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Subtropical Depression FIVE


Subtropical Depression Five Discussion Number   1
NWS National Hurricane Center Miami FL       AL052018
500 AM AST Wed Aug 15 2018

The low pressure system that the National Hurricane Center has been
monitoring over the central Atlantic Ocean has developed
sufficently organized deep convection and a well-defined center.
Since the system has a large radius of maximum wind and is
co-located with an upper-level low, it is being classified as a
subtropical depression.  The initial intensity is set to a possibly
conservative 30 kt based on an earlier ASCAT pass.  This value
is slightly below the latest satellite intensity estimate from TAFB.
It is interesting to note that so far this hurricane season, four of
the five systems have been a subtropical cyclone at some point in
their lifetimes.

Slow strengthening is possible during the next 24 hours or so while
the system remains over warm SSTs and in fairly low wind shear
conditions.  After that time, sharply colder waters, drier air, and
a significant increase in wind shear should cause extratropical
transition in about 48 hours, or sooner, and a gradual weakening.
The models show the post-tropical system merging with a frontal zone
in 3 to 4 days.

The subtropical depression is moving slowly northward at about 4
kt.  A north to north-northeastward motion is expected today as the
system moves on the western periphery of a mid-level ridge.  A mid-
to upper-level trough currently over the northeastern U.S. is
anticipated to approach the cyclone, and that feature should cause
the system to accelerate northeastward on Thursday and Friday.
The models are in good agreement, and the NHC track forecast lies
near the middle of the guidance envelope.


INIT  15/0900Z 37.6N  45.6W   30 KT  35 MPH
 12H  15/1800Z 38.5N  45.4W   35 KT  40 MPH
 24H  16/0600Z 40.2N  44.4W   45 KT  50 MPH
 36H  16/1800Z 42.5N  41.9W   45 KT  50 MPH
 48H  17/0600Z 45.1N  37.9W   45 KT  50 MPH...POST-TROP/EXTRATROP
 72H  18/0600Z 51.0N  24.0W   40 KT  45 MPH...POST-TROP/EXTRATROP
 96H  19/0600Z...DISSIPATED

Forecaster Cangialosi