[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 160301
TCDAT3

Tropical Depression Eight Discussion Number   1
NWS National Hurricane Center Miami FL       AL082021
1100 PM AST Sun Aug 15 2021

A small well-defined low pressure system located just
east-northeast of Bermuda has produced persistent deep convection
since this morning. Radar imagery from Bermuda along with
geostationary and polar orbiting microwave satellite data show that
the convection is sufficiently well organized to meet the
definition for a tropical cyclone. Therefore advisories have been
started on Tropical Depression Eight.  The initial intensity is set
at 25 kt, based on believable winds near that value in a pair of
recent ASCAT overpasses. Dvorak estimates from TAFB and SAB were a
little higher, so this estimate could be slightly conservative.

Confidence in the forecast is reasonably high for the next 48 to
72 h. The depression should make a slow clockwise turn around
Bermuda, steered by a mid-level ridge currently centered off the
coast of the Carolinas. Warm SSTs should provide ample fuel to
support at least slight strengthening during this period, despite
expected northerly shear, and this is shown by all of the intensity
guidance. The NHC forecast is very near the multi-model consensus
for both track and intensity through that period.

Beyond 72 h, the track and intensity models diverge quickly. The
HWRF and COAMPS-TC models rapidly intensify the system to major
hurricane strength. A stronger, deeper vortex would likely be
steered generally westward through the end of the forecast period,
influenced by a deeper-layer steering flow.  However, the global
models have a much weaker, shallower system which turns northward,
and then possibly accelerates northeastward by day 5. As a result,
the spread in the track guidance is nearly 800 n mi by day 5, with
the GFS and HWRF taking the extreme positions to the northeast and
southwest, respectively. The official track forecast takes a middle
approach, and is between the various consensus aids at days 4 and
5. The NHC intensity forecast is slightly more conservative, and is
below the consensus at the end of the period. Significant changes
to the forecast may be required in subsequent advisory packages.


FORECAST POSITIONS AND MAX WINDS

INIT  16/0300Z 33.2N  62.7W   25 KT  30 MPH
 12H  16/1200Z 32.4N  62.8W   30 KT  35 MPH
 24H  17/0000Z 31.5N  63.3W   35 KT  40 MPH
 36H  17/1200Z 31.1N  64.0W   40 KT  45 MPH
 48H  18/0000Z 31.0N  64.9W   45 KT  50 MPH
 60H  18/1200Z 31.0N  66.1W   50 KT  60 MPH
 72H  19/0000Z 31.2N  67.4W   55 KT  65 MPH
 96H  20/0000Z 32.0N  68.5W   55 KT  65 MPH
120H  21/0000Z 33.5N  68.0W   55 KT  65 MPH

$$
Forecaster Zelinsky
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