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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2685. GPTGUY
3:34 AM CDT on August 18, 2007
SF i would say the radar..IMO
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2684. VEROBEACHFL1
8:33 AM GMT on August 18, 2007
--ok i better leave--im scaring myself--lol
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2683. Aggie92
8:30 AM GMT on August 18, 2007
I'm thinking that Dean will take a more WNW/NW track, but I'm thinking that it won't occur until around 75. I think this movement that we're seeing is just wobble. I think the the shift is coming though.
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2682. sflhurricane
8:32 AM GMT on August 18, 2007
Which would be smarter to look at for more updated views of the storm? PR radar? or sat. views?...
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2681. stormybil
8:30 AM GMT on August 18, 2007
arent we always the last to know sf then there goes the panic .
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2680. quakeman55
8:31 AM GMT on August 18, 2007
I'm definitely noting the more NW motion too...let's see how long this persists...
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2679. sflhurricane
8:30 AM GMT on August 18, 2007
Wow the evidence is there, not only by looking at the radar, but now the Sat views too.. Dean moving WNW or maybe even NW for the moment... Jog? or Change of course? Is it feeling the affects of the ULL... interesting next few hours
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2678. GPTGUY
3:30 AM CDT on August 18, 2007
yeah VERO we'll have to see if it persists or if its just a wobble...but if it persists then you might have pegged it
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2677. sflhurricane
8:28 AM GMT 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
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2676. VEROBEACHFL1
8:29 AM GMT on August 18, 2007
well---IF the WNW holds true at 67W --i definetely pegged that 1.............
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2675. VEROBEACHFL1
8:27 AM GMT on August 18, 2007
WOW--DID I PEG IT ON THE HEAD? will still have to wait and see tho------ at least an hour--if it holds true.........
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2674. sflhurricane
8:26 AM GMT on August 18, 2007
stormybil... check out the 48 hour link I just put out... you'll see originally how far Dean, in its early stages was from the ULL, and now Cat 4 Dean's western edges are beginning to be affected by the extreme eastern edge of the ULL
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2673. katadman
8:25 AM GMT on August 18, 2007
Understood sfl, but it won't take much movement to the north to completely alter all the models, especially if it happens earlier than expected. Am I correct in assuming so?
Member Since: September 7, 2006 Posts: 0 Comments: 1081
2672. redrobin
8:17 AM GMT on August 18, 2007
Wishcasting-I have seen that word a lot on this sight. Just go for it. You just might be right.
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2671. GPTGUY
3:26 AM CDT on August 18, 2007
on the satellite you can see the beginnings of the WNW/NW movement..as soon as it approches 67w
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2670. VEROBEACHFL1
8:26 AM GMT on August 18, 2007
BUOY----Conditions at 42059 as of
(3:50 am EDT)
0750 GMT on 08/18/2007: Unit of Measure: English Metric Time Zone: Station Local Time Greenwich Mean Time [GMT] British Summer Time [GMT+1] Eastern Greenland [GMT-1] Azores [GMT-2] Western Greenland [GMT-3] Atlantic Standard [GMT-4] US/Eastern Standard US/Central Standard US/Mountain Standard US/Pacific Standard Alaska Standard [GMT-9] Hawaii-Aleutian Standard [GMT-10] Samoa Standard [GMT-11] International Date Line West [GMT-12] Western European [GMT+0] Central European [GMT+1] Eastern European [GMT+2] Moscow [GMT+3] USSR Zone 3 [GMT+4] USSR Zone 4 [GMT+5] USSR Zone 5 [GMT+6] USSR Zone 6 [GMT+7] China Coast [GMT+8] Japan Standard [GMT+9] Guam Standard [GMT+10] GMT+11 International Date Line East [GMT+12]
Click on the graph icon in the table below to see a time series plot of the last five days of that observation.

Wind Direction (WDIR): N ( 350 deg true )
Wind Speed (WSPD): 44.7 kts
Wind Gust (GST): 58.3 kts
Wave Height (WVHT): 32.8 ft
Dominant Wave Period (DPD): 13 sec
Average Period (APD): 9.9 sec
Atmospheric Pressure (PRES): 29.24 in
Pressure Tendency (PTDY): -0.36 in ( Falling Rapidly )
Air Temperature (ATMP): 78.4 F
Water Temperature (WTMP): 83.7 F
Dew Point (DEWP): 76.6 F
Heat Index (HEAT): 81.7 F
Combined plot of Wind Speed, Gust, and Air Pressure
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2669. sflhurricane
8:23 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

48 hour loop
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2668. stormybil
8:24 AM GMT on August 18, 2007
right sf thats what im seeing
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2667. VEROBEACHFL1
8:24 AM GMT on August 18, 2007
i think the NW movement now is NOT a jog and i think we are on track --the few of us that have been following this for the last hours....
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2666. VEROBEACHFL1
8:22 AM GMT on August 18, 2007
im down with that maui------ thatys what we are trying to figure our---WHERE and WHEN and HOW much of a NORTHERN turn will affect DEAN.......
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2665. sflhurricane
8:20 AM GMT on August 18, 2007
katadman...yea i agree. but it seems that the NHC doesnt seem concerned by this ULL much as they, along with the NWS believe the ULL will move west and dissipate, being replaced by a huge area of High pressure. From looking at the sat. view, that ULL looks to be holding strong while moving SW.
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2664. stormybil
8:20 AM GMT on August 18, 2007
dont think anyone or the models thought the ull will dig that south over cuba they had way above on the west coast of fla . thats why we think its going to catch dean very soon and head him more north stay tuned
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2663. GPTGUY
3:21 AM CDT on August 18, 2007
yeah bobcane I see it also..im using GRLevel3 and ive been watching it move more NW
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2662. VEROBEACHFL1
8:20 AM GMT on August 18, 2007
wel--ok ill share----------- but this is not in stone ok peeps---all it is is IMO----------
---originall i was totally into the WNW movement with a sharper N movement on,near or around 80W--but that might be too far out--------- is what im working on right now...........
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2661. MauiHawk
8:18 AM GMT on August 18, 2007
Well, no one responded to my earlier comment, so I guess I'll repeat because I would like an opinion:

Instead of focusing on the center of the ULL, look at the eastern edge of its influence. It appears to me to be retreating westward nearly as fast as Dean is moving (or at least much faster than the center of the ULL). Is that not ridging building in behind the ULL, and building in faster than the low is moving. Notice how the ULL appears to be elongating as a result? Could that not be an indicator of its impending demise?
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2660. bobcane
7:19 AM GMT on August 18, 2007
NW turn of Dean now evident on PR long range radar. I am using GR2AE and I can see the turn now.>>>

The ULL off Florida digging SW and on the move now.

I do not believe this is a dean wobble but rather a more northern component to the track has begun.
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2659. GPTGUY
3:20 AM CDT on August 18, 2007
Does anyone think with a more WNW path if the ULL is moving WSW or SW that Louisiana and maybe Mississippi need to start watching Dean?
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2658. katadman
8:18 AM GMT on August 18, 2007
Good night all. Y'all have been very informative and patient with this novice. I really appreciate you. Let's all remember to pray for our friends who are in the path of Dean.
Member Since: September 7, 2006 Posts: 0 Comments: 1081
2657. VEROBEACHFL1
8:19 AM GMT on August 18, 2007
right now---- im trying to figure out when/where the bigger NORTHWARD movement will come----------- BUT its hard because that darn ULL wont make up its mind------fast,slow,west,ssw etc.......... only time will tell.....
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2656. stormybil
8:18 AM GMT on August 18, 2007
when the ull get him hes going right over the tip of cuba
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2655. VEROBEACHFL1
8:17 AM GMT on August 18, 2007
--if u scroll down a bit (maybe the next page) MR nicevilles comments were very welcome also............ but im only going to 80W....
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2654. katadman
8:12 AM GMT on August 18, 2007
sfl-Thanks for the link. That is what I've been looking for. Looks to me that with that southward movement of the ULL, it will be effecting Dean's path sooner than later. IMHO
Member Since: September 7, 2006 Posts: 0 Comments: 1081
2653. VEROBEACHFL1
8:15 AM GMT on August 18, 2007
redrobin--:) whew-- thx i needed that---- ppl would think im wishcastin but im really trying to learn and figure this thing out---i been studying/learning religisly......
---i like to do my own opinions and see how they actually pan out--thats all it is---IMO--------stuff..
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2652. sflhurricane
8:16 AM GMT on August 18, 2007
Vero, so where do you think this storm is headed?
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2651. VEROBEACHFL1
8:15 AM GMT on August 18, 2007
ahhh--yes --i could be wrong here---time will tell :)
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2650. redrobin
8:11 AM GMT on August 18, 2007
Dont feel alone. I am sure there are more but dont want to say. Then they would have to deal with being wrong. hahaha I think a N pull will happen soon. I think the low -well in my layman thinking wont do to much to it. He is so strong he will pull a John Wayne and plow through where he wants. Big Bad Dean can do what he wants at a Cat 5.
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2649. sflhurricane
8:14 AM GMT on August 18, 2007
Ya from looking at the PR radar, it looks like its moving maybe even NW.. and thats a motion thats been going on for atleast an hour (thats how long the loop goes for)..
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2648. VEROBEACHFL1
8:13 AM GMT on August 18, 2007
stormy---i think u know how adamint i was with the more of a WNw track for like what? 2 days now?
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2647. errantlythought
8:13 AM GMT on August 18, 2007
Concur, Vero.

PR radar definately showing at least a northern wobble. Time will tell if its a jog.
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2646. GPTGUY
3:13 AM CDT on August 18, 2007
Whoa..Vero you think us here on the MS Coast might have to worry a little?
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2645. VEROBEACHFL1
8:11 AM GMT on August 18, 2007
it was my orignal track--but a little more n of what the NHC has it at now---only till about 80W then---then the fun starts......it MIGHT go more north BEFORE 80w but that remains to be seen........too early to tell......
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2644. GPTGUY
3:11 AM CDT on August 18, 2007
lol...yeah when i put the cursor where the eye starts to where it finishes in the loop its definetly WNW..but like you said maybe a woble we'll have to see if it lasts
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2643. VEROBEACHFL1
8:10 AM GMT on August 18, 2007
ok cool--that makes 2 of us--lol--whew..felt alone for a minute lol
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2642. stormybil
8:10 AM GMT on August 18, 2007
you know where i stand vero :)
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2641. sflhurricane
8:10 AM GMT on August 18, 2007
Vero, what sat. view are you looking at?
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2640. GPTGUY
3:08 AM CDT on August 18, 2007
VERO Im looking at a radar out of Puerto Rico and it does look like the eye is moving WNW in the last couple frames
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2639. VEROBEACHFL1
8:10 AM GMT on August 18, 2007
--or is e1 afraid to answer?
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2638. redrobin
8:09 AM GMT on August 18, 2007
YES==I agree
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2637. VEROBEACHFL1
8:08 AM GMT on August 18, 2007
does anyone else here agree with me on this WNW movement by 67, 68 or 69?
--or do i stand alone...........
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2636. sflhurricane
8:06 AM GMT on August 18, 2007
Yea that ULL doesnt seem to be dissipating at all. More evidence can be seen by looking at the MIA radar...There are strong, isolated storms forming off the coast.
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2635. VEROBEACHFL1
8:06 AM GMT on August 18, 2007
------i think he just started to hit WNW right at the 67W where i said it should---but it might just be a jog----lets wait about an hour.........
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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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