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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2935. Masquer08er
11:45 AM GMT on August 18, 2007
Tell me why I am wrong in thinking Mean Dean looks like it going to Haiti/DR.

Sorry about the Mean Dean. I've been dying to say it.
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2934. guygee
11:46 AM GMT on August 18, 2007
"What is the story with the CLP5 Model always seeming to have this further east?"

BiloxiGirl - CLP5 is just he climatology prediction, something like the average expected historical track. I has no relationship to current conditions...it is only shown for comparison and benchmarking the other models.
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2933. gsueagle07
11:47 AM GMT on August 18, 2007
Just like I have been saying...Mexico/Texas border...all models coming into agreement...Jamica and the Yucatan pennisula is the areas that might take the brunt of the storm....I don't believe northern Mexico is all that populated....
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2932. sunnyandshear
11:47 AM GMT on August 18, 2007
it was to stay on this shift to 303 or so...jamaica would get a direct hit...the eye would almost go over the entire island...lets just hope this is a overgrown wobble..."""

If your going to endure a Hurricane, having the eye go over you is a good thing. The Eye is nice and calm, and when it passes over, it gives you a breather and time to shore up lose or broken defenses. If a hurricane has got to come, hope you get some eye.
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2931. watchingnva
11:46 AM GMT on August 18, 2007
anyone else see the cooler water being shown and pulled north into the gulf in this SST eddy loop ???...wierd...
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2930. extreme236
11:46 AM GMT on August 18, 2007
what i have noticed though with dea is that his size has been decreasing over the past couple days. perhaps he is becoming more tight and compact
Member Since: August 2, 2007 Posts: 19 Comments: 19234
2929. watchingnva
11:41 AM GMT on August 18, 2007
no mas...just jamaica...i dont see dean going that far north....and unless something drastically changes over the next 2 days...im not seeing a north turn either...a lil further north that an emily track...but very simalar...Im not saying that things cant change...but as of right now i see a landfall in mexico...bout 50-150 miles south of the border...but that can def. change...
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2928. FMTXWMAN
11:31 AM GMT on August 18, 2007
Latest WV loops appear to show Dean undergoing a eyewall replacement cycle. Also some have said there's dry air entrainment. It looks like it, but my train of thought goes towards the wind field expanding and it's just where the storms are firing and not yet being completely pulled into the core circulation.

The good news is that as Dean becomes larger it will become more difficult to maintain cat 5 status due to the amount of mass all that moving air begins to have.


Still we have to keep in mind record THCP in Dean's path so it's also possible Dean could set a record for largest cat 5 storm.
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2927. BiloxiGirl
11:43 AM GMT on August 18, 2007
gotcha
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2926. WPBHurricane05
7:43 AM EDT on August 18, 2007
What is the story with the CLP5 Model always seeming to have this further east?

Mix between Climatology and Persistence. Not accurate.
Member Since: July 31, 2006 Posts: 56 Comments: 8112
2925. sporteguy03
11:40 AM GMT on August 18, 2007
That ULL is putting up some fight, it has slowed Dean down some but not for long.
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2924. BiloxiGirl
11:41 AM GMT on August 18, 2007
What is the story with the CLP5 Model always seeming to have this further east?
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2923. rxse7en
11:36 AM GMT on August 18, 2007
Ugh, models are showing a freight train of lows coming in after Dean. The CMC is especially creepy. I'm thinking this weekend is a good time to stock up on water, propane, food, batteries and gas. Gonna run the generator for the first time to check it out--thing's been sitting, wrapped up in my garage for two seasons unused.

B
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2922. WildHorseDesertTx
6:41 AM CDT on August 18, 2007
Good morning ya'll, south of Corpus here. I am just keeping my eye on Dean and getting plywood ready, just in case.
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2921. WPBHurricane05
7:39 AM EDT on August 18, 2007
The models slowly starting to agree

Link
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2919. Masquer08er
11:35 AM GMT on August 18, 2007
There are more Floridians on here than any other group. If there are more of us then there is a higher percentage that we will have more idiots. I think you all should cut us some slack until school starts next week. Plus everyone down here is a product of someone from the Northeast. Dare I say more.

LOL BTW,School started Aug.9 here. :::getting my slack cutting scissors:::
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2918. WPBHurricane05
7:37 AM EDT on August 18, 2007
GFDL shifting further south


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2917. waccamaw16
11:34 AM GMT on August 18, 2007
looks like the ull is holding its ground and starting to affect dean a little.
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2916. IKE
6:33 AM CDT on August 18, 2007
San Juan,PR is within 150-200 miles of the center of Dean...their winds....east-southeast at 6 mph.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
2915. Masquer08er
11:33 AM GMT on August 18, 2007
WatchingnVA,

Did you mean Haiti/DR?
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2914. eaglesrock
7:32 AM EDT on August 18, 2007
Mike Seidel is in Jamaica right now. They're probably saving Cantore for when it hits the US...LOL
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2913. PalmHarbor
11:31 AM GMT on August 18, 2007
Posted By: JupiterFL at 11:30 AM GMT on August 18, 2007.
There are more Floridians on here than any other group. If there are more of us then there is a higher percentage that we will have more idiots. I think you all should cut us some slack until school starts next week. Plus everyone down here is a product of someone from the Northeast. Dare I say more.


LOL well spoken:)
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2912. watchingnva
11:29 AM GMT on August 18, 2007
if it was to stay on this shift to 303 or so...jamaica would get a direct hit...the eye would almost go over the entire island...lets just hope this is a overgrown wobble...
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2911. bajelayman2
11:31 AM GMT on August 18, 2007
nolesjeff,

I am in Barbados.

Just a layman here, as my name says.
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2910. PalmHarbor
11:28 AM GMT on August 18, 2007
Heh I just want to make sure I can really not worry about it, I have no interest in betting on the destruction of myself or others:)If we are clear I'm happy as an undiscovered clam on Clearwater Beach and will hope for the best for those not clear.
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2909. Keys99
11:06 AM GMT on August 18, 2007
Good Morning I have to agree with Red Robins forcast NWS Key West at 4 pm yesterday forcast the ULL to be over us this am. CLoudy and Rain here now. THe ULL on WV looks to be moving West to WSW and moving faster then it was yesterday at this time. Only time will tell where the two interact. I think it will hit the Yucatan tip or shoot the gap
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2907. miken62
11:28 AM GMT on August 18, 2007
Great...now we need a blog for gambling...place your bets!!!!!
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2906. watchingnva
11:27 AM GMT on August 18, 2007
yea, i was just going to say...looks like deans already ready for an ewrc...... thats insane...hell only grom larger in size with one...might lose a lil punh for a few hours...but will strenghten again as soon as the new eye tightens up...crazy...
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2905. hurricane667
11:27 AM GMT on August 18, 2007
Dean is really stregthening and reorganized with intense convectoin. would not be surprised by a cat 5 by the 8 or 11 advisories
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2904. Masquer08er
11:21 AM GMT on August 18, 2007
Palm Harbor, If you are betting a beer at a bar, your out of the picture. If your family's safety is an issue, keep an eye on it.
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2903. sunnyandshear
11:23 AM GMT on August 18, 2007
Good thing Dean can't move South much, or there'd be a whole Southern Crowd harping about wsw wobbles. Fact that Florida or US Coast isn't South, would keep the group's numbers small though..
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2902. JupiterFL
11:22 AM GMT on August 18, 2007
That eye is starting to tighten up. I would not be surprised to see a EWRC sometime very soon.
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2901. IKE
6:20 AM CDT on August 18, 2007
Posted By: ForecasterColby at 6:20 AM CDT on August 18, 2007.
Bobby, T#s are satellite estimates. I would be suprised if Dean's were that high, given the current reorganization.


I agree.
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2900. Bobbyweather
7:21 AM EDT on August 18, 2007
ForecasterColby, right now, Dean's T# is 6.0
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2899. guygee
11:10 AM GMT on August 18, 2007
The 0Z 08/18 ensembles are showing a greater spread and uncertainty in the track then the 0Z 08/17 ensemble runs. Operational Model Guidance Products, check GFS ensemble track guidance, frame 1 and frame 5.

If a weakness has developed in the ridge as I perceive, then Dean should continue pulling north of the NHC 18/0900z track, and there will be no doubt in a very few hours.
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2898. obxrox
11:18 AM GMT on August 18, 2007
might have just been a jog...the 1045z loop trending back W...nonetheless, it's still farther N than anticipated
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2897. PalmHarbor
11:19 AM GMT on August 18, 2007
Posted By: drj27 at 11:17 AM GMT on August 18, 2007.
so the florida panhandle is pretty much out of the picture from dean hitting us


I was actually more concerned about fl al ms since it was moving more north.. Am I wrong to be concerned with the apparent change?
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2896. ForecasterColby
11:18 AM GMT on August 18, 2007
Bobby, T#s are satellite estimates. I would be suprised if Dean's were that high, given the current reorganization.
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2895. IKE
6:18 AM CDT on August 18, 2007
Posted By: drj27 at 6:17 AM CDT on August 18, 2007.
so the florida panhandle is pretty much out of the picture from dean hitting us


There's not one computer model that has Dean making landfall any closer then 600-700 miles west of the Florida panhandle.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
2894. Weather456
11:17 AM GMT on August 18, 2007
The worst is over for Saint Kitts - we had tropical storm force winds from 5 am yesterday to about 6pm...Heavy rains..some down branches and at least one landslide.
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2893. drj27
11:15 AM GMT on August 18, 2007
so the florida panhandle is pretty much out of the picture from dean hitting us
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2892. IKE
6:15 AM CDT on August 18, 2007
Posted By: obxrox at 6:13 AM CDT on August 18, 2007.
It's moving basically like the NHC said it was going to move. Just about all of the computer models take Dean into Mexico...other than the GFDL which is trending further south and is getting closer to making landfall in Mexico from what it was showing yesterday...those are the facts.

actually, it's moving N faster than forecast by NHC


It might be 10-20 miles north of the center of the projected points. I agree, that is important if you live in Jamaica and Haiti/DR.
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2891. Weather456
11:13 AM GMT on August 18, 2007
Deab claimed at least 3 lives - 2 in dominica and 1 in saint lucia
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2890. Bobbyweather
7:13 AM EDT on August 18, 2007
Dean will have a T# of 6.5, since it has winds of more than 146 mph, and it has lower pressure than 935 mb.
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2889. CajunSubbie
11:14 AM GMT on August 18, 2007
by watching the radar the lil jog keeps on next sat frame will knock it off the track forcast
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2888. IKE
6:12 AM CDT on August 18, 2007
Not trying to minimize Dean, but the wave behind it in the central-Atlantic, may be a greater threat for the northern GOM and the east coast of the US. The GFS has been on this one for awhile...

Link
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2887. obxrox
11:11 AM GMT on August 18, 2007
It's moving basically like the NHC said it was going to move. Just about all of the computer models take Dean into Mexico...other than the GFDL which is trending further south and is getting closer to making landfall in Mexico from what it was showing yesterday...those are the facts.

actually, it's moving N faster than forecast by NHC
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2886. C2News
7:11 AM EDT on August 18, 2007
That "jog" to the wnw/nw is persisting...

I will check back in about 30 minutes to see what happens
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2885. Weather456
11:09 AM GMT on August 18, 2007
Good morning, OMG! Dean is almost a cat 5 very unexpected this early
Member Since: July 24, 2005 Posts: 407 Comments: 19076

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About JeffMasters

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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