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Tropical Storm Miriam Discussion Number   5
NWS National Hurricane Center Miami FL       EP152018
200 AM PDT Mon Aug 27 2018

Miriam's cloud pattern and overall convective organization have
continued to improve, with a tight comma-cloud pattern now evident
in infrared imagery.  Two recent ASCAT passes around 0600Z indicate
that the low-level center is now located just inside the northwest
quadrant of the convective comma head, and that the radius of
maximum winds (RMW) is still about 20 nmi that was ascertained from
earlier passive microwave low-level ring data. The ASCAT data
indicated peak winds of only 37 kt, but this is likely an
underestimate of Miriam's actual intensity due to a) the data having
been located near the swath edge and b) due to Miriam's small RMW.
The advisory intensity of 50 kt is based on a average of subjective
and objective satellite intensity estimates that range from T3.0/45
kt to T3.6/57 kt, respectively. The previous advisory's 34-kt wind
radii remain unchanged based on the aforementioned ASCAT wind data.

Miriam is now moving due west or 270/13 kt. There are no significant
changes to the previous forecast track or reasoning. The cyclone is
expected to maintain a general westward motion for the next 72 h as
Miriam moves along the southern periphery of a strong ridge to its
north. By 96 h and beyond, Miriam is forecast to move northward to
north-northwestward through a break in the subtropical ridge near
created by a strong mid- to upper-level trough/low forecast to drop
southward between 140W-150W from the upper-low's current position
over the central North Pacific. The latest model guidance has come
into better agreement on both the timing and location of the
northward turn, although noticeable speed differences still exist,
with the ECMWF and UKMET being the faster models. The NHC official
forecast track lies close to the previous advisory track, and is
just a tad north of the HCCA, FSSE, and TVCN consensus models.

Steady strengthening still appears likely for the next couple of
days. Miriam's upper-level outflow has improved some and is less
restricted in the northwest quadrant, a signal that the shear is
beginning to relax, which would support steady or significant
strengthening in the short term. By 36-48 h, however, the shear is
forecast by the global models to again increase to 15-20 kt from the
northwest, which should inhibit the intensification process during
that time. Around 72 h, the shear is expected to decrease yet again,
followed by another increase in the shear. Rather than showing
roller-coaster fluctuations, the NHC intensity forecast just calls
for steady strengthening through 48-72 h, followed by a gradual
weakening trend, which mirrors the previous advisory trend and is
close to an average of the HCCA and FSSE intensity models.

FORECAST POSITIONS AND MAX WINDS

INIT  27/0900Z 13.9N 129.2W   50 KT  60 MPH
 12H  27/1800Z 13.9N 130.9W   60 KT  70 MPH
 24H  28/0600Z 13.8N 133.0W   70 KT  80 MPH
 36H  28/1800Z 13.8N 135.1W   80 KT  90 MPH
 48H  29/0600Z 14.0N 137.1W   90 KT 105 MPH
 72H  30/0600Z 14.7N 140.2W   90 KT 105 MPH
 96H  31/0600Z 17.5N 141.6W   80 KT  90 MPH
120H  01/0600Z 22.0N 142.4W   55 KT  65 MPH

$$
Forecaster Stewart

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