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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985. weatherguy03
8:09 PM EDT on August 17, 2007
New blog up! With pretty maps!..LOL Enjoy.
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984. KYhomeboy
12:06 AM GMT on August 18, 2007
Amystery...excuse me? You didn't mention the Cayman Islands...but Jamaica and the other countires are our neighbours. So yes I do take offence. Hope nothing comes knocking on your door to kill you...but thats just me...I wishcast to preserve life...not to take it. Your clearly young and immature so I will let it go :)
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983. StuccoMan
7:07 PM CDT on August 17, 2007

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Hurricane DEAN Public Advisory
Home Public Adv Fcst/Adv Discussion Wind Probs Maps/Charts Archive

US Watch/Warning

000
WTNT34 KNHC 180004
TCPAT4
BULLETIN
HURRICANE DEAN INTERMEDIATE ADVISORY NUMBER 19A...CORRECTED
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL042007
800 PM AST FRI AUG 17 2007

...CORRECTED MAXIMUM WIND SPEED TO 135 MPH...

...DEAN REACHES CATEGORY FOUR STRENGTH AS ITS CENTER PASSES SOUTH OF
THE VIRGIN ISLANDS...
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980. AnthonyJKenn
12:04 AM GMT on August 18, 2007
Not neccessarily, Donna..remember that Dean was booking it at nearly 25mph earlier, more likely, it's slowing to a more normal rate of speed. 17 mph is still rather quick for a hurricane, especially one that is rapidly intensifying as Dean is.


Anthony
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979. obxrox
11:55 PM GMT on August 17, 2007
latest GFDL shows landfall in vicinity of Freeport, TX with 150+ mph surface winds, Wed 6AM...that's the outlier model. all other model consensus is south.

will be interesting to see how the trends change and models shift. my hope is that all in this storm's path fare well and are safe, wherever it ends up.
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978. extreme236
12:07 AM GMT on August 18, 2007
REPORTS FROM AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT INDICATE
THAT MAXIMUM SUSTAINED WINDS HAVE INCREASED TO AT LEAST 135
MPH...215 KM/HR...WITH HIGHER GUSTS. DEAN IS NOW A CATEGORY FOUR
HURRICANE ON THE SAFFIR-SIMPSON HURRICANE SCALE. SOME ADDITIONAL
STRENGTHENING IS FORECAST DURING THE NEXT 24 HOURS.


Notice how they said "at least" so perhaps they are still finding higher winds
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977. tallahasseecyclone
12:04 AM GMT on August 18, 2007
Seems to be wrapping in a little dry air on the east side.
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976. quakeman55
12:08 AM GMT on August 18, 2007
Ah, okay. I get it now.
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975. Weather456
12:08 AM GMT on August 18, 2007
Dean also slowed to 19 mph
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974. Dan187
12:06 AM GMT on August 18, 2007
REPORTS FROM AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT INDICATE
THAT MAXIMUM SUSTAINED WINDS HAVE INCREASED TO AT LEAST 135
MPH...215 KM/HR...WITH HIGHER GUSTS. DEAN IS NOW A CATEGORY FOUR
HURRICANE ON THE SAFFIR-SIMPSON HURRICANE SCALE. SOME ADDITIONAL
STRENGTHENING IS FORECAST DURING THE NEXT 24 HOURS.



was corrected
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973. Weather456
12:07 AM GMT on August 18, 2007
REPEATING THE 800 PM AST POSITION...14.9 N...65.1 W. MOVEMENT
TOWARD...WEST NEAR 19 MPH. MAXIMUM SUSTAINED WINDS...135 MPH.
MINIMUM CENTRAL PRESSURE...946 MB.
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971. StormJunkie
12:06 AM GMT on August 18, 2007
We are now in the 1km view. Images will update much more often!
Member Since: August 17, 2005 Posts: 26 Comments: 15723
970. scottflorida
12:05 AM GMT on August 18, 2007
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969. VEROBEACHFL1
12:06 AM GMT on August 18, 2007
HI all-- i still think the more northernly it will go--gradually -- any of the models show this? just popped in for a second--watching the marlins and the giants.....
968. quakeman55
12:05 AM GMT on August 18, 2007
Interesting how they say "at least"...that's the first time I've seen them do that. So they know for sure they are actually greater than 130MPH...maybe that's why they are calling it a Cat 4, even though 130MPH is a high-end Cat 3.
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967. zingocat
12:05 AM GMT on August 18, 2007
When I visited Jamica about 10 years ago I asked some locals if they were worried about hurricanes. These young people said no, we are too far south to get them.
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966. extreme236
12:06 AM GMT on August 18, 2007
nvm dean has 135mph winds
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965. Xion
12:04 AM GMT on August 18, 2007
135 mph now.
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964. Weather456
12:04 AM GMT on August 18, 2007
WC saying 135 mph
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963. Crazman
12:06 AM GMT on August 18, 2007
135mph MISPRINT ON NHC
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962. OUFan919
12:05 AM GMT on August 18, 2007
NHC said winds are AT LEAST 130mph...So they could be around 135-140. My taking of what that means.
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961. efallon28
12:06 AM GMT on August 18, 2007
Posted By: cirrocumulus at 12:02 AM GMT on August 18, 2007.



One statistic I found fascinating was that of the previous ten wettest Julys on record in Texas, six had major hurricanes hit the Texas Gulf Coast the same year. Texas had one of the wettest Julys ever this year.


Did you ever think that maybe that's why they were record months? I mean, these storm do drop a lot of rain. Just a thought.
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960. waccamaw16
11:59 PM GMT on August 17, 2007
looks like the ull is not moving on the last couple of loops on the satellite. it could be stopping because of the little frontal boundry thats in s.carolina to alabama and that might be stopping the ull from moving at this time , but that could change later
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959. Skyepony (Mod)
12:05 AM GMT on August 18, 2007
Storm DEAN: Observed by AF #302
Storm #04 In Atlantic Ocean
Total Flights For Storm #04: 05
Date/Time of Recon Report: August 17, 2007 23:32:00 Zulu
Position Of The Center: 14 52 ' N 065 02 ' W (14.87 N 65.03 W)
Minimum Height Measured At Standard Level Of N/A Millibars: 2632 Meters (Normal: N/A Meters)
Maximum Surface Winds Were Estimated At: 119 Knots (136.85 MPH)
Estimated Surface Winds Were Measured At: 006 Nautical Miles (6.9 miles) From Center At Bearing 302
Maximum Flight Level Winds Near Center Were 130 Knots (149.5 MPH) From 048
Maximum Flight Level Winds Were Measured 010 Nautical Miles (11.5 Miles) From Center At Bearing 309
Minimum Pressure: 946 Millibars (27.934 Inches)
Maxium Flight Level Temperature / Pressure Altitude Outside The Eye: 8C (46.4F) / 3045 Meters
Maximum Flight Level Temperature / Pressure Altitude Inside The Eye: 18C (64.4F) / 3045 Meters
Dewpoint Temperature / Sea Surface Temperature Inside The Eye: 13C (55.4F) / NAC (NAF)
Eye Wall Was Characterized As Being: CLOSED
Eye Form Was Characterized As Being: 16
Center Fix Established Using: Penetration Radar Wind Pressure Temperature
Center Fix Established At Level(s): 700 Millibars
Navigational Accuracy Measured At: 0.02 Nautical Miles
Meteorological Accuracy Measured At: 2 Nautical Miles

Other Information:
1: Maximum Flight Level Winds Were 130 KT NW Quadrant at 23:29:00


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958. Dropsonde
11:59 PM GMT on August 17, 2007
Hi StormW. Yes I check your blog whenever I go online and see an update to it. Your views as a met are among those that I put the most confidence in. Congrats on making the featured list.
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957. adrianalynne
12:03 AM GMT on August 18, 2007
If any part of Florida of should be worried, I think its the Panhandle
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956. NASA101
12:04 AM GMT on August 18, 2007
oops...130 mph is still CAT 3. looks like some of us got it wrong..!?
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955. tallahasseecyclone
12:02 AM GMT on August 18, 2007
How much of a pressure drop is this from last check. I think that 130 mph may be a little conservative.
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954. mgreen91
11:54 PM GMT on August 17, 2007
Posted By: Amystery at 11:53 PM GMT on August 17, 2007.

I feel sorry for Jamica, Yucatan and then Mexico...wish it would just die.....but better them then us(sounds bad, but oh well)
THAT WAS VERY MEAN...
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953. WPBHurricane05
8:03 PM EDT on August 17, 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?

That wouldnt be good. The GFDL had Dean slowing down.
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952. TheCaneWhisperer
11:55 PM GMT on August 17, 2007
I feel for them as well Amy! They have been blasted over the past several years. Direct hit on Jamacia, Caymans and Cozumel and Cancun of all places, hurts. Nature of the beast and your chosen destination I guess. Floridians like myself are breathing a sigh of relief and taking a break. Especially when every credible forecaster says your the bullseye this year. Dean is days away from a major strike to landmass, sit back and observe. Take advantage, it's looking to be crazy from here on out. Family in Jamacia is evacuating as we speak and I know K-Man is up to the same.
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951. Tropicnerd13
12:01 AM GMT on August 18, 2007
i still think the storm will go more north than mexico. jmho. i also have the next set of model runs in 2 days or less. that is what they will be.
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950. Weather456
11:54 PM GMT on August 17, 2007
cat 4

REPEATING THE 800 PM AST POSITION...14.9 N...65.1 W. MOVEMENT
TOWARD...WEST NEAR 19 MPH. MAXIMUM SUSTAINED WINDS...130 MPH.
MINIMUM CENTRAL PRESSURE...946 MB.
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949. ThePainkiller
12:02 AM GMT on August 18, 2007
000
URNT12 KNHC 172356
VORTEX DATA MESSAGE AL042007
A. 17/23:32:00Z
B. 14 deg 52 min N
065 deg 02 min W
C. NA mb 2632 m
D. 119 kt
E. 302 deg 006 nm
F. 048 deg 130 kt
G. 309 deg 010 nm
H. 946 mb
I. 8 C/ 3045 m
J. 18 C/ 3045 m
K. 13 C/ NA
L. CLOSED
M. 16
N. 12345/7
O. 0.02 / 2 nm
P. AF302 0504A DEAN OB 03
MAX FL WIND 130 KT NW QUAD 23:29:00



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948. scottflorida
12:02 AM GMT on August 18, 2007
http://i28.photobucket.com/albums/c205/scottflorida/Untitled-2.jpg
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947. WPBHurricane05
8:02 PM EDT on August 17, 2007
000
URNT12 KNHC 172356
VORTEX DATA MESSAGE AL042007
A. 17/23:32:00Z
B. 14 deg 52 min N
065 deg 02 min W
C. NA mb 2632 m
D. 119 kt Surface winds 140mph from Recon!
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946. DonnaGalveston
12:01 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?
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945. Dan187
12:02 AM GMT on August 18, 2007
south a bit, 14.9
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943. Fl30258713
11:59 PM GMT on August 17, 2007
StormJunkie

Here's the link again Link for UKMET.

The 12Z UKMET was headed due west and 18Z to the NW
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942. franck
12:00 AM GMT on August 18, 2007
Ohhhh!! Pinhole eye. Think Dean's gonna do the Gilbert thing..except maybe on Brownsville?
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941. sngalla
8:01 PM EDT on August 17, 2007
NHC 8pm AT 800 PM AST...0000Z...THE CENTER OF HURRICANE DEAN WAS LOCATED
NEAR LATITUDE 14.9 NORTH...LONGITUDE 65.1 WEST OR ABOUT 800 MILES...
1290 KM...EAST-SOUTHEAST OF KINGSTON JAMAICA AND ABOUT 255 MILES...
410 KM...SOUTH-SOUTHEAST OF SAN JUAN PUERTO RICO.

DEAN IS MOVING TOWARD THE WEST NEAR 19 MPH...31 KM/HR. THIS
MOTION IS EXPECTED TO CONTINUE THROUGH SATURDAY WITH A GRADUAL
DECREASE IN FORWARD SPEED. ON THIS TRACK...THE CORE OF THE
HURRICANE WILL BE MOVING WELL SOUTH OF PUERTO RICO TONIGHT.

REPORTS FROM AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT INDICATE
THAT MAXIMUM SUSTAINED WINDS HAVE INCREASED TO AT LEAST 130
MPH...215 KM/HR...WITH HIGHER GUSTS. DEAN IS NOW A CATEGORY FOUR
HURRICANE ON THE SAFFIR-SIMPSON HURRICANE SCALE. SOME ADDITIONAL
STRENGTHENING IS FORECAST DURING THE NEXT 24 HOURS.
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940. Metallica1990
12:01 AM GMT on August 18, 2007
CAT 4 I was close with the pressure :)
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939. Melagoo
12:02 AM GMT on August 18, 2007
Charleston TV said... hits Mexico oh well...
guess what.. we are going to pay for those illegals anyway..


I always find it hidious when you think back a few hundred years and how we took everything from the native people and pretty much slaughtered them ... they lived here for 1,000s of years.
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938. adrianalynne
12:00 AM GMT on August 18, 2007
Florida seems to be out of the woods
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937. NASA101
12:00 AM GMT on August 18, 2007
Officially a CAT 4 now!!
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936. samspade
11:58 PM GMT on August 17, 2007
Newbie posting/not an expert - not in Houston area, but used to live there and have family there.

This latest GFDL run is not headed for Houston, but rather just east of Matagorda Bay about 50-100 miles from Galveston. Kind of reminds me of where Carla hit. Wouldn't be too bad for Galveston/Freeport, though they'd probably still get Cat 1/2 force storm surge etc, since they'd be on the bad side of the storm.

Actually, in the scheme of things, that's not the worst place for it to hit. Matagorda Bay is sparsely populated, with the exception of Victoria (~50,000) which is about 50 miles off the coast. The only better place for it to hit to keep loss of life/property down is the King Ranch/Kenedy County in between Brownsville and Corpus Christi where the cows live.

Hitting Central America/Mexico would be terrible for loss of life - worse than the US. Terrible to hear someone say that.
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935. OUFan919
12:00 AM GMT on August 18, 2007
AT LEAST 130mph winds
946mb pressure

Category 4
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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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