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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

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
Hurricane Dean near the island Puerto Rico
the ciclonic surge hard mind in Salinas,Puerto Rico mines the Community Las Ochenta in the south of Puerto Rico

Climate Change

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

08er, I could not tell a lot from the radar, but from current sat, and MIMIC imagery that is 3hrs old, it looks that way to me.

If any of you have not seen MIMIC imagery, you really need to check it out. A type of Microwave morphed imagery. Really cool. Only problem is it only updates every 6-12hrs I think.
8 hours plus of WNW/NW and counting.
It's probably time to take a look at the 384 hour GFS that has a hurricane going into Louisiana!
Here's mine (again)
The ULL or TUTT Low (Tropical Upper Tropospheric Trough) should not be much of a factor in the future movement of Dean.

If you believe the non-tropical models this TUTT is really evident only at the 300mb level and above (approx 30,000 ft) and would not exert much of a steering influence at that height.
If anything, it may add some VWS (vertical wind shear) into the equation and limit or reduce storm intensity farther down the track.

Then again, it could aid outflow across the top of the storm as well.

That said, we're down to a persistence track and the model consensus appears logical.
If the Canadian model is right Dean aint the end of it. Wasnt the GFS similiar a couple days ago? Jamming
cchs- you forgot to mention if the ULL is over Texas/Mexico mainland and out of picture. The scenario i think the models choose.
MN, I am all confused by this ULL and have not really been watching/keeping up with it.

I am going with "Blog Consensus Model" for now...Wunder if we could market that...lol Anwyho for now the BCM's forecast for the ULL is "who the hell knows, but there is more Nward motion then was expected..."
SJ- ya MIMIC makes for cool animation loops.
3510. Loki240
New Blog!
i pretty much think louisiana is in the clear time to move on wishcasters maybe next storm...sorry guys...and i'm not trying to be sassy cuz i dont have a problem with wishcasters they just people that really love the weather and want to be where its happening
Isn't it true that the stronger a hurricane gets, the more sensitive it becomes to steering currents in all levels of the atmosphere?

Is that true or not? I heard that from somebody on this blog a week ago...
3514. centex
I'm follwing the wobbles about 3 loops in last 6 hours. But the move to WNW is not the wobbles.
the last couple of runs of the GFS had the low going almost due west across the gulf, and staying close to shore and being pretty weak. the latest model runs of some of the other models show it dipping much farther south and strengthening. the early gfs runs had the high building back in after the low crossed into tx, and pushing dean down into mexico. while this may still happen, it looks to me as if the odds of it occuring are getting lower. right about now i would be getting queasy if i was short gasoline futures.
LOL SJ - The BCM - think FSU can incorporate that on their experimental page?

I started looking at the ULL after I got back home last night because it was drawing from as far SW as the W Caribbean. I also notice that (at the time) Dean was moving W roughly 2x the speed at which the ULL was moving W.

Now, with the apparent change in motion of Dean, I'm wondering if they might not miss each other. The ULL maintains it's "forward motion" at roughly the same speed as last night. But, Dean's "westward motion" has been reduced by roughly 20-30% with this more northerly component to it's motion.

Bottom line - wait and see, I guess...
3518. sammo
New blog.
It's kind of funny how everyone is paying attention to the wobbles. No TS/hurricane runs in a perfect straight line (that I ever seen), it will wobble and bounce a little.

Also for the formula for knots to mph..multiply the knots you have by 1.15 and round off..Viola.
3520. marker
Tropical forecast points on loops available on WU show Dean taking a straight path toward Mex. A straight path rarely happens. Anyone in a 2000 mile stretch of GOM coast should pay attention. Best scenario is Dean runs toward ULL and gets sheared to a smaller weaker storm.

This is Mother nature - anyhing can happen !
this will be a very good radar to monitor in the next hours.
What do you think would cause the ridge that is supposed to build over Florida to weaken? Some of you are throwing that possibility out there and wanted to know what would cause this and what are the possibilities of it happening.
3523. bobcane
Last night using radar returns the direction shift was noted to 303 deg WNW. In the last 30 minutes returns have showed a slightly westward return to 287 deg, 17 deg N of W.

According to Dr Steve all the models are shifting south to a mexico landfall.
3525. Dan187
URNT12 KNHC 181536
A. 18/15:02:00Z
B. 15 deg 39 min N
068 deg 43 min W
C. 700 mb 2511 m
D. 80 kt
E. 196 deg 5 nm
F. 281 deg 091 kt
G. 194 deg 005 nm
H. 930 mb
I. 11 C/ 3051 m
J. 21 C/ 3038 m
K. 10 C/ NA
M. C11
N. 12345/7
O. 0.02 / 2 nm
P. AF304 0604A DEAN OB 15
MAX FL WIND 92 KT SW QUAD 15:00:30 Z
sure looks like it is trending to a more northerly track - maybe just a big wobble.

As I understand it, you are correct, the larger the storm the more you have to move higher up in the atmosphere to determine a mean (as in average flow).

But a TUTT low or ULL is at/near the top of the atmosphere All or most of the TS's are below it.
Sometimes these ULL's can extend down around the 500mb level and can add some steering but this one does not appear to.
The lastest GFS global shows 40-50knot northeasterly winds coming from around the top of the ULL into the projected track of Dean.

yawwwwn GM ALL :)
track mark
17.2,77 C5/H/D
18,81 C4/H/D

Where does this northerly track model come from?

I know it could be a fluke that may correct itself.
whats left of erin runs nne ull rushes in to south texas to fill in the void dean follows up keep yer heads up now boys
Good Morning folks,
Dean is a very impressive entity. A beautiful storm in terms of Mother Nature's wonders, alas, not so beautiful in terms of the effects of life and property for those in his path. As far as his path and the NHC forcasting at this point, I have been plotting and tracking their forcasts from day one, and I must admit,they have nailed it on the money. The nexy few days are an almost given to be correct also: with a margin of error not so big. With over 100 Weather Professionals working there and so many up-to-date forcast technologies at their finger tips, I would give the NHC much credit in those forcast tracks we are seeing. After day 3-5, the margin of error obviously widens, but, I would still have to validate their expertise as the best one to follow in the days ahead. Just my opinion, BUT, as they say always, Mother Nature can be fickle, so be wise and prudent, and ready if need be, ciao for now, Rich
3533. pt100
Hi, look at the CMC forecast and see what it predicts just north of Cuba! CMC
Thatt ULL seems to be moving into the eastern gulf now and staying ahead of Dean. That is a good thing, right? I mean that would keep Dean on the Westward movement, right?
Does a fast moving hurricane have a greater or lesser chance of making a northerly turn, when it is recurving, than a slower moving one and why?I have heard it explained both ways on TV weather.