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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2485. Crisis57
6:42 AM GMT on August 18, 2007
i don't see the COC hitting Jamaica i honestly think it will pass north maybe even through cuba
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2484. StormJunkie
6:41 AM GMT on August 18, 2007
STOC, while that cone is not out of the question, it goes against all of the data we have right now. I doubt the right half of that is really in the cone. I would move your center line to the left edge if I were to take a stab. Although I do agree that it is looking more likely that it will go to the N of the forecast track. That is what the GFDL has been showing run after run after run. Just my two cents ☺
Member Since: August 17, 2005 Posts: 26 Comments: 16319
2483. stormybil
6:43 AM GMT on August 18, 2007
what does ncep mean
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2481. mikester
6:42 AM GMT on August 18, 2007
Go here and check out texas. Man something just boomed in texas.
http://weather.msfc.nasa.gov/GOES/goeseastconusir.html
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2480. StoryOfTheCane
6:41 AM GMT on August 18, 2007
the quicker this thing spins the more it pulls itself away from the equator, the only factor that would prevent that is an inhibitor like a strong High.
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2479. VEROBEACHFL1
6:41 AM GMT on August 18, 2007
ULL for one, all depends how fast it leaves there--and i really dont see it takin off anytime soon....
--they may meet and that'll pull dean slightly more northern and western..... WNW probably starting at about 67w and then gradually northward-----op am tirred.
--not meet---u know what i mean....
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2478. TheCaneWhisperer
6:40 AM GMT on August 18, 2007
Cat5 Andrew is the only comparison I can make and it leveled South Florida. Homestead, FL and many others were completely gone.
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2477. stormybil
6:39 AM GMT on August 18, 2007
yes story im thinkng the one over cuba . too thats when the ull picks him up
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2476. druseljic
6:41 AM GMT on August 18, 2007
Storyofthecane, can you please explain why you see Dean possibly taking one of these paths? Thanks...learning here :-)
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2474. StoryOfTheCane
6:39 AM GMT on August 18, 2007
the models always seem to predict it too far south, this thing is too big to maintain a westerly course, its going to have to go further north
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2473. VEROBEACHFL1
6:39 AM GMT on August 18, 2007
im with u storyofthecane................all the way.........mostly the first 2 tho.......
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2472. mikester
6:39 AM GMT on August 18, 2007
Ty drop. Looking at the current water vapor i dont know if that ull is going to effect dean at all. It appears to not be all that strong right now.
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2471. OUFan919
6:39 AM GMT on August 18, 2007
I'm drunk but did I see the models go a little farther north this run than the past few runs?? Besides the GFDL
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2470. JamaicanGir
1:36 AM EST on August 18, 2007
gilbert destroyed the islands vegetation and it took some places up to 4 months for power to be restored
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2469. TheCaneWhisperer
6:37 AM GMT on August 18, 2007
PR is about to get nailed by the tail.
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2468. Dropsonde
6:32 AM GMT on August 18, 2007
Wow good image Mikester, really shows the track race between the systems, the reach of the low, and how close it is to affecting Dean.
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2467. IRememberIvan
6:35 AM GMT on August 18, 2007
We're out of sat blackout now? That fast?

Geez I cant wait to see what Dean looks like now.
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2466. StormJunkie
6:35 AM GMT on August 18, 2007
Dean should under go an EWRC at a some point, and when it does it is likely to doughnut even more then now....
Member Since: August 17, 2005 Posts: 26 Comments: 16319
2465. CajunSubbie
6:34 AM GMT on August 18, 2007
another nice lil jog north seems longer this time, lets see if it shoots back west
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2464. JamaicanGir
1:35 AM EST on August 18, 2007
Thanks Stormunkie

will try to do so
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2463. StoryOfTheCane
6:35 AM GMT on August 18, 2007
hey SJ, good to see ya!
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2461. DanielPC
1:32 AM CDT on August 18, 2007
we might get a early speacial update from the nhc if dean gets to be a cat 5 before 5 am hang out people

I would expect them to issue a special update; they did so at 9:30 EDT Friday when the winds jumped to 145.
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2460. TheCaneWhisperer
6:34 AM GMT on August 18, 2007
Morning SJ!
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2459. mikester
6:32 AM GMT on August 18, 2007
I just went to the page stormjunkie told us about and last to frames of goes data showing the tail weakening which i guess is normal for such a powerful storm?
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2458. TheCaneWhisperer
6:31 AM GMT on August 18, 2007
I don't know what to say JamaicanGir, difference between a Cat3 and a Cat5 is huge.
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2457. StoryOfTheCane
6:34 AM GMT on August 18, 2007
This is what I foresee..

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2455. stormybil
6:32 AM GMT on August 18, 2007
ncep fronts sorry i dont know this but when you check this box what does the yellow line stand for under ncep thanks
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2454. JamaicanGir
1:33 AM EST on August 18, 2007
TCW, don't let this change your mind this is truly a beautiful island, if only our government could pull it together and get us out of debt and make our economy grow it would be paradise
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2452. Dan187
6:30 AM GMT on August 18, 2007
If the pressure drops to near 900, I am rather interested in what the winds end up at, considering HH winds at 930 mb would support surface winds near 140 kts.
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2451. VEROBEACHFL1
6:31 AM GMT on August 18, 2007
dont....know.....if ...ill.make it till5am....
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2450. JamaicanGir
1:28 AM EST on August 18, 2007
ok, gilbert was only a cat 3 when it came ashore, can;t imagine a cat 5 here as numerous homes of the lower income islanders are wooden structures and not enough shelters
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2449. StormJunkie
6:23 AM GMT on August 18, 2007
Morning y'all

Well, I see that the GFS, and Ukmet have shifted N some and the GFDL is just being a persistent little bugger. Not really a good sign for the oil coast...

As for Sat images, now that Dean is in the Conus imagery, the Sat black out period should not last as long. Notice the image times from here. This sector went in to blackout later then the other sectors.
Find that link and much more including models, imagery, marine data, wind data, and preparedness info.
Quick Links.

Here is a little info on using the GHCC site.
Select the first link in the imagery section. This is the Global Hydrology and Climate Data Center. There are several rows of maps. The first column is visible imagery. The second infrared and the third is water vapor. The first row is the W Atl view and the frames update most often and also has the highest max zoom. The next active row down does not have the same zoom level and the images update every 30 minutes. The Carib row uses a different color scale on the IR imagery and shows more whites and reds with weaker convection. Select the map you wish t o view. Below the image that comes up are options that will allow to change size, number of frames, zoom level, quality, etc. Set these parameters the way you choose and then click the area on the map you wish to view. Make sure you select the animate feature if you wish to see a loop. Allow loop to load and enjoy. This site releases images prior to any other site as far as I know.
Member Since: August 17, 2005 Posts: 26 Comments: 16319
2448. stormybil
6:28 AM GMT on August 18, 2007
we might get a early speacial update from the nhc if dean gets to be a cat 5 before 5 am hang out people
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2447. KoritheMan
6:25 AM GMT on August 18, 2007
Wilma's record is 882 mb. 888 mb was Gilbert's reading. Anyway, I hope they get a plane in when Dean peaks.

Yeah, I'm tired tonight, can't type.
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2446. Business
6:29 AM GMT on August 18, 2007
jesus...i go out to a bar, come back, and this thing is almost a cat5. damn, even though the whole gulf coast needs to be prepared just in case, texans better have their plans in place.
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2445. TheCaneWhisperer
6:27 AM GMT on August 18, 2007
Coming down for the first time in November this year, very exited, never been.
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2444. Twisterman555
6:28 AM GMT on August 18, 2007
Dean's tail is...wow!
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2443. Jamaicajane
1:24 AM EST on August 18, 2007
Category 5! I'm tempted to throw in some choice Jamaican bad words (and they are very, very colorful ...) But, like JamaicaGir on the other side of the island, all I can do is to prepare and hope we will be spared the worst. And, if we have to be hit, I sure hope it will be the only hit for the season and not one of a slew. Bye for now.
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2441. JamaicanGir
1:25 AM EST on August 18, 2007
ok TCW they are about 1.5 hr drive from me, have you ever been here
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2440. StoryOfTheCane
6:25 AM GMT on August 18, 2007
what an amazing system we have here

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2439. mikester
6:25 AM GMT on August 18, 2007
That ull your talking about.
pic
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2438. Crisis57
6:25 AM GMT on August 18, 2007
Posted By: Dropsonde at 6:23 AM GMT on August 18, 2007.

Crisis57: As of the last water vapor satellite before the blackout, the ULL appeared to be partially over Florida.


i do see that which explains the rain we received here but i see its slowly but the middle of the low is still off the east coast spining
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2437. forecastFlyer
6:22 AM GMT on August 18, 2007
I'll stay for a 3 but anything stronger than that and I probably will get out of Dodge.
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2436. JamaicanGir
1:23 AM EST on August 18, 2007
question?? is it possible that the Blue mountains of 7000ft could deflect the winds and shift the eye either north or south of the island
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2435. TheCaneWhisperer
6:18 AM GMT on August 18, 2007
Family in South Coast JamaicanGir, little SSW of Santa Cruz.
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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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