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Tropical Storm Miriam Discussion Number   4
NWS National Hurricane Center Miami FL       EP152018
800 PM PDT Sun Aug 26 2018

The overall cloud pattern of Miriam has continued to gradually
improve this evening.  However, light northerly shear of about 10
kt, as analyzed by UW-CIMSS, seems to be preventing the cyclone from
strengthening at a more rapid pace.  Earlier ASCAT data and a more
recent partial AMSR overpass indicated that the low-level center of
Miriam is slightly displaced to the northwest of most of the deep
convection, but is still well-embedded within the cirrus canopy.  A
consensus of objective and subjective satellite intensity estimates
suggests that Miriam has strengthened a little since the last
advisory, so the initial intensity has been increased to 45 kt.

The tropical storm is still moving westward at around 12 kt, and all
indications are that this motion will continue for the next few
days.  By day 4, Miriam should turn northward as it reaches a break
in the subtropical ridge created by an extensive mid- to upper-level
low located over the central North Pacific.  The track guidance is
in good agreement on this general scenario, but still differs on the
exact timing and location of the northward turn.  The NHC forecast
therefore remains near the middle of the guidance envelope, and is
essentially a blend of the HCCA and TVCX consensus aids and the
previous forecast.

Steady strengthening is still likely for the next day or two. While
Miriam's structure does not appear to be conducive for rapid
intensification at the moment, this could change quickly and with
little warning due to the small size of the cyclone's inner-core.
The model spread increases from 36 h onward, with the dynamical
models indicating continued intensification to major hurricane
strength, while the statistical guidance is much lower. Regardless
of Miriam's peak intensity, by the end of the forecast, steady
weakening is likely as Miriam encounters lower SSTs and higher
shear.  The NHC forecast continues to favor the higher dynamical
models for the first couple of days, and closely follows the
intensity consensus after that.

FORECAST POSITIONS AND MAX WINDS

INIT  27/0300Z 13.8N 127.9W   45 KT  50 MPH
 12H  27/1200Z 13.9N 129.7W   55 KT  65 MPH
 24H  28/0000Z 13.7N 131.9W   65 KT  75 MPH
 36H  28/1200Z 13.8N 134.0W   80 KT  90 MPH
 48H  29/0000Z 13.9N 136.0W   90 KT 105 MPH
 72H  30/0000Z 14.3N 139.5W   90 KT 105 MPH
 96H  31/0000Z 16.8N 141.3W   80 KT  90 MPH
120H  01/0000Z 21.0N 141.5W   65 KT  75 MPH

$$
Forecaster Zelinsky

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