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Tropical Storm Miriam Discussion Number  10
NWS National Hurricane Center Miami FL       EP152018
800 AM PDT Tue Aug 28 2018

A very recently arriving high-resolution microwave overpass
indicates that Miriam is a sheared tropical cyclone.  The GPM pass
shows that the center of Miriam is located well northwest of the
primary mass of deep convection due to moderate northwesterly
shear.  The center of the cyclone is located well west of the
earlier position estimates, which has also required a re-location.
The most recent Dvorak estimates from TAFB and SAB, as well as
UW-CIMSS/ADT, support an initial intensity, however, this could be
generous given the recent microwave data.

Environmental conditions are not expected to change over the next
24 hours, with possibly a slight reduction in the shear in 24 to 48
hours.  This would suggest that Miriam should only gradually
strengthen over the next couple of days and it is likely to reach
its peak intensity in 2-3 days.  After that time, increasing
southwesterly shear and cooler waters are expected to initiate
weakening.  By 96 hours, the shear is forecast to become quite
strong and Miriam will be moving over sub 26C SSTs. These
progressively hostile conditions should cause Miriam to become
a post-tropical cyclone by 120 hours. The latest intensity forecast
is close to the FSU Superensemble, and generally in between the ICON
and HCCA consensus models.

The initial motion continues to be due westward, or 270/12 kt,
as Miriam continues to be steered around the southern periphery of
a subtropical ridge.  A mid- to upper-level trough northeast of
Hawaii will weaken the western periphery of the ridge over the next
couple of days, and Miriam is forecast to turn northwest then
north-northwest, between the ridge and the aforementioned trough.
Forecast models diverge regarding the forward speed of Miriam
at days 3-5, depending how they handle the interaction of Miriam
with the the trough to its northwest. The ECMWF continues to favor
an accelerating northward track as Miriam gets caught in strong
southerly flow on the eastern side of the trough. The GFS is much
farther southwestward, as it appears to take a more shallow Miriam
westward later in the forecast period.  Since the consensus
aids remain clustered between these two scenarios, the latest
forecast NHC track is once again near the various consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT  28/1500Z 14.2N 135.4W   55 KT  65 MPH
 12H  29/0000Z 14.2N 136.9W   55 KT  65 MPH
 24H  29/1200Z 14.3N 138.9W   60 KT  70 MPH
 36H  30/0000Z 14.7N 140.4W   65 KT  75 MPH
 48H  30/1200Z 15.6N 141.3W   75 KT  85 MPH
 72H  31/1200Z 18.5N 141.8W   75 KT  85 MPH
 96H  01/1200Z 22.0N 142.6W   50 KT  60 MPH
120H  02/1200Z 26.0N 144.8W   35 KT  40 MPH...POST-TROPICAL

$$
Forecaster Brown/Latto

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