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WTNT44 KNHC 222037

Tropical Depression Lee Discussion Number  18
NWS National Hurricane Center Miami FL       AL142017
500 PM AST Fri Sep 22 2017

The mid-level circulation from Lee separated from the low-level
center two days ago and moved northward around the eastern
periphery of a large upper-level trough.  A large convective burst
over the mid-level center yesterday then caused a new low-level
center to develop while it moved on the northern side of the upper
trough. ASCAT and ship data indicate that the small circulation is
now well defined, and since deep convection has persisted near the
center, advisories on Lee are being restarted. The initial wind
speed is 30 kt, which could be conservative given the healthy
structure seen on the last-light visible and recent microwave data.

Cold upper-level temperatures from the upper trough should mitigate
the effect of marginally warm waters for the next few days while Lee
remains in a low-shear environment.  The depression is expected to
strengthen over the next few days until westerly shear is forecast
to increase. The official forecast is similar to the model guidance,
although it shows an earlier peak more coincident with the
lower-shear, warmer-water environment.  This wind speed forecast is
difficult because Lee is a very small tropical cyclone, which
notoriously have rapid changes, both up and down, in intensity.

Lee is moving northward at about 6 kt.  The depression should turn
to the northeast and east over the next couple of days while it
moves around a ridge over the tropical central Atlantic.  Lee is
forecast to become trapped beneath a mid-latitude ridge in a few
days, which should cause the cyclone to drift at long range.  It
should be noted that models have a rather weak representation of
Lee at the present time, which causes this to be an uncertain
forecast.  For now, the forecast is closest to the ECMWF, which
has the most coherent cyclone to follow.


INIT  22/2100Z 30.8N  48.9W   30 KT  35 MPH
 12H  23/0600Z 31.7N  48.7W   40 KT  45 MPH
 24H  23/1800Z 32.3N  48.2W   45 KT  50 MPH
 36H  24/0600Z 32.3N  47.2W   50 KT  60 MPH
 48H  24/1800Z 32.1N  45.8W   55 KT  65 MPH
 72H  25/1800Z 31.4N  43.2W   60 KT  70 MPH
 96H  26/1800Z 30.5N  42.0W   55 KT  65 MPH
120H  27/1800Z 29.5N  43.0W   50 KT  60 MPH

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