Subtropical Depression DON (Text)

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.
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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Page last modified: Monday, 18-Dec-2023 12:09:11 UTC