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Tropical Depression Ten Discussion Number   9
NWS National Hurricane Center Miami FL       AL102021
500 AM AST Mon Aug 30 2021

The deep convection with Tropical Depression Ten is in a sheared 
bursting pattern this morning, associated with overshooting cloud 
top temperatures below -80 C. A 0518 UTC AMSR-2 microwave pass did 
show a bit of banding associated with this activity on the 37 GHz 
low-level channel. However, Proxy-Vis satellite imagery indicates 
this convective activity remains located downshear of the low-level 
center thanks to very strong 30-50 kt northwesterly flow associated 
with a subtropical jet at 200 mb. The latest round of subjective 
Dvorak estimates were 25 kt from SAB and 35 kt from TAFB. Taking a 
blend of these estimates and the earlier ASCAT wind data supports 
keeping the intensity at 30 kt for this advisory.

The depression has resumed a northward motion this morning, with the 
latest estimate at 360 degrees at 7 kt. A deep-layer trough passing 
by well to the north is continuing to provide a weakness in the 
subtropical ridge, allowing the cyclone to escape northward. The 
system could even move a bit east of due north over the next 24-36 
hours if down-shear convective bursts help to drag the low-level 
center a bit right of the steering flow. After 36 hours, the 
deep-layer trough moves eastward, allowing the subtropical ridge to 
build back in. The net result is that the depression should turn 
leftward and begin a more northwestward motion by the latter part of 
this week. The latest track guidance has once again made another 
westward shift this cycle after 36 hours, and the NHC track 
forecast has been nudged in that direction as well. However, the 
latest track is still not as far west as the GFS & ECMWF models, and 
further westward adjustments may be needed in subsequent forecasts. 

Strong upper-level flow is the primary hindrance for the depression 
currently. In fact, both GFS & ECMWF-based SHIPS guidance shows the 
vertical wind shear remaining above 30 kt for the next 24 hours as 
the cyclone moves through the core of a subtropical jet streak. 
Interestingly, this shearing flow seems to be mostly based in the 
upper-levels, with much lower mid-level shear diagnosed by UW-CIMSS. 
This lower mid-level shear may help explain why deep-convection has 
not yet been completely stripped away from the low-level center. 
After 36 hours, most of the guidance agrees that an upper-level low 
will cut off to the southwest of the depression, providing a more 
favorable upper-level environment over the system. However, it 
remains unclear what will be left of the depression by that time, 
and the latest 00z ECMWF, HWRF, and HMON runs suggest the vortex 
will be too weak and diffuse to take advantage of the more favorable 
conditions. For now, the latest NHC intensity forecast will maintain 
the current intensity through 48 hours, with only modest 
intensification beginning after that time assuming the circulation 
is coherent enough to take advantage of the more favorable 
environment. The latest intensity forecast is just a bit lower than 
the previous forecast, and is also lower than the HCCA and IVCN 
consensus aids. It remains distinctly possible that the depression 
could become a remnant low if its convection is completely stripped 
away.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  30/0900Z 20.8N  50.6W   30 KT  35 MPH
 12H  30/1800Z 21.7N  50.6W   30 KT  35 MPH
 24H  31/0600Z 22.6N  50.3W   30 KT  35 MPH
 36H  31/1800Z 23.6N  50.1W   30 KT  35 MPH
 48H  01/0600Z 24.8N  50.7W   30 KT  35 MPH
 60H  01/1800Z 26.1N  51.7W   35 KT  40 MPH
 72H  02/0600Z 27.6N  53.1W   40 KT  45 MPH
 96H  03/0600Z 30.8N  55.3W   45 KT  50 MPH
120H  04/0600Z 35.1N  53.6W   50 KT  60 MPH
 
$$
Forecaster Papin
 
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