[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT45 KNHC 162041
TCDAT5

Subtropical Depression Don Discussion Number  11
NWS National Hurricane Center Miami FL       AL052023
500 PM AST Sun Jul 16 2023

Don is a strange system.  Satellite images show that convection has
increased in aerial coverage in the southeastern quadrant of the
cyclone, although it is still not well defined in banding features
or other organizational metrics.  The convection is also closer to
the center, and the upper-level low near Don has weakened.  In some
ways it has actually gained some tropical characteristics since
yesterday despite remaining over cool waters, but the convective
organization is still shy of a tropical cyclone.  Regardless of the
scientific curiosity, there are no signs that the winds have
strengthened, and Don is best classified as a 30-kt subtropical
depression on this advisory.

Don has turned to the east this afternoon and should gradually
execute a large clockwise loop over the central Atlantic during the
next few days, first turning southeast, then south and southwest
through Wednesday. The only change to that part of the forecast was
a slight eastward adjustment at a faster forward speed.  There
continues to be a large guidance spread on days 4/5 as Don moves
south of a blocking ridge and eventually turns to the west or
northwest.  Interestingly, the regional hurricane models are
generally on the left side of the guidance envelope, and the global
models are on the right side.  I just don't trust the regional
guidance in this set-up where the large-scale steering might matter
more than the storm structure (and the regional models struggle in
hybrid situations).  Thus, the forecast leans on the eastern side of
the guidance at long range, resulting in little change from the
previous forecast, east of the model consensus.

Don isn't likely to change much in intensity for the next few days,
with a somewhat unfavorable environment persisting.  While not
explicitly shown, it could degenerate into a remnant low during the
next few days until it reaches warmer waters.  Then, a fair bit of
the guidance brings the system back (as a tropical cyclone) due to
more instability over warmer waters in a lighter shear environment.
The forecast does show Don as a tropical storm in 4 days, but with
water temperatures still below 25C, it isn't expected to be very
strong at long range.  The new NHC forecast is close to the previous
one and remains lower than the guidance at the extended time ranges.


FORECAST POSITIONS AND MAX WINDS

INIT  16/2100Z 39.3N  47.0W   30 KT  35 MPH
 12H  17/0600Z 39.4N  45.3W   30 KT  35 MPH
 24H  17/1800Z 38.5N  42.9W   30 KT  35 MPH
 36H  18/0600Z 37.0N  41.1W   30 KT  35 MPH
 48H  18/1800Z 35.3N  40.6W   30 KT  35 MPH
 60H  19/0600Z 33.8N  40.9W   30 KT  35 MPH
 72H  19/1800Z 33.0N  41.5W   30 KT  35 MPH
 96H  20/1800Z 33.4N  44.3W   35 KT  40 MPH...TROPICAL CYCLONE
120H  21/1800Z 35.5N  48.0W   35 KT  40 MPH

$$
Forecaster Blake
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