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Tropical Storm Tara Discussion Number   6
NWS National Hurricane Center Miami FL       EP222018
400 PM CDT Mon Oct 15 2018

Tara is now a tiny tropical storm consisting of a small, circular
area of intense convection that continues to go through bursting
phases. Two recent ASCAT scatterometer passes revealed that the
persistent cold overshooting cloud tops of -85C to -90C near the
center has apparently stretched the vortex column vertically,
causing the intensity to increase and wind field to shrink. The
34-kt wind radii have contracted down to about 20 nmi, and the
outermost circulation has decreased to only about 140 nmi wide. The
highest ASCAT wind speed value measured was 40 kt on both passes.
However, actual peak winds were likely undersampled given that they
were observed at a distance of only 10 nmi from the center. The
initial intensity has been increased to 45 kt based on a Dvorak
satellite intensity estimate of T3.0/45 kt from both TAFB and SAB,
an objective UW-CIMSS ADT estimate of T3.4/53 kt and a SATCON
estimate of 48 kt, and this estimate is probably conservative.

The initial motion estimate is an uncertain 305/02 kt. Tara's
center has likely re-developed closer to the strong convective
bursts that had been occuring just north of the previous low-level
center. As such, the exact direction and speed of the tiny tropical
cyclone is difficult to ascertain. Recent scatterometer and visible
satellite data suggest that Tara may have become stationary or is
making a tight loop. All of the global and regional models now keep
the tiny tropical cyclone offshore for the entire 120-h forecast
period, and the new NHC model guidance indicates that a slow,
erratic motion toward the west-northwest or northwest should
occur for the next five days while Tara remains embedded within a
weak steering flow regime. The new official track forecast follows
the trend of the previous few advisory tracks, and lies near a blend
of the HCCA, FSSE, and TVCE consensus models. However, due to the
continued large spread in the guidance, the track forecast is of
low to moderate confidence.

The intensity forecast is problematic due to Tara's very small
circulation since tiny tropical cyclones can both spin up or spin
down very quickly. Moderate easterly to southeasterly vertical wind
shear is forecast to continue for the next 24 hours or so, followed
by a pronounced decrease in the shear until 48-72 hours. These
conditions should allow for a slow but steady increase in the
intensity. However, during the 36-to-60-hour period when the shear
will be the lowest, more significant strengthening could occur due
to Tara's small circulation. However, none of the guidance brings
the tiny cyclone to hurricane strength. By days 4 and 5,
southwesterly wind shear is forecast to increase to around 20 kt,
which is expected to induce gradual weakening. Due to Tara's small
circulation, the official intensity forecast is slightly higher than
the consensus models HCCA, IVCN, and FSSE, and closely follows the
LGEM statistical-dynamical intensity forecast aid.

Regardless of how close Tara comes to southwestern Mexico, heavy
rainfall will continue to 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  15/2100Z 18.0N 104.5W   45 KT  50 MPH
 12H  16/0600Z 18.1N 104.7W   50 KT  60 MPH
 24H  16/1800Z 18.2N 104.9W   55 KT  65 MPH
 36H  17/0600Z 18.2N 105.4W   55 KT  65 MPH
 48H  17/1800Z 18.2N 106.0W   55 KT  65 MPH
 72H  18/1800Z 18.7N 107.3W   55 KT  65 MPH
 96H  19/1800Z 18.8N 108.2W   50 KT  60 MPH
120H  20/1800Z 18.5N 108.4W   45 KT  50 MPH

$$
Forecaster Stewart

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