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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385. WPBHurricane05
5:27 PM EDT on August 17, 2007
For the short term Dean is right on track. At 5pm they moved the track west since Dean is moving west. It hasn't moved north or south between 11am and 5pm.
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384. GetReal
9:27 PM GMT on August 17, 2007
Just noting an undeniable fact of how the official forecast positions are being posted super imposed upon the satelitte loops. There is no conspiracy....
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383. DonnaGalveston
9:27 PM GMT on August 17, 2007
What would Dean do in Galveston if he hit here? Would we be safe if we just boarded up and hid in the closet? I've never been through a hurricane.
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382. VEROBEACHFL1
9:28 PM GMT on August 17, 2007
just keep watching the little 6 hr updates ---
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381. extreme236
9:28 PM GMT on August 17, 2007
i noticed that the eye is showing up again...
Member Since: August 2, 2007 Posts: 19 Comments: 19234
380. VEROBEACHFL1
9:26 PM GMT on August 17, 2007
EXACTLY as "getreal" posted it-------------
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379. Metallica1990
9:24 PM GMT on August 17, 2007
Posted By: DonnaGalveston at 9:23 PM GMT on August 17, 2007.

Hi I lives in Galveston TX and I have read comments here for over a year without posting. I come home early from work today to find my neighbors boarding up their windows. They tell me they are leaving tomorrow afternoon to beat the rush. Last time evacuating took us about 11 hours to get to Dallas. Should we go ahead and board up and get out of here? There are lines forming in gas lines already. Is it too early to leave? Should we wait? What can you tell me I am new at this?


its way too early to tell just keep an eye on it! right now you should go to the store and get what ever you need for a possible hurricane (canned foods batteries etc.) it would be a good idea now because even if dean misses you whos to say the next one wont
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378. InCozumel
9:27 PM GMT on August 17, 2007
Good afternoon all,

Have been watching Dean grow up since it was a wave. After our experience with Wilma in Cozumel, we are really cautious. Most of the fishing and diving boats have been removed from the water. The lines at the supermarkets and gas stations are quite long. And we still have customers who are visiting here who want to diving and fishing. We have crosses our fingers and toes and waiting but do not want another direct hit from a category 4 or 5 Dean.
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377. OUFan919
9:27 PM GMT on August 17, 2007
DonnaGalveston:
I would be prepared to board up. I wouldn't leave yet, but I would start preperations just in case. You should go out and get all the supplies and gas that you need just in case you need to get out of there.
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375. VEROBEACHFL1
9:26 PM GMT on August 17, 2007
the NHC has changed their track from the LAST track that they had........ now it is slightly more NNW...........doesnt anyone else see that--
---of corse it RIGHT ON TRACK becasue its the more northern UPDATED track.....
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374. TXKiwi
9:24 PM GMT on August 17, 2007
Donna
If you have an easy evacuation place to go to, i.e. I have a house in Lake Charles but live in Houston, so I'm going to Lake Charles for the weekend (so Dean will follow me and spare Houston), then go early.

I would bet money (in fact I did yesterday) that Galveston will be evacuated on Tuesday +- 24 hours. It's not about certainty of landfall, but precautions 3-4 days out as its too late to evacuate once you are certain of landfall.

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373. rwdobson
9:25 PM GMT on August 17, 2007
"The NHC in order to cover it's mistakes, erases the old forecast points, and puts up new forecast points based upon where position of where Dean is at THAT time"

yes, it's a huge conspiracy. especially since the satellite page is not actually maintained by NHC.

the track still has not changed SIGNIFICANTLY.
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372. Melagoo
9:26 PM GMT on August 17, 2007
DEAN looks MEAN!
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371. Drakoen
9:25 PM GMT on August 17, 2007
Posted By: MTJax at 9:24 PM GMT on August 17, 2007.

EULER has some 1800z model runs. Some models have moved right for the early track guidance


Those 18z is the time frame. They are actually the 12z runs set out 6 hours later.
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370. Weather456
9:23 PM GMT on August 17, 2007
A NHC hurricane center official was talking to BBC Caribbean on the radio a while ago and he said that Dean could become a dangerous cat 5 in the west caribbean.
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369. KYhomeboy
9:22 PM GMT on August 17, 2007
I wish I could say that I wasn't nervous..but thats what I certainly am. Right now...the forecast track takes Dean right over Grand Cayman....AGAIN. As if Ivan 3 years ago wasn't enough. I am hoping for some sweet relief and pray Dean takes a slightly more northerly track but it doesn't look like it's going to happen. And even if it did...it would endanger other people lives and property! Sort of a 'no win' situation.
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368. frivolousz22
9:24 PM GMT on August 17, 2007
hurricanedean2007-

why dont you want it to hit the Yucatan?

so its stronger and will kill more people when it hits a populated american area?
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367. floridaeast
9:24 PM GMT on August 17, 2007
TY GET REAL--WELL SAID!
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366. StormJunkie
9:24 PM GMT on August 17, 2007
Links work real well y'all :~)
Member Since: August 17, 2005 Posts: 26 Comments: 16874
365. HurricaneDean2007
5:24 PM EDT on August 17, 2007
TNonsense LMFAO, that picture is OLD OLD OLD OLD
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364. russh46
9:22 PM GMT on August 17, 2007
Bouy link

Link
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363. MTJax
9:24 PM GMT on August 17, 2007
EULER has some 1800z model runs. Some models have moved right for the early track guidance

http://euler.atmos.colostate.edu/~vigh/guidance/
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362. StormJunkie
9:22 PM GMT on August 17, 2007
Hey all, Just a request here, the blog is running fairly slow and we are starting to get a lot of images, some fairly large. Just keep in mind that everyone is not on cable.

Hey mel, good to see you :~)

but, umm that last image you posted is huge, file size wise. Just a heads up.

Back to the Java issue, which I will beat!
Member Since: August 17, 2005 Posts: 26 Comments: 16874
361. DonnaGalveston
9:21 PM GMT on August 17, 2007
Hi I lives in Galveston TX and I have read comments here for over a year without posting. I come home early from work today to find my neighbors boarding up their windows. They tell me they are leaving tomorrow afternoon to beat the rush. Last time evacuating took us about 11 hours to get to Dallas. Should we go ahead and board up and get out of here? There are lines forming in gas lines already. Is it too early to leave? Should we wait? What can you tell me I am new at this?
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360. TropicalNonsense
9:22 PM GMT on August 17, 2007
Posted By: sporteguy03 at 8:14 PM GMT on August 17, 2007.

My met said if ULL does not cross FL all bets are off.


This is what I have been telling people for the last
3 days. The ULL is not co-operating with the forecast.
It appears stronger and is not moving much. The Forecast
should be changed in the next 48 hours if the ULL does
not start moving Northwest. Dean will start to feel it's
pull very soon otherwise and end up north of the forecast.

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359. Drakoen
9:22 PM GMT on August 17, 2007
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358. HurricaneDean2007
5:23 PM EDT on August 17, 2007
LMFAO @ "boobless"!!!!!
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357. GetReal
9:18 PM GMT on August 17, 2007
Posted By: rwdobson at 9:18 PM GMT on August 17, 2007.

Hmm, looks like Dean is right now on the 15th parallel and exactly where NHC predicted it to be. Guys, you gotta learn not to read too much into these little wobbles.


You are mistaken, the track has been to the right of the official track. The NHC in order to cover it's mistakes, erases the old forecast points, and puts up new forecast points based upon where position of where Dean is at THAT time of the new advisory.... That is why there is the illussion that Dean is on the forecasted path.
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356. Weather456
9:21 PM GMT on August 17, 2007
Look at this

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355. Metallica1990
9:21 PM GMT on August 17, 2007
a ver good looking storm indeed it should start looking insane if it continues to strengthen
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353. JLPR
9:19 PM GMT on Agosto 17, 2007
it might be a cat 4 at 8pm with the eye clearing out
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352. StormJunkie
9:20 PM GMT on August 17, 2007
Deans eye will be clear in the next 30 to 45 min...

Already very visible as of the 20:45 frame
Member Since: August 17, 2005 Posts: 26 Comments: 16874
351. Melagoo
9:08 PM GMT on August 17, 2007
Look at Africa making waves

wide view Look at Africa again
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350. DG136
9:20 PM GMT on August 17, 2007
Those cloud tops sure are cooling off, and the eye is trying to clear out. Are there any bouys in the vacinity of Dean? I'd be interested to see what the pressure is doing.
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349. HurricaneDean2007
5:20 PM EDT on August 17, 2007
I wish it would start going WNW, I don't want it to hit the Yucatan.
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347. rwdobson
9:20 PM GMT on August 17, 2007
You can check the status of the model runs here. Looks like the 18z GFS is running now.

Link
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346. Crisis57
9:19 PM GMT on August 17, 2007
classic Hurricane look, Dean is looking awesome look at his banding
wow
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345. WPBHurricane05
5:18 PM EDT on August 17, 2007
The NHC is fairly good when the track comes to 48 hours. Once we get passed that, the error starts to become larger.

Don't focus on the wobbles so hard. You cant make the storm go WNW or NW.
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343. boobless
9:17 PM GMT on August 17, 2007
iceman
Of course the flip side of that coin is the ULL could exert a S-SSW push as they draw nearer. Then we're inline with the consensus track
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342. Weather456
9:17 PM GMT on August 17, 2007
Powerful Dean continues through the Caribbean

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341. wederwatcher555
9:18 PM GMT on August 17, 2007
doesnt the 18z gfs start to come out now?
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340. rwdobson
9:17 PM GMT on August 17, 2007
Hmm, looks like Dean is right now on the 15th parallel and exactly where NHC predicted it to be. Guys, you gotta learn not to read too much into these little wobbles.
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338. StormJunkie
9:13 PM GMT on August 17, 2007
Going to be interesting to see the 18z runs...Only three more hours for them...
Member Since: August 17, 2005 Posts: 26 Comments: 16874
337. nola70119
9:13 PM GMT on August 17, 2007
NHC forecaster Avila says the Bahamas ULL will not in itself steer but may weaken and allow a ridge to form, which (I guess) dictate the western motion after Jamaica. I think they are unsure of what is going to happen really....
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335. HurricaneDean2007
5:15 PM EDT on August 17, 2007
Tell me...when is Dean going to go NW?
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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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