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Tropical Storm Lee Discussion Number  21
NWS National Hurricane Center Miami FL       AL142017
1100 AM AST Sat Sep 23 2017

Lee remains a compact tropical cyclone as indicated by a 20-kt wind
report at 1200Z from ship LAQX5, located only 70 nmi east of the
center. A 0943Z WindSAT pass had a few 40-45 uncontaminated wind
vectors in the southwestern quadrant, so the intensity has been
conservatively increased to 40 kt, which is closer to the satellite
intensity estimates of T2.5/35 kt from TAFB and UW-CIMSS ADT.

The initial motion is a slow drift toward the north or 360/02 kt,
based primarily on microwave satellite fixes. The global and
regional models are in general agreement that Lee will move slowly
at 5 kt or less for the next 5 days, and either make a wide or a
tight anticyclonic turn back toward the west after 72 h due to a
building high pressure ridge to the north of the cyclone, and a
stationary upper-level low located a few hundred nmi to the south.
The GFS makes the widest turn, whereas the ECMWF makes the tightest
turn; the remainder of the model guidance lies somewhere between
those two extremes. Until the model guidance becomes more
convergent, the official NHC forecast track will remain basically
down the middle of the guidance envelope, close to the blend of the
TCVA, TVCX, and HCCA consensus models.

Earlier WindSAT and SSMI/S microwave data showed that Lee possessed
an 8-10 nmi diameter eye-like feature in both the low- and
mid-levels, indicating that the cyclone has a well-defined
inner-core wind field, albeit quite small. The deep-layer vertical
wind shear is forecast by both the GFS and ECMWF models to decrease
to 5-10 kt during the next 48 hours or so while Lee remains over
SSTs of about 27.5 deg C. In addition, upper-level temperatures are
currently running, and are expected to remain, about 3 deg C colder
than normal, which should ensure sufficient instability for the
generation of inner-core thunderstorm activity through the entire
forecast period. Based on the combination of these favorable dynamic
and thermodynamic conditions, Lee is forecast to steadily develop
into a hurricane by 48-72 hours, with the only inhibiting factor
being occasional intrusions of very dry mid-level air. However, the
strong instability/CAPE that will be available should allow for deep
convection to more or less persist near the center until modest
westerly shear begins to affect the cyclone by 72-120 hours, during
which time a slow weakening trend is expected. The official
intensity forecast is very similar to the previous advisory and is
close to a blend of the IVCN and HCCA intensity consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT  23/1500Z 31.9N  49.4W   40 KT  45 MPH
 12H  24/0000Z 32.2N  49.1W   45 KT  50 MPH
 24H  24/1200Z 32.3N  48.3W   50 KT  60 MPH
 36H  25/0000Z 31.9N  47.2W   55 KT  65 MPH
 48H  25/1200Z 31.3N  46.2W   60 KT  70 MPH
 72H  26/1200Z 30.5N  45.3W   70 KT  80 MPH
 96H  27/1200Z 29.9N  45.9W   70 KT  80 MPH
120H  28/1200Z 30.5N  47.1W   65 KT  75 MPH

$$
Forecaster Stewart

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