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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1135. charlesimages
12:49 AM GMT on August 18, 2007
Hello everyone!

I though I would put in my full forecast of what I think for Dean. Now a powerful CAT 4 storm.. I predict that Dean will strengthen into a dangerous CAT 5 storm, and make landfall within 20 miles both ways of Galveston, TX, and should make landfall as a strong CAT 3 storm. By the time it reaches Houston, TX it should be downgraded to high end CAT 2, still very dangerous. Just adding my 2 cents worth!!

Good luck!

My forecaster is the CAR - GF (GF = Gut Feeling lol this is my very own track plot map. thanks to Microsoft Paint!)

DEAN...MY FORECAST
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1134. DonnaGalveston
12:48 AM GMT on August 18, 2007
I am going to take a break from this for a bit. I will check back though for more information. You all are so wonderful and I have gained great information while reading here. Thank you so much!
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1133. extreme236
12:48 AM GMT on August 18, 2007
Posted By: Xion at 12:47 AM GMT on August 18, 2007.

The ADT (which has constantly underestimated Dean) has just increased T# by .3!!!!!!!!!!!!!!!!!!

CI# /Pressure/ Vmax
5.6 / 960.4mb/104.6kt


Well, the ADT is getting closer lol
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1132. paulfrmpasschristian
6:47 PM CST on August 17, 2007
SABEN wunderground..all models shifting South!
Timber ridge here
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1130. bethie
12:43 AM GMT on August 18, 2007
The I-10 construction at Tallahassee will bottleneck traffic coming from the west.
That construction will continue through 2008.
Please beware. I drive it daily and it is terrible.
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1129. melwerle
12:46 AM GMT on August 18, 2007
Evening Storm! Thank you for the updates and all your help! you are da man....
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1128. ClearH2OFla
8:45 PM EDT on August 17, 2007
Well all gonna check out some fantasy football news. For me its about the High and ULL. I dont care what the models are saying at this point.
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1127. Fl30258713
12:42 AM GMT on August 18, 2007
jamnkats

Might curb you expletives, young people and little old ladies blog in here. I understand the frustration, but, well, you know. Be nice.
Member Since: July 24, 2006 Posts: 1 Comments: 987
1126. obxrox
12:43 AM GMT on August 18, 2007
cat 5 history to consider:

23 cat 5's between 1923-1998, or about 1 every 3 years

dean will be the 7th cat 5 since 2003, or about 1.5 per YEAR (and we're not done with this year)

what could be behind this increase in intense hurricanes?.....


Questions: Why did you omit 1999 through 2002? What does it do to your averages if you factor those years into either side of your comparison?


sure, no problem...for the sake of discussion add 99-02 to the most recent period. that gives an average of .63 cat 5's per year over the past 11 years versus .32 cat 5's per year over the previous 70 yr period. looks like an increased trend to me. of course the ranges can be adjusted and so forth....
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1125. Xion
12:44 AM GMT on August 18, 2007
The ADT (which has constantly underestimated Dean) has just increased T# by .3!!!!!!!!!!!!!!!!!!

CI# /Pressure/ Vmax
5.6 / 960.4mb/104.6kt
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1124. SaBenDa
12:44 AM GMT on August 18, 2007
Paul from Pass Christian

Just finished the house...Off Minge...

Where are you getting your GFDL run?

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1123. Crazman
12:45 AM GMT on August 18, 2007
Strong front moving south over Atlanta as we speak, I wonder what that will do to the ULL and High and Even Dean in the long term, this from is coupled with a back front over chicago moving a similar path almost due south to the gulf...
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1122. cirrocumulus
12:43 AM GMT on August 18, 2007
I think it is awesome that Dr. Masters is posting on the arctic ice as this ties in with Dean. Thank you Dr. Masters! A good book called Laboratory Earth by Stephen Schneider explains different variables.
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1121. weatherguru
12:46 AM GMT on August 18, 2007
A tropical wave along 42 west is moving west at a pace of 7 degrees longitude per day and has no organizational features. A tropical wave along 34 west has a low pressure area near 14 north. This wave is moving west at 7-8 degrees longitude per day. Satellite images and computer model information suggest this feature might become an organized tropical system over the weekend or early next week
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1120. KRL
12:44 AM GMT on August 18, 2007
Wel, I agree Mel, warnings do give you anxiety. Every state has its pluses and minuses. I got used to the humity in FL after a a few months. Still in my opinion the best state to live in in the US right now. But everyone has different things they like and dislike of course. I just love the tropical weather here.
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1119. Bootsie1
7:40 PM CDT on August 17, 2007
I live in MS is it to early to start getting ready? All the weather people say its heading for TX. I hope they are right.
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1118. Metallica1990
12:44 AM GMT on August 18, 2007
are the hunters outta dean now?
and does anyone have a good site on there recon flights
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1116. louisianaweatherguy
12:42 AM GMT on August 18, 2007
Dean is currently directly on a WEST movement... no wobblesin the last few hours

If Dean moves more to the WNW or NW tonight, the models will all shift to the north...
Member Since: July 9, 2007 Posts: 0 Comments: 692
1115. extreme236
12:44 AM GMT on August 18, 2007
Posted By: ClearH2OFla at 12:40 AM GMT on August 18, 2007.

I wouldnt trust CNN


why? they said the whole truth when i watched them. they said the path and the forecasted strength
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1113. Xion
12:43 AM GMT on August 18, 2007
I wouldnt trust CNN


I like Chad Meyers though. He seems to know what he's talking about and was able to point out the GDFL track as one of the most reliable when it pointed to NOLA in its earlier run.

That has luckily changed.
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1112. weatherguru
12:43 AM GMT on August 18, 2007
hey everyone...food for thought.....this is from accuweather...



A tropical wave along 42 west is moving west at a pace of 7 degrees longitude per day and has no organizational features. A tropical wave along 34 west has a low pressure area near 14 north. This wave is moving west at 7-8 degrees longitude per day. Satellite images and computer model information suggest this feature might become an organized tropical system over the weekend or early next week
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1111. tallahasseecyclone
12:38 AM GMT on August 18, 2007
Does anyone know how crazy the I-10 Corridor in North FL will become if a hurricane strikes to the west? I just moved to FL from Wyoming and drive a route truck for a local beverage company. Increased traffic on 10 will make my life a living hell and I won't get much money for my efforts.
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1110. melwerle
12:40 AM GMT on August 18, 2007
krl - i was living in CA during the northridge quake (double quake if i remember...). Woke up and ran outside in my undies and tshirt after being out all night. finally went back to sleep only to run outside again about 2 hours later. No one sat near the overhead fans in the morning at church. I moved to the east coast and thought it would be easier to live in a place that had warning but I'm finding the warning makes me more nervous - I have days to prepare but the thought of losing my home and such is frightening. I miss the weather back on the west coast, don't miss the quakes but not fond of the stuff here either (although I do love the thunder and lightning!)
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1108. SaBenDa
12:42 AM GMT on August 18, 2007
Anyone know if/how to get shuttle pics of the storm...They are still up there..
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1107. kmanislander
12:39 AM GMT on August 18, 2007
jamnkats

Are you in Jamaica ?
Looks like the NW Caribbean is in the bulls eye for now. But, a lot can change before Sun Night/ Mon morning.

I am still hopeful that we will be spared a direct strike.
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1106. paulfrmpasschristian
6:43 PM CST on August 17, 2007
GFDL has moved back from LA to MID TX
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1105. watchingnva
12:40 AM GMT on August 18, 2007
to look at this thing and imagine it has all night during the max to do its thing.....I can just imagine what im going to wake up to....i have a feeling tonight is one of deans many bottom drop nights he will have over the next week....I have a feeling there will be a couple nights like tonight...we will watch and see...
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1103. bbesse
12:40 AM GMT on August 18, 2007
Yea lefty, the next several sets of model runs should be very interesting. Either the GFDL has to give, or everything else does.

Agreed. The proof will be in the pudding for the GFDL here soon. I'm hoping the GFDL was waaaaay off when it predicted the middle of LA.

We will know soon enough.
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1102. cirrocumulus
12:39 AM GMT on August 18, 2007
Dewpoint in Lubbock at a yearly high of 69 under the dominance of warm air and former tropical storm Erin.
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1101. ClearH2OFla
8:40 PM EDT on August 17, 2007
I wouldnt trust CNN
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1100. Fl30258713
12:33 AM GMT on August 18, 2007
Sure are a bunch of folks speaking of forecast tracks in absolutes.

I don't think we will have that much confidence until Sunday for what ever path Dean takes.

If the decrease in forward speed continues there may not be landfall until later next week.

I'm all for folks being prepared but don't panic yet. I'm waiting to see how the trends for Dean play out, especially with the variables in its path and lack of confidence, right now, in forecast track.

Nothing is set in stone.
Member Since: July 24, 2006 Posts: 1 Comments: 987
1099. KRL
12:40 AM GMT on August 18, 2007
NRL Pic - Great shot of Dean's eye.

NRL Pic Link
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1098. howarjo1943
12:35 AM GMT on August 18, 2007
The high isn't THAT weak. Dean has been speeding along at 20+mph and has recently moved in a more due west and at times slightly south of due west motion. However, Dean has decreased a little in forward speed from earlier today and wobbled a little to the north on the last frame so the high might be weakening now.
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1097. earthlydragonfly
8:40 PM EDT on August 17, 2007
Dean has slowed down over 20% in the last 24 hrs
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1096. DonnaGalveston
12:39 AM GMT on August 18, 2007
TexasWeatherNut, here in Galveston lines are forming at gas stations and grocery stores. Several in my area have already begun to board up their homes and businesses.
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1095. bbesse
12:39 AM GMT on August 18, 2007
Is there any chance that Dean might slow down?

Man, I hope not. 19 mph is cruising. It can just keep on moving right by!
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1094. StormJunkie
12:35 AM GMT on August 18, 2007
Interesting thoughts Skye!

Yea lefty, the next several sets of model runs should be very interesting. Either the GFDL has to give, or everything else does.
Member Since: August 17, 2005 Posts: 26 Comments: 16874
1093. katadman
12:26 AM GMT on August 18, 2007
Posted By: obxrox at 12:25 AM GMT on August 18, 2007.

cat 5 history to consider:

23 cat 5's between 1923-1998, or about 1 every 3 years

dean will be the 7th cat 5 since 2003, or about 1.5 per YEAR (and we're not done with this year)

what could be behind this increase in intense hurricanes?.....


Questions: Why did you omit 1999 through 2002? What does it do to your averages if you factor those years into either side of your comparison?
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1091. thelmores
12:36 AM GMT on August 18, 2007


Ih ope this scenerio does not play out...... 18z GFDL!
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1090. TexasWeatherNut
12:27 AM GMT on August 18, 2007
Thanks Dr. M for an excellent discussion.

Folks here in Houston are indeed becoming a bit tense -- the network of electronic signs along the major Houston freeways are flashing a message stating that a major hurricane is entering the Gulf and advises us to keep our gas tanks filled! With the Rita evacuation fiasco still fresh in our memories, many of my friends say they will NOT evacuate, no matter what the city officials say.
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1087. Metallica1990
12:37 AM GMT on August 18, 2007
what about allen he became a cat 5 3 times and reached min. pressure on its second time
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1086. mgreen91
12:36 AM GMT on August 18, 2007
Is there any chance that Dean might slow down?
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1085. DonnaGalveston
12:34 AM GMT on August 18, 2007
deadmandancing, I wish I could get my mind off of it, but I am trying to stay informed and learn as much as possible. I am trying not to freak out about it. Even if I go out everyone around town is talking about it already.
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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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