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Tropical Storm Miriam Discussion Number   9
NWS National Hurricane Center Miami FL       EP152018
200 AM PDT Tue Aug 28 2018

Miriam's cloud pattern has still not improved very much, with
microwave data showing the low-level center still slightly
displaced from the deep convection due to about 10 kt of
northwesterly shear.  Dvorak estimates from TAFB and SAB, as well
as the UW-CIMSS SATCON, all remain near 55 kt, which will remain
Miriam's initial intensity.  Miriam is expected to remain over warm
waters and in a relatively low-shear environment for the next 48-72
hours, which should allow the cyclone to strengthen modestly over
the next few days.  At 72 hours and beyond, Miriam will encounter
significantly stronger shear ahead of an amplifying mid- to
upper-level trough northeast of the Hawaiian Islands, and the
cyclone will also move over sub-26C waters.  These conditions should
cause a rather quick weakening trend on days 4 and 5, with Miriam
barely hanging on as a tropical storm by the end of the forecast
period.  The intensity guidance has decreased somewhat on this
forecast cycle, probably because Miriam has less time before it
reaches stronger shear.  The updated NHC intensity forecast lies
between the HCCA guidance and the IVCN intensity consensus, with the
forecast peak intensity just slightly below that of the previous
advisory.

The initial motion is due westward, or 270/10 kt, with Miriam
located along the southern periphery of the subtropical ridge.  The
amplifying trough northeast of Hawaii is breaking down the ridge and
should cause Miriam to turn sharply toward the northwest and north
between days 2 and 4.  There are significant speed differences among
the models by days 4 and 5, related to how deep Miriam is when it
interacts with the deep-layer trough.  The ECMWF, which maintains a
deeper, stronger vortex, shoots Miriam quickly northward, ending up
more than 600-700 nm north of where the GFS and HWRF models have the
cyclone by day 5.  Given where the consensus aids lie between those
two extremes, the updated NHC track forecast has been slowed down a
bit from the previous forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  28/0900Z 14.0N 133.5W   55 KT  65 MPH
 12H  28/1800Z 14.0N 135.1W   60 KT  70 MPH
 24H  29/0600Z 14.1N 137.1W   70 KT  80 MPH
 36H  29/1800Z 14.4N 138.8W   75 KT  85 MPH
 48H  30/0600Z 15.1N 140.1W   80 KT  90 MPH
 72H  31/0600Z 17.9N 141.0W   80 KT  90 MPH
 96H  01/0600Z 21.5N 141.5W   55 KT  65 MPH
120H  02/0600Z 26.0N 144.0W   35 KT  40 MPH

$$
Forecaster Berg

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