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 Weathermandan:
Are the Hurricane Hunters in Danny yet? Have they found anything that supports that swirl east of the original center being a new developing LLC? Or not yet? I feel like the track is almost impossible to pin down...

My thoughts-is this correct? (2 secenerios for the track)
1. Convection builds over the original center, the storm clips eastern North Carolina and eastern Long Island, and makes landfall in southeastern New England.
2. A new LLC develops, and the storm follows a track like that of the HWRF, passing well offshore of North Carolina, and moving over 40N/70W.


Looks like the first scenario to a point of clipping NorthCarolina. Then the storm will turn more to the Northeast and proceed to Yarmouth, part of Canada. Look at this link, itll show you what Danny may do.

It may even clip Boston, as the track says.

Here is the link. Do you want a wind shear map too? Ill give you all a link to the site!
Here is Storm Pulse:
Link

And here is the wind shear site:
Link

DOes it look good for Danny as is proceeds? No, not in my opinion. It looks as it he turned too far to the left, he may get ripped apart.
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Quoting antonio28:


Missing the caribbean. Another Fish?

It's way to early to declare 94L a fish storm. Once it makes it to 15N then we should have some idea of where it is going.
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Quoting IKE:


Could be...has model support.


Yep. Since ECMWF and UKMET saw it that way two days ago.
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Our Golden Panther is back...

Hey WS, what are your westward thoughts on danny?
Member Since: March 10, 2006 Posts: 0 Comments: 10034
Looks like it's got 2 COC's to me. Maybe it's an optical illusion?
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602. IKE
Quoting antonio28:


Missing the caribbean. Another Fish?


Could be...has model support.
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Quoting IKE:
12Z HWRF on 94L....


Missing the caribbean. Another Fish?
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Looks like wind shear might hinder the Tropical Disturbace a little too.
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Are the Hurricane Hunters in Danny yet? Have they found anything that supports that swirl east of the original center being a new developing LLC? Or not yet? I feel like the track is almost impossible to pin down...

My thoughts-is this correct? (2 secenerios for the track)
1. Convection builds over the original center, the storm clips eastern North Carolina and eastern Long Island, and makes landfall in southeastern New England.
2. A new LLC develops, and the storm follows a track like that of the HWRF, passing well offshore of North Carolina, and moving over 40N/70W.
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We now have orange on 94L
Member Since: September 2, 2007 Posts: 178 Comments: 20439
Quoting presslord:


yup KEH...DoubleBranch gets it...looks like bad lookin weather comin to us goin over the Stono Bridge onto Johns Island...
On and off here. I think most of the thunder I am hearing is from your direction.
Not worried about Danny. In any case, hurricane shelf has been supplied since June.
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Quoting leelee75k:
is it possible for Danny with his current split personality disorder separate into two completely different storms? one with the convection and one without, or does the one without the convection becomes something else?


I think the eastern rotation is in the mid-levels right now, and not at the surface.

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Quoting Weathermandan:
CODE ORANGE!!
A TROPICAL WAVE IS LOCATED OVER THE FAR EASTERN ATLANTIC OCEAN ABOUT
350 MILES SOUTH-SOUTHWEST OF THE CAPE VERDE ISLANDS. THE ASSOCIATED
SHOWER ACTIVITY CONTINUES TO SHOW SIGNS OF ORGANIZATION...AND SLOW
DEVELOPMENT OF THIS SYSTEM IS POSSIBLE OVER THE NEXT COUPLE OF DAYS
AS IT MOVES WESTWARD AT 10 TO 15 MPH. THERE IS A MEDIUM CHANCE...
30 TO 50 PERCENT...OF THIS SYSTEM BECOMING A TROPICAL CYCLONE
DURING THE NEXT 48 HOURS.


... and there is pretty good model agreement that it will be held south by a very broad high pressure ridge building across the Atlantic. I don't see this one recurving north, but it's early going.

Hopefully it will get blasted apart by shear at some point, or another low trough will come along (the models are not presently calling for that).

- WTO
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584. IKE
Quoting Chiggy007:
NHC didn't even give it an orange color at 2pm outlook!!
Seems like 94L looks very good convection wise but it might be lacking a closed surface low..!?

Throw all the models out for now...


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Quoting Chiggy007:
NHC didn't even give it an orange color at 2pm outlook!!
Seems like 94L looks very good convection wise but it might be lacking a closed surface low..!?

Throw all the models out for now...


One more time... It did get an Orange Circle and it is getting better organized.

Yes, teh mdoels are not that great on a system that hasn't formed yet so we will just have to wait.
Member Since: March 10, 2006 Posts: 0 Comments: 10034
Ok, I take it back - spoke too soon! Orange circle now..
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Quoting IKE:
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
200 PM EDT THU AUG 27 2009

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

THE NATIONAL HURRICANE CENTER IS ISSUING ADVISORIES ON TROPICAL
STORM DANNY...LOCATED ABOUT 550 MILES SOUTH-SOUTHEAST OF CAPE
HATTERAS NORTH CAROLINA.

A TROPICAL WAVE IS LOCATED OVER THE FAR EASTERN ATLANTIC OCEAN ABOUT
350 MILES SOUTH-SOUTHWEST OF THE CAPE VERDE ISLANDS. THE ASSOCIATED
SHOWER ACTIVITY CONTINUES TO SHOW SIGNS OF ORGANIZATION
...AND SLOW
DEVELOPMENT OF THIS SYSTEM IS POSSIBLE OVER THE NEXT COUPLE OF DAYS
AS IT MOVES WESTWARD AT 10 TO 15 MPH. THERE IS A MEDIUM CHANCE...
30 TO 50 PERCENT...OF THIS SYSTEM BECOMING A TROPICAL CYCLONE
DURING THE NEXT 48 HOURS.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.

$$

Break out the orange crayon...
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Quoting Chiggy007:
Shouldn't really read too much in to the models for 94L - it hasn't even deserved an orange circle from NHC yet... LOL!

All I can tell is that it's moving WEST for now...


now it got the orange circle
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The models showing a westward track for 94L toward the Lesser Antilles over the next 5-7 days are all shallower models. If the storm develops and has any depth at all, it will most likely follow a GFS/HWRF track. Interesting to note that I couldn't find it on the ECMWF. Of course, anything can happen, as it is pretty far south, but with all the weaknesses in the ridge, the usually reliable HWRF might not be all that far off.
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well thanks for all the interesting comments, im out, i have an english class to go to :P later guys!
Member Since: June 28, 2006 Posts: 24 Comments: 8201
hey everyone...enjoying some time off in Indian Rocks beach...great place!
Heading back to Cayman on Monday and wondering if 94L has a chance getting into our area?
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is it possible for Danny with his current split personality disorder separate into two completely different storms? one with the convection and one without, or does the one without the convection becomes something else?
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575. 7544
is that another jog sw in the last frame
Member Since: May 6, 2007 Posts: 0 Comments: 6690
NHC didn't even give it an orange color at 2pm outlook!!
Seems like 94L looks very good convection wise but it might be lacking a closed surface low..!?

Throw all the models out for now...
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Quoting Chiggy007:
Shouldn't really read too much in to the models for 94L - it hasn't even deserved an orange circle from NHC yet... LOL!

All I can tell is that it's moving WEST for now...


It's orange now..
Member Since: March 10, 2006 Posts: 0 Comments: 10034
Quoting btwntx08:

yeah but i really doubt that it looks to outlier for now other models have it almost due west at this point

That's what I thought, our experts have been saying that there is no weakness for at least 7-10 days to take 94L out to sea. I just wish these models would make up there minds.
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thank you
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Quoting IKE:
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
200 PM EDT THU AUG 27 2009

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

THE NATIONAL HURRICANE CENTER IS ISSUING ADVISORIES ON TROPICAL
STORM DANNY...LOCATED ABOUT 550 MILES SOUTH-SOUTHEAST OF CAPE
HATTERAS NORTH CAROLINA.

A TROPICAL WAVE IS LOCATED OVER THE FAR EASTERN ATLANTIC OCEAN ABOUT
350 MILES SOUTH-SOUTHWEST OF THE CAPE VERDE ISLANDS. THE ASSOCIATED
SHOWER ACTIVITY CONTINUES TO SHOW SIGNS OF ORGANIZATION
...AND SLOW
DEVELOPMENT OF THIS SYSTEM IS POSSIBLE OVER THE NEXT COUPLE OF DAYS
AS IT MOVES WESTWARD AT 10 TO 15 MPH. THERE IS A MEDIUM CHANCE...
30 TO 50 PERCENT...OF THIS SYSTEM BECOMING A TROPICAL CYCLONE
DURING THE NEXT 48 HOURS.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.

$$


Invest to go orange.
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94L a hurricane in 72h according to the SHIP... this explains the predicted northward movement. However a weaker system should follow a more westerly motion.
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Quoting jasoniscoolman10:
Invest 94 in 72 hours will be a hurricane i do not think so........




Care to argue your point with facts?

Not saying it will or won't be...just that the deep easterly flow around 94L is supportive of quick intensification if something were to develop...
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myrtle1...yea...I think we're gonna be just dandy...
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525 yes, there have been other days, Isabel could also count. But Bill had such good conditons for such a long time, it has to stand out as one of the best. I will say this though, with so much power, lots of waves were closeouts, and they weren't the easiest to drop in to.
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CODE ORANGE!!
A TROPICAL WAVE IS LOCATED OVER THE FAR EASTERN ATLANTIC OCEAN ABOUT
350 MILES SOUTH-SOUTHWEST OF THE CAPE VERDE ISLANDS. THE ASSOCIATED
SHOWER ACTIVITY CONTINUES TO SHOW SIGNS OF ORGANIZATION...AND SLOW
DEVELOPMENT OF THIS SYSTEM IS POSSIBLE OVER THE NEXT COUPLE OF DAYS
AS IT MOVES WESTWARD AT 10 TO 15 MPH. THERE IS A MEDIUM CHANCE...
30 TO 50 PERCENT...OF THIS SYSTEM BECOMING A TROPICAL CYCLONE
DURING THE NEXT 48 HOURS.
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Quoting KEHCharleston:
542. DoubleBranchGuy
; ) Bless your heart.
If I may add. You are either from Charleston or you are from 'off'. Doesn't matter whether it is Colombia, South American or Columbia, SC.


yup KEH...DoubleBranch gets it...looks like bad lookin weather comin to us goin over the Stono Bridge onto Johns Island...
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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.