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Tropical Storm Rosa Discussion Number   5
NWS National Hurricane Center Miami FL       EP202018
300 AM MDT Wed Sep 26 2018

After steadily intensifying yesterday, Rosa's strengthening trend
has temporarily paused.  The tropical storm has a well-defined
curved band that wraps about three-quarters of the way around the
center.  However, some dry air appears to be intruding into the core
from the northeastern quadrant, which could be the reason why Rosa
has stopped strengthening for now.  The Dvorak classifications from
TAFB and SAB remain 3.5/55 kt, so the initial intensity is held at
that value.  This wind speed is also close to the latest SATCON
estimate from CIMSS at the University of Wisconsin.

A strengthening trend is expected to resume soon, and it seems
likely that Rosa will rapidly intensify during the next couple of
days while it remains over warm SSTs and in an environment of low
wind shear of 10 kt or less.  The NHC intensity forecast closely
follows the HCCA model, and brings Rosa to hurricane strength later
today and major hurricane strength on Thursday.  Beyond a few days,
steady weakening is forecast due to cooler SSTs, dry and stable air,
and an increase in southwesterly wind shear.

The tropical storm is moving west-northwestward at 7 kt, steered by
a mid-level ridge to its north.  A west to west-northwest motion at
about the same forward speed is expected for the next couple of days
as the pattern holds.  After that time, the ridge is expected to
break down as a large, deep-layer low moves toward the U.S. west
coast.  This change in the steering flow should cause Rosa to move
northwestward late Friday and Saturday and then northward late in
the weekend.  The models have come into a little better agreement
this cycle, and a slight eastward shift has been made at 96- and
120-hours based on this guidance.  This forecast lies fairly close
to the various consensus aids.

FORECAST POSITIONS AND MAX WINDS

INIT  26/0900Z 15.5N 110.4W   55 KT  65 MPH
 12H  26/1800Z 15.9N 111.5W   70 KT  80 MPH
 24H  27/0600Z 16.2N 113.2W   85 KT 100 MPH
 36H  27/1800Z 16.5N 115.0W  100 KT 115 MPH
 48H  28/0600Z 16.8N 116.7W  105 KT 120 MPH
 72H  29/0600Z 18.3N 119.0W  105 KT 120 MPH
 96H  30/0600Z 20.7N 120.3W   85 KT 100 MPH
120H  01/0600Z 23.0N 120.0W   65 KT  75 MPH

$$
Forecaster Cangialosi

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