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 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.
Member Since: August 28, 2006 Posts: 6 Comments: 2899
Dannys models have trended westward today, figured that would be the case because of the center jog. I'm still confused on why North Carolina is not under at least a Tropical Storm Watch. Somebodies slacking lol.
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Quoting futuremet:
This will be the new pattern soon...




Which is what exactly? Sorry, do not know how to read all these maps....What does it mean?
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Quoting WeatherStudent:
It takes it into the Carib. as I suspected.


They already got Florida DOT running signs on the freeway telling you where its going, so they suspect it as well.
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Once again, it appears Danny is shedding its exposed center and trying to reform a new center further to the northeast. Bess guesstimate would be near 28°N 71°W for the location of this newly forming center.

The exposed circulation can be seen in the last several frames of the satellite loop to become more elongated SW-NE and it also losing velocity.
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NWS Melbourne...

FRI NIGHT-SUNDAY...BOTH DANNY AND THE UPPER LEVEL LOW TRANSITING
ACROSS THE DEEP SOUTH WILL BE WELL TO THE NORTH OF THE AREA BY
FRIDAY NIGHT...
LEAVING THE MEAN MIDWEST/EAST COAST TROUGH IN PLACE
FOR THE WEEKEND. TROUGH WILL BE SUFFICIENT TO KEEP THE WRN
ATLANTIC RIDGE SUPPRESSED SOUTH OF THE STATE...KEEP EC FLORIDA IN
DEEP SWRLY FLOW. AS WE HAVE SEEN OFTEN THIS SUMMER...THIS FLOW
WILL LIMIT THE INLAND PROGRESSION OF THE ECSB AND FAVOR THE EAST
COAST FOR AFTERNOON CONVECTION. TEMPERATURES WILL CLIMB A FEW
DEGREES ABOVE NORMAL...ESPECIALLY FOR COASTAL LOCATIONS WITH A
LATE STARTING SEA BREEZE.

LOL!!! Danny better get a move on if it's supposed to be out of here in 24 hours. :-)
Member Since: May 27, 2008 Posts: 0 Comments: 459
This will be the new pattern soon; the pattern will shift, and the troughing will be more predominant at the west coast.


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Thank you Presslord!!
Ya'll have a good day now.....
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Good afternoon, everyone!

I will have a graphics update shortly.
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Oh no...no no no no...not gonna do it.

wimp...

;)
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AL 94 2009082718 BEST 0 107N 270W 25 1009 DB
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well im gonna blindessly bet that 94l is going to hit the carolinas....

that is my third offense press i am now waiting on my flogging.
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by granny...went to same school as press...moved to fl in 2004 and have enjoyed WU ever since
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Quoting fmbill:
If Danny is a depression at 5pm, how will that affect the forecast path?


I would assume a much more western track.
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Is Danny stalled?
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Quoting Elena85Vet:


All I know is it locked up dictionary.com


LOL
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Quoting DaytonaBeachWatcher:


Ok Press, lets add them up, how many points did she score in the first half of the post, as oppopsed to how many she lost at the end....?


lol...she's still way ahead...
Member Since: August 13, 2007 Posts: 0 Comments: 10492
If Danny is a depression at 5pm, how will that affect the forecast path?
Member Since: May 27, 2008 Posts: 0 Comments: 459
Danny 18Z model update


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LLC of Danny trying to establish itself back eastward around 28N/70W, imo
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Quoting futuremet:
Schools great WS...last year in high school.


You wear a suit to school?
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This is now old news, but I had to run out for a bit, but thanks for the nice words KEHCharleston and Presslord, even if you don't like my football loyalties:-) As a former outside salesmen from North Carolina who had to try to get business in Charleston, I KNOW how y'all feel about your beautiful city...lol...I know all to well. So happy I don't have to beat my head up against that wall everyday anymore. I absolutely love the Charleston area though, and if there was ever a place to feel so smug about, that would be it. I would sure like to end my day today sitting on the deck at Red's in Shem Creek, watching the dolphins and drinking several cold beers. Then I'd go hang out at the Blind Tiger downtown...that place has one of the best outside courtyards to hang at I've ever been to. LOVE IT. Sorry to be off topic. I'll shut now Mr. Blog
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12z ECMWF....

ECMWF
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Quoting Floodman:


No, he meant blindlessly...


LOL
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Quoting jasoniscoolman10:
look at the models now.. some of the models of tropical storm danny hitting NC NOW..

Definitely a bit of a shift west. In addition, please, show some respect. I have an MBA and I stayed at a holiday inn express last night.
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Wow. Thats the worst comment I have ever seen written on this blog.

really? ok, guys, can we post the Top Ten put-downs from the main blog to catch this guy up to speed?
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Schools great WS...last year in high school.
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How's this for and "extrapolated"?

SFWMD
Member Since: May 27, 2008 Posts: 0 Comments: 459
Quoting cchsweatherman:


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.


Some of these comments are WAYYY over the top.
Happy Birthday StormW !!! I always look forward to reading your blog :) I hope that the rest of this evening is awesome for your birthday. Go have some fun!!!!
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Quoting ncleclerc:
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!!!!


Ok Press, lets add them up, how many points did she score in the first half of the post, as oppopsed to how many she lost at the end....?
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Quoting ncleclerc:
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!!!!


OK...now I'm blushing...xoxoxoxo
Member Since: August 13, 2007 Posts: 0 Comments: 10492
"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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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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