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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3485. hurrycain
2:54 PM GMT on August 18, 2007
BuffaloSnow77....TWC had also hedged their bets with buying up stocks for roofers in south Texas.
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3484. StormJunkie
3:25 PM GMT on August 18, 2007
08er, yu have a link to the radar?
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3483. atmoaggie
3:26 PM GMT on August 18, 2007
Posted By: weatherboykris at 3:25 PM GMT on August 18, 2007.

atmoaggie....CSU doesn't make that map.


right, RAMMB in CIRA at CSU, staffed by CSU folks, sorry.
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3481. littlefish
3:22 PM GMT on August 18, 2007
Agreed SJ, WNW and N of track slightly. The GFS model takes this track and still parks it in Mexico. Interesting to see the spewing of 'atmosphere' off west coast Africa into CATL in a WNW direction. Almost looks like it is 'pushing' the CATL ULL toward the Dean ULH and in turn Dean ULH pushing ULL all westward. Just my untraind eye watching the force of that African wave that evaporated. I don't see s surface circ any more with the LB from yesterday now aorund 17N 38W. I think it fell apart.
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3480. weatherguru
3:25 PM GMT on August 18, 2007
he north of the track right now..its probably just a wobble
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3479. Masquer08er
3:23 PM GMT on August 18, 2007
I just have a feeling that theres more to it then meets the eye.........

Pun intended.

I don't jump at "wobbles". This has been constant all morning (at least 6 hours).
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3478. FEEDERBAND
3:20 PM GMT on August 18, 2007
actually steve lyons said that if they moved the track at all they would probalby move it further sout. hard to believe for me but who knows.
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3477. finnadat
3:22 PM GMT on August 18, 2007
you can also use google calc for knots in mph - search "137 knots in mph" or whatever and it'll tell you - google is a good thing..
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3476. weatherboykris
3:24 PM GMT on August 18, 2007
atmoaggie....CSU doesn't make that map.
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3474. atmoaggie
3:24 PM GMT on August 18, 2007
So far this season, these CSU guys have pretty well called it when formation probability spikes. The last 2 spikes resulted in systems and we are now exceeding the probability present when the previous systems formed.

prob time series
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3473. weatherboykris
3:21 PM GMT on August 18, 2007
Just looked at the dropsonde in the eye...had 930mb with 8kt surface winds.So surface minimum pressure's around 927-929.
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3472. MrNiceville
3:21 PM GMT on August 18, 2007
Morning all - just popped in for a moment - busy day...

What's the scoop? Is the current motion a change in direction, or some sort of wobble? With ULLs out of range to the NE and NW of the storm, I assume that they are not affecting it.
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3471. StormJunkie
3:21 PM GMT on August 18, 2007
wbk, that would be somewhat expected if Dean is starting to under go a EWRC. There is usually some weakening associated with that.
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3470. fldoughboy
3:11 PM GMT on August 18, 2007
I still think Florida is safe. I don't recall any models pointing to Florida. It would literally have to cut at least 90 degrees to hit us from the SW point of the GOM.
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3469. Labayourambler
3:22 PM GMT on August 18, 2007
I noticed a more northward jog with Dean. Is this a temporary job or one that stay permanent and could make go further east toward La.?
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3468. Masquer08er
3:21 PM GMT on August 18, 2007
SJ, Are those concentric eye walls on the PR radar? Thanks
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3467. centex
3:19 PM GMT on August 18, 2007
Is the change to more WNW on time with the models? It had been moving almost due W. The timing and degree and these changes is what I'm concentrating on.
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3466. weatherboyfsu
3:14 PM GMT on August 18, 2007
Just from past experience...........I seem to be picking up on a unsettleness from all the communication processes(NHC, TWC,etc)....I know we are dealing with a major hurricane but it just seems like there is alot of uncertainty behind the scenes......just something that i am sensing.....I just have a feeling that theres more to it then meets the eye.........
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3465. CycloneBoz
3:17 PM GMT on August 18, 2007
Thanks BtnTx.

I made the decision to hang out in Corpus Christi for Erin. I asked here and everyone who responded pretty much agreed.

Doug put me on the exact coordinates at landfall since I was busy driving.

I'll have to be careful about getting too close to the beach next week for Dean when it makes landfall. Had a storm the size of Dean hit Padre Island on Thursday, that entire part of the island would have been underwater.

I think I'll stay somewhat inland for Dean.
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3464. Melagoo
3:21 PM GMT on August 18, 2007
Conversion table for
knots to miles per hour
KTS to MPH

5 Knots = 5.8 MPH
10 Knots = 11.5 MPH
15 Knots = 17.3 MPH
20 Knots = 23.0 MPH
25 Knots = 28.8 MPH
30 Knots = 34.6 MPH
35 Knots = 40.3 MPH
40 Knots = 46.1 MPH
45 Knots = 51.8 MPH
50 Knots = 57.6 MPH
55 Knots = 63.4 MPH
60 Knots = 69.1 MPH
65 Knots = 74.9 MPH
70 Knots = 80.6 MPH
75 Knots = 86.4 MPH
80 Knots = 92.2 MPH
85 Knots = 97.9 MPH
90 Knots = 103.7 MPH
95 Knots = 109.4 MPH
100 Knots = 115.2 MPH
105 Knots = 121.0 MPH
110 Knots = 126.7 MPH
115 Knots = 132.5 MPH
120 Knots = 138.2 MPH
125 Knots = 144.0 MPH
130 Knots = 149.8 MPH
135 Knots = 155.5 MPH
140 Knots = 161.3 MPH
145 Knots = 167.0 MPH
150 Knots = 172.8 MPH
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3462. finnadat
3:17 PM GMT on August 18, 2007
i think we are looking at an allen analogue - http://www.nhc.noaa.gov/archive/storm_wallets/atlantic/atl1980-prelim/allen/prelim02.gif
i would like to see the september analogues and cat 1 and 2s in the area as well, but given the atmospheric conditions and upper level currents i'm hoping it is like allen. hits kingsville where nobody lives and looses steam before hitting
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3460. weatherboykris
3:18 PM GMT on August 18, 2007
Dan....that is the highest 10 second wind,so essentially a flight level gust of 138,not a sustained wind.
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3458. weatherboykris
3:18 PM GMT on August 18, 2007
Recon just went through the eye...extrapolated 924mb.
Oddly enough...the SFMR only had 115kt surface winds on the NE eyewall.
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3457. A4Guy
3:12 PM GMT on August 18, 2007
Drak - good advice about not paying attention to the wobbles. Many on here make it seem like the models can't even predict what will happen in the next 12 hours...and that the NHC mets (some of the best in the world) can't figure it out either!! lol.

Now having said that...I remember models and NHC keeping Charley moving towards Tampa...and we clearly saw the bend away from the track that put him right into Punta Gorda.
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3456. StormJunkie
3:14 PM GMT on August 18, 2007
This movement could be good for Jamaica, not so great for DR, Haiti, and Cuba. It is pretty well N of the forecast points, and still moving that way WNW with a little more N in it.
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3455. Dan187
3:16 PM GMT on August 18, 2007
HH just went through the eye

150530 1546N 06836W 6962 02749 9567 +118 +118 143135 138 110 008 00

max flight winds found 138 kts.
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3454. nola70119
3:11 PM GMT on August 18, 2007
No one living on the Gulf Coast is going to make a decision based on a 5 day computer model.....if you don't like here its all theoretical.
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3453. Masquer08er
3:15 PM GMT on August 18, 2007
LAWXguy, If that trend continues, it would put the eye within 70 mi of Key West.
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3452. scwindsaloft
3:15 PM GMT on August 18, 2007
The ULL over Fla seems to be on track..a slow wastward movement...thats wht the NHC holds to their forecast.
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3451. boobless
2:50 PM GMT on August 18, 2007
WAG time 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.

Still see some model inconsistencies in the location of the western periphery of the ridge next week so the possibility of some small changes to the right of consensus could occur.
OR
just might be my evil-inner-wishcaster-self rearing its ugly head

(nuttin like a 2-3 day adrenaline rush just prior to and during a landfall in your neck of the woods)

Course the 2-3 or more weeks of primitve conditions that follow really suck.



I had guessed mid-upper TX coast on Friday next a couple of days ago as landfall/time.


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3449. atmoaggie
3:13 PM GMT on August 18, 2007
The same product. I've been watching this...the moderately favorable area has been growing and growing with each update.

TC prob
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3447. weatherboykris
3:15 PM GMT on August 18, 2007
Oddly enough...the SFMR only had 115kt surface winds on the NE eyewall.
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3445. louisianaweatherguy
3:13 PM GMT on August 18, 2007
For 7 hours now, Dean has shown a reall track change towards the WNW with some wobbles to the NW...

If this motion continues for the rest of today, expect the models to shift towards Central Texas Coast to extreme SW Louisiana...

THIS is why New Orleans needs to get their ducks in a row this weekend JUST IN CASE...
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3444. weatherboykris
3:14 PM GMT on August 18, 2007
Recon just went through the eye...extrapolated 924mb.
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3443. weatherboyfsu
3:13 PM GMT on August 18, 2007
No GOM state is ruled out completely..........Just the chances are lowered..............
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3442. weatherboykris
3:11 PM GMT on August 18, 2007
Beentherinmiami....they are basically just little circular wobbles that the eye goes around as it moves along it's path.They have something to do with the spin created by the winds,and only happen in strong storms.Not relevant to the long term track,but at landfall it can be the difference between going through the eyewall or a rainband.
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3440. CajunSubbie
3:07 PM GMT on August 18, 2007
this blog & nhc saw katrina aiming for nola , 24 hrs before they changed that forcast.
get your news here.. cause it happens here first.
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3439. i12BNEi
3:12 PM GMT on August 18, 2007
I don't see how any gulf states can be ruled out.
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3438. StormJunkie
3:09 PM GMT on August 18, 2007
CMC is known for its cyclogenesis hyperactivity

lol, now that is putting it a little mildly don't yo think lf ;~)
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3437. BuffaloSnow77
3:02 PM GMT on August 18, 2007
Link If Dean continues on this course, without moving any more N than this track, it hits N. Texas .... extrapolate it. This would bring anything East of N. Texas to the FL. Panhandle into play
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3436. Meteorologist1969
3:09 PM GMT on August 18, 2007
Drakoen, Point take, however it hints at the Tropics remaining active.
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3435. scaldisnoel
3:09 PM GMT on August 18, 2007
cyclone boz,

I may be alone in this, but I would appreciate it if you provided links to the video, not embed the video in your post. It just eats up more bandwidth and makes it take longer to refresh.

Thanks
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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.