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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2735. stormybil
8:59 AM GMT on August 18, 2007
looks like the eye has just went over 15n now
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2733. hurricanehamster
8:56 AM GMT on August 18, 2007
whats deans strenth?
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2732. samiam1234
8:58 AM GMT on August 18, 2007
I am from Galveston and am somewhat prepared.. but I had a gut feeling that we here will not see any signs of this storm... and the more I see the latest the more confident I feel about this.. I am getting more and more happier for us, but the poor ppl have Mexico will pay the price for this.. I think this will end up going a bit more in the central mexico region and not effect even the Southern Texas part, other then a little rain and some wind.
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2731. katadman
8:59 AM GMT on August 18, 2007
What time will the new computer models come out? The NHC apparently relies heavily on them for their own track. Also, they will have all the new data factored in, si'?
Member Since: September 7, 2006 Posts: 0 Comments: 1081
2730. VEROBEACHFL1
9:00 AM GMT on August 18, 2007
DEAN IS MOVING TOWARD THE WEST NEAR 17 MPH...28 KM/HR. A GENERAL
WESTWARD TO WEST-NORTHWESTWARD MOTION IS EXPECTED DURING THE NEXT
24 HOURS. ON THIS TRACK...THE CORE OF THE HURRICANE WILL BE MOVING
SOUTH OF THE DOMINICAN REPUBLIC LATER TODAY AND SOUTH OF HAITI
TONIGHT.
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2729. VEROBEACHFL1
8:59 AM GMT on August 18, 2007
ahhh the 5am-- do they hold my WNW movement? or did they keep it at WEST?
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2728. redrobin
8:58 AM GMT on August 18, 2007
Good Luck Vero- I am off to work...:(
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2726. MauiHawk
8:52 AM GMT on August 18, 2007
Maui.. I doubt the NHC would discuss how they were wrong about the movement of the ULL

I really don't think the NHC takes things so personally. They are constantly discussing what happens that isn't expected and how it influences their forcast. For example here:

http://www.nhc.noaa.gov/archive/2007/al04/al042007.discus.016.shtml?

The discuss why their intenisty forcast came up short owing to dry air, and why they thought the track needed to be moved N because of more interaction with the ULL than previously thought.
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2725. VEROBEACHFL1
8:57 AM GMT on August 18, 2007
well--its all IMO---- like i said before we ALL have 1 thing on ALL our sides and opinions------------

----TIME! :)
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2724. redrobin
8:53 AM GMT on August 18, 2007
Slow down---south
Speed up---North
So far that is what they are saying here in Houston
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2723. MauiHawk
8:49 AM GMT on August 18, 2007
im sorry--maui---i agreed with you up until that comment-----
No need to be sorry :D I'm sure Cancun is rooting for you.

My feeling is simply that as much as I like to poor over this stuff and pretend I can understand what's going on and what it all means, the NHC is pretty darn good at this stuff and I'll always be inclined to believe them over my own musings.
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2722. sflhurricane
8:50 AM GMT on August 18, 2007
Maui.. I doubt the NHC would discuss how they were wrong about the movement of the ULL...The ULL, according to them was supposed to move across Southern FL and into the Gulf and dissipate.. Its clear that that the ULL isn't showing signs of weakening and is moving SW.. Im just wondering why the NHC doesnt believe this feature will be a player in the future path of Dean
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2721. VEROBEACHFL1
8:52 AM GMT on August 18, 2007
WNW WILL STICK---JUST WAIT---ITS A WEEBLE WOOBLEY STORM -------not a sharp N movement --but gradually to the west NW --is that y all of u put like 5 w's in front of it --to express a west but slight movement north?
-----example---WWWWWNW?
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2720. GPTGUY
3:50 AM CDT on August 18, 2007
Posted By: katadman at 3:48 AM CDT on August 18, 2007.

Hard to tell if that is just a wobble or not. Seemed to me to dip a tad back SW at the very end of the loop.?


yeah kat I agree..thats what i was looking at..wanted to make sure before I posted but you beat me too it lol
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2719. samiam1234
8:48 AM GMT on August 18, 2007
guys dean has gone through an entire day of slowing down.. it stated this morning motoring at 25 mph and has come down to 17 mph in a matter of 24 hours.. will this speed reduction continue, and stay this way, is something reducing it for the short term and will it pick up speed again.. or is this the speed it will stay at.. also if its going to keep slowing down, will that help it go North or does speed have nothing to do with the track.

thanks
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2718. redrobin
8:49 AM GMT on August 18, 2007
150 mile winds-15.1 n
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2717. silverstripes
1:43 AM PDT on August 18, 2007
Amystery,

I like your thinking. HAHA. I came up with my new descriptions earlier today since everybody kept saying it was moving W-NW. IF it was truly moving W-NW then that can mean anywhere from 271 degrees all the way to 314. Big difference there so.

NW = 315
WNW = 292.5
WWNW = 281.25
WWWNW = 275.6

HAHA. SO it has been moving mostly W with some WWWWNW and sometimes WWNW throughout it's life.
Ok I must be bored.
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2716. VEROBEACHFL1
8:48 AM GMT on August 18, 2007
its too early for a nailed down landfall-- but i respect your opinion 100%
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2715. MauiHawk
8:45 AM GMT on August 18, 2007
In the new discussion for Dean, the NHC still believes the ULL will be moving west.. but one can clearly see the ULL moving SW... Hmmm?

SF, but your argument was that you thought the ULL wasn't doing what the NHC expected it to. They certainly would have mentioned that if it were the case. And the long term track would have been nudged N not S if it were the case.

Whether or not it appears like it to you, logic says the ULL must be doing what the NHC expects it too-- and they have a heck of a lot more experience at interpreting these maps than us...
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2714. katadman
8:45 AM GMT on August 18, 2007
Hard to tell if that is just a wobble or not. Seemed to me to dip a tad back SW at the very end of the loop.?
Member Since: September 7, 2006 Posts: 0 Comments: 1081
2713. redrobin
8:46 AM GMT on August 18, 2007
NOT wishcasting- The hit will be in TX, by what is current at this time.
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2712. VEROBEACHFL1
8:46 AM GMT on August 18, 2007
--its too early------ for a mexico predict------
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2711. VEROBEACHFL1
8:45 AM GMT on August 18, 2007
im sorry--maui---i agreed with you up until that comment-----
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2710. VEROBEACHFL1
8:45 AM GMT on August 18, 2007
at 67w
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2709. MauiHawk
8:41 AM GMT on August 18, 2007
*YAWN* lots of wishcasting going on tonight.....yall have watched too many loops, yall are going loopy.....Dean is goin W into Mexico.

That seems to me the most likely bit of conjecture I've seen all evening. (Though a cat 5 storm hitting *anywhere* is not a yawn... and remember the spring breakers need somewhere to go next year)
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2708. sflhurricane
8:42 AM GMT on August 18, 2007
Katadman, here, you'll see the Northern movement


PR radarLink
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2707. VEROBEACHFL1
8:43 AM GMT on August 18, 2007
--i think we are going to see a slight change in the next0----maybe 11am update-------
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2706. katadman
8:42 AM GMT on August 18, 2007
Makes the GFDL of yesterday morning look pretty accurate.
Member Since: September 7, 2006 Posts: 0 Comments: 1081
2704. bekroweather
8:33 AM GMT on August 18, 2007
Hello everyone,

indeed it looks that with the last few satellite frames the center of Dean has move decisively to NW. It's first time really that the storm has been N of 15.

Given the Forecast Points, however, it's still on track according to the latest NHC prediction.
2703. VEROBEACHFL1
8:42 AM GMT on August 18, 2007
omg---and im not getting paid? lol
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2702. sflhurricane
8:40 AM GMT on August 18, 2007
In the new discussion for Dean, the NHC still believes the ULL will be moving west.. but one can clearly see the ULL moving SW... Hmmm?
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2701. katadman
8:37 AM GMT on August 18, 2007
Well, if there is any movement toward the north, I can't tell it.
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2700. redrobin
8:39 AM GMT on August 18, 2007
TWC Just admitted a slight move to the north. Vero you may be right
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2699. silverstripes
1:36 AM PDT on August 18, 2007
Satellite pictures overlayed with the lat/lon lines are best to use. It has defintely moved moved towards the WWWNW or maybe even WWNW. THe entire eye is above the 15 degree line now. Where as for many hours it was running right down the 15 line.
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2698. VEROBEACHFL1
8:39 AM GMT on August 18, 2007
guys--"WNW" ---maybe not a sharp NW but a gradual WNW movement-----IMO------ if DEAN pulls a sharp NW ill be surprised as of right now----too early--- just a WNW movement for a while..........
---just wanted to clarify lol/////
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2696. bobcane
8:36 AM GMT on August 18, 2007
We need 45 minutes of radar to confirm the change in direction. That will be or not to be on the NW shift. True on the west motion on the last radar return. I am getting updates every 5 minutes and the last frame jogged west again.
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2695. GPTGUY
3:38 AM CDT on August 18, 2007
yeah SF i agree
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2694. sflhurricane
8:37 AM GMT on August 18, 2007
GPTGUY, I see the bend back too.. but the bend is still a little north of due west
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2693. ebzz
4:37 AM EDT on August 18, 2007
Posted By: sflhurricane at 4:30 AM EDT on August 18, 2007.
katadman, I think you're right.. It just scares me being in South FL... I dont wanna wake up tomorrow and see this monster getting pulled towards Cuba

Well, I'm not saying it's out of the question but probably won't happen. I will say that Charley's track was kinda similar to this until it jogged to the south of Jamaica.

Image and video hosting by TinyPic
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2692. MauiHawk
8:26 AM GMT on August 18, 2007
MauiHawk, check this loooong loop out and you'll see how quickly Dean is gaining on that ULL... its getting close and the concern is that the ULL, which was supposed to be entering the Gulf and weakening by now, is holding on and moving SW towards the straights

Hmm, my browser seems to choke after loading 33 images, so I couldn't watch them all at once. But I did watch it all in pieces. I see what you are saying, but it also seems to me that earlier on, the ULL had more space to itself. As time goes on, it looks like ridging is squeezing up against it which I suspect is what will weaken it and help force it westward.

If you look at the ridging ahead of Dean, it is shriking, yes... but not *that* fast. Anyway, we should know in a few mins whether you are correct, because if you are, I'm sure it will be mentioned in the 5am discussion...
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2691. GPTGUY
3:37 AM CDT on August 18, 2007
I think we all are SF lol
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2690. sflhurricane
8:36 AM GMT on August 18, 2007
5 AM update is out... Dean moving west with winds at 150 mph
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2689. Jaxen
4:33 AM EDT on August 18, 2007
Hey gang ... has anyone noticed that all during Four/Dean's life the NHC's 72-hour forecasts have been spot on? Without discussing ULLs or ridging highs, I'm inclined to go with them at 72, which as of 2am put it on Cancun/Cozumel's doorstep, meaning it would have to really turn on a right-turn signal to miss the Yucutan.

Now if the NHC moves the dots north in 20 minutes, I'll be inclined to put faith in a northern track into the southcentral GOM.
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2688. GPTGUY
3:35 AM CDT on August 18, 2007
ok updated radar looks like its bending back west!!
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2687. stormybil
8:35 AM GMT on August 18, 2007
wait till 5 vero hang out
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2686. sflhurricane
8:34 AM GMT on August 18, 2007
Hm might just be a "jog" to the NW... not sure... the LAST frame on the PR radar has the storm "jog" back west. I might just be looking at this too closely
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2685. GPTGUY
3:34 AM CDT on August 18, 2007
SF i would say the radar..IMO
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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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