[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 222045
TCDAT4

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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