Tropical Storm POLO

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TROPICAL STORM POLO DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP172014
800 PM PDT THU SEP 18 2014

There has been little change in organization since the Hurricane
Hunters investigated the storm this afternoon.  A blend of
subjective Dvorak intensity estimates from TAFB and SAB, as well as
ADT numbers from UW-CIMSS, supports holding the intensity at 60 kt
for this advisory.  Not much change in strength is likely in the
short term.  However east-northeasterly shear, associated with a
large upper-level anticyclone anchored over northwestern Mexico, is
forecast to increase over Polo during the next several days.  This,
along with less favorable thermodynamic conditions, should lead to
gradual weakening of the storm through the forecast period.  The
official wind speed forecast is the same as the previous one.  This
is not far from the Decay-SHIPS guidance for the first couple of
days of the forecast period, and a little above it thereafter.

The motion continues northwestward and, at least temporarily, a
little faster or 320/8 kt.  Global models indicate that a ridge is
forecast to develop westward from a mid-troposphere high pressure
system over northern Mexico during the next several days.  This
should cause the track of Polo to gradually bend toward the
west-northwest. By late in the forecast period, the weakening
cyclone should be drifting westward in the shallow low-level flow.
The official track forecast has been nudged just a little northward
on account of the recent faster northwestward motion.  This is close
to a consensus of the GFS and ECMWF predictions, and slightly south
of the multi-model consensus TVCE.

Although the northward shift is not large, given the uncertainties
in the track and future wind radii, it is prudent to issued a
tropical storm watch for the extreme southern Baja California
peninsula at this time.

FORECAST POSITIONS AND MAX WINDS

INIT  19/0300Z 18.4N 106.6W   60 KT  70 MPH
 12H  19/1200Z 19.0N 107.1W   60 KT  70 MPH
 24H  20/0000Z 19.8N 108.1W   55 KT  65 MPH
 36H  20/1200Z 20.6N 109.3W   50 KT  60 MPH
 48H  21/0000Z 21.3N 110.5W   45 KT  50 MPH
 72H  22/0000Z 22.2N 113.1W   40 KT  45 MPH
 96H  23/0000Z 22.5N 115.5W   30 KT  35 MPH
120H  24/0000Z 22.5N 117.0W   25 KT  30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Pasch

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