Danny disorganized, but generating strong winds

By: Dr. Jeff Masters , 1:55 PM GMT on August 27, 2009

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Tropical Storm Danny continues to look disorganized this morning. The low level circulation center is exposed to view, with the heaviest thunderstorms lying several hundred miles east of the center. The center has undergone several relocations over the past 12 hours, and may do so yet again this morning, in order to position itself nearer to the heaviest thunderstorm activity. Despite its disorganization, Danny continues to generate strong winds, with the Hurricane Hunters and QuikSCAT both reporting winds in the 55 - 60 mph range early this morning. There is plenty of dry air in Danny's vicinity interfering with development, thanks to an upper-level trough of low pressure.


Figure 1. Morning satellite image of Danny showing the exposed swirl of clouds where Danny's center is, well displaced from the heaviest thunderstorm activity to the east.

The intensity forecast for Danny
The upper-level low that has been keeping Danny disorganized has weakened and separated from the storm, leaving Danny in a region with moderate wind shear of 10 - 15 knots and a modest amount of dry air. These environmental conditions will remain roughly constant through Friday night. Slow to moderate strengthening of Danny to a Category 1 hurricane should result, and is called for by all of the reliable intensity models. By Friday night, a trough of low pressure will approach the U.S. East Coast and bring high wind shear of 20 - 35 knots through Saturday. Danny will be close enough to this trough that the trough may be able to feed energy to Danny as the trough converts Danny to an extratropical storm. As a result, Danny may not weaken as fast as one might ordinarily expect, given the high levels of wind shear expected on Saturday. A landfall in Cape Cod, eastern Maine, or Nova Scotia with 55 - 75 mph winds is a good bet.


Figure 2. Performance of the main models used to forecast Hurricane Bill. Forecasts for the time periods 24, 48, 72, 96, and 120 hours are shown, with the track errors for each models' forecasts in nautical miles (nm). The statistics are shown for the regular interpolated version of the models used by the NHC forecasters in real time to make their forecasts. The "Consensus" model is the NHC's TVCN consensus, which is the average of at least two of the other models shown here (but not including the Canadian model). The Canadian model had the best performance of any model for Bill, surpassing even the Official NHC forecast. The next best performing models were the GFDL and GFS. Last year's best performing model, the European Center model, was not available for this analysis. Image credit: Dr. Jim Goerss, Navy Research Lab, Monterey.

The track forecast for Danny
Wunderground provides a computer models plot showing the hurricane track forecasts of most of the major models used by NHC to formulate their official forecast (one notable exception: we can't show the European Center ECMWF model, since this model is not freely available). One of the most frequently asked questions I get is, "which model do you trust?" This morning we have several models like the NOGAPS and Canadian calling for Danny to pass very near Cape Hatteras, North Carolina, then over Cape Cod, Massachusetts. The rest of the models foresee Danny missing Cape Hatteras, and continuing on to an encounter with Nova Scotia instead of Cape Cod. According the 2008 NHC forecast verification report, the best performing model during the 2008 hurricane season was the European Model (ECMWF), by a wide margin. The ECMWF out-performed the official NHC forecast, and it is very rare for an individual model to do this. The next best models were the GFDL and HWRF, while the NOGAPS, UKMET, and GFDN did the worst of the major models. The Canadian model was not analyzed, but historically has been among the worst of the models for forecasting hurricanes.

So far this year, the ECMWF has also done well. Unfortunately, the European Center group does not make the output of their hurricane tracking module publicly available, so I cannot present any statistics of their model's performance. Somewhat surprisingly, the Canadian model has also done very well this year. The model received a major upgrade in its physics of the past year, and has performed extremely well in hurricane track forecasts for both the Eastern Pacific and the Atlantic so far this year. In fact, for Hurricane Bill, the Canadian model gave better track forecasts then the NHC did (Figure 2). Danny is a different storm than Bill, and it is possible that the Canadian model will do less well with a storm that is disorganized, like Danny is. Nevertheless, with the Canadian model consistently keeping a Danny's track close to Cape Hatteras and going over southeastern Massachusetts, residents of these areas need to be prepared for possible hurricane conditions from Danny. Given the recent reformation of Danny's center, and the possibility of yet another reformation later today, all of the track models must be viewed with more than the usual amount of doubt. Since the center reformations have been moving Danny's center to the north and east, it may be that the Canadian model's prediction lies too close to the U.S. coast.

For those of you wondering about specific probabilities of getting tropical storm force or hurricane force winds, consult the NHC Wind Probability Product. The 11 am EDT NHC forecast gave Cape Hatteras a 4% chance of seeing hurricane force winds from Danny, and Nantucket, MA, a 7% chance.

For more information on computer models used by NHC
Basics of hurricane forecast models (Dr. Jeff Masters, wunderground.com, updated 2007)
Description of computer models used for hurricane forecasts (NHC, updated 2009)
Description of computer models used for hurricane forecasts (NOAA/AOML)


Figure 3. Tropical wave 94L off the coast of Africa.

Invest 94L off the coast of Africa
A well-organized tropical wave lies a few hundred miles southwest of the Cape Verdes Islands, near the coast of Africa. This wave was designated 94L by NHC this morning. Shear is low, about 10 knots, and waters are warm enough to support development. The dry Saharan Air Layer is relatively limited in extent and intensity, so dry air may have only a small inhibiting effect on the wave. Expect some slow development of this wave as it moves westward over the next few days. NHC is giving this system a low (less than 30% chance) of developing into a tropical depression by Saturday. This system is moving rather slowly, 10 - 15 mph, and it will be at least a week before it approaches the Lesser Antilles Islands. The GFS model develops 94L into a tropical depression early next week.

Special note on using the Canadian model
While the Canadian model has been doing well with hurricane track forecasts this year, the model still does a poor job forecasting the genesis of new tropical cyclones. The Canadian model has a false alarm rate perhaps three times higher than any other model, so one should not believe the Canadian model's regular predictions of new tropical cyclones springing up. You can access output from the Canadian model at Environment Canada or at Florida State University or Penn State.

I'll have an update later today.

Jeff Masters

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Quoting A4Guy:


You had better callthe NHC and tell them to update their forecast.


they won't update their forecasts unless this is more of a trend than what it has been, it has only been doing this since early this morning and it's only noon now. Definitely need to update the track if this continues for another 6 hours, assuming a new LLC doesn't form.
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311. 7544
happy bithday strom w
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Quoting stormygace:
Noooooo! keep Myrtle Beach & Wilmington outta the cone! too many maters that need another 10 days! TS Hannah did ugly things to my tomatoes & peppers last year!
LOL....looks like lots of spaghetti sauce meals for you this fall.....muhahaha!:)
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307. A4Guy
Quoting TampaSpin:


Thats the difference in a weak system like DANNY as it moves West as apposed to a stronger System that DANNY was to be.....SECONDLY Danny is moving WEST because of the High Pressure to its North built from the HIGH centered off the WEstern tip of Cuba! I seen this coming last night.


You better call the NHC and tell them to update their forecast.
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303. 7544
Quoting StormChaser81:
If Danny makes it past 75w I dont think thats a wobble...


naked swril aproching 74 prettey close if that part of danny and is west of the symbols
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Member Since: September 14, 2007 Posts: 3 Comments: 3963
Quoting alaina1085:
So whats the steering looking like for this 94L? Everyone said Bill was going to be a carib-GOM storm and it def wasnt...lol.



Ridging is "SUPPOSED" to build in early next week behind Danny and increase risk to Caribbean, SE Coast & GOM but, Troughing has been the predominant force time and time again this year so I will believe it when I see it.
Is this a sports blog or a political blog? Oh no wait its a tropical weather blog. Really? coulda fooled me!
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I remember a tropical storm, which I believe was in 1984. It had been forecast to move northward and actually struck somewhere in Central FLA. I was in N. Miami at the time. It was in a similar position as Danny. All I remember is that we only had a few hours warning, but luckily we only got strong winds and I do not remember getting any rain at all since the convection had been well removed from the circulation. A close scenario to what Danny might be trying to do.
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So whats the steering looking like for this 94L? Everyone said Bill was going to be a carib-GOM storm and it def wasnt...lol.
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If Danny makes it past 75w I dont think thats a wobble...
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Quoting Elena85Vet:


I agree. They likely don't see this as a trend and as long as they stay in their cone they're good to go.


They were out of the 5am cone this morning. Which is unusual. But, again, the cone is 67%, so Danny was one of the 33% this particular time.

5am Cone

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I can only see half of the Cape Verde system on the NHC's Atlantic Wide View satellite loop, but there are slight signs of rotation. Anyone confirm this?
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Quoting P451:
Might have a new LLC trying to form under the convection.

HH's showing a wind shift at 27.5N 69.8W:

If that is actually the case, it would be no suprise! It'll be interesting to see what the offical NHC word will be on TS Danny!!
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Dont sing"Danny Boy" or he will come to you.
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Quoting CycloneOz:


I wish I knew!

Hurricanes are no more than thunderstorms rotating rapidly around a low pressure system. Perhaps it has something to do with how a hurricane pulls wind from the surface and shoots it skyward, and that disrupts the formation of leaders.

There has to be some research on the subject, but I have yet to find any.


feel free to correct me if i'm way off, this is an educational guess but I hope I'm on the right track here. This happens to be something I have never really looked into, so I tried to come up with my own consensus for part of the question to see how far off i might be.

there is usually a lot of lightning and severe thunderstorm activity in the outer bands of a hurricane due to the fact that those storms have more "breathing" room to be discrete supercells. The convection near the center of a hurricane is not necessarily, and rarely ever is traditional thunderstorm activity. Although winds are more intense, rains are heavier, the nature of it is more in the rain storm category (albeit it's much more than just a rain storm) than something you'd see out of a cold front. Lightning is actually still fairly common even in the eye wall, however due to several factors it is usually not cloud to ground

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268. snark! LOL
Member Since: September 14, 2007 Posts: 3 Comments: 3963
Quoting stormpetrol:
I don't like the model tracks for 94L , if this trend continues could spell bad news for the windward/leewards, western caribbean and probably the gulf states,jmo.


Just give it time, we're all are very aware of how all of these tracks change back & forth and forth and back especially when its still only a yellow colored circle.
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Quoting Vortex95:


it is not going to hit florida it is a wobble. The NHC knows what it is doing.


I agree. They likely don't see this as a trend and as long as they stay in their cone they're good to go.
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Quoting stormpetrol:
I don't like the model tracks for 94L , if this trend continues could spell bad news for the windward/leewards, western caribbean and probably the gulf states,jmo.


The one thing these two men shared was a love, a passion for their work and their duties. They don't come like these anymore.

If the ULL pulls away from Danny and a new COC appears than the models can become more credible.
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Hmmm, the 12z NAM has this little bugger festering in the BOC on Sat. at about noon. After how well it did with 93l, I am going to be interested to see what happens over the next few runs. Boy, we certainly don't want ANYTHING in that bathtub!!

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Approximately how many days away is 94L from the Winward Islands?
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I don't like the model tracks for 94L , if this trend continues could spell bad news for the windward/leewards, western caribbean and probably the gulf states,jmo.
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Quoting NOVArules:


Or Ted Kennedy *sniff*


I liked John Hope better lol but Teddy was a good guy too, minus his little screwup back in the day
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Quoting Becca36:
Can you please tell me why lightning is so rare in a hurricane? I've been through several over the past 13 years here in Boca Raton,FL and often wondered about that. Tia


I wish I knew!

Hurricanes are no more than thunderstorms rotating rapidly around a low pressure system. Perhaps it has something to do with how a hurricane pulls wind from the surface and shoots it skyward, and that disrupts the formation of leaders.

There has to be some research on the subject, but I have yet to find any.
Member Since: Posts: Comments:

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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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