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Tropical Storm Iselle Discussion Number   9
NWS National Hurricane Center Miami FL       EP142020
800 AM PDT Fri Aug 28 2020
 
GOES-16 shortwave infrared imagery and a recent AMSR2 microwave
overpass indicate that deep convection has been developing near the
surface center during the past several hours.  In fact, the
microwave image showed a small, compact inner core defined by a
partially closed eye-like feature.  My initial thoughts were that
this cloud feature is in the mid-portion of the atmosphere, but the
lower 37 GHZ frequency confirmed very little vertical tilt.
Subjective and objective satellite intensity estimates are based on
the shear scene-type which would yield a slightly lower intensity
estimation.  Consequently, the initial intensity is held at 50 kt,
but it could certainly be a little stronger based on the
aforementioned polar low-orbiter pass.
 
The FV3/GFS and ECMWF SHIPS statistical-dynamical intensity models
show 20 to 30 kt of northeasterly shear persisting through the next
few days, however, the UW-CIMSS shear analysis reveals less than 20
kt.  For now, based on what the large-scale models and the SHIPS
models agree on, gradual weakening should begin by early Saturday
morning and continue through Monday morning as the cyclone
traverses decreasing oceanic temperatures and moves into a more
thermodynamically stable surrounding environment.  The NHC intensity
is an update of the previous advisory, and calls for Iselle
to weaken to a tropical depression on Sunday and degenerate into a
remnant low on Sunday evening.
 
Based on the 0920 UTC AMSR2 pass, the initial position was adjusted
to the northwest of the previous position and the forward motion is
estimated to be northeastward, or 035/4 kt.  A northward direction
should commence by early Saturday morning, then a turn
north-northwestward to northwestward is forecast during the 48-60
hr period.  As Iselle continues to weaken and become a more
shallower system, a turn toward the west-northwest, well offshore
of the southern coast of the Baja California peninsula, is forecast
to occur Monday morning.  The NHC forecast is nudged to the left of
the previous track forecast due the adjusted position, and is based
on the HCCA and TVCE multi-model consensus aids.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  28/1500Z 18.3N 115.3W   50 KT  60 MPH
 12H  29/0000Z 19.2N 114.8W   50 KT  60 MPH
 24H  29/1200Z 20.6N 114.6W   45 KT  50 MPH
 36H  30/0000Z 22.2N 114.9W   35 KT  40 MPH
 48H  30/1200Z 23.5N 115.7W   30 KT  35 MPH
 60H  31/0000Z 24.1N 116.6W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  31/1200Z 24.4N 117.3W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  01/1200Z...DISSIPATED
 
$$
Forecaster Roberts
 
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