Tropical Depression Eleven Forecast Discussion

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000
WTNT41 KNHC 120900
TCDAT1

Tropical Depression Eleven Discussion Number   3
NWS National Hurricane Center Miami FL       AL112020
500 AM AST Wed Aug 12 2020

After an earlier burst of deep convection near the center and in 
the western semicircle of the circulation, overall thunderstorm 
activity has decreased somewhat. This is likely due to the 
entrainment of mid-/upper-level dry air as noted in GOES-16 
high-resolution mid- and upper-level water vapor imagery, in 
conjunction with some modest southeasterly vertical wind shear.
The latest subjective and objective Dvorak satellite 
classifications from TAFB/SAB and UW-CIMSS ADT of 30 kt and 33 kt, 
respectively, support maintaining an initial intensity of 30 kt.

The depression continues moving a little north of due west, or 
280/13 kt. This general motion is expected to continue through 
tonight due to easterly flow on the south side of a strong 
subtropical ridge situated to the north of the cyclone. By 24 h, all 
of the global models are in decent agreement that a slight weakness 
will develop within the ridge, allowing the cyclone to turn more 
toward the west-northwest and then continue that motion through the 
end of the forecast period. The latest NHC model guidance is tightly 
packed about the previous forecast track, so no significant track 
changes were made.

The aforementioned unfavorable conditions of dry air entrainment 
and southeasterly shear are expected to give way to more conducive 
environmental conditions by 24 h when the cyclone will begin to move 
underneath the center of a synoptic-scale upper-level anticyclone. 
This will result in the shear decreasing to near zero, along with a 
pronounced improvement in the upper-level outflow pattern. The much 
lower shear conditions should also reduce the amount of dry air 
entrainment, while allowing for some moistening of the surrounding 
environment to occur. These more favorable conditions are expected 
to persist through at least the 60-h period, and thus slow but 
steady strengthening is forecast during that time. By 72 h and 
beyond, the global models and regional models show the system 
moving out from underneath the positive influence of the 
upper-level anticyclone, encountering moderate southerly to 
southwesterly vertical wind shear, which is expected to induce 
gradual weakening. It should be noted that during the 48-60 h 
period when the shear will be the lowest and sea-surface 
temperatures will be near 28.5C, there is a narrow window of 
opportunity where the intensity could peak higher than what is 
currently indicated. The new NHC intensity forecast is very similar 
to the previous advisory, and is slightly higher than the 
consensus intensity consensus models IVCN and HCCA due to 
anticipation of the very favorable low-shear conditions.


FORECAST POSITIONS AND MAX WINDS

INIT  12/0900Z 12.2N  42.9W   30 KT  35 MPH
 12H  12/1800Z 12.6N  44.8W   35 KT  40 MPH
 24H  13/0600Z 13.2N  47.3W   40 KT  45 MPH
 36H  13/1800Z 14.0N  49.6W   45 KT  50 MPH
 48H  14/0600Z 15.1N  52.1W   50 KT  60 MPH
 60H  14/1800Z 16.3N  54.6W   50 KT  60 MPH
 72H  15/0600Z 17.6N  57.0W   45 KT  50 MPH
 96H  16/0600Z 20.1N  61.8W   40 KT  45 MPH
120H  17/0600Z 22.9N  65.6W   35 KT  40 MPH

$$
Forecaster Stewart



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