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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963. WXHam
7:07 PM GMT on August 28, 2009
Danny's COC drifting a tad bit west of the track as of 1815 UTC

img src="Danny 8-29 1815 UTC" alt="" />
Member Since: July 1, 2008 Posts: 0 Comments: 33
962. jeffs713
8:07 PM GMT on August 27, 2009
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?

For intensity, I use a combo of SHIPS, GFDL, and HWRF, while keeping in mind that the GFDL and HWRF both tend to overdevelop storms before they are established.
Member Since: August 3, 2008 Posts: 16 Comments: 5875
960. StormChaser81
8:00 PM GMT on August 27, 2009
Quoting Hurricane009:
Why have they not updated 94 to a medium chance for development? There is low winds shear, not much dry air, and warm waters.


It is a medium potential.
Member Since: August 11, 2008 Posts: 0 Comments: 2315
959. klaatuborada
7:53 PM GMT on August 27, 2009
Earlier this morning I read through 400 posts here and could confirm with certainty that Danny was forming in the Atlantic.

Is this still the consensus of opinion? Have we finished the "West, East, West, East" debate and moved on to the "94L, African Wave" conversation? Are we still on the "ugly, where's the eye" point?

And most importantly, do I need to tie everything down for the weekend here on Cape Cod?

(Please note, this is an excellent site to "learn" sarcasm as well as met)
Member Since: August 15, 2004 Posts: 23 Comments: 392
957. nolacane2009
7:51 PM GMT on August 27, 2009
Quoting Floodman:


I'll do that! Landfalls are bad...been in one, Jeanne, a CAT3 and I won't stick around for a 4...I'll run with the rabbits!


After Katrina I run for a 2
Member Since: August 19, 2009 Posts: 0 Comments: 248
956. Floodman
7:48 PM GMT on August 27, 2009
Quoting justalurker:
if or any make land fall here..look me up will have a beer, show you my volunteer work i do, in my sake i hope we never get hit with one.


I'll do that! Landfalls are bad...been in one, Jeanne, a CAT3 and I won't stick around for a 4...I'll run with the rabbits!
Member Since: August 2, 2006 Posts: 10 Comments: 9922
955. IKE
7:48 PM GMT on August 27, 2009
NEW BLOG!!!!!!!!!!!!
Member Since: June 9, 2005 Posts: 23 Comments: 37858
954. justalurker
7:47 PM GMT on August 27, 2009
Quoting Floodman:


I can't say for sure...we get a landfall in SFLA I might be there in a forward office or soemthing...
if or any make land fall here..look me up will have a beer, show you my volunteer work i do, in my sake i hope we never get hit with one.
Member Since: August 18, 2009 Posts: 0 Comments: 157
953. fmbill
7:47 PM GMT on August 27, 2009
I find this very hard to believe. If this plays out, 2009 truly will be the year of the troughs!

Member Since: May 27, 2008 Posts: 0 Comments: 428
952. IKE
7:46 PM GMT on August 27, 2009
System moving through me here in the Florida panhandle looks better than Danny....

Member Since: June 9, 2005 Posts: 23 Comments: 37858
950. nolacane2009
7:45 PM GMT on August 27, 2009
Quoting atmoaggie:

If you want to look up a weather term, the AMS weather glossary is here: http://amsglossary.allenpress.com/glossary
Very useful resource for anyone, n00b through met.


Thank you so much. That is what I really like about this site everyone is so nice and informative.
Member Since: August 19, 2009 Posts: 0 Comments: 248
949. atmoaggie
7:45 PM GMT on August 27, 2009
He may not look like much at the moment, but just a few hours ago NHC said a 10% chance Danny is a cat 2 on Sat.

Member Since: August 16, 2007 Posts: 6 Comments: 12461
948. hurricanejunky
7:43 PM GMT on August 27, 2009
Quoting jipmg:


and it wont win, thats a mid level circulation and the surface low to the west which is DANN's COC is far too strong to lose against w.e forms under those T storms


makes sense...that naked coc is sure moving west though. any thoughts on when it'll make the turn? Just a delay of the inevitable?
Member Since: August 28, 2006 Posts: 6 Comments: 2895
947. pandora1783
7:43 PM GMT on August 27, 2009
Quoting jasoniscoolman10:
i am laught so hand right now look at crazy computer models some of the models saying moving west and some computer moving wnw and nw...this is tell you that the computer models are having a hard time telling you where the storm is going..i do not think in 102 having winds over 110 mph the modles are way off.


Looks like a three year old with some crayons
946. jipmg
7:42 PM GMT on August 27, 2009
Quoting hurricanejunky:
Danny is looking rather strange. It looks like the circulation in the east-side convection is not winning the battle with the naked COC. What an odd storm this is turning out to be.


and it wont win, thats a mid level circulation and the surface low to the west which is DANN's COC is far too strong to lose against w.e forms under those T storms
945. Floodman
7:41 PM GMT on August 27, 2009
Quoting ConchHondros:
Danny still looks sad


Yeah, like a deflating balloon, kinda
Member Since: August 2, 2006 Posts: 10 Comments: 9922
944. Dodabear
7:41 PM GMT on August 27, 2009
Quoting BrockBerlin:
started my first met class today (I'm a met major) so maybe in 2-4 years I can add more to the blog than lame sarcasm.


If nothing else, this is a good place to learn how tough a job forecasting really is. More art than science for the good ones.
Member Since: July 28, 2001 Posts: 0 Comments: 2279
943. Floodman
7:40 PM GMT on August 27, 2009
Quoting justalurker:


coming down to SFLA any time soon?


I can't say for sure...we get a landfall in SFLA I might be there in a forward office or soemthing...
Member Since: August 2, 2006 Posts: 10 Comments: 9922
941. hurricanejunky
7:39 PM GMT on August 27, 2009
Danny is looking rather strange. It looks like the circulation in the east-side convection is not winning the battle with the naked COC. What an odd storm this is turning out to be.
Member Since: August 28, 2006 Posts: 6 Comments: 2895
940. ConchHondros
7:39 PM GMT on August 27, 2009
Danny still looks sad
Member Since: October 1, 2007 Posts: 0 Comments: 551
938. Floodman
7:38 PM GMT on August 27, 2009
Quoting 21N71W:
Bits of Danny are spreading south, all the way down to the Turks and Caicos...why is he still here ???


Because he's strong enough to present with winds in the 50-60 range; disorganised in appearance or not, the energy is there
Member Since: August 2, 2006 Posts: 10 Comments: 9922
937. justalurker
7:38 PM GMT on August 27, 2009
how accurate do you think NHC has danny going right now? they sure hit the nail on the head with bill!!
Member Since: August 18, 2009 Posts: 0 Comments: 157
936. fmbill
7:38 PM GMT on August 27, 2009
Quoting OSUWXGUY:
18Z SHIPS shows virtually no further strengthening with Danny as a tropical system, and only slightly when it makes the extratropical transition.

Shear holds favorable through tomorrow afternoon (<15knots) but then rapidly increases!

SHIPS DANNY


18Z SHIPS takes 94L to an 84 knot hurricane and moves it on a mostly western track over the next 5 days.
SHIPS 94L


Looks like high pressure has now built in above Danny.

Member Since: May 27, 2008 Posts: 0 Comments: 428
935. IKE
7:38 PM GMT on August 27, 2009
Quoting OSUWXGUY:
18Z SHIPS shows virtually no further strengthening with Danny as a tropical system, and only slightly when it makes the extratropical transition.

Shear holds favorable through tomorrow afternoon (<15knots) but then rapidly increases!

SHIPS DANNY


18Z SHIPS takes 94L to an 84 knot hurricane and moves it on a mostly western track over the next 5 days.
SHIPS 94L


Danny's days are numbered.

94L seems to slow to a crawl on the SHIPS data at the end of the run.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
934. bluewaterblues
7:37 PM GMT on August 27, 2009
Quoting AllStar17:
Be careful what you say about "disorganized, sloppy, etc." Danny. He still has the chance to strengthen when the environment gets more favorable.


That is a good point
Member Since: August 6, 2006 Posts: 0 Comments: 164
933. stormsurge39
7:37 PM GMT on August 27, 2009
I can see the naked swirl on the visible but its not going NW. Also all the storm action is to the east. This storm is starting to remind me of one of those that arent going to cooperate with computer models.
932. futuremet
7:37 PM GMT on August 27, 2009
Classic NHC

Member Since: July 19, 2008 Posts: 43 Comments: 4051
931. pinehurstnc
7:37 PM GMT on August 27, 2009
some here spoke of hannah last year,, here in pinehurst nc,if any 1 here follows golf,, it washed out the road to my mothers house,, detour 4 3 months,, and a bridge was washed out in a golf community ,they had to either swim or boat to get across,, so little storms can drop 8 inches of rain , in a short time..
Member Since: August 13, 2009 Posts: 0 Comments: 59
930. justalurker
7:37 PM GMT on August 27, 2009
Quoting Floodman:


**bows deeply**

The pleasure is mine!


coming down to SFLA any time soon?
Member Since: August 18, 2009 Posts: 0 Comments: 157
928. 21N71W
7:36 PM GMT on August 27, 2009
Bits of Danny are spreading south, all the way down to the Turks and Caicos...why is he still here ???
Member Since: August 1, 2006 Posts: 0 Comments: 180
927. StormChaser81
7:36 PM GMT on August 27, 2009
This is on the latest satellite imagery page whats this about?

Danny(AL05)

Latest Satellite Imagery
Member Since: August 11, 2008 Posts: 0 Comments: 2315
925. Floodman
7:35 PM GMT on August 27, 2009
Quoting justalurker:
hey flood,

good to hear from you today..as usual.


**bows deeply**

The pleasure is mine!
Member Since: August 2, 2006 Posts: 10 Comments: 9922
924. OSUWXGUY
7:35 PM GMT on August 27, 2009
18Z SHIPS shows virtually no further strengthening with Danny as a tropical system, and only slightly when it makes the extratropical transition.

Shear holds favorable through tomorrow afternoon (<15knots) but then rapidly increases!

SHIPS DANNY


18Z SHIPS takes 94L to an 84 knot hurricane and moves it on a mostly western track over the next 5 days.
SHIPS 94L
923. justalurker
7:35 PM GMT on August 27, 2009
hey flood,

good to hear from you today..as usual.
Member Since: August 18, 2009 Posts: 0 Comments: 157
922. Dodabear
7:34 PM GMT on August 27, 2009
Quoting reedzone:


Oh I dunno.. Reedsville... Reedzone? sounds alot like my screename. People on here always bash me for advising people to watch a certain system.

Just in case you didn't know, Reedville is a real town located on the central coast of Virginia.
Member Since: July 28, 2001 Posts: 0 Comments: 2279
921. jipmg
7:34 PM GMT on August 27, 2009
Quoting saintsfan06:
Without looking back through all of the posts what is the current position of the COC for Danny. You could see the nekkid swirl this morning but my untrained eye cannot pick it out now.


Its so easy to pick out on visible satellite.. anyhow based on my estimation its around:

27.3N and I think 73.4W
920. largeeyes
7:34 PM GMT on August 27, 2009
That TS Warning is for marine.
Member Since: August 22, 2006 Posts: 0 Comments: 1451
919. Floodman
7:33 PM GMT on August 27, 2009
Quoting nolacane2009:


Without using google I would have to say Continental US


BINGO!
Member Since: August 2, 2006 Posts: 10 Comments: 9922
918. willdunc79
7:33 PM GMT on August 27, 2009
I understand that the COC is aimlessly floating around and all and Danny also is somewhat farther west but I still feel like the current NHC track is absolutely on the money/on point/valid and still see Danny effecting areas/states north of Va. Still see NC & SC as being safe especially since Danny is getting ready to start moving towards the North so thats why I'm not surprised to see and advisories at all for NC and/or SC.
Member Since: June 27, 2009 Posts: 0 Comments: 152
917. atmoaggie
7:33 PM GMT on August 27, 2009
Quoting nolacane2009:


Thank you very much. I will see if I can find it. No one here will ever have to worry about me making stupid comments though.

If you want to look up a weather term, the AMS weather glossary is here: http://amsglossary.allenpress.com/glossary
Very useful resource for anyone, n00b through met.
Member Since: August 16, 2007 Posts: 6 Comments: 12461
915. futuremet
7:32 PM GMT on August 27, 2009
A more Stagnant-vigorous Bermuda high in the near future.

Link
Member Since: July 19, 2008 Posts: 43 Comments: 4051
914. saintsfan06
7:32 PM GMT on August 27, 2009
Without looking back through all of the posts what is the current position of the COC for Danny. You could see the nekkid swirl this morning but my untrained eye cannot pick it out now.
Member Since: September 18, 2007 Posts: 0 Comments: 171
913. justalurker
7:32 PM GMT on August 27, 2009
where do i get a ticket for the show?
Member Since: August 18, 2009 Posts: 0 Comments: 157

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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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