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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1485. KnowYourRole
10:03 PM EDT on August 17, 2007
Posted By: TexasRiverRat at 10:02 PM EDT on August 17, 2007.
I just dont see the Yucatan knocking that much breath out of Dean?


No way to know. Depends on how long it is over the Yucatan for one, and where exactly it crosses.
1484. FatPenguin
2:00 AM GMT on August 18, 2007
FYI
Strongest Atlantic storm on record for August was Allen (1980) at 899mb with winds at 190mph.

I remember that one. A big high pressure system and drought in Texas ate that storm up before it made landfall.

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1483. ChuckieTodd
9:02 PM CDT on August 17, 2007
Does anyone know when the next GFDL run will be released?
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1482. bluehaze27
2:02 AM GMT on August 18, 2007
TWC mentioned the ULL possibly pulling the storm north.
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1480. kmanislander
2:01 AM GMT on August 18, 2007
Drak

Does that image mean that Dean is still below 15N?
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1479. rodrigo0
2:01 AM GMT on Agosto 18, 2007
Some pictures here from Martinica...

http://www.flickr.com/photos/ebrmx/tags/martinica/

Live blogging coverage here: http://ebrmx.com
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1478. szqrn1
2:00 AM GMT on August 18, 2007
anybody see this thing getting to MS or New Orleans?
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1477. TexasRiverRat
2:00 AM GMT on August 18, 2007
I just dont see the Yucatan knocking that much breath out of Dean?
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1476. tallahasseecyclone
1:58 AM GMT on August 18, 2007
I think that people are getting a little carried away about this being stronger than wilma tomorrow morning. But after an EWRC....
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1475. bappit
1:49 AM GMT on August 18, 2007
Mr. Guerra:
Don't compare Dean to Katrina--like comparing apples and cactus--but you can talk about how Texas has been saturated with rain this summer, how flat the land is, how poor the drainage is. On the other hand, if the storm continues to move quickly, like Erin before it, that will lessen rain totals.

Inland flooding from rain is the largest killer historically, but it is not the main culprit with Katrina.

In Texas' history at least, the worst inland flooding has occurred with slow moving tropical storms, not intense hurricanes, let alone fast moving ones.
Member Since: May 18, 2006 Posts: 10 Comments: 6011
1474. littlefish
1:57 AM GMT on August 18, 2007
Dean I think is being pulled south just a hair by the southerly low and that might be what shifted models, not the ULL or the High to the north. Possibly. As for the wave off Africa, as I mentioned it has 'spin' but almost no convection and is further north so is over cooler waters for awhile and will therefore struggle.
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1473. VegasRain
1:58 AM GMT on August 18, 2007
I wonder what happened to the concentric eyewalls that Dean had earlier?

The eye is about 5 or 6 miles it looks like. Its very small, and it probably wont be able to sustain an eye that small for long. On the flip side, Dean will probably become a strong Catagory 4 or Catagory 5 before the eye collapse and undergoes an ERC.
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1472. Inyo
1:57 AM GMT on August 18, 2007
Posted By: SarahFromFLA at 1:52 AM GMT on August 18, 2007.
If I made a graft of my height showing every year from 1979 until now, you would conclude that I had always been 5.6, nothing ever any shorter. I was 5.6 in 1979 and still am.

That’s my issue with the Arctic ice survey; 28 years is geological time is nothing. It doesn’t even seem that much in my own life.


but what if you did that for 10 years, and then on the 11th and 12 years you grew 5 inches, and on the 13th year you grew 10 inches. Youd figure something was up, even if you didnt know how fast you grew before 1979
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1471. Baybuddy
2:00 AM GMT on August 18, 2007
I hear ya moonlight!
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1470. bluehaze27
2:00 AM GMT on August 18, 2007
Dean is already at 145 so cat 5 is definitely going to happen.
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1469. Drakoen
2:00 AM GMT on August 18, 2007
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1468. comtrader
1:54 AM GMT on August 18, 2007
hi cajunsub
this stuff is really fascinating. its scary to think that a lot of lives and hundreds of billions of dollars probably hinge on whether or not one system latches onto another in a small window of time and space.
stay safe.
david
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1466. Fl30258713
1:59 AM GMT on August 18, 2007
that cmc run was 12 Z
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1465. DonnaGalveston
2:00 AM GMT on August 18, 2007
I'm sorry, I guess things are posting so quickly I wasn't sure if I was showing up or not.

How is Galveston looking? Any good news yet?
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1464. KnowYourRole
9:59 PM EDT on August 17, 2007
Posted By: Faethe at 9:58 PM EDT on August 17, 2007.
Has anyone heard from any of the people in the Lesser Antilles yet?



I was doing some searching on the internet for various info from the islands. Appears a lot of places are without power because they have not updated their info since this morning.
1463. TexasRiverRat
1:57 AM GMT on August 18, 2007
I can guarntee that Gas and Water sales are up today around the houston area! I saw a guy with at new generator and 4 cases of water right down the road. My gas and water is full to!
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1461. Metallica1990
1:55 AM GMT on August 18, 2007
Posted By: FatPenguin at 1:54 AM GMT on August 18, 2007.

Posted By: Metallica1990 at 1:49 AM GMT on August 18, 2007.

25 mb in 4 1/2 hours very rapid strengthening

++++++++++++++++++++++++++++++++++++++

I believe Wilma has the record at 90mb over 12 hours and 98mb over 24 hours??


it was 53 in 6 hours
and 99 in 24
1460. WPBHurricane05
9:58 PM EDT on August 17, 2007
WPB, remember the GFDL is showing winds at the 35m level. You have to reduce that some and get the 10m winds. That said, it is still showing a very strong system. 165+mph

165, 195 either way that would scare the crap out of me. lol
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1458. bluehaze27
1:57 AM GMT on August 18, 2007
moonlightcowboy, talking about only Dean isn't going to change things out in the rest of the atlantic and I'm sure we can all walk and chew gum at the same time.
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1457. KnowYourRole
9:58 PM EDT on August 17, 2007
Posted By: moonlightcowboy at 9:57 PM EDT on August 17, 2007.
Look, I don't think this is the time to be talking too much other than where this storm is going, and who it's going to injure and kill. Right now some of friends in Jamaica and several in the Cayman's are the next target!

...give K'man, his family and his home some respect, please!


Don't know him, but hope he and his family are doing okay.
1455. 7544
1:56 AM GMT on August 18, 2007
the ull is diging south to cuba what will this mean for dean is this anticipated by the models
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1453. weathersp
9:57 PM EDT on August 17, 2007
Dean
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1452. StoryOfTheCane
1:58 AM GMT on August 18, 2007
it looks like the models are just trying to cause less panic by taking it into the yucatan, we should all know this is not going to happen
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1451. Xion
1:55 AM GMT on August 18, 2007
The ONLY things that will stop Dean from being upgraded to Cat 5 at this point are:

1.) Eyewall replacement cycle
2.) Lack of HH data (possibly)
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1450. StormJunkie
1:56 AM GMT on August 18, 2007
WPB, remember the GFDL is showing winds at the 35m level. You have to reduce that some and get the 10m winds. That said, it is still showing a very strong system. 165+mph
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1449. moonlightcowboy
1:55 AM GMT on August 18, 2007
Look, I don't think this is the time to be talking too much about anything other than where this storm is going, and who it's going to injure and kill. Right now some of our friends in Jamaica and several in the Cayman's are the next target!

...give K'man, his family and his home some respect, please! TIA
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1448. windsock
1:56 AM GMT on August 18, 2007
Has Dean grown in size? Looks like it is huge now.
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1447. deadmandancing
1:56 AM GMT on August 18, 2007
Welcome back, Donna.
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1446. kev22
1:55 AM GMT on August 18, 2007
Can we say this is rapid deepening? The NHC had Dean's pressure at 961 a few hours ago, and now it's 936 and falling. I think the NHC may have underestimated the pressure.
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1445. CorpusChristiKid
1:56 AM GMT on August 18, 2007
01:15:00Z 14.87N 65.45W 696.7 mb
(~ 20.57 inHg) 2,623 meters
(~ 8,606 feet) 937.8 mb
(~ 27.69 inHg) - From 145° at 17 knots
(From the SE at ~ 19.5 mph) 01:15:30Z 14.85N 65.48W 697.6 mb
(~ 20.60 inHg) 2,607 meters
(~ 8,553 feet) 936.3 mb
(~ 27.65 inHg) - From 136° at 4 knots
(From the SE at ~ 4.6 mph) 01:16:00Z 14.83N 65.50W 696.5 mb
(~ 20.57 inHg) 2,617 meters
(~ 8,586 feet) 935.8 mb
(~ 27.63 inHg) - From 306° at 17 knots
(From the NW at ~ 19.5 mph)


Hey folks, new to the blog and I was looking at the live decoder results (captured above)from the HH and interesting it appears they went into the eye. Notice the low pressures and the wind direction from the SE at first and in about a minute they are hitting the other side of the eyewall (winds now from NW). Guess there is no room to fly in that eye (2 nm?).
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1444. bluehaze27
1:55 AM GMT on August 18, 2007
Here's the CMC loop. I know some don't trust it, but it's interesting nonetheless


Link
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1443. CajunSubbie
1:55 AM GMT on August 18, 2007
Posted By: comtrader at 1:50 AM GMT on August 18, 2007.

its interesting to look at the latest GFS and to understand why it is taking Dean due west over the yucatan and on into mexico. the key feature is still the ULL which is currently off the southeast coast of florida. The GFS moves this low thru the gulf and into the texas coast. it does this fast enough so that dean does not get pulled along. if you look out about 60 hours from now, dean will, according to GFS, make its closest pass to the pull of this low, but will still miss its attraction by one or two hundred miles. this explains the huge track differences in the models. if the low tracks farther south thru the gulf or doesnt make it to texas in time then dean will latch on to its coattails and get sucked into the gulf. this is an extreme case of model bifurcation. either dean is going to miss the gulf coast by 500 - 1000 miles or its going to get there. so far the gfs has overestimated the movement of the low, which leads me to think that the gulf coast is by no means out of the woods even though the current model shows a 500 - 1000 mile miss. the key interaction or lack thereof is going to come in about 60 hours.

Thanks man, great post. People like you make this blog great.
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1442. nolasoci
1:55 AM GMT on August 18, 2007
FYI
I am a New Orleans guy and I am taking no chances. I booked a hotel tonight and believe me. Sherveport, la Monroe,la Jackson, ms

are all completely book due to the storm...
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1441. foofi
1:46 AM GMT on August 18, 2007
Even though NO got all of the media hype from Katrina, the MS and AL coasts were devastated. There were hundreds of structures that survived Camille that Katrina reduced to matchsticks. It wasn't the wind it was the surge that caused the devastation. A cat 5 storm pushes a lot of water ahead of it even if the winds decline to cat 3 before landfall. Flooding in NO was caused by levee breaches, French Quarter stayed dry because the land there wasn't "created" by levees.
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1438. BiloxiGirl
1:52 AM GMT on August 18, 2007
Starting to have a panic attack. There is just so much to think about and potentially do. When and where do you start with a storm that is so bad and yet still so far away?
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1437. atmoaggie
1:53 AM GMT on August 18, 2007
Take a look aa the loop in Link

Convection being pulled and forming out of ITCZ in last couple of frames.
Member Since: August 16, 2007 Posts: 6 Comments: 12461
1436. nrtiwlnvragn
1:52 AM GMT on August 18, 2007
Anyone know why the 18z Ukmet only goes out 48hrs? And has any one seen it anywhere else?

SJ

I think they only run the UKMET two times a day, 0000 and 1200. If you look back at previous 0006 and 0018 runs on the FSU site, they are only 48 hours. Maybe 0006 and 0018 are "interppolated" LOL
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1435. WPBHurricane05
9:53 PM EDT on August 17, 2007
Just checked the GFDL, 195mph winds? And I see that they updated Dean. This storm is crazy.
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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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