Tropical Depression Fifteen Forecast Discussion

WTNT45 KNHC 010239

Tropical Depression Fifteen Discussion Number   2
NWS National Hurricane Center Miami FL       AL152020
1100 PM EDT Mon Aug 31 2020

Tropical Depression Fifteen is a sheared tropical cyclone, with the 
edge of the deep convection displaced nearly 30 n mi to the east of 
the exposed low-level center. An Air Force Reserve Hurricane Hunter 
mission earlier this evening provided SFMR instrument measurements 
of nearly 30 kt in the southeast quadrant. And, a recent ASCAT 
overpass showed several wind vectors in that same general area 
between 25-30 kt. Therefore, the initial intensity is being held at 
30 kt. 

The depression is moving northeast at 12 kt around the northwestern 
periphery of the subtropical ridge. A turn to the east-northeast is 
expected to occur on Tuesday as the cyclone moves along the 
northern periphery of the ridge. Late in the forecast period, what 
is left of the cyclone is forecast to get caught in the flow 
between building high pressure to its northeast, and an approaching 
trough to its northwest, which should induce a more northerly 
component of motion. The latest NHC forecast is little changed from 
the previous one, and is near the tightly clustered track guidance. 

The cyclone has moved into an environment with moderate westerly 
shear of about 20 kt, as indicated by the latest UW-CIMSS deep-layer 
shear analysis product. The SHIPS guidance by both the GFS and ECMWF 
indicate that the shear magnitude will more than double between 18 h 
and 36 h from now.  So it is anticipated that the only window of 
time for the depression to strengthen will be tonight through 
tomorrow, while the system is over warm waters and in a moist 
atmospheric environment. After 48 h, when the vertical wind shear is 
forecast to be between 40 and 50 kt, the cyclone is forecast to 
weaken, and by 96 h the NHC forecast now shows the system 
degenerating into a remnant low. Exactly what transpires with the 
cyclone later in the forecast period remains uncertain, as some of 
the global models suggest that the cyclone will open into a trough 
in a few days, while others indicate that the system could be 
absorbed by a larger mid-latitude trough around day 5. Regardless of 
what occurs by those time frames, the models all suggest that the 
system should not gain much more strength than it currently has. The 
official NHC intensity forecast is in good agreement with the 
various intensity consensus aids.


INIT  01/0300Z 33.2N  75.7W   30 KT  35 MPH
 12H  01/1200Z 34.2N  74.0W   35 KT  40 MPH
 24H  02/0000Z 35.1N  71.2W   35 KT  40 MPH
 36H  02/1200Z 36.0N  68.6W   35 KT  40 MPH
 48H  03/0000Z 36.9N  65.9W   35 KT  40 MPH
 60H  03/1200Z 37.7N  63.3W   30 KT  35 MPH
 72H  04/0000Z 38.1N  60.5W   30 KT  35 MPH
 96H  05/0000Z 38.7N  55.8W   30 KT  35 MPH...POST-TROP/REMNT LOW
120H  06/0000Z 41.6N  50.4W   30 KT  35 MPH...POST-TROP/REMNT LOW

Forecaster Latto


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