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Tropical Storm Miriam Discussion Number  11
NWS National Hurricane Center Miami FL       EP152018
200 PM PDT Tue Aug 28 2018

Northwesterly shear has taken a toll on Miriam, with visible
satellite imagery depicting an exposed low-level center located
northwest of the deep convection.  Subjective Dvorak T-numbers have
decreased to 3.0 by both SAB and TAFB, and a blend of the T- and
CI-numbers yields an initial intensity of 50 kt for this advisory.

Model guidance suggests that the northwesterly shear that is
currently affecting the cyclone will continue for at least the
next 24 hours, but it could diminish slightly by late Wednesday,
allowing for some modest strengthening in 36 to 48 hours.
Thereafter, a mid- to upper-level trough to the northwest of the
cyclone is expected to bring about increasing southwesterly shear
over the system, which is expected to cause steady weakening in 3
to 4 days.  The cyclone is then expected to become a post-tropical
cyclone by 120 hours as shear increases even further, and the system
crosses cooler SSTs.  The official intensity forecast has been
lowered for much of the forecast period, reflecting both the
lowering of the initial intensity and the expected continuation of
the shear.  Although not explicitly shown in the official forecast,
there is still some chance that Miriam could reach hurricane status
in a couple of days, before the shear increases.

The initial motion continues to be due west, or 270/11 kt,
as Miriam continues to be steered around the southern periphery of
a subtropical ridge. The track forecast philosophy for the next few
days remains unchanged as the aforementioned mid- to upper-level
trough northeast of Hawaii weakens the western periphery of the
ridge, and Miriam is forecast to turn northwestward then
north-northwestward between the trough and ridge.  There continues
to be a large spread in the guidance by day 5, with the ECMWF and
UKMET taking a more vertically coherent system northward, while the
GFS, HWRF, and HMON turn Miriam westward as the system weakens and
becomes a more shallow system.  The latter scenario is beginning to
seem more plausible and the NHC track forecast was adjusted
southward and westward, but additional changes may be required
if future forecasts shown a faster rate of demise.


FORECAST POSITIONS AND MAX WINDS

INIT  28/2100Z 14.1N 136.4W   50 KT  60 MPH
 12H  29/0600Z 14.2N 138.0W   50 KT  60 MPH
 24H  29/1800Z 14.4N 139.8W   50 KT  60 MPH
 36H  30/0600Z 14.9N 141.1W   55 KT  65 MPH
 48H  30/1800Z 16.1N 141.7W   60 KT  70 MPH
 72H  31/1800Z 19.3N 142.0W   55 KT  65 MPH
 96H  01/1800Z 22.6N 143.1W   40 KT  45 MPH
120H  02/1800Z 26.0N 146.0W   30 KT  35 MPH...POST-TROP/REMNT LOW

$$
Forecaster Brown/Latto

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