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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"blindlessly...
is such a lonely word...
everyone just doesn't have a clue...

Blindlessly..
Is hardly ever heard...
unless you went to FIU......"
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Quoting JupiterFL:


I think you meant blindly.
Blindlessly is a double negative.


All I know is it locked up dictionary.com
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Quoting FLdewey:
Listen JFV since you're obviously new to gambling I'll break the bad news to you. You have to bet with something that's actually worth something.


Wow. Thats the worst comment I have ever seen written on this blog. No matter how annoying someone may be, there's no reason to make such a comment. I hope you get banned for that.
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Presslord You are like a ray of sunshine. I look forward to everyday to read what you are going to say and you take just as much dish out. A great guy with a awesome sense of humor!!
p.s. a central North Carolina girl!!!!
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Good Evening folks...Just popping in...Happy Birthday StormW. Enjoy ur day. Is there any chance Erika coming to the West Indies? Or is it too soon to tell?
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Quoting OSUWXGUY:
Taken from the 11AM NHC Discussion:

DANNY IS CURRENTLY IN AN AREA OF UPPER-LEVEL CONFLUENT FLOWASSOCIATED WITH THE PERSISTENT CYCLONIC SHEAR AXIS SEEN IN WATER
VAPOR IMAGERY. THE LARGE-SCALE MODELS FORECAST THIS FEATURE TO BE
REPLACED BY AN UPPER-LEVEL ANTICYCLONE DURING THE NEXT 24-30 HR...
AND IF THIS VERIFIES IT WILL GIVE DANNY ITS BEST CHANCE TO
STRENGTHEN.

Right now, it's this confluent flow that has really buggered up Danny...

You can see this confluent (or coming together) flow aloft on the satellite loop linked below.

First find the naked swirl. Now look at the bright white wispy clouds (cirrus) to the west and see them streaming east... Now look for the same type of clouds streaming to the west off of the big area of convection to the east of the LLC.

So these two areas of Cirrus are converging over or just to the east of the LLC. This converging causes sinking air and prevents convection. It is the opposite of the anticyclone which is typically over a tropical system that provide divergent flow aloft.


Now one final point from the satellite. You can see the cirrus streaming east and southeast on the east side of the area of deep convection. Strong divergence over this feature is helping support the deep convection.


Link


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

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Blindlessly, 150%!!
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For old time sake

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Quoting JupiterFL:


I think you meant blindly.
Blindlessly is a double negative.


No, he meant blindlessly...
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Quoting WeatherStudent:
Alright, ladies and gents. It's official, I'll bet my life on here this afternoon that 94L, soon to be Ericka, will 150% be an eventual US land threater, because I just do not see this one being a fish storm. Thanks, all.



150%?????
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Quoting DestinJeff:


ROFL!!!!!
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Taken from the 11AM NHC Discussion:

DANNY IS CURRENTLY IN AN AREA OF UPPER-LEVEL CONFLUENT FLOW ASSOCIATED WITH THE PERSISTENT CYCLONIC SHEAR AXIS SEEN IN WATER
VAPOR IMAGERY. THE LARGE-SCALE MODELS FORECAST THIS FEATURE TO BE
REPLACED BY AN UPPER-LEVEL ANTICYCLONE DURING THE NEXT 24-30 HR...
AND IF THIS VERIFIES IT WILL GIVE DANNY ITS BEST CHANCE TO
STRENGTHEN.

Right now, it's this confluent flow that has really buggered up Danny...

You can see this confluent (or coming together) flow aloft on the satellite loop linked below.

First find the naked swirl. Now look at the bright white wispy clouds (cirrus) to the west and see them streaming east... Now look for the same type of clouds streaming to the west off of the big area of convection to the east of the LLC.

So these two areas of Cirrus are converging over or just to the west of the LLC. This converging causes sinking air and prevents convection. It is the opposite of the anticyclone which is typically over a tropical system that provide divergent flow aloft.


Now one final point from the satellite. You can see the cirrus streaming east and southeast on the east side of the area of deep convection. Strong divergence over this feature is helping support the deep convection located to the east of the LLC. All the strong bands of surface convergence are streaming into this area instead of the LLC. This is why the LLC is not taking over and strengthening...


Link
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Quoting presslord:
All will be forgiven when you dawgs beat the Gators for me...please tell me they will do that!!

from your lips to God's ear, my friend...


Not gonna happen, keep dreamin
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Quoting WeatherStudent:
Alright, ladies and gents. It's official, I'll bet my life on here this afternoon that 94L, soon to be Ericka, will 150% be an eventual US land threater, because I just do not see this one being a fish storm. Thanks, all.


You should bet your Weather Underground account. Man it was nice this morning, well thats gone.
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Quoting WeatherStudent:
Alright, ladies and gents. It's official, I'll bet my life on here this afternoon that 94L, soon to be Ericka, will 150% be an eventual US land threater, because I just do not see this one being a fish storm. Thanks, all.


The ULTIMATE wishcast...
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Quoting WeatherStudent:
Alright, ladies and gents. It's official, I'll bet my life on here this afternoon that 94L, soon to be Ericka, will 150% be an eventual US land threater, because I just do not see this one being a fish storm. Thanks, all.


Do you promise to leave the blog forever if you are wrong, cuz i will contact bill gates, to see if he can help us here.
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ROTFLMAO!!!! This is just getting too good to miss out on.
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Would you people quit quoting WS? There's a reason we have him on ignore.
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All will be forgiven when you dawgs beat the Gators for me...please tell me they will do that!!

from your lips to God's ear, my friend...
Member Since: August 13, 2007 Posts: 0 Comments: 10479
Quoting WeatherStudent:
Alright, ladies and gents. It's official, I'll bet my life on here this afternoon that 94L, soon to be Ericka, will 150% be an eventual US land threater, because I just do not see this one being a fish storm. Thanks, all.


I would say it has a 75% chance of not going out to sea, because the anomalous troughing pattern is expected to halt by the second week of September. It is too early if this thing will affect the U.S. or if it will degenerate like Ana-Banana did.
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Quoting WeatherStudent:
My fault. I meant to write blindlessly.


I think you meant blindly.
Blindlessly is a double negative.
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That's a real live sign in Florida folks, WS take cover
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Quoting WeatherStudent:
Alright, ladies and gents. It's official, I'll bet my life on here this afternoon that 94L, soon to be Ericka, will 150% be an eventual US land threater, because I just do not see this one being a fish storm. Thanks, all.


at long last...some hope...
Member Since: August 13, 2007 Posts: 0 Comments: 10479
Quoting NRAamy:
actually...StormW told me privately that he is 100% certain that 94 is gonna become a Cat 5 and obliterate SEFLA...

better start putting up that plywood on the ol' outhouse now...

don't go there Amy - you know what happened last time ;)
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Quoting KEHCharleston:

Let's see if I have this right
CMC for tracking (once something has developed - CMC too quick to develop cyclones that are not there)
SHIPS - for intensity?
?????- For pinning down where a storm will develop?


The CMC is cyclogenically biased, but handles synoptic scale features with great efficacy. The GFS is mediocre at all fields. The ECMWF seems to be well balanced, and performs relatively well at all fields. The UKMET and the NOGAPS tend to be too conservative, and the UKMET has the propensity to push storms further west.
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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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Good afternoon all! Just wanted to stop in this afternoon while at campus to wish someone who I have come to respect and look up to for guidance a Happy Birthday to StormW. Continue the excellent work and enjoy your special day.

In regards to Danny, its really surprising that the storm has been holding on considering all the hostile conditions coming from various directions with the ULL to the SW, a deep dry air surge from the SE, and dry air to the NW. In my mind, I don't envision this becoming a hurricane in the future given the current conditions and the fact that I'm thinking that it will interact with the OBX this weekend.

In regards to Invest 94L, its too early in the lifetime for the computer models to be taken with serious consideration. Allow the system to develop and progress before really giving much credence to the models. But, I must say this shows great promise for development as there are really no inhibiting factors in the disturbance's environment.
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27/1745 UTC 27.3N 73.4W T2.0/2.5 DANNY -- Atlantic

Rhut-Rho, somebody's in twouble.
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Quoting DestinJeff:




LOL had to jump out of lurker mode for this one!!!
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I am gonna need depends, to continue reading this blog.
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Quoting DestinJeff:




LMAO!!!!
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Quoting presslord:
...ah...you DID write "blindlessly"


LOL
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Quoting HurricaneCavalier:


Same for me.. Which over-acheiving school did you drunkenly pass through PressLord?


The University of By God Georgia...and let the record show...I am not necessarily disagreeing with your description of me...
Member Since: August 13, 2007 Posts: 0 Comments: 10479
Quoting futuremet:
Back from school

Dr M: 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.

Yes, the Canadian model performs well on tracking. I still think the ECMWF is more reliable, however.

Let's see if I have this right
CMC for tracking (once something has developed - CMC too quick to develop cyclones that are not there)
SHIPS - for intensity?
?????- For pinning down where a storm will develop?
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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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