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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where do i get a ticket for the show?
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Quoting IKE:


It looks absolutely terrible and NC does not deserve a TS watch based on how pitifully awful it looks.



Looks dont mean everything.
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i leave for an hour and all of a sudden comedy hour starts..LOL
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908. jipmg
Quoting stormsurge39:
Is Danny being sheared? It looks worse than ever! It looks like its being stretched out like Taffy.


I think it has more to do with dry air? I really dont know, there isn't any significant shear over DANNY based on satellite...
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Quoting reedzone:


Oh I dunno.. Reedsville... Reedzone? sounds alot like my screename. People on here always bash me for advising people to watch a certain system.


Reedville, VA on the west shore of the Chesapeake, about 70 miles from Richmond
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Quoting NARCHER:
94l forecast to cane but a fish thanks to bill and danny. bermuda high has holes in it.


Ahhhh....look again.
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902. IKE
Quoting stormsurge39:
Is Danny being sheared? It looks worse than ever! It looks like its being stretched out like Taffy.


It looks absolutely terrible and NC does not deserve a TS watch based on how pitifully awful it looks.
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Quoting fmbill:


Okay...so regardless of strength, Dannny is not going to keep moving due west for the next 2 days.


Correct!
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Quoting carolinamomma:
Most NC news stations are reporting little or no effects expected from Danny.
So, yea.

Don't go near walmart if TS warnings are issued tonight or tomorrow.

We're close .... ....



[Link]
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Quoting BrockBerlin:


I originally thought it meant cone of uncertainty but now I realize the acronym would have made no sense.


I'm afraid I must blindlessly disagree with you...it actually makes PERFECT sense....

CONe of UnSertainty...
Member Since: August 13, 2007 Posts: 0 Comments: 10484
Quoting futuremet:


Careful, you might get a ban. Me, NRAmy, P451 got banned 24hrs ago for posting the Stormtop's weather center with director JFV image
I got banned too and I didn't even post an image. I just responded conveying my amusement! Somebody has an itchy trigger finger...
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Quoting SaoFeng:


I was pretty close to the Blue Ash tornado maybe 5 years ago but thats about it


Thanks for posting!

I actually went to Sycamore High School which was across the street from the F4 that roared through at like 545 in the morning. Always interested in the weather, I managed to sleep through the storm, as it passed about 2 miles north of my house and there was no wind or bad weather to speak of... Scary time to have a tornado that strong.
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Is Danny being sheared? It looks worse than ever! It looks like its being stretched out like Taffy.
Quoting carolinamomma:


Just don't ask what the CONUS is... you will be flogged. Not flAgged, flOgged.

(ps.... I had to google it... SO GLAD I didn't ask!)


Without using google I would have to say Continental US
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Quoting OSUWXGUY:


To answer your question, it's not so much the strength of Danny that will determine its East Coast impact...but rather if the CoC reforms further east or not.

The steering for all levels will yank this thing starting tomorrow mornign RAPIDLY to the NNW and the North. The upper level low over the Southeast currently is forecast to eject to the east as ridging over the western US starts to dig a trough over the Midwest.


Okay...so regardless of strength, Dannny is not going to keep moving due west for the next 2 days.
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Quoting Bayside:


irregardless!
LOL!!!!
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94l forecast to cane but a fish thanks to bill and danny. bermuda high has holes in it.
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888. jipmg
Danny's COC is drifting around!
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Quoting Dodabear:

Now what made you think he/she was mocking you????


Oh I dunno.. Reedsville... Reedzone? sounds alot like my screename. People on here always bash me for advising people to watch a certain system.
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Looks like the intensity models are in pretty close agreement (hey, doesn't happen often) up through about a day and a half...so are they all right or all wrong? We shall see.



Wonder how much they will change once the 18 Z comes out later.
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Its latitude to me is a big plus for this system and kinda reminds me of Hurricane georges back in 1998.
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stormsurge39

You have mail
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Quoting presslord:


aw...go ahead...I do it all the time...


Just don't ask what the CONUS is... you will be flogged. Not flAgged, flOgged.

(ps.... I had to google it... SO GLAD I didn't ask!)
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Quoting BrockBerlin:


winds of 20kts... what newspaper uses kts are they writing for boaters?


Yes, they write for boats in their Marine forecasts...
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Quoting presslord:


aw...go ahead...I do it all the time...

I will say this. I was always taught that if you don't know about something it is best not to even comment on it.
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Quoting atmoaggie:

Ohio gets their share in spring, but that particular outbreak was The tornado outbreak of the century. (Somewhat like the way your university puts "The" before their name lest anyone confuse Ohio State with another Ohio State...happens up there, I guess).


Ha Ha. "The" before Ohio State is admittedly goofy... There is an Ohio University that could make it a bit confusing I suppose...but now I think its more of a schtick thing than anything.

I lived down in Big 12 country when I went to grad school at Oklahoma. I have a question for you, when are the schools in the Big 12 going to learn to play defense??? Admittedly OSU's offense has sucked lately, but we at least slowed Texas down in the Fiesta last year, and Florida did the same to Oklahoma in the NC game.
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Quoting reedzone:


Nice try wise guy! I'll report you if you ever think of mocking me again. I'm not a wishcaster and models have proved my point of it moving a bit west.

Now what made you think he/she was mocking you????
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Danny looks like a squall line

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Everybody gets 40mph gust every winter when strong cold fronts blast through
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somewhere on WU is a pretty good glossary...if you can find it, it's a good resource...

just don't read the section on "blindlessly"....
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Quoting SaoFeng:


I was pretty close to the Blue Ash tornado maybe 5 years ago but thats about it

That one we are talking about was 148 nadoes over the course of about 18 hours. A number of them went through towns. 300+ killed. Like I said, The nado outbreak.

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Be careful what you say about "disorganized, sloppy, etc." Danny. He still has the chance to strengthen when the environment gets more favorable.
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Quoting nolacane2009:


Thank you very much. I will see if I can find it. No one here will ever have to worry about me making stupid comments though.


aw...go ahead...I do it all the time...
Member Since: August 13, 2007 Posts: 0 Comments: 10484
WCTI in eastern NC said 20-30mph east of 17...
Member Since: August 22, 2006 Posts: 0 Comments: 1455
Quoting presslord:


somewhere on WU is a pretty good glossary...if you can find it, it's a good resource...


Thank you very much. I will see if I can find it. No one here will ever have to worry about me making stupid comments though.
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Quoting presslord:
he may not have any blinds...but he has a really cute shower curtain...


I'd say that is a BURN...
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Quoting MILLERTIME1:
GFLD model develops Danny into cat cat 3? Just off the coast of Cap Code.


aint gonna happen with cold waters up there, common sense
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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.