Photobucket
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3289. ClearH2OFla
10:26 AM EDT on August 18, 2007
Drak no coords just looking at the CMC unless the last run has changed
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3288. weatherboykris
2:25 PM GMT on August 18, 2007
The SFMR has 26kt surface winds.
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3287. sporteguy03
2:24 PM GMT on August 18, 2007
Drak,
Whats behind Dean?
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3286. weatherboykris
2:24 PM GMT on August 18, 2007
The hurricane hunters are on the west side,heading south and getting ready for their run through the SW and NE quadrants."Red 6,standing by..."
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3285. extreme236
2:24 PM GMT on August 18, 2007
what i noticed with the wave at 35w is that the gfs developed it in the itcz convection part of the storm, and thats where QS picked up some cyclonic turning as well
Member Since: August 2, 2007 Posts: 19 Comments: 19234

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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.

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