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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I know everyone is focusing on Danny and with its proximity to the US Mainland they should, however 94L looks to be a potential beast that could be a major concern for the U.S. for Labor day weekend. At this point it looks more impressive than Bill did at a similar location and is also a large sized storm like Bill was.
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Quoting tibor:
Some gneral questions:

1)Which model(s) should I put most of my trust in?

2) And how are they weighed when the cone of probabilty is calculated?


1) Don't put your trust on no one. You have to take all available information and make an educated decision... but if you're to listen to someone... primarily listen to NHC (weather wise) guidance.

2) As the Doc has posted above... this link should help answer it: Link
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Ready to Sea Mike Vick back in action tonight!!! Will be real interesting to see if he's still got it. Back to the tropics...
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Quoting ChrisCone:


i believe early in the daylight period of the visible sat images (i forget what exactly this scan is called) there was evidence of two or three frames worth of south westerly ly jog, but it is since gone.


sorry, quoted this instead of modified lol i keep doing that
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Quoting Acemmett90:

hey drakster
whos goona win tonight phins or bucs


Fins, of course...

--

Hey Presslord - I feel your pain, I got co-workers planning on a scuba diving trip here shortly. Will you go an help me rescue them when they get swept out to sea?
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I'm supposed to drive the beer cart at a Knights of Columbus golf tournament Saturday...Is the prevailing wisdom here that it will be cancelled?
Member Since: August 13, 2007 Posts: 0 Comments: 10484
Quoting iluvjess:
"How many miles is Danny from FL. ?"

Not sure but I bet I could sling a grub on a quarter ounce jig from Miami beach and hit that COC!


Woah... I don't want what your smoking...
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GOM here i come.
Live from "Danny"
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Quoting VAbeachhurricanes:


the thunderstorms are barely moving..
There is no northerly component to Danny,s movement either.
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Noooooo! keep Myrtle Beach & Wilmington outta the cone! too many maters that need another 10 days! TS Hannah did ugly things to my tomatoes & peppers last year!
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Quoting VAbeachhurricanes:



its you haha, umm its moving around 275 degrees


i believe early in the daylight period of the visible sat images (i forget what exactly this scan is called) there was evidence of two or three frames worth of southerly jog, but it is since gone.
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Surprisigly... not much on here about 94L - that thing looks massive on visible sat...
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193. 7544
Quoting hurricanedave:
just me or is danny moving west southwest


happy birthday storm

yeap dave im glad to see others seeing it too thought i was going nuts but looks like danny is lol
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Quoting P451:


I just think it's a very messed up system. Sometimes those LLCs will dissipate and a new one will form closer to or under the heavy convection but I just don't know. It's so far removed - and everything involved is in such a poor environment - that I don't really see how anyone could (A- say this is a strenthening 55mph TS) and (B- know where it is going, whether it will even survive, or what intensity it could be in the near or far future).

As I suggested last night, it's a big wait and see system. I think I said 36 hours last night - so that'd be 24 more hours from "now" - before this system showed any real changes be it intensification or dissipation.



Thanks again! Think that is why it is referred to as the "Cone of Uncertainty". The models have performed well, so far this season. I guess we shall just have to wait. They should be speaking about "Our Erika" soon. That should prove interesting.
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.


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Quoting NOVArules:
Does anyone think danny is going insane?


Seeding a TC with either Prozac or Ritalin would probably not be cost effective...

...but you never know! The drug companies are sometimes very generous and dole out their wares for free to those in real need! :)
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Ok, let's be realistic here. Danny is NOT going into the GOMEX. Having said that, I do believe that there is an equal chance that Danny affects SC as well as the OBX.
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187. tibor
Some gneral questions:

1)Which model(s) should I put most of my trust in?

2) And how are they weighed when the cone of probabilty is calculated?
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Link


pretty darned good NOLA post Katrina story...
Member Since: August 13, 2007 Posts: 0 Comments: 10484
Quoting StormW:


Thank you!


OH!! well a Happy Birthday is in order for you StormW...AND a Hello to you too....:)
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Quoting hurricanedave:
just me or is danny moving west southwest


You are right.Danny is going to the GOM.
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This one may have an IKE surprise up it's sleeve.
Quoting hurricanedave:
just me or is danny moving west southwest



its you haha, umm its moving around 275 degrees
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Does anyone think danny is going insane?
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Quoting Acemmett90:

LETS GO DOLPHINS !!!!!!!!!!!!


OMG! The Dolphins are going to dominate this year! I can feel it!

GO FINZ! 1st Home Team in SuperBowl History!!!

JUST DO IT, GUYS!
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just me or is danny moving west southwest
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Here's an animation created about hurricanes that everyone here is sure to find interesting and educational. It is definitely a "must-see!"

http://hosted.ap.org/specials/interactives/_national/hurricanesHistoryNew/index_abc.html
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Good Evening/Morning all.
Happy Birthday StormW.
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The current flow appears to be zonal in nature. Not sure if this trough is gonna be quite as amplified as the previous one that steered Bill away. Just don't see it yet.
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Quoting VAbeachhurricanes:


the thunderstorms are barely moving..

By viewing the satellite pictures you are 100% correct! The question is, "What is going on underneath the AOC"? Best bet to see movement is to use the visible satellite and follow the COC which is currently exposed to the W!
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Quoting DestinJeff:


Press at the NHC:



"It is SOUTH CAROLINA and/or NORTH CAROLINA! It is NOT The Carolinas!"


Heeyyy Yaa'lll
wut's thate thingy out in the wuter, we gittin uh storrrm?
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Geez Danny make your mind up, Carolinas strike (oops am I not supposed to say that LOL) or out to sea. It was moving nw but now it has made more of a westerly shift.
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Quoting P451:
From the end of the other blog:




I have no problem with them classifying the system as tropical. They stated clearly that it was somewhere between a sub-tropical and tropical system but it leaned just slightly enough toward tropical for them to call it tropical.

What I do have a problem with is the intensity it has been given. There are tiny samples of wind nearing 55mph in thunderstorms far FAR removed from the COC of a very poorly disorganized system.

So why name it a 55mph system when at most 1% of the entire system has such a wind speed? Of which is probably rain contaminated in those thunderstorms anyway.

A novice would see the track of the system and assume the entire system is a 55mph threat when in fact the tiniest region under one or two intense thunderstorms has that wind speed.

I think it's a valid point of contention given the satellite representation of this system and the distance from the COC that such a value can be found and in such a tiniest of parts of the storm.

Danny's wind field by HH aircraft and highest wind barb posted:





While this may be a tropical entity I have a very hard time taking anyone serious that says this is a 55MPH TS.

This thing wouldn't blow a paper cup off a picnic table.



you definitely have some good points there. This is the kind of storm that will make us all pull our hair out really if we read too much into things like that though. For all we know, a broad area of strong tropical force winds could be "pulsing" in and out, on and off, and they might know that thus not want to make any changes according to their findings unless it is consistent. However, I wouldn't be surprised if they were considering calling this a sub-tropical storm... it also wouldn't surprise me if it was sub-tropic/extra-tropical by the time it gets to New England's latitude thus its effects may be quite a bit away from the actual center. So I personally feel that overall people are focusing too much on the "tropical" possibilities and not enough on the "extra tropical" possibilities, which may do the same kind of damage under the right circumstances.

me saying this makes me realize i was a little harsh on the NHC's cone of uncertainty, but I still agree that the 1 2 3 model is most reasonable for situations like this.
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the thunderstorms are barely moving..
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Quoting DestinJeff:
Posted this earlier .... low level steering wind barbs show indicate wsw - wnw fwd motion for Danny in the short-term, at least:


Eyes might be decieving me, but is Danny moving around the high which would start moving it SW? No hint of NW.
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Gotta run....Gonna take the WaveRunners out before going to the BUCS game tonite.......everyone have a good day.

I have explained it in my Tropical Update as to why Danny is moving WEST if anyone would like to view.
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Quoting P451:
I think this speaks for itself...



That can't be good for the east coast. I live in Richmond Va and I'm ready for whatever happens.
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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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