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 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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Quoting ChrisCone:


in other words the northern most cloud deck has not made much northward progress, the same area of convection is being maintained as opposed to moving to the north even though the direction of the clouds themselves are going north (as they should around that right side of any circulation), it's all still pretty well netted near the LLC - and the last hour or so it's catching back up to the LLC.


oh alright, thanks again, i really think its developing a new coc near 26N and 70, or 71 west
Member Since: June 28, 2006 Posts: 25 Comments: 8360
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --AS OF 10 AM THURSDAY...WATER VAPOR IMAGERY SHOWING UPPER LOW ACROSS
LA/MS THIS MORNING WITH MOISTURE PLUME SURGING N AHEAD OF THIS
FEATURE AND ACROSS THE SOUTHEASTERN UNITED STATES. SHORTWAVES ARE
ROTATING AROUND THE UPPER LOW AND ONE OF THESE SHORTWAVE TROUGHS
WILL PULL NORTH TODAY...PASSING TO OUR W. PRECIPITABLE WATERS WILL
BE ON THE INCREASE THROUGH THE PERIOD. AS WEAK CAP ERODES TODAY AND
INSTABILITY RAMPS UP...EXPECT SHOWERS AND THUNDERSTORMS TO
DEVELOP...ESPECIALLY MID TO LATE AFTERNOON HOURS ACROSS OUT
SOUTHWEST AND WESTERN ZONES...PRIMARILY IN SOUTH CAROLINA. ISOLATED
THUNDERSTORMS ARE POSSIBLE FURTHER ELSEWHERE INCLUDING ALONG THE
RESULTANT SEABREEZE BOUNDARY. THUNDERSTORMS THAT DO DEVELOP SHOULD
PERSIST THROUGH THE EARLY EVE BEFORE DIMINISHING. INVERTED-V
PROFILES INDICATE THERE IS A POTENTIAL FOR AN ISOLATED WET
MICROBURST WITH STRONG GUSTY WINDS. STORM MOTIONS SHOULD BY AROUND 5
KT...AND SO ANY THUNDERSTORM COULD PRODUCE LOCALLY EXCESSIVE RAIN
AND SIGNIFICANT PONDING. FLOODING IS POSSIBLE ACROSS LOW LYING AND
POOR DRAINAGE AREAS.

TROPICAL STORM DANNY IS NOT WELL STRUCTURED THIS MORNING WITH THE
CENTER NOT COLLOCATED WITH THE CONVECTION. THE CENTER IS LOCATED W
AND SW OF THE DEEPER CONVECTION. THIS SHOULD MEAN DANNY WILL BE SLOW
TO STRENGTHEN. IT ALSO MEANS THAT THE CENTER COULD RELOCATE...
PERHAPS UNDER THE DEEPER CONVECTION WITH TIME AS THE SHEAR
DIMINISHES. AT THIS TIME...THE HURRICANE CENTER IS STILL EXPECTING
THE STORM TO PASS TO OUR E. HOWEVER...UNTIL THE STORM PASSES N OF
OUR LATITUDE...SOMETIME OVERNIGHT FRI OR EARLY ON SAT...WE WILL
CONTINUE TO CLOSELY MONITOR AND THERE IS A SMALL CHANCE THAT
TROPICAL CYCLONE WATCHES WILL BE ISSUED FOR A PORTION OF OUR AREA
LATE THIS AFTERNOON.-- End Changed Discussion --

Good afternoon everyone..from Wilm NWS..still an uncertanity with the forecast..and just last night the local mets were saying nothing to worry about..only high seas to worry about..this would catch a lot of people off guard if this storm has a mind of its own!
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409. slavp
Quoting tornadodude:
rotation near 26 n 70 w?
link
Don't see one there, but looks like maybe 27.5N 71.5W
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Quoting tornadodude:


ok, makes sense, thanks!


in other words the northern most cloud deck has not made much northward progress, the same area of convection is being maintained as opposed to moving to the north even though the direction of the clouds themselves are going north (as they should around that right side of any circulation), it's all still pretty well netted near the LLC (although disorganized by tropical standards) - and the last hour or so it's catching back up to the LLC. However there is definitely some sheer blowing the cloud tops a bit to the northeast, but not the convection as a whole.

Link

check it out, the coldest/highest cloud tops have been consistent in the same general location for hours, moving slightly west... with warmer/lower cloud tops being sheered away. This is a sign of disorganization, but the main area of most intense convection is remaining fairly consistent.

sorry, perhaps I was a little bit of a troll myself when i commented you? Didn't mean it like that.
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Invest 94L:
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It looks like both. Like its trying to form a new LLC while spinning into the old?

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405. 900MB
Quoting tornadodude:
ok, so i think im seeing some rotation in the middle of the convection on the last couple of frames, thoughts?


Yep, kinda looks that way. Back to 71 or 71.5 W. Will they be talking jog East next?
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Anyone familiar with track of Betsy '65. I don't know how to post pic's but due to her crazy track, I wonder if she was pulled and pushed around by similiar low level steering. NOT saying that Danny is or will follow the same or similiar path. Thanks! :)
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Without a vortex message, center appears to be at 27.267N 73.250W according the HH.
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Quoting FlyinFish:


Agreed, or is it just wrapping around the Low level circulation?


not sure, might be forming a new coc
Member Since: June 28, 2006 Posts: 25 Comments: 8360
Quoting tornadodude:
ok, so i think im seeing some rotation in the middle of the convection on the last couple of frames, thoughts?


Agreed, or is it just wrapping around the Low level circulation?
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cycloneoz - could I get a link to the base tanker pic?

That's a keeper for ribbing friends and family.
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Those are my concerns as well. I hope you are wrong.
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SARCASM
Member Since: September 14, 2007 Posts: 3 Comments: 3963
396. 7544
danny wooble to the sw
Member Since: May 6, 2007 Posts: 0 Comments: 6874
rotation near 26 n 70 w?
link
Member Since: June 28, 2006 Posts: 25 Comments: 8360
Quoting TampaSpin:


Started rainning so no Waverunning today.....NICE update STormW. Ya, Danny is moving WEST because the High Pressure built overhead from the high Centered off the Western tip of Cuba! This trend needs to stop or Virginia, South Carolina, and North Carolina could become a real threat as i expect Danny to begin some very quick intensification as conditions are improving and its approaching the Gulf Stream.....just my opinion.


A well educated opinion based on current observations that include it's almost due West movement against what the forecast points have predicted.
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Quoting Acemmett90:

you know why it a blog it easier to hear sarcasum verbely


clueless.
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Quoting TampaSpin:


Started rainning so no Waverunning today.....NICE update STormW. Ya, Danny is moving WEST because the High Pressure built overhead from the high Centered off the Western tip of Cuba! This trend needs to stop or Virginia, South Carolina, and North Carolina could become a real threat as i expect Danny to begin some very quick intensification as conditions are improving and its approaching the Gulf Stream.....just my opinion.
Hey Tampa - you've been saying west for a while....Danny could be one of those sleepers. :(
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Danny is a tough storm to look at. I try to focus on what I "think" is the center and follow it to see which way it is moving. Unfortunately too much other stuff (clouds, rainstorms, convection, etc..) is moving in different directions.

Once Danny gets his act together we may finally know which way he is going and which way he *should* go. Until then, I gotta go with StormW, Dr. M., and the NHC.
Member Since: March 10, 2006 Posts: 0 Comments: 10754
387. jpsb
Quoting 69Viking:


I have to agree with you when you look at this visible with the forecast pts! He looks to be moving almost due West with very little movement to the North at this time. See for yourself.

Link


Sure looks like west movement to me.
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ok, so i think im seeing some rotation in the middle of the convection on the last couple of frames, thoughts?
Member Since: June 28, 2006 Posts: 25 Comments: 8360
Quoting StormW:
TROPICAL WEATHER SYNOPSIS /DANNY/ /94L/ AUG. 27, 2009 ISSUED 12:00 P.M. EDT


Started rainning so no Waverunning today.....NICE update STormW. Ya, Danny is moving WEST because the High Pressure built overhead from the high Centered off the Western tip of Cuba! This trend needs to stop or Virginia, South Carolina, and North Carolina could become a real threat as i expect Danny to begin some very quick intensification as conditions are improving and its approaching the Gulf Stream.....just my opinion.
Member Since: September 2, 2007 Posts: 179 Comments: 20448
HH is at 27.443 and not to the center yet... travelling S.
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Quoting caneluver:
Danny still is not following the forcast points. He wants to keep west.


I have to agree with you when you look at this visible with the forecast pts! He looks to be moving almost due West with very little movement to the North at this time. See for yourself, make sure you put a check mark for Trop Pts.

Link
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Quoting ChrisCone:


while the individual convection is streaming north, the area of as a whole has been nearly stationary for several hours and is now slowly moving back over the LLC. The convection moving to the north of the LLC is simply a result of the motion around the rotation and the upper level winds.

In other news, another very SLIGHT southwesterly jog may have taken place within the last hour, based on the sat image - but it's almost impossible to say for sure could just be the looks of the rotation.


ok, makes sense, thanks!
Member Since: June 28, 2006 Posts: 25 Comments: 8360
lets see what it does when the convection catches up
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376. MahFL
Darkster......kids don't understand sarcasm....he he he.
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372. thanks! that's pretty funny :)
Member Since: September 14, 2007 Posts: 3 Comments: 3963
Quoting cattlebaroness:
At what cat level do they request evacuation. I would think some of the areas that have dense population would start considering thinning out the folks due to Danny's potential landfall.


It's a matter of likelihood and proximity to landfall. Call for an evacuation for an area that doesn't get effected and those people that left will think long and hard about getting out again...
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Quoting pearlandaggie:
so what was CycloneOz's funny about? the picture is blocked by the corporate firewall...


Said "you're b-day candles have arrived" and pic is of a huge cargo ship.
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Quoting StormW:
TROPICAL WEATHER SYNOPSIS /DANNY/ /94L/ AUG. 27, 2009 ISSUED 12:00 P.M. EDT


Thanks Storm! Great analysis.
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so the coc is moving west but the convection appears to be moving north?

link
Member Since: June 28, 2006 Posts: 25 Comments: 8360
thats at danny i hope

nope... hurricane hunter report shows 60kt winds at the surface of the invest... unless this is some sort of glitch?
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so what was CycloneOz's funny about? the picture is blocked by the corporate firewall...
Member Since: September 14, 2007 Posts: 3 Comments: 3963
Happy birthday stormW.

That was definitely LOL cycloneoz.
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Quoting WXHam:
Do I see convection moving to Danny's COC?

Link


I believe so. Have to see if he continues to get his close back on.
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364. 789
Quoting StormW:
Good afternoon!

Hot off the press!!

Thanks!

TROPICAL WEATHER SYNOPSIS /DANNY/ /94L/ AUG. 27, 2009 ISSUED 12:00 P.M. EDT
its your birthday .... thanks stormW
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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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