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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3035. 1900hurricane
7:41 AM CDT on August 18, 2007
Katrina had a pressure at 920 mbs, and it was a CAT 3!
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3034. watchingnva
12:44 PM GMT on August 18, 2007
jam....we will be praying for ya...and hoping for the best...unfortunantly...it doesnt look good for ya...
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3033. sunnyandshear
12:44 PM GMT on August 18, 2007
Ready made, just copy and paste

It's too early to call, we'll just have to wait and see where it goes,lol. Nobody is out of the woods yet, it's too early to relax, and nobody should let their guard down,lol. The next few days should be interesting,lol. Five day forecasts are so unreliable,lol. This thing could go anywhere from Maine to Mexico, so the next model runs will be critical,lol. Am I the only one, or does anyone else see this? lol.
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3032. BlowCLevel
12:36 PM GMT on August 18, 2007
I miss Nash Roberts, too. He was like a blogger before the internet.

Camille took a pretty quick turn into the MS coast when she hit and Nash called it before the NHC. In fact, he insisted that people evacuate and the NHC threatened to revoke his license if he didn't preach the official line. Nash refused, people left and the storm hit. If they would have listened to the NHC, tons more people would have died. That's the story, anyway.

After that, everyone listened to Nash first. He was no-nonsense and really, really good at his job.

It killed me when they had him on for Rita, I think, and the snotty little then meteorologist treated him like a joke. So undignified. Those of us who knew realized what an ignoramus she was but it was sad to see him treated that way in his twilight years. Of course, he was a gentleman about it.
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3031. jamweather
12:43 PM GMT on August 18, 2007
here in jamaica, some folks are having a laid back approach to preparing for the hurricane. i am just coming off the road (last of my preparation) and there are persons who still do not believe that we will be affected by Dean. They are crazy. i will not be surprised if a lot of my fellow citizens are displaced. i pray for them and myself cause its gonna be bad.
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3030. Masquer08er
12:39 PM GMT on August 18, 2007
Biloxi,

Did you see where Gov. Blanco declared a state of emergecy in LA because of Mean Dean.
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3029. WPBHurricane05
8:39 AM EDT on August 18, 2007
I thought the stronger the hurricane got, the more north it would turn?

Not always true


Member Since: July 31, 2006 Posts: 56 Comments: 8112
3027. BiloxiGirl
12:34 PM GMT on August 18, 2007
Steel - That is my problem I think. I remember deciding that Katrina wasn't really coming to MS, and the next thing I know my parents are calling me and making hotel reservations to evacuate. Just when you think you know what a storm is doing based on these models.... everything changes. To me, we don't know what any storm will do until about 48 hours out from landfall.
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3026. dmaddox
12:39 PM GMT on August 18, 2007
http://icons.wunderground.com/data/images/at200704_hd.gif

920mb PRESSURE! Thats a CAT 5 folks, no matter how you slice it!
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3025. Randyman
12:35 PM GMT on August 18, 2007
Someone please explain to me what is this on the Houston/Galveston radar loop...I see this every now and then...it always appears to dissipate most of the rain within it's vicinity...


Link
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3024. srada
12:37 PM GMT on August 18, 2007
I thought the stronger the hurricane got, the more north it would turn?
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3023. castnblast
12:33 PM GMT on August 18, 2007
Sherry B, I couldn't agree with you more...Still 5 days out...I can and continue to forecast with 100% probabability the following: Gas prices are going up! LOL...aside from that we are still dealing w/ a 5 day forecast...But, notice how they have shifted north, than back south to their original points? I'm not calling this one. but a guess puts it in the cone...LOL
3022. srada
12:27 PM GMT on August 18, 2007
good morning,

steve lyons just said more of a mexico landfall, looks like the GFDL is sliding back towards the other models but texas still looks to get a lot of rain and surge...
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3021. weathersp
8:33 AM EDT on August 18, 2007
Does this happen in only the strongest of hurricanes? And does it mean there will be reintensification when the other eyewall forms?

1st: Mainly above Cat 2 Status.

2nd: Yes, Likely stronger than the first.
Member Since: January 14, 2007 Posts: 17 Comments: 4140
3020. JRRP
12:22 PM GMT on Agosto 18, 2007
WE ARE PREPARED AS IF THE HURACANE
NOS FUERA A GOLPEAR
THAT IS
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3019. extreme236
12:33 PM GMT on August 18, 2007
well with Dean, even though it is going under an EWRC the pressure continues to fall. so when the new wall forms, then perhaps this could re-intensify to a very powerful category 5
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3018. steelmagnolia44
12:14 PM GMT on August 18, 2007
BiloxiGirl,

I think we can relax a little while still keeping an eye on it.

IMO if there were any chance that the cone of uncertainty might include the Katrina ravaged areas of Mississippi, I think the local officials would be SCREAMING at everyone right now to get ready!

We all remember the way the NHC kept moving the projected path for Katrina and other storms. We should not forget that conditions change, but we can look at what seems to be the clear model trend and base our anxiety levels on that for the moment! This level of hype for days before landfall in an area that is not included in the cone or the models at the moment can be exhausting!

steel
Member Since: August 24, 2005 Posts: 0 Comments: 43
3017. Tampawxgirl
12:30 PM GMT on August 18, 2007
Posted By: weathersp at 12:26 PM GMT on August 18, 2007.
Basically the eyewall falls apart and another one forms out side of that and moves in closer to the center. During this time both eyewalls are weak so the hurricane weakens just for a bit.


Does this happen in only the strongest of hurricanes? And does it mean there will be reintensification when the other eyewall forms?
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3016. EllistonVA
12:29 PM GMT on August 18, 2007
Biloxigirl - you will be fine no matter what happens. Why? Because you are smart enough to be paying attention ahead of time and not just waiting until it's on top of you!
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3015. ForecasterColby
12:30 PM GMT on August 18, 2007
Everyone hop in the tropics chat.

I'm very interested to know what the NW quadrant holds when they get there.
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3014. SherryB
8:20 AM EDT on August 18, 2007
Morning all! IMHO...with Dean still 5 days out from landfall, it is still to early to tell who is out of the woods. From reading for the last 3 days, we would be naive to think that anyone in the GOM is out of the woods. The ULL still has not made the movement that was predicted by the NHC so who the heck really knows!
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3013. 1900hurricane
7:27 AM CDT on August 18, 2007
Hail was also reported in Hurricane Emily in '05.
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3012. GPTGUY
7:26 AM CDT on August 18, 2007
BiloxiGirl I would just keep watching it like Elliston said the accuracy of the models tend to drop off further in time...but once its looking clear for our area we need to shift our attention back out to the Atlantic
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3011. 1900hurricane
7:23 AM CDT on August 18, 2007
Most thunderstorms produce hail, but it will melt in a short period of time normmaly, and form heavy rain. Normally in a tropical cyclone, the air is too warm for hail last very long at all, even with very strong updrafts. What hail in a tropical cyclone means is that the updrafts in the EW are extreme.
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3010. Keys99
12:21 PM GMT on August 18, 2007
WOW the ULL is packing a little punch in the storms over us Official guest to 43 mph just 15 min ago
Member Since: August 16, 2007 Posts: 0 Comments: 132
3009. Tampawxgirl
12:26 PM GMT on August 18, 2007
Thanks, WeatherSpotter. I'll check it out.
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3008. KnowYourRole
8:23 AM EDT on August 18, 2007
Posted By: WPBHurricane05 at 8:21 AM EDT on August 18, 2007.
Never would have guessed hail in a hurricane. Never seen it myself, but I don't go out side during storms.


it's not common, but it does occur. the reason you don't see it is because of both warm air and the fact that the wind would blow the hail horizontally and not vertically which allows it more time to melt before reaching the surface.
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3007. weathersp
8:23 AM EDT on August 18, 2007
Wow, I was away for much of yesterday evening and come back to a massive cat 4! Yikes.

This thing will be a massive monster after this replacement cycle.

What is the replacement cycle? This is a new term for me.


Basically the eyewall falls apart and another one forms out side of that and moves in closer to the center. During this time both eyewalls are weak so the hurricane weakens just for a bit.
Member Since: January 14, 2007 Posts: 17 Comments: 4140
3006. SLU
8:23 AM AST on August 18, 2007
Posted By: Weather456 at 8:20 AM AST on August 18, 2007.

Hail was reported in a 1995 hurricane...cant remember the name..I think it was the "m" storm


Hurricane Marilyn in the US Virgin Islands
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3005. Skyepony (Mod)
12:19 PM GMT on August 18, 2007
Sonde to the eyewall

Dropsonde Observations
Storm Name: DEAN (04L)
Mission Number: 06
Flight ID: AF304
Observation Number: 06
Time: 1200Z
Latitude: 15.3N
Longitude: 67.9W
#NAME?
Surface: 942 mb; Temp: 80F; Dewpt: 80F; N (N/A) @ N/A mph
1000mb height: Unavailable
925mb height: 525 ft; Temp: 77F; Dewpt: 77F; WSW (250) @ 107 mph
850mb height: 2969 ft; Temp: 77F; Dewpt: 69F; W (275) @ 107 mph
700mb height: 11804 ft; Temp: 53F; Dewpt: 53F; NW (310) @ 124 mph
500mb height: Unavailable
400mb height: Unavailable
300mb height: Unavailable
250mb height: Unavailable
200mb height: Unavailable
EYEWALL 225 SPL 1529N06785W 1156 LST WND 033 MBL WND 25094
AEV 00000 DLM WND 27590 939715 WL150 24101 108 =

942mb winds: N (N/A) @ N/A mph
939mb winds: SW (235) @ 115 mph
929mb winds: WSW (245) @ 118 mph
919mb winds: WSW (250) @ 93 mph
909mb winds: WSW (245) @ 106 mph
896mb winds: WSW (255) @ 113 mph
894mb winds: W (260) @ 119 mph
887mb winds: WSW (255) @ 105 mph
872mb winds: W (260) @ 108 mph
864mb winds: W (265) @ 97 mph
850mb winds: W (275) @ 107 mph
823mb winds: W (280) @ 123 mph
771mb winds: WNW (295) @ 109 mph
699mb winds: NW (310) @ 124 mph
#VALUE!

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3004. WeatherSpotter
12:21 PM GMT on August 18, 2007
http://en.wikipedia.org/wiki/Eyewall_Replacement_Cycle#Eyewall_replacement_cycles

Info about EWRC's
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3003. dean2007
12:12 PM GMT on August 18, 2007
EWRC is occuring right now, but the hurricane continues to strengthen regardless of EWRC. Pressure down to 924mb per hurricane hunters.
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3002. floridafisherman
12:22 PM GMT on August 18, 2007
456 i believe floyd had hail and lightning
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3001. weathersp
8:21 AM EDT on August 18, 2007
Odd but I can see it happening. Massive updrafts bring up to the very to where satilights have recorded -66C
Member Since: January 14, 2007 Posts: 17 Comments: 4140
3000. ForecasterColby
12:22 PM GMT on August 18, 2007
Hail happens, but it's quite rare in tropical systems. I have no idea what it means, though.

I'm very surprised not to see concentric eyewalls in that report.
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2999. Tampawxgirl
12:17 PM GMT on August 18, 2007
Wow, I was away for much of yesterday evening and come back to a massive cat 4! Yikes.

This thing will be a massive monster after this replacement cycle.

What is the replacement cycle? This is a new term for me.
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2998. BiloxiGirl
12:19 PM GMT on August 18, 2007
I think I am looking for an excuse to stop driving myself crazy monitoring this storm. I check on it frequently throughout the day and I think I am trying to talk myself into thinking that I really shouldn't worry about it. But it sounds like maybe we aren't to that point just yet.
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2997. WPBHurricane05
8:20 AM EDT on August 18, 2007
Never would have guessed hail in a hurricane. Never seen it myself, but I don't go out side during storms.
Member Since: July 31, 2006 Posts: 56 Comments: 8112
2996. FLWeatherFreak91
12:18 PM GMT on August 18, 2007
JRRP, how are the people preparing for The Storm? I remember when I was down there for Georges no one did anything, many people didn't even know what was coming.
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2995. Weather456
12:18 PM GMT on August 18, 2007
Hail was reported in a 1995 hurricane...cant remember the name..I think it was the "m" storm
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2994. guygee
12:14 PM GMT on August 18, 2007
I see that Dean wobbled north and in the last few frames through 09:45Z is moving parallel and just north of the NHC track. I will henceforth remove my name from the list of wobblecasters and do my wobblewatching in private until I see a significant trend.
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2993. EllistonVA
12:11 PM GMT on August 18, 2007
Biloxigirl - If you look at Dr. Master's post you will see that the accuracy of the models drops off rapidly the further out you go in time. To make things worse, the land around the GOM makes a circle which means that even a small jog left or right can change the landfall location by 100 miles. Everyone needs to stay vigilant.
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2992. WPBHurricane05
8:18 AM EDT on August 18, 2007
where are you guys getting those measurements

Link
Member Since: July 31, 2006 Posts: 56 Comments: 8112
2991. GPTGUY
7:18 AM CDT on August 18, 2007
Hail is rare..if at all common when they move ashore...but can be quite common when there over open water
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2990. jj292
12:15 PM GMT on August 18, 2007
Storms this big and strong can in a way create their own environment and can go where they want. No they don't blast through a giant high pressure system or anything like that, but they tend to have a few tricks up their sleeves sometimes.
2989. Weather456
12:12 PM GMT on August 18, 2007
Dean

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2988. WPBHurricane05
8:17 AM EDT on August 18, 2007
VORTEX DATA MESSAGE AL042007
A. 18/11:51:20Z
B. 15 deg 23 min N
067 deg 52 min W
C. 700 mb 2437 m
D. 121 kt
E. 5 deg 008 nm
F. 137 deg 145 kt
G. 050 deg 008 nm
H. 926 mb
I. 11 C/ 3049 m
J. 23 C/ 3041 m
K. 11 C/ NA
L. CLOSED
M. C12
N. 12345/ 7
O. 0.02 / 2 nm
P. AF304 0604A DEAN OB 03
MAX FL WIND 145 KT NE QUAD 11:48:40 Z
SMALL HAIL INBOUND NE QUAD
RADAR PRESENTATION EXCELLENT

Hail in a hurricane?..??....??
Member Since: July 31, 2006 Posts: 56 Comments: 8112
2987. hurricane667
12:16 PM GMT on August 18, 2007
where are you guys getting those measurements
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2986. JRRP
12:09 PM GMT on Agosto 18, 2007
FLWeather
IN SANTO DOMINGO
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2985. JPV
12:09 PM GMT on August 18, 2007
Posted By: BiloxiGirl

JPV - so if all the models are converging much further south, what scenario would ever bring it up to say LA and MS?


5 day forecasts aren't very reliable.

And hurricanes can be VERY unpredictable.

Not saying that it will make a US landfall, but I think that it's way too early to call it, either way.
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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.