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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Were having some squally weather here in Mobile AL.
OSUWXGUY-

hey, what is your take on a possible new llc forming?
Member Since: June 28, 2006 Posts: 24 Comments: 8200
One other thing to chomp on. Highest winds (Stronger Circulation) were in the lower levels. Could it be that as conditions improve your looking at a better defined MLC of an unstacked system. What level were the HH flying when they caught the new wind shift?
anyone who doesn't get that I'm pokin' fun at MYSELF hasn't been payin' attention...
Member Since: August 13, 2007 Posts: 0 Comments: 10479
Quoting DestinJeff:


looks at least like a MLC due east of last reported LLC


could be since its not vertically stacked at all
Member Since: September 6, 2007 Posts: 0 Comments: 5999
How would you like to be in SE Virginia we always get crammed up in there with the Carolinas and we are not a Carolina at all.
Member Since: Posts: Comments:
Be interesting to see what the NHC has to say in the next discussion. Am anxiously awaiting it.
Member Since: June 29, 2007 Posts: 0 Comments: 1136
not at all dear, i didnt slam anybody, just stated a fact. there it is.
Member Since: June 29, 2007 Posts: 0 Comments: 1136
Quoting DaytonaBeachWatcher:
Funny to see the same ole "faces" jumping in on an argument by someone else


probly what you just did as well
Member Since: April 26, 2006 Posts: 5 Comments: 3073
K8...SJ's around...he'll turn nup...
Member Since: August 13, 2007 Posts: 0 Comments: 10479
Member Since: September 6, 2007 Posts: 0 Comments: 5999
501. OSUWXGUY
5:19 PM GMT on August 27, 2009
Back from lunch and boy do I have a lot to say...


First off, in regards to the question of lightning and hurricanes:


Within a typical thunderstorm over land, area of strong updrafts and strong downdrafts are small and are in close proximity. Ice particles within the storm bumb into each other, leading to charge separation and the upper portions of a storm becomes a different polarity than the lower portions of the cloud. Lightning then occurs either within the cloud or between the cloud and the ground based on the charges built up.


Within a hurricane, regions of strong updrafts and strong downdraft are much larger in size. Alternating large regions of the storm (eyewall for instance) head upward and downward.

The fact that these regions ARE so broad is what allow hurricane hunters to fly into intense hurricane. Airplanes cannot safely fly into the core of intense thunderstorms and try to avoid this whenever possible...

The broader areas of updrafts and downdrafts leads to less ice particle collision and preferential separation within a hurricane. No charge separation = no lightning.

Now in the rain bands in the periphery of the hurricane, individual storms have the charateristics that allow lightning to form - and is why folks have reported seeing lightning with rainbands but not usually with the hurricane itself.


One final note to complicate things... Sometime within the hurricane, small areas of very strong rotation (mesocyclones) can form causing supercell like structures to rotate around the eye or inner structure of the storm. These areas cause localized (small) areas of intense vertical motion - more like a normal thundestorm over land - and can cause lightning!
Cross Section of a Hurricane
500. stormsurge39
5:19 PM GMT on August 27, 2009
Why does the NHC see NW movement?
499. DVG
5:19 PM GMT on August 27, 2009
I also have observed apparent circulation with the heavy convection east of the llc.

Is it possible this is a mid level circulation?
With regards to the comments of the convection qrapping around the llc, I am considering that if the convection does indeed have circulation at mid levels, it may stack up??
Member Since: August 29, 2007 Posts: 0 Comments: 259
498. presslord
5:18 PM GMT on August 27, 2009
Quoting StormChaser81:


Get over it, even the nhc uses carolinas. This is getting old.


feel free to ignore...in fact, I'd be honored if you did...
Member Since: August 13, 2007 Posts: 0 Comments: 10479
497. tornadodude
5:18 PM GMT on August 27, 2009
Quoting PcolaDan:


carolinas




hahahahaha
Member Since: June 28, 2006 Posts: 24 Comments: 8200
496. K8eCane
5:18 PM GMT on August 27, 2009
Hi Press, Storm ,Ike and others
i'm mostly in lurk mode till i see what this darned thing is gonna do and what has happened to StormJunkie??
Member Since: April 26, 2006 Posts: 5 Comments: 3073
495. VAbeachhurricanes
5:18 PM GMT on August 27, 2009


the REAL coc is beginning to be obscured by clouds... trying to organize itself
Member Since: September 6, 2007 Posts: 0 Comments: 5999
494. DaytonaBeachWatcher
5:17 PM GMT on August 27, 2009
Funny to see the same ole "faces" jumping in on an argument by someone else
Member Since: June 29, 2007 Posts: 0 Comments: 1136
493. PcolaDan
5:17 PM GMT on August 27, 2009
Quoting presslord:


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


carolinas


Member Since: August 22, 2008 Posts: 12 Comments: 6010
492. 7544
5:17 PM GMT on August 27, 2009
watch the high he may just stall out soon
Member Since: May 6, 2007 Posts: 0 Comments: 6676
490. hydrus
5:16 PM GMT on August 27, 2009
Quoting caneluver:
Danny still is not following the forcast points. He wants to keep west.
Not even close.
Member Since: September 27, 2007 Posts: 1 Comments: 20319
489. TheCaneWhisperer
5:17 PM GMT on August 27, 2009
Case and point as to why the NHC won't change on a whim.. Reform happens, were back to where we were this morning lol.
488. Cavin Rawlins
5:15 PM GMT on August 27, 2009
94L

Member Since: July 24, 2005 Posts: 407 Comments: 19076
487. 900MB
5:14 PM GMT on August 27, 2009
Quoting cheezemm:
Here's my take....shear is relaxing and you're starting to see the convection attempt to wrap around the current LLC but it's (the entire storm) stacked more like this:

/

Instead of this:

|

Don't make fun of my poor graphics!


:)
Member Since: June 11, 2006 Posts: 0 Comments: 660
486. presslord
5:14 PM GMT on August 27, 2009
Quoting SCAnne:



really i thought we only had 49 states

geez it was just a general observation would you like for me to go pick out and list the specific storms so i can list which carolina they hit


yes...that would be nice...
Member Since: August 13, 2007 Posts: 0 Comments: 10479
485. FlyinFish
5:14 PM GMT on August 27, 2009
Quoting popartpete:
Jersey Shore here. Who thinks first hurricane-force gusts in 24 years?


LBI here... I doubt it... 3% chance ill say.
Member Since: June 8, 2008 Posts: 0 Comments: 43
484. Dakster
5:13 PM GMT on August 27, 2009
Ok. Well it looks like the next few hours or so will be critical to the Carolinas. People int eh Carolina's should really watch this closely.. Heck from the Carolina's South are in a little danger now, but from the Carolina's Northward are in even greater danger.

As my good friend Caroline said you can't be too careful.
Member Since: March 10, 2006 Posts: 0 Comments: 10020
483. TampaSpin
5:13 PM GMT on August 27, 2009
Quoting presslord:
then you should know: there is no such place as "the Carolinas"...it's either North or South Carolina....


Presslord....think of the travel expense we will save with a the relief efforts in North Carolina. It will be in your backdoor....LMAO Notice i did not say the Carolina's
Member Since: September 2, 2007 Posts: 178 Comments: 20439
482. SCAnne
5:13 PM GMT on August 27, 2009
Quoting presslord:
then you should know: there is no such place as "the Carolinas"...it's either North or South Carolina....



really i thought we only had 49 states

geez it was just a general observation would you like for me to go pick out and list the specific storms so i can list which carolina they hit
481. largeeyes
5:12 PM GMT on August 27, 2009
4 degrees left for my longitude....
Member Since: August 22, 2006 Posts: 0 Comments: 1451
480. tramp96
5:12 PM GMT on August 27, 2009
Quoting IKE:
12Z CMC...


I may be wrong but I have been watching the CMC for a few days now and it seems pretty consistant.
Member Since: August 15, 2009 Posts: 0 Comments: 1524
479. pearlandaggie
5:12 PM GMT on August 27, 2009
Member Since: September 14, 2007 Posts: 3 Comments: 3963
478. StormChaser81
5:12 PM GMT on August 27, 2009
Quoting presslord:
then you should know: there is no such place as "the Carolinas"...it's either North or South Carolina....


Get over it, even the nhc uses carolinas. This is getting old.
Member Since: August 11, 2008 Posts: 0 Comments: 2315
477. hydrus
5:11 PM GMT on August 27, 2009
Quoting popartpete:
Jersey Shore here. Who thinks first hurricane-force gusts in 24 years?
Did Jersey have 75 mph gusts in 1992?
Member Since: September 27, 2007 Posts: 1 Comments: 20319
476. cheezemm
5:11 PM GMT on August 27, 2009
Here's my take....shear is relaxing and you're starting to see the convection attempt to wrap around the current LLC but it's (the entire storm) stacked more like this:

/

Instead of this:

|

Don't make fun of my poor graphics!
Member Since: August 13, 2008 Posts: 0 Comments: 7
475. tornadodude
5:11 PM GMT on August 27, 2009
Caneluver,

oh its fine, we are all here to express our thoughts, so anyone is allowed to disagree
Member Since: June 28, 2006 Posts: 24 Comments: 8200
474. WXHam
5:10 PM GMT on August 27, 2009
I'm no expert BUT it seems Danny's getting its act together ... from the satellite view anyway and COC is moving West.

Link
Member Since: July 1, 2008 Posts: 0 Comments: 33
473. presslord
5:10 PM GMT on August 27, 2009
then you should know: there is no such place as "the Carolinas"...it's either North or South Carolina....
Member Since: August 13, 2007 Posts: 0 Comments: 10479
472. Cavin Rawlins
5:09 PM GMT on August 27, 2009
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.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
470. pearlandaggie
5:09 PM GMT on August 27, 2009
467. glad you liked that
Member Since: September 14, 2007 Posts: 3 Comments: 3963
469. VAbeachhurricanes
5:08 PM GMT on August 27, 2009
Member Since: September 6, 2007 Posts: 0 Comments: 5999
467. presslord
5:09 PM GMT on August 27, 2009
pearland...cute...
Member Since: August 13, 2007 Posts: 0 Comments: 10479
466. tornadodude
5:07 PM GMT on August 27, 2009
caneluver,
thanks, but we are all entitled to an opinion so its alright
Member Since: June 28, 2006 Posts: 24 Comments: 8200
465. popartpete
5:07 PM GMT on August 27, 2009
Jersey Shore here. Who thinks first hurricane-force gusts in 24 years?
Member Since: July 10, 2007 Posts: 0 Comments: 418
464. SQUAWK
5:07 PM GMT on August 27, 2009
I used to drive through the carolinas on a regular basis. But now with the storms heading for the carolinas, I probably won't. But I did live in the carolinas for a while and still have relatives in the carolinas to this day.

What's my count??????

LOL
Member Since: December 9, 2008 Posts: 0 Comments: 2498
463. SCAnne
5:07 PM GMT on August 27, 2009
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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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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