Tropical Depression TWENTY (Text)

Tropical Depression Twenty Discussion Number   1
NWS National Hurricane Center Miami FL       AL202020
500 PM AST Sat Sep 12 2020
The tropical wave and associated area of low pressure that NHC has 
been tracking since it emerged off of Africa a couple of days ago 
has become sufficiently organized to be designated as a tropical 
depression. A curved band of deep convection developed early this 
morning and persisted just to the west of a well-defined low level 
circulation throughout the day. An earlier ASCAT overpass showed 
that 25-30 kt winds over the northwestern portion of the 
circulation, which is the basis for the initial intensity being 
set at 30 kt. 

The depression has a rather large circulation, with the radius of 
maximum winds nearly 100 n mi from the center and the overall wind 
field appearing to extend outward over 300 n mi. The environment 
surrounding the cyclone over the next 36 h is characterized by 
moderate northeasterly vertical wind shear and plenty of warm water 
and atmospheric moisture. These factors are supportive of gradual 
strengthening, however, due to the large size of the system, it may 
take some time for it to consolidate. The NHC intensity forecast 
shows only slight strengthening through 36 h as the system 
consolidates, and that portion of the forecast is well below the 
intensity guidance. By early next week, the wind shear is expected 
to decrease to under 10 kt and a faster rate of intensification is 
indicated from 36-96 h in anticipation of the cyclone having a 
better structure to take advantage of the lower shear. After 96 h 
the intensity is held steady as northwesterly shear is forecast to 
increase while the system encounters some slightly drier air and 
moves over lower oceanic heat content. The NHC intensity forecast 
beyond 36 h starts well below most of the guidance, and trends close 
to the IVCN/ICON later on in the forecast period. 

The depression is moving toward the west-northwest at 8 kt, steered 
by a mid-level ridge to its north. This ridge is forecast to build 
westward over the next few days, which should result in a continued 
general west-northwest motion, perhaps at a slightly faster forward 
speed early next week. By the middle of next week, a weakness is 
forecast to develop in the ridge, partially due to interaction of 
Paulette and a mid- to- upper level trough over the northern 
Atlantic at that time, and the cyclone should turn to the northwest 
into this weakness. Overall, track guidance from the global and 
regional models is in decent agreement on this scenario, and the 
NHC forecast is between the HFIP corrected consensus HCCA and the 
TVCN multimodel consensus. 

INIT  12/2100Z 11.4N  33.5W   30 KT  35 MPH
 12H  13/0600Z 11.5N  34.5W   30 KT  35 MPH
 24H  13/1800Z 12.1N  36.1W   35 KT  40 MPH
 36H  14/0600Z 12.8N  38.3W   40 KT  45 MPH
 48H  14/1800Z 13.3N  40.7W   50 KT  60 MPH
 60H  15/0600Z 14.0N  42.8W   60 KT  70 MPH
 72H  15/1800Z 15.0N  44.5W   70 KT  80 MPH
 96H  16/1800Z 17.7N  47.3W   80 KT  90 MPH
120H  17/1800Z 21.0N  49.7W   80 KT  90 MPH
Forecaster Latto

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Page last modified: Thursday, 31-Dec-2020 12:10:07 UTC