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Tropical Storm Tara Discussion Number   4
NWS National Hurricane Center Miami FL       EP222018
400 AM CDT Mon Oct 15 2018

The satellite presentation of the tropical cyclone has improved
somewhat overnight, with convection continuing to burst over the
western and northwestern portions of the circulation.  The center
is slightly more embedded within the southeastern portion of the
cold cloud tops, but easterly shear continues to restrict
convection over the eastern portion of the system.  A couple of
ASCAT passes around 0400 UTC were helpful in locating the center of
the cyclone and both indicated peak winds of 25 to 30 kt to the
west of the center.  Dvorak classifications at 0600 UTC were
T2.5 (35 kt) from both SAB and TAFB, and given the increase in
organization, the system has been upgraded to a 35-kt tropical
storm. The nineteenth named storm of the 2018 eastern North Pacific
hurricane season.

Recent microwave imagery and the ASCAT data show that Tara has not
moved much, and the initial motion estimate is stationary for
this advisory.  Although the tropical storm is located within an
area of weak steering currents and little overall motion is
expected during the next couple of days, the track guidance has
become much more divergent this cycle.  The GFS and UKMET models
have trended northward and now bring Tara near the southwestern
coast of Mexico within the next day or so.  On the other hand, the
ECMWF takes Tara southeastward and southward away from the coast.
The various consensus aids have trended northward in the
short-term, and the new NHC track forecast has been nudged in that
direction through 36 hours, but it is not as far north as the GFS,
GFS ensemble mean, and UKMET models.  After 48 hours, a ridge is
forecast to build westward across central Mexico, which should cause
Tara to turn west-northwestward at a slightly faster forward speed.
Given the spread in the guidance and its recent trends, the track
forecast is of low confidence.

The intensity forecast is also very challenging and of low
confidence this morning. Tara is expected to remain under the
influence of easterly shear during the next day or two.  The shear,
however, is not expected to be strong enough to prevent gradual
strengthening.  After that time, the intensity forecast depends on
how close to the coast the cyclone moves.  If Tara gets too close to
southwestern Mexico, the small cyclone is likely to weaken.  If it
stays offshore, however, the shear is forecast to decrease which
should allow for additional strengthening.  Since the NHC track
forecast keeps Tara offshore, it calls for gradual strengthening
through 72 hours, but it is not as aggressive as before since most
of the guidance is lower than before.  Increasing southwesterly
shear by days 4 and 5 should lead to weakening, and although the NHC
forecast keeps Tara a tropical storm trough the period, most of the
global models weaken and dissipate the system by the end of the
week.

Due to the uncertainties in the track forecast, the government of
Mexico has issued a Tropical Storm Watch for a portion of the coast
of southwest Mexico.  Regardless of how close Tara tracks to
southwestern Mexico, heavy rainfall will be the primary threat along
the coast of southwestern Mexico due to the system's slow motion,
and life-threatening flash flooding will be possible in mountainous
areas.


FORECAST POSITIONS AND MAX WINDS

INIT  15/0900Z 17.5N 104.2W   35 KT  40 MPH
 12H  15/1800Z 17.8N 104.4W   40 KT  45 MPH
 24H  16/0600Z 17.9N 104.5W   45 KT  50 MPH
 36H  16/1800Z 17.9N 104.7W   45 KT  50 MPH
 48H  17/0600Z 18.0N 105.3W   50 KT  60 MPH
 72H  18/0600Z 18.3N 106.5W   55 KT  65 MPH
 96H  19/0600Z 19.0N 108.0W   50 KT  60 MPH
120H  20/0600Z 19.0N 108.0W   45 KT  50 MPH

$$
Forecaster Brown

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