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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1035. KEEPEROFTHEGATE (Mod)
12:17 AM GMT on August 18, 2007
it will achieve 165 mph winds at its peak then the slow decrease in wind an overall storm intensity will begin to come down it funny about a week ago of all the gripe as to where was the season well i guess its here and in a big way
Member Since: July 15, 2006 Posts: 178 Comments: 56059
1034. obxrox
12:17 AM GMT on August 18, 2007
cat 5 history to consider:

23 cat 5's between 1923-1998, or about 1 every 3 years

dean will be the 7th cat 5 since 2003, or about 1.5 PER YEAR over the admittedly short five year period (and we're not done with this year). perhaps a small sample size, but....

what could be behind this increase in intense hurricanes?.....
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1033. waccamaw16
12:14 AM GMT on August 18, 2007
fox news just said that it was going to strike from mexico to the top coastal areas of texas. it is funny how they know for sure that is where it is going,i hate that because it could turn and hit someone else and catch them off guard,because of weather men that says it is going right in a location that is 96 to 120 hrs away according to the speed of the storm
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1032. tallahasseecyclone
12:21 AM GMT on August 18, 2007
To me outflow looks to be increasing on the west side and decreasing a bit on the east anyone know what this might indicate as far as dean's movement as far as speed might be concerned.
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1031. extreme236
12:23 AM GMT on August 18, 2007
raw ADT T# is 6.7 for dean. that would be about 127kts...
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1030. ebzz
8:23 PM EDT on August 17, 2007
So anyone want to start taking bets on when it will become a cat5? I'll guess either by noon tomorrow or by midnight sunday.
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1029. bbesse
12:21 AM GMT on August 18, 2007
If you live in Texas, you should begin hurricane prep NOW!!! GFDL has it in for Texas.

Well, I'm just glad that the GFDL changed its tune from its earlier model smacking LA. I think this is much better news for TX as well. I personally have no faith in any model that rejects the norm wildly, and then goes back drastically adjusts itself. To me it appears out of control, and I'm staying with the majority of other models. I think the GFDL anticipated something that didn't pan out. Based on the data we now have in, I would conclude their earlier model was just flat out wrong.

I'm hoping this whole system just stays due west and shoots straight across Mexico.
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1028. kmanislander
12:21 AM GMT on August 18, 2007
Wherever you live on the planet there is mother nature waiting with some unpleasantness.
Tornados,earthquake,flood, drought,ice storm,hurricane,etc etc.

At least with hurricanes there is more than ample warning
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1027. paulfrmpasschristian
6:22 PM CST on August 17, 2007
HH reported gust of 150 at 9331 feet
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1026. MaxTempest
12:22 AM GMT on August 18, 2007
StormW - concerning the intensity of Dean, according to the NHC forecast track on this loop, it wasn't supposed to be a CAT4 until around 70-71W. What are the implications, if any, of the early strengthening?

Link
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1025. DonnaGalveston
12:20 AM GMT on August 18, 2007
Here is a question for someone with good experience: On television they said watch the shape of the eye for hints on change of course. They looked at the radar for Puerto Rico and said the eye appears to have a bit of a northwest slant, meaning the storm could turn in that direction shortly. Does that make sense?
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1024. Skyepony (Mod)
12:21 AM GMT on August 18, 2007


This is just starting to intensify into something ugly, by morning he may just be a monster.
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1021. H2OMaker
12:20 AM GMT on August 18, 2007
From the NHC Discussion

The GFDL shows a more northwesterly track across extreme
western Cuba and the central Gulf of Mexico. While this solution is
outside the overall model guidance envelope...it can not be rule
out completely since this model has an excellent forecast track
history.
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1020. JLPR
12:17 AM GMT on Agosto 18, 2007
so what could happen if it stalls south of PR?
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1019. paulfrmpasschristian
6:17 PM CST on August 17, 2007
Sorry,I was speaking about the ULL and the ridge answer you gave...what they are really really doing versus the model predictions
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1018. DonnaGalveston
12:18 AM GMT on August 18, 2007
Another one of my friends just called me to say they were going to board up and head out of town on Sunday.
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1017. thelmores
12:17 AM GMT on August 18, 2007




amazing...... 5 days from the cape verdes, and look at Dean now! Well on his way to cat 4, and beyond!
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1016. howarjo1943
12:15 AM GMT on August 18, 2007
Dean has moved just south of due west the past 6 hours.
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1015. Crazman
12:18 AM GMT on August 18, 2007
The winds are 135mph not 130...
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1014. Dropsonde
12:14 AM GMT on August 18, 2007
Whoa this blog's load time is slowing to a crawl for me. I'm out for awhile. It seems that Dean's bomb out phase has begun. I think there's a decent chance of an EWRC tomorrow, however.

Considering it hasn't yet hit the really hot water, this intensity is disturbing. I haven't wanted to say it but we could seriously be looking at a Gilbert, Wilma level system before this is finished.
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1013. DonnaGalveston
12:16 AM GMT on August 18, 2007
They keep stating on television that when storms slow down, they change course. They said if the forward motion slows again on the next update, look for the course to change. They did say it is possible it is just slowing down too though. I'm not really sure they know what is going to happen.
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1012. kmanislander
12:12 AM GMT on August 18, 2007
Good evening everyone

From early this morning everyone here has been in preparation mode with the intention of having everything complete by early Sun afternoon. Long lines at gas stations but no shortages of fuel. Supermarkets are well stocked as are hardware stores.
We don't want Dean here but I doubt that anyone will be caught off guard.

Still hoping for a NW turn but would hate for it to inflict pain on others. I see that the UKMET has joined the GFDL with a pass NE of Jamaica . It will be interesting to see what happens near 70W as these are two very reliable models

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1011. EvanKisseloff
8:14 PM EDT on August 17, 2007
If you live in Texas, you should begin hurricane prep NOW!!! GFDL has it in for Texas.
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1010. KEEPEROFTHEGATE (Mod)
12:15 AM GMT on August 18, 2007
130MPH GUSTS 145
Member Since: July 15, 2006 Posts: 178 Comments: 56059
1008. hosweather
12:15 AM GMT on August 18, 2007
The Dr's afternoon update has some fascinating information. Seems the NHC hurricane path forecasts would be slightly improved if they simply adopted the consensus model (CONU). All the individual forecasting experience at NHC makes them better than individual models but not as good as the consensus. Models have come a long way. Most fascinating was that individual models were not as good at 120 hours as climatology/persistence (CLP5) and the CONU was only a little better.

If you put up CONU and CLP5 on the TropicalAtlantic site , you will see that the two plots begin to diverge strongly around Jamaica. Any forecast of where Dean will go after Jamaica looks to be very problematic at this point. Unfortunately for the people of Jamaica it is highly likely that Dean will do terrible damage to the island. JMVHO
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1007. wunderwog
12:14 AM GMT on August 18, 2007
With all the discussions of the historical accuracy of the various models, I was wondering if the models are tweaked from season to season. I would imagine that they are in an effort to improve accuracy.
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1006. tallahasseecyclone
12:13 AM GMT on August 18, 2007
The question for NOLA is if this does make landfall to the west as a cat 4, how close will the eye need to be to push up some wicked surge on the dirty side?
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1005. DocBen
12:15 AM GMT on August 18, 2007
If this bad boy can shoot the channel and enter the Gulf as a Cat 5 ... could we be needing to define Cat 6 by landfall? 175 mph?
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1004. StuccoMan
7:15 PM CDT on August 17, 2007
This hurricane is about to stall
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1003. Crisis57
12:14 AM GMT on August 18, 2007
you can see here and if guys want you can loop it the ULL has been there all day long
ULL
1002. westernmob
12:12 AM GMT on August 18, 2007
GFDL has shifted farther south. Better news for here in Mobile, AL. Lets hope the ridge holds and puts it into sparsely populated areas of the Mexican desert
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1001. errantlythought
12:08 AM GMT on August 18, 2007
Not sure where people are getting SW movement, radar out of PR definately points at a roughly 280 movement still.
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1000. ClearH2OFla
8:11 PM EDT on August 17, 2007
hey Storm W how goes it.
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999. Crisis57
12:14 AM GMT on August 18, 2007
nice graphic Tropic
998. paulfrmpasschristian
6:12 PM CST on August 17, 2007
Storm w ..what do you thinK? is gfdl more wrong than the rest??
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997. Tropicnerd13
12:11 AM GMT on August 18, 2007
gfdl has it as a cat 5 soon. but that is the old model run. they need to update the gfdl run video thing.
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996. MysteryMeat
12:05 AM GMT on August 18, 2007
Gotta love how every cone drawn by commenters manage to include New Orleans, even though the official ones, at most, get to the Texas-Louisiana border.
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994. Crisis57
12:11 AM GMT on August 18, 2007
Also Storm i think whats left of Erin is playing a part in that as well
993. cirrocumulus
12:10 AM GMT on August 18, 2007
So much for the GFS on the remnants of Erin. The storm is pulled north rather than west as the GFS forecasted. Remnants of Erin are centered over Reagan County according to Midland/Odessa radar.
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992. Tropicnerd13
12:04 AM GMT on August 18, 2007
ok my models are coming soon. yes, they are just lines on paper. that is what models are. lol. but seriously, i think the modles will change to this soon. now they say 135 mph winds. sounds like rapid intensification is occuring.....
d
again not wishcasting. you can see my prediction getting more true with the recent info. i am not trolling or wishcasting. i am not trying to make anyone feel bad either, or make anyone afraid or scared or worried in any way. just stating what i think on paper.
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991. DonnaGalveston
12:08 AM GMT on August 18, 2007
Thank you StormW for that very detailed answer! Half of it did go over my head though!
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990. Metallica1990
12:10 AM GMT on August 18, 2007
Posted By: Metallica1990 at 10:59 PM GMT on August 17, 2007.

Posted By: ThePainkiller at 10:58 PM GMT on August 17, 2007.

Recon is on its way out to Dean now. It will be intresting to see what they find.

I say CAT 4 winds of 140<
pressure <945


I was close :)
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989. dean2007
12:06 AM GMT on August 18, 2007
The winds are 135mph and it is a category four hurricane the pressure has dropped from 961mb to 946mb thats a 15mb drop in the last three hours.
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988. Weather456
12:09 AM GMT on August 18, 2007
Wow dean maybe the first cat 5 since wilma 2005
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987. Crisis57
12:08 AM GMT on August 18, 2007
Posted By: StormW at 12:07 AM GMT on August 18, 2007.

Posted By: DonnaGalveston at 12:03 AM GMT on August 18, 2007.

On television they say the fact that the storm is slowing in forward motion means it may be about to have a change in course. Is that true?

Most of the time yes...or it means steering currents are getting weak. I feel the slowdown is from that ULL we've been talking of. If you look at water vapor, the big ridge over the U.S. is centered over Texas. The flow on the east sied of it is from NNE TO SSW. Right now, Dean is in a position to where it can't get steered right away by the ridge until it deals with the flow around the ULL. The ridge will have to slide east and repalce that ULL. Go to water vapor, loop it and do a closeup. You should be able to see what I'm talking about.


Thanks Storm lets me know im not going crazy that the ULL isn't moving guys
986. presslord
8:09 PM EDT on August 17, 2007
it appears the wave @ 35w x 14n is doing push ups and taking it's vitamins....
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985. weatherguy03
8:09 PM EDT on August 17, 2007
New blog up! With pretty maps!..LOL Enjoy.
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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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