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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T.S Danny is overhyped....he looks to be a mess on imagery and I will be shocked if he proves to make a landfall anything stronger than what Hannah was.
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Quoting stormsurge39:
If Danny keeps up with the West track, our conditions favorable for strengthing?


Still seeing a fair amount of shear to his west; he'd have to drop WSW towards SEFL. Not much chance of really rapid development in that environment
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GFLD model develops Danny into cat cat 3? Just off the coast of Cap Code.
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SaoFeng. That was 10 years ago, wasn't it? F4 that killed 5 people 1999. I remember because I skipped school that day not wanting to drive through that on my way down to Cincy(7 Hills).
Member Since: August 22, 2006 Posts: 0 Comments: 1462
Quoting BrockBerlin:
I am pretty sure (about 1.03 % sure) 94L will hit Reedville on the central coast of Virginia with winds of 134.567 mph in 12 days and 4 hours.


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.
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Quoting nolacane2009:
I just want to say that I really half the time do not understand what I am reading because I do not understand weather Terminology. I will say I do enjoy coming here and getting some incite to what is going on being that I live in New Orleans. I like to be prepared to leave before our lazy government tells us to leave. So basically I just want to thank all of you.


somewhere on WU is a pretty good glossary...if you can find it, it's a good resource...
Member Since: August 13, 2007 Posts: 0 Comments: 10492
Virginia newspaper have said little effects, perhaps winds of 20kts.
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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).


I was pretty close to the Blue Ash tornado maybe 5 years ago but thats about it
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I just want to say that I really half the time do not understand what I am reading because I do not understand weather Terminology. I will say I do enjoy coming here and getting some incite to what is going on being that I live in New Orleans. I like to be prepared to leave before our lazy government tells us to leave. So basically I just want to thank all of you.
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Danny is still a mess....I've looked in 3 ways and I still can't tell where a COC is gonna be.
Member Since: August 22, 2006 Posts: 0 Comments: 1462
Quoting fmbill:


So...if Danny is declared a depression at 5pm, how will that affect the forecast path?



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.
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If Danny keeps up with the West track, our conditions favorable for strengthing?
Texas should be good this year, as long as these troughs consistently are coming down. Now later in the fall, systems in the BOC can spin up but cold fronts will save us and push whatever is out there to Florida.

Supposed to be some kind of moisture surge into TX next week models keep picking up on, be nice to get rain.
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Quoting OSUWXGUY:


Nope. I missed out on that tragic day. Born in 1982. Ohio always seems to get the right combo of strong wind shear and just enough returning moisture from the Gulf in early to mid-April to cause some really bad weather...

My parents lived in Cincy and saw the tornado skipping across the hills of Cincy on that day though!

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).
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Looking back some days ago, didn't those earlier model runs have Danny hugging close to the Florida coast and then tracking up toward NC and the northeast?

Quoting weatherman4189:
Meterologist Denis Phillips in Tampa. Just posted on his twitter that the weaker danny stays the more likely it will head west and may eventually affect FLA.
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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.
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Quoting DestinJeff:
This is meant in good fun ONLY to our friend WS:

img src="
Free Myspace Generators" alt="" />


Careful, you might get a ban. Me, NRAmy, P451 got banned 24hrs ago for posting the Stormtop's weather center with director JFV image
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2pm

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since models tend to be polewardly biased in long range runs. We will just have to wait and see.

espcially the GFS, which is the main one that goes out that far.



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Quoting jasoniscoolman10:
maybe at 5pm North Carolina will have Tropical Storm Watch


It's been a bit aggravating, people in North Carolina need to know what is "possibly" coming to them. Also a Tropical Storm Watch should be posted all the way up to Cape Cod, MA. Would not surprise me that the watch would be a Hurricane Watch on the Carolinas. They need some sort of watch on the coastline.
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Quoting atmoaggie:

I don't know how old you are, but wow, that is the day the "Super Outbreak" started...
I'll wager you weren't around and in Xenia.


Nope. I missed out on that tragic day. Born in 1982. Ohio always seems to get the right combo of strong wind shear and just enough returning moisture from the Gulf in early to mid-April to cause some really bad weather...

My parents lived in Cincy and saw the tornado skipping across the hills of Cincy on that day though!
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Quoting futuremet:


It means there will be ridging at the east coast, which favors westward movement. BUT, as the Bermuda high strengthens, the models are forecasting the Azores high to weaken (typical for negative NAO), and slight troughing may occur in the mid-Atlantic. This may cause storms to move farther north. However, I don't think it will be sufficient to send something out to sea, since models tend to be polewardly biased in long range runs. We will just have to wait and see.



Thank you!
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Quoting TexasHurricane:


Which is what exactly? Sorry, do not know how to read all these maps....What does it mean?


It means there will be ridging at the east coast, which favors westward movement. BUT, as the Bermuda high strengthens, the models are forecasting the Azores high to weaken (typical for negative NAO), and slight troughing may occur in the mid-Atlantic. This may cause storms to move farther north. However, I don't think it will be sufficient to send something out to sea, since models tend to be polewardly biased in long range runs. We will just have to wait and see.

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Quoting iluvjess:
"If Danny is a depression at 5pm, how will that affect the forecast path?"

The declaration of a system as a depression or a storm has no effect on the forecast path. Ttopical cyclones are not steared by declarations.


I believe he was hinting as to if Danny continues to fall apart, how would that affect the steering. Obviously, the storm does not respond to how it is classified by the NHC.
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It looks like someone thinks baby Danny will go a little more west, than earlier thought?

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Quoting TexasHurricane:


Hi There local...or I think you are pretty local.... What is your take on this possible soon to be Erika?


I don't worry about systems that far out, only concern when in NW Carribbean, Gulf, or Florida Straits moving in our general direction
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Jeff you are killing me man.
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Wave across eastern atlantic looks like its has continue to organize.I looked at the GFS ensembles and they have consistently been showing a trough rebuilding over the western Atlantic in the 11-15 day range so we'll see what happens.Not really buying into a quick recurve as the HWRF suggest.
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"If Danny is a depression at 5pm, how will that affect the forecast path?"

The declaration of a system as a depression or a storm has no effect on the forecast path. Ttopical cyclones are not steared by declarations.
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latest NWS discussion for PR


THE GFS MODEL GUIDANCE SUGGESTS THAT
BY LATE MONDAY/EARLY TUESDAY NEXT WEEK...A TROPICAL WAVE WILL
APPROACH TO THE REGION INDUCING A GENERAL INCREASE IN CLOUDINESS
AND SHOWER ACTIVITY ACROSS THE LOCAL ISLANDS. THE SJU-GFS LAYER
PRECIPITABLE WATER IMAGE SUGGESTS UP TO 2.32 INCHES OF PWAT ACROSS
THE REGION BY THAT TIME. GOOD INDICATION THAT THE PEAK OF THE WET
SEASON WILL BE WITH US VERY SOON. STAY TUNED.



Are they talking about 94L? I have some doubts
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819. N3EG
Quoting HurricaneCavalier:
...ooops... I should have done it blindlessly instead...where's a towel?


I can't hardly understand what he said.
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Quoting OSUWXGUY:


Happy Birthday Storm!

Kind of fun having stuff to track on your birthday...

No TCs on my birthday usually (April 3)...but we've been known have a few 'nados on that day in the past!

I don't know how old you are, but wow, that is the day the "Super Outbreak" started...
I'll wager you weren't around and in Xenia.
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Quoting RitaEvac:


They already got Florida DOT running signs on the freeway telling you where its going, so they suspect it as well.


Hi There local...or I think you are pretty local.... What is your take on this possible soon to be Erika?
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Ive got an ailing Grandmother and a Aunt that are up in age,living together in Melbourne Fl. My point is there seems to be alot of doubt where the COC is and what direction it is going. Should the Fl. coast have anything to worry about? Danny seems to be getting closer and closer!!
he has a shower curtain in the outhouse?
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he may not have any blinds...but he has a really cute shower curtain...
Member Since: August 13, 2007 Posts: 0 Comments: 10492
Quoting DaytonaBeachWatcher:

Does anyone think that he types his post and then uses a thesaurus??

Now you see why he is an ignore list favorite. Unbelievable how he is allowed on here.
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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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