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TROPICAL STORM LISA DISCUSSION NUMBER   6
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL132016
1100 PM AST TUE SEP 20 2016

Deep convection has increased in coverage near and to the northeast
of the center of Lisa this evening.  Although the exact center has
been difficult to pinpoint in infrared satellite images, it appears
that it is located near the southwestern edge of the convection due
to some southwesterly shear.  Satellite intensity estimates still
support a wind speed of around 40 kt, so the initial intensity is
unchanged for this advisory.  Lisa has about 24 hours over warm
water and in a marginally conducive upper-level wind environment in
which to strengthen.  After that time, an upper-level low to Lisa's
northwest is expected to produce strong southwesterly shear over
the cyclone, which should cause weakening.  As a result, the NHC
intensity forecast calls for peak intensity in about 24 hours,
followed by steady weakening thereafter.  The official forecast is
near the upper-end of the guidance at 24 hours, but is slightly
lower than the consensus after that time.

The somewhat uncertain initial motion estimate is 315/9 kt.  The
track forecast appears to be fairly straight forward as Lisa
should continue moving northwestward during the next several
days toward a break in the subtropical ridge.  Late in the forecast
period, the cyclone should turn northward ahead of a deepening mid-
to upper-level trough over the west-central Atlantic.  The dynamical
models are in good agreement on this scenario, and the NHC forecast
is near a blend of the typically better performing ECMWF and GFS
models.

FORECAST POSITIONS AND MAX WINDS

INIT  21/0300Z 17.1N  31.7W   40 KT  45 MPH
 12H  21/1200Z 17.8N  32.7W   45 KT  50 MPH
 24H  22/0000Z 18.6N  34.0W   50 KT  60 MPH
 36H  22/1200Z 19.4N  35.2W   45 KT  50 MPH
 48H  23/0000Z 20.6N  36.7W   40 KT  45 MPH
 72H  24/0000Z 23.6N  39.8W   30 KT  35 MPH
 96H  25/0000Z 26.5N  42.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  26/0000Z 31.0N  43.5W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Brown

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