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Tropical Storm Miriam Discussion Number   6
NWS National Hurricane Center Miami FL       EP152018
800 AM PDT Mon Aug 27 2018

The convective structure of Miriam has become better organized
this morning, with a continued increase in overall banding.  An
earlier AMSR2 microwave overpass also revealed a well-defined band
over the southeastern and southern portions of the circulation but
there was some evidence of northwesterly shear with the center near
the northwestern edge of the primary convective band.  The initial
intensity has been increased to 55 kt, which is supported by a
Dvorak Data T-number of 3.5 from TAFB, earlier AMSU data, and
recent UW/CIMSS ADT estimates of T3.7 (59 kt).

Satellite fixes indicate that Miriam is moving westward or 270
degrees at 12 kt.  The cyclone should remain on a westward heading
over the next 48 hours while it is steered by a large deep-layer
ridge to the north.  After that time, a large mid- to upper-level
low to the northeast of the Hawaiian Islands is expected to produce
a break in the ridge between 140W-150W.  This should cause Miriam to
turn northwestward, then north-northwestward between days 3 through
5.  The track guidance is in good agreement on this scenario, but
there are some differences in Miriam's forward speed after
recurvature begins.  The ECMWF and UKMET take Miriam much faster
northward than the remainder of the dynamical models, and the NHC
track forecast is once again close to the various consensus aids at
72 h and beyond to account for the speed differences.

The global models suggest that there will be a gradual increase in
northwesterly shear over Miriam during the next day or so, however,
this is not expected to prevent the cyclone from becoming a
hurricane within the next 24 hours.  The moderate shear is forecast
to relax by Tuesday night, which should allow for additional
intensification.  The intensity guidance is not quite as aggressive
as before, and the NHC intensity forecast, which lies between the
IVCN intensity consensus model and the HFIP corrected consensus, has
been adjusted slightly downward.  Increasingly southwesterly
vertical wind shear and cooler waters are expected to cause Miriam
to weaken late in the forecast period.


FORECAST POSITIONS AND MAX WINDS

INIT  27/1500Z 13.9N 130.5W   55 KT  65 MPH
 12H  28/0000Z 13.9N 132.3W   65 KT  75 MPH
 24H  28/1200Z 13.8N 134.3W   70 KT  80 MPH
 36H  29/0000Z 13.9N 136.3W   75 KT  85 MPH
 48H  29/1200Z 14.1N 138.1W   85 KT 100 MPH
 72H  30/1200Z 15.5N 140.8W   85 KT 100 MPH
 96H  31/1200Z 18.7N 141.8W   75 KT  85 MPH
120H  01/1200Z 23.3N 143.2W   55 KT  65 MPH

$$
Forecaster Brown

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