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 presslord:


OK...that's 2 "carolinas" offenses in one post...the next infraction shall be met with a flogging...



hey i'm from SC i can commit "carolina offenses" if i want to. its the non-natives that aren't allowed. plus i'm too lazy to type out both names.
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Quoting P451:
Earlier the HH's found a wind shift within the heavy convection. Here's an illustration.



Now I'd swear I am seeing that region of the heavy convection develop rotation. Here's an illustration followed by the loop:







Higher Res Loop

The Current LLC also seems to have stalled and become less defined. Seemingly going from west, to a jump south, then stalled?

So, something is changing now with Danny in several places at once. Will be interesting to see what plays out this afternoon. Whether the current LLC meets back up with convection or the new twist in the convection becomes a dominant feature.

..and the HH's continuing to probe Danny.



thats exactly what ive been trying to point out, but you did it a lot more effectively
Member Since: June 28, 2006 Posts: 25 Comments: 8347
Member Since: September 14, 2007 Posts: 3 Comments: 3963
458. 7544
sw of the last forcast point hmm
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only 3 degrees till it gains my longitude... and looks like nothing will stop it from doing that.
Member Since: September 6, 2007 Posts: 0 Comments: 6506
456. IKE
12Z CMC...
Member Since: June 9, 2005 Posts: 23 Comments: 37858
Quoting presslord:


OK...that's 2 "carolinas" offenses in one post...the next infraction shall be met with a flogging...


and don't confuse flogging with flagging lol
Member Since: June 28, 2006 Posts: 25 Comments: 8347
Quoting VAbeachhurricanes:
456 your quick thoughts on the LLC?


Its not as vigorous as this morning, and it appears to be reforming again as it meanders west. It seems a consistent track with this comes when the feature is more organize.
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What-stay out of the water?? The season is the reason, you haven't lived unless you've been out on the outside (on a board) on a 6 foot plus day. With Bill at 10-12 feet and clean, it was an experience of a lifetime for me. Can't wait for Danny, need to make that turn, otherwise it won't be good. But if you are a bather, then yes, you have no business in the water.
Member Since: August 28, 2006 Posts: 0 Comments: 381
Flyinfish,

yes i see that, i am probably wrong, but i think its relocating
Member Since: June 28, 2006 Posts: 25 Comments: 8347
000
URNT12 KNHC 271656
VORTEX DATA MESSAGE AL052009
A. 27/16:39:30Z
B. 27 deg 16 min N
073 deg 14 min W
C. NA
D. 39 kt
E. 036 deg 131 nm
F. 098 deg 48 kt
G. 035 deg 124 nm
H. EXTRAP 1007 mb
I. 21 C / 456 m
J. 24 C / 461 m
K. 17 C / NA
L. NA
M. NA
N. 1345 / 01
O. 0.02 / 2 nm
P. AF307 0505A DANNY OB 07
MAX FL WIND 50 KT NE QUAD 15:51:00Z
SLP EXTRAP FROM BELOW 1500 FT


isnt this it?
Member Since: June 29, 2007 Posts: 0 Comments: 1136
Quoting SCAnne:
Watching this storm reminds me of how with most storms that hit the carolinas, everyone says they will go north or out to sea. but then just a day or so before landfall they say oops its gonna hit the carolinas.


ouch twice in the same post... LOL
Member Since: September 6, 2007 Posts: 0 Comments: 6506
Quoting FlyinFish:
It looks like both. Like its trying to form a new LLC while spinning into the old?



It's pulling a reverse fujiwara!
Member Since: March 10, 2006 Posts: 0 Comments: 10314
Quoting caneluver:


Its still moving due west also.


Actually slightly S of W if you go by the 11am position, but could just be a wobble or mis-calculation on the 11am based on sat.

From 27.5 to 27.27
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Quoting tornadodude:
ok, you can quote me on this later, but i really think the old coc is beginning to diminish and a new one is forming near 26N and 70-71W
link

Ok. I quoted you. Yeah, this looked like it was happening in the last frame of the last update, now there's a few that are showing some spin in the cloud tops on the East convection. And the "old" LLC looks like its fraying.
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just wait for the vortex message from the HH it shouldnt be much longer
Member Since: September 6, 2007 Posts: 0 Comments: 6506
Watching this storm reminds me of how with most storms that hit the carolinas, everyone says they will go north or out to sea. but then just a day or so before landfall they say oops its gonna hit the carolinas.
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you can see 94L's waves in the bottom right corner of the last frame of NOAA's wave model..
Member Since: September 14, 2007 Posts: 3 Comments: 3963
425. CycloneOz 12:53 PM EDT on August 27, 2009 Hurricane Bill Imagery - from 8/21/2009

Great pictures Oz, keep 'em coming.
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Quoting Weather456:
Good Afternoon

Tropical Update


Thanks, nice update.
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456 your quick thoughts on the LLC?
Member Since: September 6, 2007 Posts: 0 Comments: 6506
Good Afternoon

Tropical Update
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Quoting TampaSpin:
I believe a new LLC may be forming at 26.9N 71.8W IMO


agreed, what i was referring to
Member Since: June 28, 2006 Posts: 25 Comments: 8347
.
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I believe a new LLC may be forming at 26.9N 71.8W IMO
Member Since: September 2, 2007 Posts: 178 Comments: 20439
Danny looks as if he's heading West. What's going to happen when the moisture from the GOM (which has been creating rain and storms over Northern and Central Florida) and Danny collide?? Is that when it's supposed to head North? Looks 'iffy' right now.
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ok, you can quote me on this later, but i really think the old coc is beginning to diminish and a new one is forming near 26N and 70-71W
link
Member Since: June 28, 2006 Posts: 25 Comments: 8347
Quoting Seastep:
cycloneoz - could I get a link to the base tanker pic?

That's a keeper for ribbing friends and family.


http://njscuba.net/artifacts/ship_freighter.html
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Quoting tornadodude:


oh alright, thanks again, i really think its developing a new coc near 26N and 70, or 71 west


definitely can see that, I think it is a little bit of column a and a little bit of column b... or so it would appear.
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It worked. Cool.
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Quoting GainesvilleGator:
It looks like the COC of Danny is due North of the Haiti/Dominican Republic border & due East of Lake Okechobee. Danny looks to be going NW & the swirl that was the center looks like it is going bye bye.


Um ... I don't think so
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Hurricane Bill Imagery - from 8/21/2009

CycloneOz gets slammed by crashing wave (sequence of 4 frames)





CycloneOz uses underwater camera to get right into the malestrom! (sequence of 2 frames)



Safe Harbor? I don't think so!!!
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.
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Quoting FlyinFish:
Did it change....

Yup. Looks good.
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Quoting caneluver:
the current COC is strong with 55mph winds. A new center is not reforming. Covection trying to catch up with the center and will likely do so before the day is out.

I hope not, that would mean quite a westward shift in the models, or it might just do a spinabout, i hope not.
Member Since: June 29, 2007 Posts: 0 Comments: 1136
It looks like the COC of Danny is due North of the Haiti/Dominican Republic border & due East of Lake Okechobee. Danny looks to be going NW & the swirl that was the center looks like it is going bye bye.
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Did it change....
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Quoting FlyinFish:
SQUAWK, i meant to change it to a different new one. Thats old. Its a cobia.

OK. Good looking fish. I would have been proud of that catch.
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its gonna be hard to replace this coc it seems pretty well established.
Member Since: September 6, 2007 Posts: 0 Comments: 6506
SQUAWK, i meant to change it to a different new one. Thats old. Its a cobia.
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Quoting Seastep:
Without a vortex message, center appears to be at 27.267N 73.250W according the HH.


that is exactly where it is. If it relocates that will be another story, but we have to go on the actual llc not where it might relocate to later.
Member Since: June 29, 2007 Posts: 0 Comments: 1136
Quoting FlyinFish:
It looks like both. Like its trying to form a new LLC while spinning into the old?

BTW like the new portrait/avatar?

Doesn't look like any flying fish I ever saw.
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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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