Tropical Storm KATE Advisory lun, 30-08

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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