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Subtropical Depression Four Discussion Number   1
NWS National Hurricane Center Miami FL       AL042020
500 PM AST Mon Jun 22 2020
 
The non-tropical low pressure system that the National Hurricane 
Center has been following for the past couple of days off of the 
U.S. east coast has developed enough organized convection near the 
center to be classified as subtropical depression. The subtropical 
status is due to the low-level circulation center being co-located 
beneath an upper-level cold low as seen in water vapor imagery. The 
initial intensity of 30 kt is based on earlier ASCAT wind data 
indicating numerous surface wind vectors of 26-28 kt in the 
southern semicircle, along with a TAFB subtropical satellite 
classification of ST1.5/25-30 kt.

The initial motion estimate is 075/08 kt. Subtropical Depression 
Four is located north of a deep-layer ridge and is being influenced 
by weak westerly mid- to upper-level flow.  The cyclone is forecast 
by all of the global and regional models to move east-northeastward 
tonight and then turn northeastward on Tuesday.  A northeastward 
motion at a faster forward speed is expected Tuesday night through 
Thursday when the system is forecast to be located over the cold 
waters of the far north Atlantic.  The cyclone should be absorbed by 
a larger extratropical low or dissipate on Thursday. The NHC track 
forecast is close to a blend of the simple consensus aids TVCN and 
GFEX and the NOAA-HCCA corrected consensus. 

The cyclone is beginning to move over a ridge of higher SSTs of 
26.0-26.5 deg C in the northern extent of the Gulf Stream. The 
forecast track takes the subtropical depression down the length of 
the axis of warmer water during the next 12-18 h, so there is the 
potential for the cyclone to become a subtropical storm during that 
time, especially given the large pool of cold air aloft, with 200-mb 
temperatures of near -58 deg C and 500-mb temperatures of about -10 
deg C which is creating a lot of instability. After the system moves 
off of the warm ridge and into sharply cooler water around 36 hours 
or so, gradual weakening is expected, with transition to an 
extratropical cyclone forecast by 48 hours. The NHC intensity 
forecast closely follows a blend of the consensus models ICON, IVCN, 
and HCCA.

FORECAST POSITIONS AND MAX WINDS
 
INIT  22/2100Z 38.2N  65.7W   30 KT  35 MPH
 12H  23/0600Z 38.5N  64.0W   35 KT  40 MPH
 24H  23/1800Z 39.0N  62.0W   30 KT  35 MPH
 36H  24/0600Z 40.5N  59.3W   30 KT  35 MPH
 48H  24/1800Z 42.5N  56.7W   30 KT  35 MPH...POST-TROP/EXTRATROP
 60H  25/0600Z 45.0N  53.3W   30 KT  35 MPH...POST-TROP/EXTRATROP
 72H  25/1800Z 47.8N  50.4W   30 KT  35 MPH...POST-TROP/EXTRATROP
 96H  26/1800Z...DISSIPATED
 
$$
Forecaster Stewart
 
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