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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wave projections......
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Quoting CycloneOz:


I have footage from Hurricane Bill of a young couple who had been drinking and then decided to head out into the big waves.

A really big wave hit them and drove them underwater. The guy popped up right away and for several seconds, he frantically looked for his girl friend. After several anxious moments, she popped up some ways down the beach.

Wise words, press...those near the water should heed them.

I remember back when I was a teenager on Long Beach on Long Island, back in 1977, a weak TS was 200 E of Long Island. My 16 yr old friend and I went body surfing in the 6-7' surf. After 10 minutes of getting SLAMMED into the sand, and pulled in every position imaginable, we got out and STAYED OUT of the water!! PLEASE stay OUT of the SURF from NC through NEW ENGLAND!!
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storm.....60mph at the surface of 94l?
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Quoting Dakster:


I don't think many ppl on the blog got that comment...

I DID LOL
Member Since: June 29, 2007 Posts: 0 Comments: 1136
Quoting yonzabam:


Check out the link on post 185.


Katrina rider here from Biloxi! I'm still Katrina riding, if you know what I mean, and would love to get off this roller coaster!
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357. WXHam
Do I see convection moving to Danny's COC?

Link
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Quoting cattlebaroness:
Thanks was wondering why there is more activity regarding evacuation in GOM than the east. Guess elevation is much greater in east.


depends on where in the East you are but there is surely more shoreline areas with steeper elevation increases than along the Gulf.... but there is also some areas of the east coast which are very similar. New York City and the surrounding shoreline communities is actually not too far off from that of New Orleans and its surrounding areas... and Long Island does not have many hills what so ever (many of Long Island's streams and flood channels were created by hurricanes of the past)
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Quoting TheCaneWhisperer:


My pleasure. Of note, there are more signs pointing to the ridging this time around as opposed to a crap-shoot.


Ill def be keeping my eye on that. So far we have been very lucky here in the GOM.
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@345, it has nothing to do with the elevation of the east vs. the gulf. it has to do with danny. the storm is weak, very lopsided, and there is a lot of forecast uncertainty. so evacuations probably aren't necessary in any case, and even if they were, you wouldn't know where to order them.
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Quoting CycloneOz:


Agreed. Senator Kennedy had a wealth of knowledge about driving in severe weather across bridges. He shall be missed...



I don't think many ppl on the blog got that comment...
Member Since: March 10, 2006 Posts: 0 Comments: 10534
Quoting presslord:
If you are a MARINER or BEACH GOER...STAY OFF AND AWAY FROM THE WATER!!!!! I CAN'T STRESS THIS ENOUGH!!!

from the birthday boys' blog...and well worth repeating...

again...and again...and again...


I have footage from Hurricane Bill of a young couple who had been drinking and then decided to head out into the big waves.

A really big wave hit them and drove them underwater. The guy popped up right away and for several seconds, he frantically looked for his girl friend. After several anxious moments, she popped up some ways down the beach.

Wise words, press...those near the water should heed them.
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Thanks was wondering why there is more activity regarding evacuation in GOM than the east. Guess elevation is much greater in east.
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Quoting presslord:
Link


pretty darned good NOLA post Katrina story...


Thanks Press...
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Quoting presslord:
If you are a MARINER or BEACH GOER...STAY OFF AND AWAY FROM THE WATER!!!!! I CAN'T STRESS THIS ENOUGH!!!

from the birthday boys' blog...and well worth repeating...

again...and again...and again...


Agree .....we lost 3 people last week from Bill ...let's not lose any this weekend......
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I can't imagine they'd be ordering evacuations for an asymmetrical tropical storm like Danny.
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Quoting alaina1085:


Thanks :)

And thanks Storm for the update as usual!


My pleasure. Of note, there are more signs pointing to the ridging this time around as opposed to a crap-shoot.
If you are a MARINER or BEACH GOER...STAY OFF AND AWAY FROM THE WATER!!!!! I CAN'T STRESS THIS ENOUGH!!!

from the birthday boys' blog...and well worth repeating...

again...and again...and again...
Member Since: August 13, 2007 Posts: 0 Comments: 10492
Quoting cattlebaroness:
At what cat level do they request evacuation. I would think some of the areas that have dense population would start considering thinning out the folks due to Danny's potential landfall.


evacuations are generally decided based on not just the strength of the storm, in fact in some situations this is one of the lesser concerns. Evacuations are usually based on the expected flooding and storm surge, which can be effect by the size of the storm as much as the strength. Therefor, a large Cat 2 can require as much evacuation as a small Cat 4. Of course, the elevation of any given area is also one of the primary concerns. I'm not 100% sure but I don't think expected wind speeds have much to do with the evacuations.
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336. 900MB
Quoting StormW:
Good afternoon!

Hot off the press!!

Thanks!

TROPICAL WEATHER SYNOPSIS /DANNY/ /94L/ AUG. 27, 2009 ISSUED 12:00 P.M. EDT


Thanks Storm and HAPPY BIRTHDAY!
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Naked swirl has electrical meaning towards organization of storm. Click my blog above for details.

MM
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Happy Birthday StormW!!! Thanks for the update.

For those of you who are presenting as heartless, have some respect for the Kennedy family, and any family who looses a loved one. Leave it alone and get back to the weather or get off the BLOG. Simple.
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2009 Storms
Active


Atlantic
94L.INVEST
05L.DANNY

East Pacific
12E.IGNACIO

Central Pacific
95C.INVEST
11E.HILDA

West Pacific
98W.INVEST
91W.INVEST
90W.INVEST

Indian Ocean

Southern Hemisphere


CHGHUR

TROPICAL CYCLONE GUIDANCE MESSAGE

NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL

1253 UTC THU AUG 27 2009



DISCLAIMER...NUMERICAL MODELS ARE SUBJECT TO LARGE ERRORS.

PLEASE REFER TO NHC OFFICIAL FORECASTS FOR TROPICAL CYCLONE

AND SUBTROPICAL CYCLONE INFORMATION.



ATLANTIC OBJECTIVE AIDS FOR



DISTURBANCE INVEST (AL942009) 20090827 1200 UTC



...00 HRS... ...12 HRS... ...24 HRS. .. ...36 HRS...

090827 1200 090828 0000 090828 1200 090829 0000



LAT LON LAT LON LAT LON LAT LON

BAMS 10.5N 25.6W 11.3N 28.3W 12.1N 31.2W 12.9N 34.4W

BAMD 10.5N 25.6W 10.7N 28.1W 11.0N 30.6W 11.4N 33.0W

BAMM 10.5N 25.6W 11.0N 28.4W 11.3N 31.3W 11.6N 34.1W

LBAR 10.5N 25.6W 10.8N 28.6W 11.4N 31.9W 12.0N 35.4W

SHIP 25KTS 29KTS 36KTS 47KTS

DSHP 25KTS 29KTS 36KTS 47KTS



...48 HRS... ...72 HRS... ...96 HRS. .. ..120 HRS...

090829 1200 090830 1200 090831 1200 090901 1200



LAT LON LAT LON LAT LON LAT LON

BAMS 13.3N 37.9W 13.4N 45.6W 13.3N 52.5W 13.4N 58.2W

BAMD 12.0N 35.3W 13.1N 39.0W 13.8N 41.8W 15.1N 44.1W

BAMM 12.0N 36.7W 12.8N 41.6W 12.9N 45.6W 12.7N 48.2W

LBAR 12.5N 38.9W 13.6N 45.3W 13.2N 44.6W 13.7N 46.3W

SHIP 58KTS 74KTS 76KTS 81KTS

DSHP 58KTS 74KTS 76KTS 81KTS



...INITIAL CONDITIONS...

LATCUR = 10.5N LONCUR = 25.6W DIRCUR = 270DEG SPDCUR = 15KT

LATM12 = 10.9N LONM12 = 22.6W DIRM12 = 263DEG SPDM12 = 15KT

LATM24 = 11.4N LONM24 = 19.5W

WNDCUR = 25KT RMAXWD = 0NM WNDM12 = 25KT

CENPRS = 1009MB OUTPRS = 1011MB OUTRAD = 200NM SDEPTH = D

RD34NE = 0NM RD34SE = 0NM RD34SW = 0NM RD34NW = 0NM



$$

NNNN
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318. I got it! :)
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Tropical storm watches will be posted for the Carolinas on the next advisory.
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While llc relocation is certainly a possibility, looking at vis loop, looks like the convection is catching back up to the current llc. Don't think it will relocate, but, as stated, certainly could.

Kind of like, I think Hanna did. Undressed and dressed three times, if I recall. Being that the convection was shearing out to the E and then shear would relax and the convection would move back over. As long as the mlc doesn't detach from the llc and is only tilted, current llc should hold. If it gets decoupled, then a new center could form.
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Quoting TheCaneWhisperer:



Ridging is "SUPPOSED" to build in early next week behind Danny and increase risk to Caribbean, SE Coast & GOM but, Troughing has been the predominant force time and time again this year so I will believe it when I see it.


Thanks :)

And thanks Storm for the update as usual!
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At what cat level do they request evacuation. I would think some of the areas that have dense population would start considering thinning out the folks due to Danny's potential landfall.
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Goodnight all, Will try to catch Barometer Bob's show in the morning.
Stay safe all.
Have a great birthday StormW
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Quoting usa777:
well 2 days from now is the 4 year anniversary of Katrina. Although I do find these storms fascinating, I would NEVER wishcast even a tropical storm on someone. I lived through Katrina in Bay St Louis Miss. and would love to hear from anyone else from the blog about thier experience. In my wildest dream I would of never thought a storm could turn so many people's lives around. Everytime I see an invest pop up it brings me back to that day. One of these days I would love to write about the things I witnessed for those few days. One of the wierdest memories of that day was a cat I saw clinging to a tree and you could just see how scared it was. Very surreal. I personally cant wait until hurricane season winds down. Hope everyone stays safe from Danny.


Check out the link on post 185.
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Quoting StormW:
Good afternoon!

Hot off the press!!

Thanks!

TROPICAL WEATHER SYNOPSIS /DANNY/ /94L/ AUG. 27, 2009 ISSUED 12:00 P.M. EDT

Enjoyed the update as usual StormW, in language anyone can understand.
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Something may be hapeneng around 28N, 78-79W.
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Quoting Acemmett90:

thanks


...wasted sarcasm
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well 2 days from now is the 4 year anniversary of Katrina. Although I do find these storms fascinating, I would NEVER wishcast even a tropical storm on someone. I lived through Katrina in Bay St Louis Miss. and would love to hear from anyone else from the blog about thier experience. In my wildest dream I would of never thought a storm could turn so many people's lives around. Everytime I see an invest pop up it brings me back to that day. One of these days I would love to write about the things I witnessed for those few days. One of the wierdest memories of that day was a cat I saw clinging to a tree and you could just see how scared it was. Very surreal. I personally cant wait until hurricane season winds down. Hope everyone stays safe from Danny.
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Quoting A4Guy:


You had better callthe NHC and tell them to update their forecast.


they won't update their forecasts unless this is more of a trend than what it has been, it has only been doing this since early this morning and it's only noon now. Definitely need to update the track if this continues for another 6 hours, assuming a new LLC doesn't form.
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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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