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 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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Maybe I'm crazy, but I don't see how Accuweather stay in buss. here is a post from Jesse Ferrell:

Some points from our morning meteorologist meeting here at AccuWeather:

- We will be shifting our track further eastward later this morning based on the models
- We will be shifting the track to at least 100 miles off of the Outer Banks
- There is still a chance the center could go over Cape Cod; but will probably be east
- The wind field is poor; most of the action will be east of the track
- The storm is tilted, almost like a hybrid
- The track is west of Bills so it won't be affected by his upwelling cooler water
- Strengthening is possible off the NC coast but right now the storm is not looking healthy
- The upper level disturbance moving through could cause rain people will blame on Danny

Link
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Gotta go, Navy calls. I'll be back later, wumail me if Charleston gets a TS watch later today.
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Good morning, all!

Can anyone provide me NHC's definition of "wobble"? LOL!!!
Member Since: May 27, 2008 Posts: 0 Comments: 455
Quoting rwdobson:
Hmmm... As the coc moves west it gets even more separated from the convection. I wouldn't be surprised to see a re-formed center closer to the convection.
I agree, TS Danny seems to have a mind of its own so far!!
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Quoting stormsurge39:
Why is NHC still saying NW? I also noticed something on the red line the WTC channel uses to track storms. It showed a jog to the west, but no mention.


I believe NHC called it a Wobble.....That's a little more than a WOBBLE IMO......
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Is there a simple explanation as to why the models are not developing 94L when all the conditions seem very ripe for development?
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Quoting TampaSpin:


Thats the difference in a weak system like DANNY as it moves West as apposed to a stronger System that DANNY was to be.....SECONDLY Danny is moving WEST because of the High Pressure to its North built from the HIGH centered off the WEstern tip of Cuba! I seen this coming last night.
Thanks, Yes, I see what you mean. Steering layers and all. Only becaue I'm familiar with it but Betsy in '65 was a crazy track. (Sorry, Ignorant of posting pictures) but I'm not sure why. Weak system or strong high pushing it down back to Fla. after heading up the east coast. Or both. Intersting.
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Remember that the NHC pretty much followed Katrina's lead.
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I think we need a longer westerly trend before we completely throw the NHC under the bus. I agree it looks to be moving west now but that COC could reform again or sharply turn north. Who knows? Or it could pull a Katrina. DOH! You know, being forecast WNW then going SW. The NHC isn't beyond reproach but they're generally pretty good. Wait and see...I know I'd like to wishcast it west over SW Florida because we need the rain!!!
Member Since: August 28, 2006 Posts: 6 Comments: 2898
Quoting P451:
I think this speaks for itself...



Interesting... we'll have to see if it keeps that up, and there is a good chance it may keep moving W or WNW for some time before an eventual turn.
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Still seems like an NC storm to me.
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Quoting Grothar:
TO: StormW

You keep calling them!! We were on late last night and very early this morning discussing the westerward motion on the exposed Circulation of Danny. As usual, some comments were not so kind. Can not wait to hear your analysis. When do you think you shall have a fix on 94L. Just put that birthday cake down and get to work!!!


so how long until stormt0p comes in here to say that Storm W is wrong? LOL
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Quoting TampaSpin:
This steering map show a WEST move....
and the one below it shows where it is coming from.....the High off Cuba.

Why is NHC still saying NW? I also noticed something on the red line the WTC channel uses to track storms. It showed a jog to the west, but no mention.
This center is really well defined. Is it that Danny is relocating or bouncing around like a pinball between the mess it's embedded in?
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Quoting presslord:
Chuck...thanks...let's go offshore and do a little sailing...


Thanks, but no thanks. I have a hard enough time forecasting the weather on land. Looks like a pretty stormy afternoon setting up with that slow moving ULL. Just heard some thunder here in West Ashtray.
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High moving into New England today so kind of hard to see too much northward movement until tonight or tomorrow.
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144. 7544
hmm we said west all day yesterday now with the high buliding in maybe we might see some wobbles to the sw looks like danny has a mind of his own hope he dosent surprise us and pull a jean stay tuned
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Quoting TampaSpin:
This steering map show a WEST move....
and the one below it shows where it is coming from.....the High off Cuba.

Definitely looks like a due west coarse at this time..
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Quoting rwdobson:
Hmmm... As the coc moves west it gets even more separated from the convection. I wouldn't be surprised to see a re-formed center closer to the convection.


I was wondering the same thing. Also is that an ull south of Mobile, AL.
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This steering map show a WEST move....
and the one below it shows where it is coming from.....the High off Cuba.

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Hmmm... As the coc moves west it gets even more separated from the convection. I wouldn't be surprised to see a re-formed center closer to the convection.
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Quoting FLSWEDE:
A lot of storms tend to stair step . that is why the NHC goes by a 12 hr track, kind of a wait and see.
Especially in the formative stages.
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Quoting VAbeachhurricanes:


how are they right on the money if the COC is 60 nm south of the forecast position?
I believe the NHC was allowing a 60NMI window of error yesterday, I agree that they are winthin "their window of error"! I am not perfect, neither are they, noone is!
ZCZC MIATCMAT5 ALL
TTAA00 KNHC DDHHMM
TROPICAL STORM DANNY FORECAST/ADVISORY NUMBER 3
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL052009
0300 UTC THU AUG 27 2009

INTERESTS IN THE BAHAMAS AND FROM THE CAROLINAS NORTHWARD TO NEW
ENGLAND SHOULD MONITOR THE PROGRESS OF DANNY.

TROPICAL STORM CENTER LOCATED NEAR 26.0N 71.6W AT 27/0300Z
POSITION ACCURATE WITHIN 60 NM
By the way, today they're using a 30NMI window of error!
000
WTNT25 KNHC 271433
TCMAT5
TROPICAL STORM DANNY FORECAST/ADVISORY NUMBER 5
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL052009
1500 UTC THU AUG 27 2009

INTERESTS FROM THE CAROLINAS NORTHWARD TO NEW ENGLAND SHOULD MONITOR
THE PROGRESS OF DANNY. A TROPICAL STORM WATCH MAY BE REQUIRED FOR
PORTIONS OF THIS AREA LATER TODAY.

TROPICAL STORM CENTER LOCATED NEAR 27.5N 73.1W AT 27/1500Z
POSITION ACCURATE WITHIN 30 NM

PRESENT MOVEMENT TOWARD THE NORTHWEST OR 310 DEGREES AT 11 KT

ESTIMATED MINIMUM CENTRAL PRESSURE 1006 MB
MAX SUSTAINED WINDS 50 KT WITH GUSTS TO 60 KT.
50 KT.......120NE 0SE 0SW 0NW.
34 KT.......180NE 120SE 0SW 135NW.
12 FT SEAS..180NE 180SE 180SW 150NW.
WINDS AND SEAS VARY GREATLY IN EACH QUADRANT. RADII IN NAUTICAL
MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

REPEAT...CENTER LOCATED NEAR 27.5N 73.1W AT 27/1500Z
AT 27/1200Z CENTER WAS LOCATED NEAR 27.5N 72.7W
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I think the NHC 11am track is way off, too far east and north.
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Chuck...thanks...let's go offshore and do a little sailing...
Member Since: August 13, 2007 Posts: 0 Comments: 10490
the east coast of FL should get some of the outer bands from danny this afternoon enhanced by daytime heating over FL w/should see some severe wx w/possible isolated tornados!!!!
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Quoting P451:
I think this speaks for itself...



I remember you calling this last night and some of us were in agreement with you. Other comments were not as kind. Do you think it is a trend or just a messed-up system?
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With all the uncertanty of the track right now, we need to keep our eyes on the sat pics and use our COMMEN SENSE.
130. LBAR
I love how the coral in the Bahamas reflects off the satellite pictures. Always stunning.

Danny: Watch out MYRTLE BEACH and WILMINGTON!
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So what's the thinking? That the COC has POSSIBLY reformed more west? or what?
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Quoting midgulfmom:
Good Morning everyone. HELLO 94L... Boy, that coc dance of Daddy is...interesting.


Thats the difference in a weak system like DANNY as it moves West as apposed to a stronger System that DANNY was to be.....SECONDLY Danny is moving WEST because of the High Pressure to its North built from the HIGH centered off the WEstern tip of Cuba! I seen this coming last night.
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danny could easily be pulled west by the ULL near AL,IMO......danny might pull a suprise and get very close to the east coast of FL and GA...
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We sent Bill Read a Portlight/Weather Underground t-shirt....wonder if that entitles me to call him and complain about the geogrphical ignorance of his staff...
Member Since: August 13, 2007 Posts: 0 Comments: 10490
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Morning Press. Looks like we'll be fine here in the beautiful Lowcountry. Danny is very disorganized and all the convection remains well displaced from the center. We will be on the left side of the storm through the weekend. Surf may get a bit churned up, but thats about all I'm expecting from Danny. Keep an eye on the next Cape Verde wave. That looks better organized than Danny.
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Something about the name Danny doesnt conjure up strength or severity..The name Erika however does..
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Quoting P451:
I think this speaks for itself...


whoa....way off
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Quoting StormW:


I still say they read this blog! LOL! (j/k)


Happy Bday Storm! I read your analysis yesterday, but with the center reformed and 60 nm south of the last forecast point, and with Danny's current shallow, disorganized state, how far west do you think he could go before turning NNW?
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Quoting P451:
I think this speaks for itself...



yeah it does
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117. 900MB
Quoting P451:
I think this speaks for itself...



I think you are right! This Danny is a joker isn't he?
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Link

i think this is a much more reasonable cone of uncertainty lol. The ships 1-2-3 model allows for a more realistic margin of error, which I'm pretty sure in this case is the best way to go. NHC didn't use to be conservative enough, now they are TOO conservative. Their official cone of uncertainty doesn't even cover an area of the average margin of error for the most accurate models.
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Quoting WXHam:



Concur. I saw that woble in the GOES update. Still think NE NC and VA Beach need to be wary.


Eh, little wind, little rain, reminds me of a typical Nor Easter we get every other week during our winters. I guess I could bring my kids sand buckets in the garage so they don't blow into my neighbors yard.
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TO: StormW

You keep calling them!! We were on late last night and very early this morning discussing the westerward motion on the exposed Circulation of Danny. As usual, some comments were not so kind. Can not wait to hear your analysis. When do you think you shall have a fix on 94L. Just put that birthday cake down and get to work!!!
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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.