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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135. MichaelSTL
3:17 PM CDT on August 17, 2007
What ax are you grinding?....Do you not see that this is a significant issue, whether relative to 28 yrs ago or many more?...What truly matters is how this impacts the future, not how it stacks up against positions based on interpretations of the past.

That is correct; and what is happening now has nothing to do with whatever happened before humans or modern society existed (and in the past things like changes in the orbit of the earth and continental drift caused climate change).
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134. waccamaw16
8:16 PM GMT on August 17, 2007
also looking at the visible loop with the forecast points on it ,it shows dean at 15n at the next point of where it will be and it seems it is already at 15n.
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133. C2News
4:18 PM EDT on August 17, 2007
Posted By: Relix at 4:18 PM EDT on August 17, 2007.
Hmm.. is Dean still going WNW? It's definitely higher than the expected track.


I believe so, according to the wunderground flash tracker
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132. extreme236
4:18 PM EDT on August 17, 2007
it appears dean is moving west but not due west, so perhaps a bit north of due west
Member Since: August 2, 2007 Posts: 19 Comments: 19234
131. Randyman
8:13 PM GMT on August 17, 2007
Posted By: Michael at 8:12 PM GMT on August 17, 2007.

Right but one thing is being left out...a big anticylone is building over Dean and that can overpower the ULL, that's what my met. thats what he told me confidently.


You can see it building on the link I provided...

Posted By: Crisis57 at 8:12 PM GMT on August 17, 2007.

Posted By: Randyman at 8:04 PM GMT on August 17, 2007.

Dean beginning to dictate the environment around him...

Link

look at the last frame of that loop and see the distance between Dean and the trouf


Yea, it should be interesting the next 48 hours...I'm sure we will all learn something new this forthcoming week...


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130. Relix
8:17 PM GMT on August 17, 2007
Hmm.. is Dean still going WNW? It's definitely higher than the expected track.
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129. C2News
4:15 PM EDT on August 17, 2007

Posted By: sporteguy03 at 4:14 PM EDT on August 17, 2007.
My met said if ULL does not cross FL all bets are off.


All bets are off of what?
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128. SherryB
8:15 PM GMT on August 17, 2007
Any time frame on when we might see (or not see) the effects of the ULL on Dean? Learning here....
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127. staggalee
8:13 PM GMT on August 17, 2007
The ULL on all models is forecast to be directly over Florida by midday Sat
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126. guygee
8:14 PM GMT on August 17, 2007
Note in the graph above that the CONU and GUNA ensembles showed more "skill" than the other models in 2006, better than even the Official NHC forecast (OFCL).
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125. MichaelSTL
3:16 PM CDT on August 17, 2007
And that report on the polar ice is very scary to me.

The 5ºC+ anomalies in the Arctic are also scary:

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124. JFLORIDA
8:12 PM GMT on August 17, 2007
benirica I donno - looks to be a bit of a direction change on Vis to with the building in the NW eyewall quad – slowdown build e then uptick NW. Also the Huge high in the SW Caribbean looks to be moving WSW as another smaller high builds to the S below dean. Top to bottom L to R the banding too seems to be building more northward.
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123. nolesjeff
8:15 PM GMT on August 17, 2007
Posted By: obxrox at 8:15 PM GMT on August 17, 2007.

Unfortunately, since satalite images where not available before 1979, it's misleading to say that "truly unprecedented melting is occurring in the Arctic". IMO

For perspective, the world is 5 billion years old and we have measured the ice for 28.

What ax are you grinding?....Do you not see that this is a significant issue, whether relative to 28 yrs ago or many more?...What truly matters is how this impacts the future, not how it stacks up against positions based on interpretations of the past.

Take this to the gw blog please!
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122. obxrox
8:10 PM GMT on August 17, 2007
Unfortunately, since satalite images where not available before 1979, it's misleading to say that "truly unprecedented melting is occurring in the Arctic". IMO

For perspective, the world is 5 billion years old and we have measured the ice for 28.


What ax are you grinding?....Do you not see that this is a significant issue, whether relative to 28 yrs ago or many more?...What truly matters is how this impacts the future, not how it stacks up against positions based on interpretations of the past.
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120. extreme236
4:13 PM EDT on August 17, 2007
yes i heard an anticylcone was going to build over dean and provide a low shear enviroment for dean as he treks across the caribbean
Member Since: August 2, 2007 Posts: 19 Comments: 19234
119. sporteguy03
8:13 PM GMT on August 17, 2007
My met said if ULL does not cross FL all bets are off.
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118. waccamaw16
8:10 PM GMT on August 17, 2007
i have seen the nudge to the north a little and looking at water vapor the ull is moving a little slower than dean and the storm could catch up and turn more north than what the forecast says. will have to wait and see
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117. barbadosjulie
8:11 PM GMT on August 17, 2007
Can anyone tell me when the next wave is supposed to develop and if its meant to affect Barbados?
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116. gsueagle07
8:12 PM GMT on August 17, 2007
Exactly....the more powerful the storm...the more it creates its own environment....the more a weak ULL would have on it...that is what I have always heard over the years....
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115. SherryB
8:10 PM GMT on August 17, 2007
Thank you all for sharing your wealth of knowledge. I am learning so much on this blog. I have been reading for the past 2 days and wanted clarification on a few things..At this point, is it safe to say that the ULL HAS NOT moved as far west as anticipated and that DEAN is further North than it was forecast to be at this time?
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113. guygee
7:51 PM GMT on August 17, 2007
From our very own weatherguy03 a good description of the CONU and GUNA ensembles that Dr. Masters shows in his graph above:

Posted By: weatherguy03 at 3:19 AM GMT on September 15, 2006.
GUNA. Once the "Aviation" run of the NCEP's global spectral model (now called the GFS - Global Forecast System) was improved in 1998 it became one of the best track guidance models. As a result, Jim Goerss added the Aviation model to the GUNS ensemble, renaming it GUNA for GFDL, UKMET, NOGAPS and AVN.

CONU. More recently Jim has created an ensemble called "CONU" (pronounced "CON-you"). This "consensus" ensemble is computed when track forecasts from at least two of the five models (the NCEP GFS and GFDL models, the Navy's version of the GFDL model and NOGAPS, and the UK Met office model) are available. The CONU tries to take advantage of the ensemble forecasts even if one or a few of the members is not available. The skill of CONU is comparable to that of the GUNA ensemble


When they talk about GUNA and CONU that are talking about the model ensemble of forecasts.

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111. NoMeteorsInOlogy
8:13 PM GMT on August 17, 2007
I have three emotions watching such an enormous covection engine as a hurricane forming--the first is awe at its power and beauty, the second is humbling in that I am reminded how tiny we all are, and the thirsd is sadness.

This storm will kill people, even if the Yucatan breaks it into a tropical depression (Texas is already inundated, and flooding from weak little Erin)...

I feel like a rubber-necker at a catastrophe premier in some ways, but the weather is too interesting to ignore--I've felt like that since I saw thundersnow as a little kid.

NMIO
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110. benirica
8:12 PM GMT on August 17, 2007
JFLORIDA why do you think PR and the DR should worry?
its far enough south, isnt it? atleast for PR
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109. nola70119
8:08 PM GMT on August 17, 2007
I don't think the NHC buys the due west after Jamaica either....they have the storm farther east than the models at 20N, which really leaves most of the gulf in play.
Member Since: June 16, 2006 Posts: 0 Comments: 1565
108. cantstopthinking
7:57 PM GMT on August 17, 2007
LOL, people are so funny, go ahead and listen to the so called experts. Predicting weather is about as accurate as predicting the stock market, were you can also find many so called experts. I have been in the computer field for 15 or so years and the models used to predict storm paths are only as good as the programmers (some more so called experts). There are no experts in life just well educated guessers!! My novice advice says be prepared for the worse, stock up on supplies and keep in contact with family. Keep a close eye on exact storm positions not someone’s prediction.
Most of all be safe!!!
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107. sporteguy03
8:12 PM GMT on August 17, 2007
Nash,
Dean continues to ever so slightly North of NHC track and the ULL still has not crossed FL
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106. Crisis57
8:10 PM GMT on August 17, 2007
Posted By: Randyman at 8:04 PM GMT on August 17, 2007.

Dean beginning to dictate the environment around him...

Link


look at the last frame of that loop and see the distance between Dean and the trouf
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105. Rainwalker
3:08 PM EST on August 17, 2007
Once again I ask, does any one have any idea when the outer bands of Dean will start affecting Jamaica. Thanks
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104. Melagoo
8:12 PM GMT on August 17, 2007
NW CARIBBEAN N OF 15N W OF 75W
1130 AM EDT FRI AUG 17 2007

...HURRICANE CONDITIONS EXPECTED SAT NIGHT THROUGH EARLY TUE...

.THIS AFTERNOON AND TONIGHT...S OF 18N E OF 80W NE TO E WINDS 15
TO 20 KT. SEAS 6 TO 8 FT. ELSEWHERE NE TO E WINDS 10 TO 15 KT.
SEAS 4 TO 6 FT.
.SAT...NE TO E WINDS 10 TO 15 KT. SEAS 4 TO 6 FT EXCEPT 2 TO 3
FT NEAR CUBA.
.SAT NIGHT...WINDS INCREASING TO TROPICAL STORM FORCE OR GREATER
E OF 78W. HIGHEST WINDS 120 KT GUSTS 145 KT. SEAS BUILDING TO 9
TO 14 FT. ELSEWHERE E OF 80W N TO NE 15 TO 20 KT INCREASING TO
20 TO 33 KT. SEAS 4 TO 6 FT BUILDING TO 6 TO 8 FT. W OF 80W NE
TO E WINDS 10 TO 15 KT. SEAS 3 TO 5 FT.
.SUN AND MON...HURRICANE CONDITIONS EXPECTED. HIGHEST WINDS 120
KT GUSTS 145 KT INCREASING TO 130 KT GUSTS 160 KT MON.

.TUE...HURRICANE CONDITIONS EXPECTED EARLY NW PART...DIMINISHING
TO SE TO S 20 TO 25 KT. SEAS SUBSIDING TO 9 TO 12 FT. ELSEWHERE
N OF 18N E TO SE WINDS 15 TO 20 KT. SEAS 6 TO 8 FT. S OF 18N E
TO SE WINDS 10 TO 15 KT. SEAS 3 TO 5 FT.


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101. rlpellegrin
3:07 PM CDT on August 17, 2007
Should anyone in Louisiana start to get nervous or should we wait until Monday to see what the NHC and Models say?
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100. nash28
8:07 PM GMT on August 17, 2007
Hey guys. Been gone all day. Had lunch with a local Met and had a nice time.

So, I see the GFDL shifted another 180 miles to the east on the 12z run.

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98. RadarRich
8:08 PM GMT on August 17, 2007
The ULL will probably affect Dean around 80, if so, at all
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97. Randyman
8:07 PM GMT on August 17, 2007
Posted By: Crawls at 8:07 PM GMT on August 17, 2007.

Do gut feelings count? Louisiana?


After the 2005 season, I would not dare get on here and eliminate anyone at this point...I still feel Lousiana is just as fair game as Texas is at this juncture...we shall see...
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96. benirica
8:09 PM GMT on August 17, 2007
JFLORIDA... how so?
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95. canesurf
8:07 PM GMT on August 17, 2007
I am intrigued as to how Dean and that ULL/weakness will interact. We'll see in a day or 2. I live in East Central Fl., and I am trying not to be nervous.
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94. Crisis57
8:07 PM GMT on August 17, 2007
Posted By: Michael at 8:07 PM GMT on August 17, 2007.

If your florida you want this thing to haul butt, get to the Yucatan quick as possible, if it was going slow then yea Florida could be in trouble but it's moiving too fast...your lucky


true but that forward speed can also let Dean catch up to the ULL or weakness between the highs we all need to just monitor it
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93. JFLORIDA
8:06 PM GMT on August 17, 2007
Perhaps PR and the Dominican Republic should worry a bit.
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92. cchsweatherman
8:04 PM GMT on August 17, 2007
That is a very inquisitive note there canesurf. I have been of the opinion all day long that the ULL will pull Hurricane Dean much farther north than the forecast models have been putting out. Hurricane Dean has been gaining significant ground on this ULL and it is only a matter of time and physics until Dean starts to feel the almost magnetic pull of the ULL.
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91. Crisis57
8:06 PM GMT on August 17, 2007
Posted By: canesurf at 8:04 PM GMT on August 17, 2007.

What I don't understand is why the NHC and models seem to ignore the ULL off Florida. It seems very possible that Dean could exploit that weakness and actually skirt N in between the 2 high preseeure areas, skirting the east coast of Florida. Isn't that even remotely possible?


it is possible look at Hurricane David
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90. seflagamma
4:07 PM AST on August 17, 2007
Thank you for your input and the new thread!
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88. Crawls
8:06 PM GMT on August 17, 2007
Do gut feelings count? Louisiana?
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87. Pachanga
7:57 PM GMT on August 17, 2007
Unfortunately, since satalite images where not available before 1979, it's misleading to say that "truly unprecedented melting is occurring in the Arctic". IMO

For perspective, the world is 5 billion years old and we have measured the ice for 28.
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86. leftovers
8:06 PM GMT on August 17, 2007
Anyone with the link for that Jamaican web radio.

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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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