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

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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1684. bappit
2:32 AM GMT on August 18, 2007
ForecasterColby: Dean is moving along pretty fast. Unusual to have an intense storm moving so quickly. Maybe it accounts for some of the raggedness.

I remember the eye of Hugo being stretched when it was moving so fast as it approached South Carolina. I think it was elongated along the direction of movement though.
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1681. ForecasterColby
2:40 AM GMT on August 18, 2007
"GFDL sez Cat 5 in a few days."

Dean might very well sez Cat 5 in a few hours.
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1679. ForecasterColby
2:39 AM GMT on August 18, 2007
He's taking a jog due to the west, which is really surprising to me.
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1678. DocBen
2:38 AM GMT on August 18, 2007
Around 35W - the next invest?
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1677. Relix
2:38 AM GMT on August 18, 2007
I say one of those ATL waves will be the annoying one for us here in Puerto Rico. Dean so far... is boring, with minor winds and not even a inch of rain. Meeeeh
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1676. Wundermobay
2:37 AM GMT on August 18, 2007
Question to the experts

Do you think the 11am edt track will be nudged south of the current
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1675. moonlightcowboy
2:37 AM GMT on August 18, 2007
Keeper, no turn at 70w. I thought so, too, earlier this morning. Can't catch the ull!
Member Since: July 9, 2006 Posts: 184 Comments: 29682
1674. JRRP
2:17 AM GMT on Agosto 18, 2007
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1671. kmanislander
2:35 AM GMT on August 18, 2007

Remember, you only need to keep the water out for about 2 to 3 hrs until the wind shifts to offshore. I agree that it is impossible to keep all water out but if you slow the ingress then by the time the winds shift offshore you only have a few inches and not a few feet to worry about inside
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1669. Metallica1990
2:37 AM GMT on August 18, 2007
yeah lets wait to have the global warming chats till the off season right now we have a category 5 hurricane forming
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1668. Demopolite
9:31 PM CDT on August 17, 2007
GFDL sez Cat 5 in a few days.

Then we have three more unorganized tropical waves out there:
From: http://www.wunderground.com/tropical/AXNT20.html

...Tropical waves...
a tropical wave is moving through the E Atlc along 32w moving W near 20 kt. This wave continues to exhibit a broad envelope of
cyclonic turning in the surrounding low to mid cloud field...especially from 12n-19n between 28w-37w. However...deep convection remains minimal near this circulation with only one
small cluster of moderate convection noted near 16n32w. A more concentrated area of scattered moderate/isolated strong convection is near the srn portion of the axis from 6n-10n
between 24w-34w...which may be more associated with speed convergence S of the ITCZ.

An Atlc ocean tropical wave is along 44w south of 20n. This wave is generally moving W near 15-20 kt...but the main area of cyclonic turning has had a SW component over the last couple of days. The wave...and this SW motion...shows up well in the 48 hr
animation of the UW-CIMSS SSMI/amsre total precipitable water product. Regardless...deep convection remains minimal near the

A wrn Caribbean Sea tropical wave is along 80w south of 19n moving west 20-25 kt. An inverted v pattern remains evident with this wave. Scattered moderate convection is over the SW
Caribbean S of 11n. Isolated showers and tstms are elsewhere within 120 nm of the axis. Cloud tops over this activity are being sheared to the NE with upper SW flow ahead of an upper
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1667. comtrader
2:35 AM GMT on August 18, 2007
its going to be a very important race.
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1666. JamaicanGir
9:36 PM EST on August 17, 2007
ok, stay safe
1665. DonnaGalveston
2:36 AM GMT on August 18, 2007
I am not an expert, but I have the radar from Puerto Rico up, and do see a north west jog at present. Am I blind?
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1663. ForecasterColby
2:34 AM GMT on August 18, 2007
Guys, keep the politics and religion to your own blogs, please. Unless congress has a massive space-borne hurricane-destroying laser I don't know about.

In any event, I think Dean's having dry air problems again on the sat. We'll see if the next recon pass shows continued RI.
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1662. AnthonyJKenn
2:31 AM GMT on August 18, 2007
WOW...I just noticed that the GFS has the remnants of Dean emerging into the Pacific.

Perhaps, it reforms into a TS again??


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1661. Baybuddy
2:35 AM GMT on August 18, 2007
That is a smart drink presslord....quinine...prevents malaria.
Member Since: June 26, 2006 Posts: 0 Comments: 1115
1660. Prgal
2:35 AM GMT on August 18, 2007
I believe we will have another storm forming soon. Just off the african cost is a huge wave that might have a chance.
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1659. A4Guy
2:35 AM GMT on August 18, 2007
He's just ridin' that 15N line.............
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1658. Wundermobay
2:35 AM GMT on August 18, 2007
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1656. Wundermobay
2:34 AM GMT on August 18, 2007
Good thing we in NW Jamaica dont have to worry about storm sturge from Dean
It would have to take a path like Allen.
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1655. JamaicanGir
9:34 PM EST on August 17, 2007
as best as can be where in mobay are you - im am in brandon hill
1653. LSU79
2:25 AM GMT on August 18, 2007
I am by far no expert here but looking at the latest loop it appears as though Deans is slowly but surely catching up with the ULL which appears to be on the move and moving a bit to the SW. The high over Texas also appears to be slowly to the west. It seems that would create a funnel leading to the central gulf coast. Any opinions?
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1652. Caymanite
2:29 AM GMT on August 18, 2007
Hi Stuccoman, I appreciate your concern but believe me we also wrote the book on foundations. With regard to 60 Ft above sea level however, that would be a real challenge as that is about the highest peak on the island. Average hight for homes above sea level is 4-6 feet.
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1651. comtrader
2:31 AM GMT on August 18, 2007
the gfdl had dean hitting la. one run ago. la. is still definitely in play.
imagine a marble sitting on the top of a pyramid. just a few inches in initial condition could make for a huge change in final solution. so it is with this particular storm.
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1650. mikester
2:33 AM GMT on August 18, 2007
Looks like africa has a nice storm coming off shore soon with little wind sheer this could be the next big hurricane of the season.
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1649. StuccoMan
9:28 PM CDT on August 17, 2007
Posted By: kmanislander at 9:28 PM CDT on August 17, 2007.

Not from the inside my friend, the outside !
You would be surprised how many here kept the ocean out with silicone and duct tape.
As long as a door or window does not break you can in fact keep water out of a home surrounded by water by sealing dryer vents,caulking around doors and windows etc. I would rather spend a week removing silicone than 6 months rebuilding a flooded home

I waterproof hoses for a living and have been for over 25 years.I would think that making a home submersible would be impossible due to permeable concrete,cracks in floors under tile,carpet,window frame joints,Etc.Just seems a little far fetched to me.let me know how it goes.
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1648. JLPR
2:27 AM GMT on Agosto 18, 2007
got a gust right now i would say 35mph
or close and sustained at 20-25mph
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2:20 AM GMT on August 18, 2007
are u ready its coming and hell follows with it u in jamica aand caymans diablo is coming tell as many as u can help urself get out if u can this is the real deal it will start nw move at 70
track mark
15.8,68 C5/H/D
21,85 C4/H/D
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1645. JamaicanGir
9:32 PM EST on August 17, 2007

Thanks, but not possible at this point to leave, will try to be as safe as possible
1644. Baybuddy
2:32 AM GMT on August 18, 2007
Dean...A rogue Hurricane who plays by his own rules.
Member Since: June 26, 2006 Posts: 0 Comments: 1115
1643. Wundermobay
2:32 AM GMT on August 18, 2007
So am I,prepared for Hurricane Dean
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1642. SkulDouggery
2:31 AM GMT on August 18, 2007
Presslord if you consume those faster the icecap will be minimized.
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1641. ChuckieTodd
9:28 PM CDT on August 17, 2007
Posted By: kmanislander at 9:28 PM CDT on August 17, 2007.
You would be surprised how many here kept the ocean out with silicone and duct tape.

I wouldn't doubt this either. In Katrina, several friends reported that they saw the waterline on the windows outside meaning that the water wasn't coming in as fast as it was rising. If the house is better sealed, I would expect less intrusion - if the walls can handle the pressure.
1640. presslord
10:31 PM EDT on August 17, 2007
and...actually...as a good Catholic...I am compelled to correct you: Jesus is eternal...now ...Can we talk about the waves in the e Atlantic?!?!?!/
Member Since: August 13, 2007 Posts: 0 Comments: 10617
1639. JamaicanGir
9:31 PM EST on August 17, 2007

I am in Mobay
1637. MelbourneTom
2:28 AM GMT on August 18, 2007
Thanks for the links, all. I do monitor all of these storms but not as fast as all of you. that is the main thing I like about this blog.
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1636. moonlightcowboy
2:30 AM GMT on August 18, 2007
Thanks, Drakoen! You calling it a night? If you are, have a good sleep! I really appreciate all of your posting, all your charts, pics...brings things to a better level of understanding! You're the best here at that, NO DOUBT!!!!
Member Since: July 9, 2006 Posts: 184 Comments: 29682
1635. nolasoci
2:30 AM GMT on August 18, 2007
Good luck Jamacian Grl.. Please get to higher ground and stay very safe. If you can leave the island please do
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