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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TO: StormW

You keep calling them!! We were on late last night and very early this morning discussing the westerward motion on the exposed Circulation of Danny. As usual, some comments were not so kind. Can not wait to hear your analysis. When do you think you shall have a fix on 94L. Just put that birthday cake down and get to work!!!
Member Since: Posts: Comments:
Quoting Acemmett90:

NOT NICE


homicide watch is more like it...
Member Since: August 13, 2007 Posts: 0 Comments: 10492
Quoting DestinJeff:
INTERESTS FROM THE CAROLINAS NORTHWARD TO NEW ENGLAND SHOULD MONITOR
THE PROGRESS OF DANNY


in related news .... Presslord is now on sucicide watch


It's confirmed. NHC does read this blog and presslord has an antagonist there. :)
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Ok, off to work :(
I will check in later...

Blog Update
Reflector site for those at work, which includes Weather456, daily update.


AOI

AOI

AOI

AOI
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108. P451
I think this speaks for itself...

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Quoting TheCaneWhisperer:


Depends on how long the motion continues.


NHC won't change much @ 11 in waiting to see if the center reforms or if this is just a wobble.
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If this storm effects northern virginia and knocks over my weak tree thats about to fall on my roof then there's going to be some problems....
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Good Morning everyone. HELLO 94L... Boy, that coc dance of Daddy is...interesting.
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Press, remember the thorazine you had for JFV, I think you may need it as the phrase will be repeated often, unfortunately.
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103. MahFL
Well Danny is moving at roughly 45 degrees from the NHC track....I wonder if they even looked at the visible sat pics ?
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A lot of storms tend to stair step . that is why the NHC goes by a 12 hr track, kind of a wait and see.
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If SC/NC are supposed to keep an eye on it, why didn't the cone change more than the ~10-15m that it did?
Member Since: August 22, 2006 Posts: 0 Comments: 1459
100. P451
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this image says it all...
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Quoting StormW:


I still say they read this blog! LOL! (j/k)

Maybe you could slip a link on their website to here! lol
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Quoting DestinJeff:
INTERESTS FROM THE CAROLINAS NORTHWARD TO NEW ENGLAND SHOULD MONITOR
THE PROGRESS OF DANNY


in related news .... Presslord is now on sucicide watch

NOT NICE
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Quoting TampaSpin:


Big TIME.......Virgiana and South Carolina could come into play.....

Yikes! I hope the folks along the east coast are keeping an eye on this. Hopefully people prepared before Bill came along.
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"really?? really nhc? its going to move that much to the north starting.... NOW? i doubt that."

REmebmer. The 11:00AM stamp does not mean that the forecast was generated at 11:00 AM. This is the release time. The info used to produce this forecast is most like 2 hours old. It takes several hours to compile all of the data before release. More than likely the visible that we are seeing now was not even available when this forecast was generated.
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Quoting TampaSpin:


Big TIME.......Virgiana and South Carolina could come into play.....

Come on now?
With a name like Danny he's a wimp.
He's afraid of the coast.
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Quoting Elena85Vet:


Go to upper right of this page. See 'Previous entries for 2009'. Click 'August'. Scroll down to the blog you were looking for (the one previous to this one). Go to the bottom of the blog and you'll see where you can retrieve entries.


Thanks Elena, it worked!
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Quoting VAbeachhurricanes:



really?? really nhc? its going to move that much to the north starting.... NOW? i doubt that.


still is a mystery to me how the cone of uncertainty can be so small at this point. i'm starting to think that they ONLY use computer models now to determine this, when I use to think they used at least a little bit of human knowledge as well.
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morning all, just want to say that i live on the coast here in MS and the local radar loops are very suspicious, its skips a lot of frames but i can still see a circulation just south of here in the gulf.
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Quoting hurricanehanna:

How MAJOR are we talking?


Depends on how long the motion continues.
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Depending on the eventual outcome of the final COC, SC may very well be in the mix here.
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Quoting StormW:


Don't know yet...ya'll gotta give me a chance to perform some analysis so I can post a forecast.


come on, come on, come on, come on
are ya done yet, are ya done yet?
Sorry that was my 8 year old typing
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Quoting hurricanehanna:

How MAJOR are we talking?


Big TIME.......Virgiana and South Carolina could come into play.....
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Agree Danny will not make the next TFP at 28.5N and 73W. Current heading won't allow it.
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Quoting pearlandaggie:
INTERESTS FROM THE CAROLINAS NORTHWARD

DOH!


the struggle to educate the masses is never ending...
Member Since: August 13, 2007 Posts: 0 Comments: 10492
Something just moved over head......it looked like it was white with a black line down the middle. Daggum it, I think we're goin back in the cone!
Member Since: August 22, 2006 Posts: 0 Comments: 1459
I know I've heard an explantion before, but for the life of me can't remember. What causes a COC to reform?
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94l looks a potential threat to Barbados and the Lesser Antilles early next weak. conditions in the EATL is very conducive for a major storm to develop. most of the global models are developing this system. all the factors are in place for a major storm. warm sst low vertical wind shear for the next seven days, very weaK sal and the A/B that is strenghening that will keep the system on a west track for a few days.
all interest in the lesser antilles should monitor the progress of this developing system. 94L is moving very slowly at 12-15 mph over warm waters. which gives lots of time to develop into a major hurricane
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Quoting VAbeachhurricanes:


how are they right on the money if the COC is 60 nm south of the forecast position?



Concur. I saw that woble in the GOES update. Still think NE NC and VA Beach need to be wary.
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Seeing the SST's approach 30 in that area, what is the total heat potential? I know we're a little west of Bill's upwelling, but we don't need a Claudette situation? Fortunately the trough coming along will provide some shear!
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INTERESTS FROM THE CAROLINAS NORTHWARD

DOH!
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Quoting StormW:


I still say they read this blog! LOL! (j/k)


I agree StormW...LOL
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really?? really nhc? its going to move that much to the north starting.... NOW? i doubt that.
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StormW - But what are the odds that that LLC just drops off and a new one forms to the NE under the convection? That would put it much closer to the track.

If the current COC ends up being the final one, I'd think the track would have to move substantially to the west.
Member Since: August 22, 2006 Posts: 0 Comments: 1459
Add roughly 100 miles to the distance Danny is from the Bahamas and that will give you a rough idea. The Bahamas is a chain of islands, some islands being closer than others, but the shortest distance is from Palm Beach to Nassua, which is between 50-75 miles.
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Quoting TheCaneWhisperer:


If that center doesn't reform they are going to need a MAJOR track adjustment soon.
Ya know what? Dr Masters just posted that the center keeps reforming in different locations. It has been doing this for days now.How can you REALLY tell which direction it is going? Last advisory Danny was moving N.W. LOL
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000
WTNT35 KNHC 271433
TCPAT5
BULLETIN
TROPICAL STORM DANNY ADVISORY NUMBER 5
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL052009
1100 AM EDT THU AUG 27 2009

...CENTER OF DANNY WOBBLES WESTWARD...

INTERESTS FROM THE CAROLINAS NORTHWARD TO NEW ENGLAND SHOULD MONITOR
THE PROGRESS OF DANNY. A TROPICAL STORM WATCH MAY BE REQUIRED FOR
PORTIONS OF THIS AREA LATER TODAY.


dang it NHC! sigh knew i was right to get my supplies!!
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"How many miles is Danny from FL. ?"

Not sure but I bet I could sling a grub on a quarter ounce jig from Miami beach and hit that COC!
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Quoting VAbeachhurricanes:


how are they right on the money if the COC is 60 nm south of the forecast position?

LOL, really
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"Got news for ya...he ain't movin NW now...last 3 hours of satellite loop imagery show a W motion, and he is south of the forecast point by at least 60nm."

Yup, in fact, the last frame (wobble) is slightly South of due West. I think he is being steared by lower level stearing currents than the models are recognizing.
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Quoting Bordonaro:
Well TS Danny is coming along nicely, although the LLC is oh, 125 miles to the W of the major convection. Looks like the DR and the NHC are right in the money! Let us hope that Danny does NOT undergo a period of rapid intensification. Looks like we're gonna have a Cat 1 Hurricane with 75-95MPH winds off of N Carolina. Everyone from New England to Nova Scotia, please heed to the advisories and warnings when they're issued! STAY OUT of the SURF!! And for 94L is looking like it will develop into a TD within about 48 hrs, everyone needs to keep an eye on this for next weekend.


how are they right on the money if the COC is 60 nm south of the forecast position?
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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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