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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84. fldude999
8:04 PM GMT on August 17, 2007
Posted By: canesurf at 8:04 PM GMT on August 17, 2007.

What I don't understand is why the NHC and models seem to ignore the ULL off Florida. It seems very possible that Dean could exploit that weakness and actually skirt N in between the 2 high preseeure areas, skirting the east coast of Florida. Isn't that even remotely possible?


I don't think the east coast of FL is a possibility, but i think its possible for a complete u turn similar to elena in '85
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81. OUFan919
8:03 PM GMT on August 17, 2007
I have dean at 15.1N at the very moment
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79. Randyman
8:04 PM GMT on August 17, 2007
Dean beginning to dictate the environment around him...

Link
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77. Rlennon
8:03 PM GMT on August 17, 2007
We have to remeber this is a young storm the wind field has not had time to spread out. It will before this is over.
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76. canesurf
8:00 PM GMT on August 17, 2007
What I don't understand is why the NHC and models seem to ignore the ULL off Florida. It seems very possible that Dean could exploit that weakness and actually skirt N in between the 2 high presseure areas, skirting the east coast of Florida. Isn't that even remotely possible?
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74. Crisis57
8:02 PM GMT on August 17, 2007
Posted By: stormybil at 8:01 PM GMT on August 17, 2007.

dean was at 14.8 at 2pm

where do you see him now


i would say 15.0N or 15.2N around there
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73. TerraNova
3:58 PM EDT on August 17, 2007
Thanks Doctor.
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71. gsueagle07
7:59 PM GMT on August 17, 2007
If the hurricane force winds never go out more than 25 miles....very few people if any would ever experience the bad stuff....that would be a blessing if they don't extend any further than that......
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70. cchsweatherman
7:52 PM GMT on August 17, 2007
Based upon my latest satellite observations, Hurricane Dean is presently on a WNW course at about 280 to 285 degrees while the ULL has migrated farther south than west throughout the day. Having done my own personal calculations based on the current data on both the forward momentum of Dean and the ULL, I would have to hazard a strong guess to state that Hurricane Dean will come into contact with the ULL in about 54 to 60 hours. I cannot say with great certainty what outcome will result of this interaction with the ULL. That is the reason why I have not been on for the past hour and a half, I was making those calculations.
Member Since: April 14, 2007 Posts: 8 Comments: 5163
67. stormybil
8:00 PM GMT on August 17, 2007
dean was at 14.8 at 2pm

where do you see him now
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64. fldude999
7:59 PM GMT on August 17, 2007
Posted By: Michael at 7:55 PM GMT on August 17, 2007.

ULL will decide where Dean goes period. So watch the ULL like a hawk.

In what way/direction will the ULL influence Dean?
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62. Randyman
7:53 PM GMT on August 17, 2007
Posted By: Michael at 7:52 PM GMT on August 17, 2007.

Also he said he's very concerned with Erin leaving the weakness over TX. He told me this baby is going to race all the way to the Yucatan then it's a whole nuther ballgame in the Gulf.

He echoes my concerns almost exactly...the remnants of Erin are'nt moving fast enough for me - I'm afraid he may end of right, though I hope I will be proven wrong in the end and this indeed end up being a Mexico storm...strangely, I've watched many storms roll off of Africa over the past several years...this one has had me the most nervous of them all, even since last Friday, 08/10/2007!
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61. NoMeteorsInOlogy
7:57 PM GMT on August 17, 2007
Travel speed seems to affect strength at landfall due to kless time to lose steam.
NMIO
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60. drusierDMD
7:42 PM GMT on August 17, 2007
thanks for the update
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59. Metallica1990
7:54 PM GMT on August 17, 2007
wilmas hurricane force winds extended only 15 miles from the center when it became a catagory 5 and the tropical strom winds went out 155 very similar to dean
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58. bucsfan0713
7:52 PM GMT on August 17, 2007
Hi all...long time lurker. I live on the west coast of Florida, thanks for all of the interesting discussion and opinions.
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57. o311
3:53 PM EDT on August 17, 2007
The NHC was off about 400 miles when Jeanne hit Ft. Pierce intead of Jacksonville, and that was less then 36 hrs out. Not to mention Charley. No one should take their pack off.
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55. C2News
3:55 PM EDT on August 17, 2007
Posted By: JFLORIDA at 3:55 PM EDT on August 17, 2007.
C2News heheh, nice way of putting it, and its true.


Thank you, someone sees it as well. Wouldn't that mean that the NHC track was a tad off? I think so...
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53. TX
7:39 PM GMT on August 17, 2007
indeed, houston/galveston area, let's get those gas price photos uploaded...and let the gouging games begin..
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52. Crisis57
7:54 PM GMT on August 17, 2007
Only thing i'm going to say is no matter what Dean is going to go and do what Dean wants to do
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51. Relix
7:52 PM GMT on August 17, 2007
Yup, winds are strong in PR (well gusts). I recorded a 27MPH peak at the roof of my house a few minutes ago, and dark clouds are covering us. This is exciting I'll admit it, but the WNW path its taking scares me =P. Closer it gets, the more winds we get.
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49. sporteguy03
7:54 PM GMT on August 17, 2007
Posted By: fldude999 at 7:53 PM GMT on August 17, 2007.

Does anyone "in the know" here think that Dean could actually be pulled north and possibly even east after entering the gulf?

Nope shouldn't Couldn't Wouldn't but I can be wrong too, but I'm more right then wrong :)
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46. fldude999
7:43 PM GMT on August 17, 2007
Does anyone "in the know" here think that Dean could actually be pulled north and possibly even east after entering the gulf?
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45. Metallica1990
7:51 PM GMT on August 17, 2007
Posted By: gsueagle07 at 7:48 PM GMT on August 17, 2007.

I'm still amazed on how small a wind radii this storm has...a Cat 3 with only hurricane force winds extending 25 miles??????...isn't that highly unusual???


well didnt wilmas hurricane force winds only go out 25 miles just before it rapidly intensified?
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43. weathermanwannabe
3:50 PM EDT on August 17, 2007
Thanks Savanna...Yeah, in the big scheme of things, it looks like it's making a beeline towards the upper level low off the coast of Florida but who am I to question the models...
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41. littlefish
7:49 PM GMT on August 17, 2007
The African wave looks to have some spin convection from the east side, but the thing looks exposed. Maybe it can suck some of the ITCZ moisture into it?

http://oiswww.eumetsat.org/SDDI/cgi/listImages.pl?m=prod,a=1,sa=9,pr=RGB,f=1,c=FOG,se=4,n=6,d=1,v=4 00,pp=0,t=200708171500#controls

Woops, how do I put it as a link? TIA!
40. C2News
3:50 PM EDT on August 17, 2007
Dean is already at the latitude it was supposed to be at at 1 am according to wunderground flash tracker
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39. welshcayman
7:48 PM GMT on August 17, 2007
Posted By: o311 at 7:45 PM GMT on August 17, 2007.

Don't the computer models run at different times? Wouldn't the latest model probably be the most accurate because of the new info.


No.

You should read Dr. M's blog at the top of this page - full of great information.

The NHC forecast track is more reliable (that is a fact not JMHO) than any single model. Unless you have a better grasp of the data than the NHC (and most of us do not) then you would be wise to believe the NHC track.
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35. JLPR
7:47 PM GMT on Agosto 17, 2007
well some decent wind guts here i PR i would say 20-25 mph or maybe a 30mph one
its on 15n so there is some wind around this system
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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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