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 Bordonaro:
Well TS Danny is coming along nicely, although the LLC is oh, 125 miles to the W of the major convection. Looks like the DR and the NHC are right in the money! Let us hope that Danny does NOT undergo a period of rapid intensification. Looks like we're gonna have a Cat 1 Hurricane with 75-95MPH winds off of N Carolina. Everyone from New England to Nova Scotia, please heed to the advisories and warnings when they're issued! STAY OUT of the SURF!! And for 94L is looking like it will develop into a TD within about 48 hrs, everyone needs to keep an eye on this for next weekend.


how are they right on the money if the COC is 60 nm south of the forecast position?
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Storm W nailed it. Danny moving west and he is WSW of the forecast track. Canadian model may have Danny pegged.

Also I don't understand Dr M comment about ECMWF not being avaialble for public??? You can go to the website and see the graphic output.

http://www.ecmwf.int/products/forecasts/d/animate/catalog/products/forecasts/medium/deterministic/m sl_uv850_z500!Wind%20850%20and%20mslp!72!North%20America!pop!od!oper!public_plots!2005101800!!!step/
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Quoting TheCaneWhisperer:


If that center doesn't reform they are going to need a MAJOR track adjustment soon.

How MAJOR are we talking?
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happy birthday!! and get to work!! ha im just giving you a hard time, you do a great job!
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How many miles is Danny from FL. ?
Quoting cheezemm:
Am I seeing the ULL or the actual center LLC on the visible diving in a goofy southeastwest whatchamacallit direction?

Looking at DestinJeff's steering maps and the current erratic motion has me a little puzzled?

What's Danny up to?


See the tiny swirl at 27.5N and 73W? That's Danny's center. It's heading NW according to the NHC.
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Happy Birthday Storm!!!!!! Remember to have some fun today!
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Quoting StormW:
Got news for ya...he ain't movin NW now...last 3 hours of satellite loop imagery show a W motion, and he is south of the forecast point by at least 60nm.


If that center doesn't reform they are going to need a MAJOR track adjustment soon.
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Well TS Danny is coming along nicely, although the LLC is oh, 125 miles to the W of the major convection. Looks like the DR and the NHC are right in the money! Let us hope that Danny does NOT undergo a period of rapid intensification. Looks like we're gonna have a Cat 1 Hurricane with 75-95MPH winds off of N Carolina. Everyone from New England to Nova Scotia, please heed to the advisories and warnings when they're issued! STAY OUT of the SURF!! And for 94L is looking like it will develop into a TD within about 48 hrs, everyone needs to keep an eye on this for next weekend.
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The Bahamas is lucky that Danny is still naked on his left side or they would probably be under some type of warning now. It's so eerie watching this swirl form so close to Florida and not be affected by it.
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Quoting pearlandaggie:

Morning all.
That is an amazing photo - of what Bill sucked up.
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Thanks Storm w, How long do you expect this to last?
pearl--thanks for that. Just goes to show that the GOM isn't that much hotter than normal. Quite interesting teh impact a cane like Bill has on SST.
Member Since: August 22, 2006 Posts: 0 Comments: 1451
Quoting StormW:


Morning!


Hi Storm are you going to be calling in tonight again on Barometer Bob's show?
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Quoting presslord:
Greetings all!

I'm writing to remind you of what great good we have all done through our work with Portlight.org over the last year in the wake of Ike and the other disasters to which we've all responded...and to share with you, in a general way, why it is so important that we continue...especially as relates to the post disaster needs of people with disabilities...

An old and dear friend of mine was named FEMA Disability Affairs Coordinator in the early Spring of this year...I'm not going to name her here...but she is a wonderful woman...and has been a tough advocate for people with disabilities...I, and many others, had high hopes that she would bring much needed change to that outfit...alas, the bungling ineffectiveness of FEMA seems to have been more than even the best intentioned soul can withstand...in short: ain't nuthin' changed...

We have been in intense communication for months about the post disaster needs of people with disabilites...specifically, we proposed a national conference at which all the players and stakeholders could engage in productive conversation about needs, expectations, etc...my hope was that FEMA would take the lead in this...silly me...I just received my latest bi-weekly e-mail from this person assuring me that "...there's a lot of good talk..." going around about the issue...well...as I am going to avoid profane invective here, this is gonna be short:

Four years after Katrina, we've had enough talk...and I give up trying to effect any change in government or big institutional agencies...Portlight is the ONLY disaster relief initiative to focus on people with disabilities...and I now believe we're out on this limb alone...

so..please be extra proud of what we've done...and please stay as engaged as you can with our work...it is not hyperbole to say: Lives depend on us...


www.portlight.org



It's a good thing you are doing, glad to be a contributer and hope to be able contribute again in the future.
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looks like under dvorak image getting it's act together some more...
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respect at last for the CMC thanks Doc :)
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and it looks like florida could get it first stormby erika if it does become erika
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Thanks Dr. M,would have expected Danny to more organized by now,but I guess too much to contend with.It this starts to go extratropical as it moves by us here in eastern mass. I assume the windfields will tend to expand.
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Danny can't go anywhere but NW because its piched between the Bermuda High and the very slow moving ULL in the southeast US, regardless of its strength.
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Am I seeing the ULL or the actual center LLC on the visible diving in a goofy southeastwest whatchamacallit direction?

Looking at DestinJeff's steering maps and the current erratic motion has me a little puzzled?

What's Danny up to?
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Greetings all!

I'm writing to remind you of what great good we have all done through our work with Portlight.org over the last year in the wake of Ike and the other disasters to which we've all responded...and to share with you, in a general way, why it is so important that we continue...especially as relates to the post disaster needs of people with disabilities...

An old and dear friend of mine was named FEMA Disability Affairs Coordinator in the early Spring of this year...I'm not going to name her here...but she is a wonderful woman...and has been a tough advocate for people with disabilities...I, and many others, had high hopes that she would bring much needed change to that outfit...alas, the bungling ineffectiveness of FEMA seems to have been more than even the best intentioned soul can withstand...in short: ain't nuthin' changed...

We have been in intense communication for months about the post disaster needs of people with disabilites...specifically, we proposed a national conference at which all the players and stakeholders could engage in productive conversation about needs, expectations, etc...my hope was that FEMA would take the lead in this...silly me...I just received my latest bi-weekly e-mail from this person assuring me that "...there's a lot of good talk..." going around about the issue...well...as I am going to avoid profane invective here, this is gonna be short:

Four years after Katrina, we've had enough talk...and I give up trying to effect any change in government or big institutional agencies...Portlight is the ONLY disaster relief initiative to focus on people with disabilities...and I now believe we're out on this limb alone...

so..please be extra proud of what we've done...and please stay as engaged as you can with our work...it is not hyperbole to say: Lives depend on us...


www.portlight.org

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looks like we may possibly have another great labor day storm in the making
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31. yep, pretty impressive, huh?
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Quoting pearlandaggie:


Bill used up some heat, sheesh.
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thanks!
goodmorning Storm!
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Thanks much for the update! I've been lurking and realize how much I have to learn. Back to reading :)
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Quoting DestinJeff:
Based on Danny's current apparent COC along 73W, 27.5N .... he could experience even a wsw fwd motion accoring to Low Level Steering winds

click pic to enlarge ... notice the wind barbs pointing wsw out in front of him



If the center stays exposed like that... erratic motion could definitely be a possibility.
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Quoting Grothar:
Does anyone know how to retrieve this morning's comments. There were a few I was still reading when the update came in?


Go to upper right of this page. See 'Previous entries for 2009'. Click 'August'. Scroll down to the blog you were looking for (the one previous to this one). Go to the bottom of the blog and you'll see where you can retrieve entries.
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Thanks Doc. An interesting season.
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I might have totally missed something here about ECMWF, but if output from it isn't freely available, who CAN use it? Is it only for use by governmental meteorological organizations?

I understand that sophisticated software has licensing requirements and intellectual property concerns. And it's not like "We know exactly where this hurricane is going to go and who it will effect...BUT WE WON'T TELL YOU! Muahhahaha!"

But I guess it seems a little odd to me that there's this great awesome model out there and the public can't see its output. I mean, one of the purposes of hurricane forecasting is to warn people of dangerous storms that might affect their health, well-being, and property. There has to be SOME use for the general public, right?

I'm sorry if I'm not making this clear. And obviously, models can be prone to large errors. Don't feel bad, ECMWF, I'm not miffed at you.
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Quoting StormW:
Thanks Dr. Masters.

Good day all!



StormW, can you shed any light on the potential track of 94L. Your opinion is always appreciated.
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quick poll guys which models has been the best so far this seasons and which had been the worst
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Does anyone know how to retrieve this morning's comments. There were a few I was still reading when the update came in?
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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.