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Tropical Storm Miriam Discussion Number  12
NWS National Hurricane Center Miami FL       EP152018
800 PM PDT Tue Aug 28 2018

There's been little change in the overall cloud pattern of Miriam
during the past several hours.  An earlier Advanced Technology
Microwave Sounder (ATMS) overpass and a 2053 UTC GCOM-W1 AMSR2 image
indicate that the surface center is still sheared to the northwest
of the deep convective banding feature encompassing the southern
portion of the cyclone.  Dvorak satellite intensity estimates from
TAFB and SAB remain unchanged, and a 2213 UTC SATCON analysis
showed 51 kt.  Based on these data, the initial intensity is held
at 50 kt.

The intensity forecast is rather problematic this evening.
Decay SHIPS guidance (both GFS and ECMWF background fields) shows
the moderate northwesterly shear increasing after 48 hours, while
indicating little change in strength up to that period.  After that
time, these statistical/dynamical guidance models quickly degenerate
the cyclone into a remnant low in 4 days.  The NOAA-HCCA, HWRF, and
the IVCN consensus model, on the other hand, all show Miriam
becoming a hurricane in 36-48 hours.  For this advisory, I've
elected to maintain continuity and show gradual strengthening to
just below hurricane strength in 36 hours, with a weakening trend
commencing in 3 days.  All the deterministic models agree with
Miriam becoming a remnant low in 5 days, and this is reflected in
the NHC forecast.

The initial motion is estimated to be westward, or 270/11 kt,
within the easterly mid-level steering flow produced by a
mid-tropospheric high to the north of Miriam.  The cyclone is
expected to turn toward the northwest in 36 hours, then
rather abruptly north-northwestward in 2 days and continuing on
through day 4, in response to a large mid- to upper level cutoff low
digging southwestward toward the Hawaiian Islands.  A large spread
in the models still exists in the latter portion of the forecast, as
mentioned in the previous advisory.  The track forecast philosophy
remains unchanged with a nudge more toward a blend of the TVCN
consensus and the global models that show a gradual turn back toward
the northwest due to a shallower vertical structure near the end of
the period.

FORECAST POSITIONS AND MAX WINDS

INIT  29/0300Z 14.1N 137.4W   50 KT  60 MPH
 12H  29/1200Z 14.2N 138.9W   50 KT  60 MPH
 24H  30/0000Z 14.5N 140.4W   55 KT  65 MPH
 36H  30/1200Z 15.4N 141.4W   60 KT  70 MPH
 48H  31/0000Z 16.7N 141.8W   60 KT  70 MPH
 72H  01/0000Z 20.2N 142.0W   55 KT  65 MPH
 96H  02/0000Z 23.4N 143.6W   40 KT  45 MPH
120H  03/0000Z 26.5N 146.7W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Roberts

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