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We now have TD4.

..

TROPICAL DEPRESSION FOUR DISCUSSION NUMBER 1

NWS NATIONAL HURRICANE CENTER MIAMI FL AL042015

1100 AM AST TUE AUG 18 2015

The low pressure system that has been moving westward across the

eastern tropical Atlantic the past few days has finally acquired

enough organized deep convection to be classified as a tropical

cyclone. A 1232 UTC ASCAT-B overpass indicated the system has a

well-defined circulation, and there was also a large field of 30-kt

and greater wind vectors in the eastern semicircle. Dvorak satellite

intensity estimates are a consensus T2.0/30 kt from TAFB and SAB, so

the initial intensity is set at 30 kt for this advisory. Upper-level

outflow is good to the south and fair to the north.

The initial motion estimate is 280/11 kt. The global and regional

models are in good agreement on the tropical cyclone moving

west-northwestward along the southern periphery of a deep-layer

subtropical ridge for the next 48-72 hours, accompanied by a

decrease in forward speed as the system approaches a weakness in

this ridge. After that time, the ridge is expected to build back in

as a trough to the north lifts out, forcing the cyclone to turn more

westward and accelerate through the remainder of the forecast

period. The official forecast track lies close to but a little

faster than the multi-model consensus TVCN due to the much slower

GFS model creating a slow bias in the consensus.

The overall atmospheric and oceanic environments surrounding the

cyclone appear conducive for slow but steady strengthening

throughout the forecast period. The only inhibiting factor will be

occasional brief intrusions of dry mid-level air associated with the

Saharan Air Layer that lies just to the north of the depression.

However, given the very low vertical wind shear of less than 5 kt,

the convective structure is expected to steadily increase in

organization, allowing the dry air intrusions to be mixed out. The

official intensity forecast closely follows the intensity consensus

model IVCN through 72 hours, and then leans closer to a blend of the

Decay-SHIPS and LGEM models at 96 and 120 hours.

FORECAST POSITIONS AND MAX WINDS

INIT 18/1500Z 10.6N 36.5W 30 KT 35 MPH

12H 19/0000Z 11.0N 37.9W 35 KT 40 MPH

24H 19/1200Z 11.3N 39.5W 40 KT 45 MPH

36H 20/0000Z 11.6N 40.9W 50 KT 60 MPH

48H 20/1200Z 12.1N 41.9W 60 KT 70 MPH

72H 21/1200Z 13.2N 44.2W 70 KT 80 MPH

96H 22/1200Z 13.7N 47.8W 80 KT 90 MPH

120H 23/1200Z 14.0N 52.4W 85 KT 100 MPH

.......

144842W5_NL_sm.gif

rgb0-lalo.gif

​

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Blimey, something decent on the way for the Atlantic? The only negative factor for 04L appears to be dry air, but it looks like the depression will remain far enough south to not be impacted too much. A category 2 hurricane moving into the Caribbean? Something we've not seen in a while. Needs closely watching.

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Certainly a good looking system too, very good structure.

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Prob of RI for 25 kt RI threshold= 48% is 4.0 times the sample mean(11.9%)
Prob of RI for 30 kt RI threshold= 37% is 4.9 times the sample mean( 7.6%)
Prob of RI for 35 kt RI threshold= 22% is 4.8 times the sample mean( 4.6%)
Prob of RI for 40 kt RI threshold= 12% is 3.9 times the sample mean( 3.0%)​

 

48% chance that it's a hurricane in 24 hours then according to SHIPS.

 

Are we seeing a CDO on the pic above or is that just a vigorous tower.

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I posted this very distinct possibility yesterday, my post seems to have been removed for some reason.

It has been quiet in the Atlantic, however conditions are fairly good for a slow build of this one.

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Winds are up to 45kts currently. There seems to be a good amount of convection over the LLCC, with banding features evident. Further intensification appears likely.

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Given the latest trends, it appears Danny is struggling to fight off the dry air. NHC intensity forecast may change.

post-12275-0-38670400-1439992473_thumb.p

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Deep convection over the center but dry air doing a number elsewhere..

 

rbtop0-lalo.gif

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Given the latest trends, it appears Danny is struggling to fight off the dry air.

 

Yep, Danny really does look quite ill, with a total lack of convection. In fact, if you look at the satellite loop below you can see the LLCC (low level circulation center) becoming exposed at around 11 N 41 W. Recently, however, a new thunderstorm has developed right over the LLCC, suggesting that Danny may finally be reversing the downward trend. 

 

post-20885-0-97811200-1440004910_thumb.g

Visible satellite loop of Danny. Courtesy: NOAA.

 

What could be the reason for this weakening? At first hand, you would say dry air entrainment, as there is a large area of dry air lurking to the north of the system. Coamps-TC model output seems to be agreeing with this idea:

 

post-20885-0-57151200-1440005587_thumb.g

Coamps-TC 06Z T+12 Relative humidity (RH) values at 500 hPa height. Courtesy: Coamps-TC

 

The black cross shows the approximate location of the center of the cyclone. What we can see here is that there is a band of dry air at 500 hPa entraining into the cyclone from the west (follow the low RH values to the west of the cyclone; the black arrow).

Note that this is just a model forecast, so the situation shown here may not perfectly correlate with the current envrionment around the storm.

 

Unless Danny is able to fend off this dry air, it may have a tough time sustaining convection, resulting in possible weakening. Therefore, the NHC forecast showing steady strengthening for the next four days may well be too agressive if current trends continue.

 

Sources:

http://www.ssd.noaa.gov/PS/TROP/floaters/04L/04L_floater.html

http://tropic.ssec.wisc.edu/#

http://www.nrlmry.navy.mil/TC.html

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To its credit, convection is strong over the center so any relaxation of dry air will allow it to strengthen.

Euro wants to kill it, last two GFS have had strong canes in the Bermuda Triangle (circling round) and Panhandle.

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Atlantic Tropical Storm Danny Model Tracks (0000 UTC 20 August 2015):

 

post-12275-0-94212000-1440058971_thumb.p

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Fantastic structure even if dry air is holding it back.

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Yup, warm spot and eye now clear on some radar types.

It appears we have Hurricane Danny.

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I'd hold fire before declaring it a hurricane, tropical storms can sometimes have eye features. But yes, Danny is looking much better, with deep centralised convection and an eye feature. The storm is very small however, indicative of the dry air it has traversed through. It does mean that Danny is more vunerable to shear and dry air driven into the core by shear. At the moment, shear is low, so Danny should continue to strengthen in the near term as the small inner core is shielded from the dry air.

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Looking like it may be downgraded to a TS later.

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I know I saw the comment that Danny could almost fit into the eye.

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