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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2785. WeatherSpotter
9:26 AM GMT on August 18, 2007
Yep,

I agree with Vero its turning NW/WNW direction especially in the last couple frames of this.

Link

Very evident of that in the Infrared/Satellite views.
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2784. stormybil
9:28 AM GMT on August 18, 2007
nite vero and sf and all this has been fun see ya all tonight we might be seeing a differnt picture latters :)
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2783. GPTGUY
4:26 AM CDT on August 18, 2007
Mississippi here and I'm definetly not wishcasting...just stating what I see on radar and satellite..if its a wobble or not...definetly not wishcasting it to come up this way..dealt with Katrina 2 yrs ago..and dont wanna deal with another one...watching Dean like this loop by loop reminds me of 2 yrs ago doing this with Katrina
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2782. sflhurricane
9:28 AM GMT on August 18, 2007
ya stormybil, its real time
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2780. sflhurricane
9:26 AM GMT on August 18, 2007
Storms still firing around Florida...wheres that so-called High the NHC is talking about to stabilize the air?
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2779. stormybil
9:25 AM GMT on August 18, 2007
sf is that link of the floater in real time
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2778. VEROBEACHFL1
9:26 AM GMT on August 18, 2007
sf? stormy? is she still here?----- anyhow--- that was my IMO for the night and proud of it---looks like i pegged the 67W pretty darn close---- lets see the next 24 hours and DEANS WNW movement and ill be back.......................
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2777. bobcane
9:26 AM GMT on August 18, 2007
Florida here also
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2775. sflhurricane
9:25 AM GMT on August 18, 2007
stormybil, where are you located if u dont mind me asking
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2773. sflhurricane
9:24 AM GMT on August 18, 2007
Yup, Im a florida guy too :)
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2772. stormybil
9:23 AM GMT on August 18, 2007
Amystery at 9:22 AM GMT on August 18, 2007.

if Dean was going to FL...Drak, jp, the whole Florida gang would be here, and this blog would be double....have you noticed all the Florida regulars are nowhere to be found? lol(they are looking for a spin in the wave off africa)

we are lol
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2771. VEROBEACHFL1
9:22 AM GMT on August 18, 2007
we are florida people amy--------------
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2770. sflhurricane
9:21 AM GMT on August 18, 2007
It looks like this ULL is fooling with us all and not doing what its supposed to do
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2769. hurrybird
9:10 AM GMT on August 18, 2007
Well - I wouldn't be to happy on Thunderhorse about this storm if I were Shell Oil. I think this one is going to about Morgan City. New Orleans is going to get its new repaired dikes tested. At least that is what I think. But all forecasts are errors until they happen anyway.

The reason for this thinking is the heat patterns on the continent. They actually feed the storm towards the East. If these come into play they storm may actually go far to the east and hit about Panama City, Fl. I have sort of been Averaging and I think about 60 miles to the west of N.O.
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2767. stormybil
9:20 AM GMT on August 18, 2007
yes it rubbling here too lost my cable connection hehehe but back now

notice the nhc is now saying a wnw in thex 24 hours not due west hmmmmm
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2766. samiam1234
9:20 AM GMT on August 18, 2007
LOL Katdaman I dont think we can call the Mexicans wishcasters, since this is clearly going right at them... unless you mean those wishcasters are some how wishing this storm miraculously goes to Texas.
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2765. VEROBEACHFL1
9:20 AM GMT on August 18, 2007
amy did u think dean was gonna ride close to the 15? till at least 80w? is that your opinion? or is it different? let me know ok---its all IMO i am posting--thats all :)
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2764. sflhurricane
9:19 AM GMT on August 18, 2007
Amystery, you dont see a northern component to Dean's movement here... Here
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2763. bobcane
9:19 AM GMT on August 18, 2007
The NHC is excellent.
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2762. sflhurricane
9:17 AM GMT on August 18, 2007
Ya I guess Im just out to prove those NHC people wrong... But those guys are good and they deserve a lotta credit
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2761. VEROBEACHFL1
9:18 AM GMT on August 18, 2007
whatever AMY---lets see what it does for the next 24 hours ok? then u can bash me :)
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2759. bobcane
9:16 AM GMT on August 18, 2007
I just finished a rose analysis of the storm track on GR2AE and the motion over the last our has been 307 deg (13 deg shy of due NW)

The NW movment has resumed after a brief jog back to the west.
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2757. samiam1234
9:13 AM GMT on August 18, 2007
just because it heads North at a quick pace doesnt mean it will hit Texas.. say if the scenario is such that the eye hits quite a bit North of Jamaica, further north then predicted by all the models.. wouldnt the high forming in the GOM still take it back down South or at the very least keep it due West... the high that is forming starts on monday and the temps in Houston will be in the high 90's again, after a brief relief from Erin.
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2756. SWFLdrob
9:12 AM GMT on August 18, 2007
hurricane wobble all the time...needs more time to see if this is a real shift or just a wobble. Also, Dean moved pretty decently to the N yesterday until hitting the 15N line...then straight west for 8-10 hours. Seems like he likes to step north every once in a while before heading back west.

Also, the models and NHC track all have some northern movement in its tracks, not straight west. Will have to wait to see, but too early to call a prolonged NW motion IMO.
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2755. VEROBEACHFL1
9:16 AM GMT on August 18, 2007
--it will go back west after 80w IF IF IF IF that ULL goes away-BUT u know WHAT? i hasnt so far.............
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2754. sflhurricane
9:15 AM GMT on August 18, 2007
If this WNW to NW movement continues, Im curious as to what the NHC will say in their 8 AM advisory
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2753. sflhurricane
9:13 AM GMT on August 18, 2007
From looking at the radar, I would say this storm has more-or-less been moving WNW to even slighly NW for the past 2 hours. I guess the NHC believes this is just temporary and a turn back to the west will pursue? (I dont see that happening though)
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2752. katadman
9:11 AM GMT on August 18, 2007
Posted By: samiam1234 at 9:10 AM GMT on August 18, 2007.

I dont think this is hitting Texas.. there will be Texas wishcasters all day tommorow.. who just like to see drama unfold and have something to talk about.. but this storm is clearing heading due West, the eye might not even hit Jamaica, rather go south of it and it looks like it will hit further south on the Yucatan as the models are showing.. this will not only reduce the storm once it enters the gulf... but will clearly make this a Mexico Cane.


Although you have a ring of certainty to your statement, not all of the models agree with that forecast. I think we will know much more by Sunday morning. I am particularly interested to see the new computer models.
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2751. sflhurricane
9:12 AM GMT on August 18, 2007
stormybill, you still here the rumblings out there?... That ULL has the air really unstable out here
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2750. GPTGUY
4:12 AM CDT on August 18, 2007
yeah SF I see it too..it looks like its bouncing
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2749. sflhurricane
9:10 AM GMT on August 18, 2007
Samiam, dont be so sure with this, or any storm...They all find a way to fool us all
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2748. VEROBEACHFL1
9:11 AM GMT on August 18, 2007
well----to the people who think their WEST movement will stick--- good luck---cause i sure as heck DO NOT--------
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2747. stormybil
9:11 AM GMT on August 18, 2007
the cone will be alot further north today i say at 5pm if the motion continues
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2746. sflhurricane
9:09 AM GMT on August 18, 2007
Uh oh Vero... Checking out PR's radar again, and the "wobble" back to the NW continues.. very interesting
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2745. samiam1234
9:05 AM GMT on August 18, 2007
I dont think this is hitting Texas.. there will be Texas wishcasters all day tommorow.. who just like to see drama unfold and have something to talk about.. but this storm is clearing heading due West, the eye might not even hit Jamaica, rather go south of it and it looks like it will hit further south on the Yucatan as the models are showing.. this will not only reduce the storm once it enters the gulf... but will clearly make this a Mexico Cane.
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2744. VEROBEACHFL1
9:09 AM GMT on August 18, 2007
---- i guess some people didnt see this coming??? -----
--i think that last report might peed some peeps off but i dont know y...........
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2743. stormybil
9:07 AM GMT on August 18, 2007
thanks sf you can really see that north jog with that one .
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2742. VEROBEACHFL1
9:06 AM GMT on August 18, 2007
jaxen=== not sure what u mean but the NHC seems to agree........
--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.
-the key here is within the next 24 hours...........
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2741. katadman
9:05 AM GMT on August 18, 2007
Certainly they're cautious. They don't want to appear as flighty as we are! LOL
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2740. sflhurricane
9:05 AM GMT on August 18, 2007
If you extrapolate Deans last few frame movements, the storm would be heading towards Hispanolia.. kinda crazy..
Here, check
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2739. MauiHawk
9:03 AM GMT on August 18, 2007
I'll see tomorrow I guess
Indeed... we all will. I'm calling it a night. Later... (and gl Jamaica)
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2738. Jaxen
5:04 AM EDT on August 18, 2007
Dean rode 15N from 64W to past 67 ... how can you say that moving "all the way up" to 15.1 is the start of WNW movement?

As for the slowdown from 25 to 17mph movement is normalization to me ... 25 is like breakneck speed. Once it settles in at 15-17 for a while, nobody will think anything of it again.

As I sign off for a while, here's an ugly thought ... even IF it misses Jamaica AND IF if it misses the Yucatan, THEN it will hit Texico as a Cat 5. *Somebody* is getting a Cat 5 hurricane. *damn*
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2737. VEROBEACHFL1
9:03 AM GMT on August 18, 2007
NHC being cautious-- and rightlfully so--- pretty good info at the 5am :)
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2736. sflhurricane
9:00 AM GMT on August 18, 2007
I guess.. but I still wonder about this ULL lol And I agree, the NHC does an amazing job and I take their word over mine anyday, but I know the basics of weather and how it applies to hurricanes so Im wondering why they aren't stating how the ULL is moving SW, creating a close call between it and Dean.. rather than continue to discuss how its moving West, when its definitely not... I'll see tomorrow I guess
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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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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.