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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2184. atmoaggie
4:47 AM GMT on August 18, 2007
box 1 is the one to watch box 2 only comes into play in like october when fronts come down and sweep systems northeast into florida like wilma

Yup
Member Since: August 16, 2007 Posts: 6 Comments: 12463
2183. Metallica1990
4:46 AM GMT on August 18, 2007
Posted By: StSimonsIslandGAGuy at 4:46 AM GMT on August 18, 2007.

I remember when a buoy south of Alabama measured the largest wave ever generated by weather (this excludes tsunamis) at 91 feet. The buoy stopped operating 3 minutes later and was never found. But like the klingons would say, it 'died with honor'


lmao
2181. C2News
12:45 AM EDT on August 18, 2007
The PR radar does pick up a jog to the nw or wnw
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2180. intunewindchime
4:44 AM GMT on August 18, 2007
hebert box
I think I read that 9 out of 10 which passed therough the box made a Fl landfall.
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2179. stormybil
4:44 AM GMT on August 18, 2007
yeap vero and c2 news also the ull is suppose to stall thats what im also watching . i think fla. has its eyes close on dean but if he make that move evryone will get wide awake fast .
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2178. Metallica1990
4:44 AM GMT on August 18, 2007
box 1 is the one to watch box 2 only comes into play in like october when fronts come down and sweep systems northeast into florida like wilma
2177. drj27
4:43 AM GMT on August 18, 2007
i guess anything is possible
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2174. Daveg
4:44 AM GMT on August 18, 2007
definitely a bit of a WNW shift...just barely. Can see it lifting form the bottom of the frame. Link

Just ever so slight...definitely not much.
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2173. KnowYourRole
12:43 AM EDT on August 18, 2007
Posted By: Pachanga at 12:43 AM EDT on August 18, 2007.
Any chance that the ULL stays stationary for another day or so? And if so, would it be influential enough to pull Dean considerably to the north? Say over the middle of Cuba?


The ULL is not stationary right now.
2171. Hellsniper223
4:44 AM GMT on August 18, 2007
Posted By: Metallica1990 at 4:44 AM GMT on August 18, 2007.

didnt ivan have 90 foot waves off pensacola as a borderline 3/4


Yup.
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2170. VEROBEACHFL1
4:43 AM GMT on August 18, 2007
thats a wake up call for the buoy------- its gonna blow up in about 10 hours
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2169. i12BNEi
4:43 AM GMT on August 18, 2007
Could this do an Ernesto?
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2168. TheCaneWhisperer
4:41 AM GMT on August 18, 2007
Posted By: Hellsniper223 at 4:35 AM GMT on August 18, 2007.
Nearly every storm entering the gulf from the carrib goes through the herbert box... That's a really realllly dumb way to predict the path of a system.


Common mis-conception! It states that every major Hurricane that struck South Florida passed through this box, not every major hurricane. basically means that if a storm is in the box you need to keep a closer eye on the progress.
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2167. C2News
12:43 AM EDT on August 18, 2007
Posted By: getalife at 12:43 AM EDT on August 18, 2007.
Yes, Florida will not be hit.


I don't think it is right to post that. No one can say that for certain.
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2166. Canenut
4:43 AM GMT on August 18, 2007
There are 2 Hebert boxes, named after Paul Hebert. Yes Dean passed by #1, but may go smack thru #2 Link
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2165. weathersp
12:41 AM EDT on August 18, 2007
Poor 42059 buoy.
Poor Buoy 42059
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2164. Metallica1990
4:42 AM GMT on August 18, 2007
didnt ivan have 90 foot waves off pensacola as a borderline 3/4
2163. Pachanga
4:39 AM GMT on August 18, 2007
Any chance that the ULL stays stationary for another day or so? And if so, would it be influential enough to pull Dean considerably to the north? Say over the middle of Cuba?
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2162. VEROBEACHFL1
4:41 AM GMT on August 18, 2007
im with ya stormy------- but a gradual NW shift first......... till at least 80W :)
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2161. getalife
4:39 AM GMT on August 18, 2007
Yes, Florida will not be hit.
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2160. Hellsniper223
4:41 AM GMT on August 18, 2007
lol Verobeachfl1, just wait till one of dem bouies catches a rogue. lmao. 20ft Ain't nothing
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2159. nola70119
4:40 AM GMT on August 18, 2007
I think Pensacola is still in play.....NHS says north and west gulf all have same probability right now.
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2158. C2News
12:41 AM EDT on August 18, 2007
Posted By: stormybil at 12:41 AM EDT on August 18, 2007.
that ull is still around dean is a cat 4 too close for comfort if the ull wasnt there i say relax but fla . not yet dean can pull the wool right from uner us and the nhc

oh remember last night i said dean likes to start jogging north at 3 am
and he did alot stick around the show will begin soon


I completely concur...let me go get my popcorn ready :)...Florida is not out of the question YET
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2157. atmoaggie
4:41 AM GMT on August 18, 2007
2OFT WAVES--HANG ON BUOY!! more to come lol

I love you, terra firma.
Member Since: August 16, 2007 Posts: 6 Comments: 12463
2156. CajunSubbie
4:41 AM GMT on August 18, 2007
a lil jog north on PR radar
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2155. drj27
4:40 AM GMT on August 18, 2007
if dean hits the florida panhandle i will pay everyone in the room a 100 bucks lol
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2154. VEROBEACHFL1
4:39 AM GMT on August 18, 2007
2OFT WAVES--HANG ON BUOY!! more to come lol
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2153. stormybil
4:38 AM GMT on August 18, 2007
that ull is still around dean is a cat 4 too close for comfort if the ull wasnt there i say relax but fla . not yet dean can pull the wool right from uner us and the nhc

oh remember last night i said dean likes to start jogging north at 3 am
and he did alot stick around the show will begin soon
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2152. treehuggingsister
4:38 AM GMT on August 18, 2007
"Pensacola is in Florida, no?"

That would be us, yes.
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2151. Metallica1990
4:39 AM GMT on August 18, 2007
lets keep the hebert box outta this dean has passed the hebert box to the south
2150. Hellsniper223
4:40 AM GMT on August 18, 2007
It's because the Herbert box is utter nonsense.
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2148. C2News
12:39 AM EDT on August 18, 2007
Posted By: KnowYourRole at 12:38 AM EDT on August 18, 2007.
Posted By: drj27 at 12:35 AM EDT on August 18, 2007.
so florida is now out of the picture

there is nothing supporting a Florida landfall.


I wouldn't say anything is out of the question just yet
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2146. KnowYourRole
12:37 AM EDT on August 18, 2007
Posted By: drj27 at 12:35 AM EDT on August 18, 2007.
so florida is now out of the picture


there is nothing supporting a Florida landfall.
2145. atmoaggie
4:37 AM GMT on August 18, 2007
GFS has that ULL still going strong at 12 UTC Sunday. Only weakens after then. I think shear will keep Dean from cat5 status.

GFS
Member Since: August 16, 2007 Posts: 6 Comments: 12463
2144. VEROBEACHFL1
4:36 AM GMT on August 18, 2007
well, at least we all have one thing on ALL OUR sides........................................................
=TIME-----------------
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2143. nola70119
4:36 AM GMT on August 18, 2007
Pensacola is in Florida, no? South Beach is out of the picture....
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2142. C2News
12:35 AM EDT on August 18, 2007
Posted By: drj27 at 12:35 AM EDT on August 18, 2007.
so florida is now out of the picture


No
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2139. IRememberIvan
4:33 AM GMT on August 18, 2007
Dean will go through an ERC before the afternoon tomorrow. This will weaken him to a low 4/high 3 for a while, but he'll easily restrengthen and then some, probably to a cat 5.

Furthermore, like Katrina and most intense hurricanes, ERCs allow the windfield to grow immensely. With Dean so far from landfall right now, an ERC just makes him THAT much worse.
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2138. drj27
4:35 AM GMT on August 18, 2007
so florida is now out of the picture
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2137. cirrocumulus
4:33 AM GMT on August 18, 2007
Has anybody noticed the dramatic shift from the previous GFS. The high pressure that was supposed to send Dean to Mexico has been projected farther east on the latest run. And low pressure appears to be invading from the west and encroaching on the high pressure.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
2136. Hellsniper223
4:33 AM GMT on August 18, 2007
Nearly every storm entering the gulf from the carrib goes through the herbert box... That's a really realllly dumb way to predict the path of a system.
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2135. VEROBEACHFL1
4:33 AM GMT on August 18, 2007
thx c2
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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.