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Tropical Storm Miriam Discussion Number  13
NWS National Hurricane Center Miami FL       EP152018
200 AM PDT Wed Aug 29 2018

At face value, Miriam seems to be getting a little better
organized.  Its convective canopy has been expanding during the
past few hours, and continuous lightning strokes have been detected
within a well-defined mid-level circulation.  However, a 0637 UTC
METOP-B microwave pass shows that Miriam remains a sheared cyclone,
with the low-level center running out ahead of the deep convection.
Still, with the expanding convection, Dvorak estimates from TAFB and
SAB have risen to T3.5, and the initial intensity is set at 55 kt.

Miriam is moving westward, or 275/11 kt, near the western end of
the subtropical ridge.  A deep-layer low located northeast of the
Hawaiian Islands is expected to cause Miriam to slow down and turn
sharply northwestward and northward during the next 48 hours.  The
models are in excellent agreement on this scenario.  After day 3,
however, there are significant differences among the guidance, with
the GFS taking a shallow cyclone almost due westward and the ECMWF
continuing to accelerate a deeper Miriam northward on the east side
of the deep-layer low.  With these models being the most acute
outliers, the NHC forecast track thinking continues to lie close to
the tight clustering of the Florida State Superensemble, HCCA, and
TVCX consensus aids.

There is also greater-than-normal uncertainty in the intensity
forecast.  The SHIPS and LGEM models show northwesterly shear
persisting over Miriam and only strengthen the cyclone slightly
during the next 24-36 hours.  The HWRF and HCCA models are still
showing more significant strengthening, making Miriam a hurricane
over the next day or two.  I was tempted to explicitly show Miriam
becoming a hurricane in the official forecast, but I decided to hold
off since the cyclone just hasn't been able to display an improved
structure as of yet.  Even if some strengthening does occur,
vertical shear is expected to increase substantially from 48 hours
onward, which will cause fast weakening and Miriam likely
degenerating into a remnant low by day 5.

FORECAST POSITIONS AND MAX WINDS

INIT  29/0900Z 14.2N 138.4W   55 KT  65 MPH
 12H  29/1800Z 14.4N 139.8W   60 KT  70 MPH
 24H  30/0600Z 14.9N 141.1W   60 KT  70 MPH
 36H  30/1800Z 15.8N 141.9W   60 KT  70 MPH
 48H  31/0600Z 17.2N 142.0W   60 KT  70 MPH
 72H  01/0600Z 20.4N 142.5W   50 KT  60 MPH
 96H  02/0600Z 23.0N 144.5W   35 KT  40 MPH
120H  03/0600Z 26.5N 148.0W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Berg

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