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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1435. WPBHurricane05
9:53 PM EDT on August 17, 2007
Just checked the GFDL, 195mph winds? And I see that they updated Dean. This storm is crazy.
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1434. KnowYourRole
9:54 PM EDT on August 17, 2007
Posted By: 53rdWeatherRECON at 9:52 PM EDT on August 17, 2007.
This is a CAT 5 storm right now

Actually it's not.
1433. DG136
1:53 AM GMT on August 18, 2007
Posted By: swFLboy at 1:53 AM GMT on August 18, 2007.

Posted By: atmoaggie at 1:51 AM GMT on August 18, 2007.

Also, the shear is low off Africa, anomalously low.

Wow, that's quite low for that region

Wow, theres low shear all the way across..
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1432. littlefish
1:53 AM GMT on August 18, 2007
SJ any thoughts on the southerly convection and possible south low pulling Dean 'down' a bit?
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1431. TexasRiverRat
1:52 AM GMT on August 18, 2007
If Dean comes to texas as even a cat3 we will be in major trouble. The Ground is so wet we will have major trees down after the last 4 months of constant rain. Not a good set up for any wind storm!
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1429. FatPenguin
1:50 AM GMT on August 18, 2007
Posted By: Metallica1990 at 1:49 AM GMT on August 18, 2007.

25 mb in 4 1/2 hours very rapid strengthening


I believe Wilma has the record at 90mb over 12 hours and 98mb over 24 hours??
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1428. H2PV
1:52 AM GMT on August 18, 2007
Posted By: atmoaggie at 12:59 AM GMT on August 18, 2007.
There's no denying the reality, of 40 named storms since January 1st 2007 and 27% of them peaked at, or passed, category 4.

We all have to admit that whole storms went undetected in our history and that from ~35 to ~65, to last active period in the 30 year cycle, we knew next to nothing about the storms, except that the wind blew and the pressure 400 miles to the east was XXXmb

NO storms went undetected in 2007. 11 out of 40 are cat 4 or higher. You are witnessing a Cat-4 on Taiwan which peaked at cat-5 yesterday, and a cat-4 turning into a cat-5 today.

All attempts to obfuscate and deceive have to ignore the reality of 2007 by trying to revise history of yesteryear. The same science which predicted that Sepat would intensify to 170 knots peak gusts (which did happen) has predicted DEAN would be cat 4 by it's present position. This same science makes it possible for you to send your blah-blah-blah at the speed of light through fiber optics, bounced off satellites, relayed by microwaves and conducted on copper wires. That same science SEE GREENHOUSE GASES AND INFRARED EMISSIONS. That's what you are looking at: Greenhouse gases and Infrared Emissions. It's science and it trumps your history lessons. This is your US NAVY picture of greenhouse gases by infrared emissions -- something wrong about your eyes?

SEPAT was cat 5 within last day.

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1424. 53rdWeatherRECON
1:46 AM GMT on August 18, 2007
This is a CAT 5 storm right now. By morning this eye will have shrunk up just like Wilma's and most of the records that wilma set will be brokenLink

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1423. 7544
1:50 AM GMT on August 18, 2007
oh dean is now a cat4 i was told when storms get that strong they start to go where ever they want is this true . hope he dosent start to jog north like he did last night for hours that would scare everyone
Member Since: May 6, 2007 Posts: 0 Comments: 6881
1422. wederwatcher555
1:49 AM GMT on August 18, 2007
the gfs was the leading model in saying taht dean would go to north mexico-south texas. but now with practically every model except the gfdl targetting n.mexico-s.texas, the gfs is way more south now. these models are crazy.
1421. kenscanes
1:43 AM GMT on August 18, 2007
a 43 milibar drop from 11 PM last night (979) to 936 as of 9:30 tonight...scary awesome...
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1419. StormJunkie
1:52 AM GMT on August 18, 2007
Nothing here press. Looks like it mostly stayed S of us
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1418. bluehaze27
1:51 AM GMT on August 18, 2007
The started off with the low at 30w but of course they went straight to Dean.
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1417. ChuckieTodd
8:51 PM CDT on August 17, 2007
Posted By: StuccoMan at 8:39 PM CDT on August 17, 2007.

Did you not read my post.Katrina hit a gambling town hence much more populated and ruptured the levees that flooded new orleans.

Just exactly what do you mean by "hit"?
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1416. SarahFromFLA
1:48 AM GMT on August 18, 2007
If I made a graft of my height showing every year from 1979 until now, you would conclude that I had always been 5.6, nothing ever any shorter. I was 5.6 in 1979 and still am.

Thats my issue with the Arctic ice survey; 28 years is geological time is nothing. It doesnt even seem that much in my own life.
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1412. atmoaggie
1:51 AM GMT on August 18, 2007
Also, the shear is low off Africa, anomalously low.

shear anom
Member Since: August 16, 2007 Posts: 6 Comments: 12463
1411. extreme236
1:50 AM GMT on August 18, 2007
Posted By: bluehaze27 at 1:50 AM GMT on August 18, 2007.

Dean now 145. TWC mentioned the low at 30 west and the CMC has two storms lining up to strike south florida

what did TWC say about the wave at 30W?
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1410. Xion
1:48 AM GMT on August 18, 2007
How much longer can the HH stay out there?

Because at this rate we will have a Cat 5 by very early morning hours.
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1409. littlefish
1:50 AM GMT on August 18, 2007
All this talk about the ULL and the ULH, but the south low may be pulling Dean south from what I'm seeing... Look at the band of convection along there. Interaction I'd say...
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1408. kimpy
1:40 AM GMT on August 18, 2007

They'll use the southbound lanes too beginning two days out to evacuate the entire area. Also, both hwy 37 and 281 will be used. I have full confidence that you won't see in TX what we saw in LA during Katrina.
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1406. comtrader
1:40 AM GMT on August 18, 2007
its interesting to look at the latest GFS and to understand why it is taking Dean due west over the yucatan and on into mexico. the key feature is still the ULL which is currently off the southeast coast of florida. The GFS moves this low thru the gulf and into the texas coast. it does this fast enough so that dean does not get pulled along. if you look out about 60 hours from now, dean will, according to GFS, make its closest pass to the pull of this low, but will still miss its attraction by one or two hundred miles. this explains the huge track differences in the models. if the low tracks farther south thru the gulf or doesnt make it to texas in time then dean will latch on to its coattails and get sucked into the gulf. this is an extreme case of model bifurcation. either dean is going to miss the gulf coast by 500 - 1000 miles or its going to get there. so far the gfs has overestimated the movement of the low, which leads me to think that the gulf coast is by no means out of the woods even though the current model shows a 500 - 1000 mile miss. the key interaction or lack thereof is going to come in about 60 hours.
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1405. coffeecrusader
1:41 AM GMT on August 18, 2007
The only thing that will effect its strength in the short term is the eye replacement cycle. Right now the eye is very small and I am expecting a replacement cyle through the night or early morning. Once this happens and the new eye forms we could be looking at a pressure in the high 800's.
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1403. bluehaze27
1:49 AM GMT on August 18, 2007
Dean now 145. TWC mentioned the low at 30 west and the CMC has two storms lining up to strike south florida.
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1402. atmoaggie
1:49 AM GMT on August 18, 2007
Posted By: bluehaze27 at 1:41 AM GMT on August 18, 2007.

Hello all, while everyone is focused on Dean (and rightfully so) there is another storm about to form around 30 longitude

The TC formation probability has been consistently good:

TC prob
Member Since: August 16, 2007 Posts: 6 Comments: 12463
1401. Metallica1990
1:47 AM GMT on August 18, 2007
25 mb in 4 1/2 hours very rapid strengthening
1400. catjojo
1:48 AM GMT on August 18, 2007
Hey does anyone know the correct answer but I believe katrina was renamed 3 times correct ?
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1399. guerra2100
8:45 PM CDT on August 17, 2007
Babbit, I guess there are never floods in land that's above sea level, huh?
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1398. presslord
9:47 PM EDT on August 17, 2007
Stormjunkie.....y'all got hammered pretty hard tonight, right? Nary a drop out here on the islands tho...
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1397. extreme236
1:47 AM GMT on August 18, 2007
so perhaps by morning we could wake up to a invest in the catl
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1396. louisianaweatherguy
1:45 AM GMT on August 18, 2007
Posted By: boozetraveler at 1:45 AM GMT on August 18, 2007.

Bob Breck is an idiot. He boldly predicted Dean would goes East and miss the US Mainland, a fish storm.

now, now be nice... say nice things... I would bet he's a heck of a lot smarter than you, are you a head meterologist anywhere with an AWESOME history that he has? heck no...

Now before you call Bob Breck an idiot again, I suggest you look at the one in the mirror, first.
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1395. KnowYourRole
9:46 PM EDT on August 17, 2007
Posted By: JFLORIDA at 9:43 PM EDT on August 17, 2007.
Here is the big loop out of PR hold an index card up horizontal and you can see the resumed NW component (small) of its forward motion. (Oh it only shows six if you are not loged in.) - trust me.

the loop I see from PR shows it going almost directly West.
1394. tharsheblow
1:31 AM GMT on August 18, 2007
It's amazing the confluence of perfect conditions for Dean right now. I think it's going to set pressure and wind speed records before too long. If Jamaica gets immolated, that might be good news for Mexico and/or the US. Possible the mountains could really wreak havoc on its circulation.
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1393. StormJunkie
1:46 AM GMT on August 18, 2007
WIH, at that point I am not sure ti matters too much as that tchp still shows temps well above normal and close to 05 temps...Just my take though
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1392. DonnaGalveston
1:46 AM GMT on August 18, 2007
This place is busy. What is the latest and how does Galveston look? I see winds have increased to 145 mph. Very frightening!
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1391. bluehaze27
1:45 AM GMT on August 18, 2007
It looks like it's getting it's act together and another one is about to roll off of Africa.
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1390. moonlightcowboy
1:42 AM GMT on August 18, 2007
The folks here at Tropical Lagniappe have a good grip on what's happening. Check them out, good, smarties with a good outlook on all that's going on right now!

...awareness, preparedness and safety is everything, and I'm gonna stay on that plan for now, instead of casting. It's too serious for playing weatherman!!!!

Please everyone in the potential path, be prepared and help someone else be prepared, too!
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1389. cirrocumulus
1:41 AM GMT on August 18, 2007
This hurricane is developing because of warmer waters induced by global warming.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
1388. StormJunkie
1:44 AM GMT on August 18, 2007
Anyone know why the 18z Ukmet only goes out 48hrs? And has any one seen it anywhere else?
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1387. presslord
9:43 PM EDT on August 17, 2007
Bluehaze....that thing has been bothering me all week....What's its status?
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1386. StuccoMan
8:41 PM CDT on August 17, 2007
Posted By: fldude99 at 8:40 PM CDT on August 17, 2007.
Posted By: StuccoMan at 1:37 AM GMT on August 18, 2007.
Posted By: DG136 at 8:34 PM CDT on August 17, 2007.
Stuccoman, are you in the FL panhandle?

yeah i live in gulf breeze.I was 6 miles from the center of dennis when it cam ashore.3 miles from the coast.Storm was powerful but brief..Not much surge.Seems like smaller compact cyclones have a much smaller surge.Ivan put Tiger point under 8 feet of water.Put okaloosa Island under 6 feet of water and that was over 140 miles from where the center came ashore.

One reason for that is that you were on the west side of the eye of Dennis.

I was in holly by the sea when dennis came ashore.
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1385. WatchingInHouston
1:23 AM GMT on August 18, 2007
StormJunkie - ForecasterColby... or anybody intimately familiar with the models - Do you have any specifics as to how TCHP contributes to intensification?

I created this loop: TCHP LOOP

It shows a considerable depreciation in TCHP values over the past two weeks through the Western Caribbean/Yucatan Strait. With all other variables being equal - any guesstimates (I won't hold you to a hard figure!!) on the difference in potential intensification between the peak on the 13th and what we're seeing now?

Thanks in advance.
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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