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Tropical Storm POLO Forecast Discussion

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WTPZ42 KNHC 160909

200 AM PDT TUE SEP 16 2014

Scatterometer data from earlier this evening indicated that the
circulation of the low pressure area south of Mexico has become
better defined.  The ASCAT data showed several 35-kt wind vectors
over the southeastern portion of the circulation.  In addition, a
ship about 50 n mi south-southeast of the center reported winds to
near 30 kt, which was in good agreement with the ASCAT data.
Based on these data and the increase in organization of the
convection overnight, advisories are being initiated on Tropical
Storm Polo.  Polo becomes the sixteenth tropical storm in the
eastern Pacific basin this season.

The center of the tropical storm is located near the northeastern
edge of the deep convection due to about 15 to 20 kt of
northeasterly shear.  The shear is forecast to decrease somewhat
during the next couple of days, which should allow for gradual
strengthening.  After 72 hours, the global models suggest that
upper-level easterly winds could increase resulting in increasing
shear.  As a result, the NHC forecast shows little change in
strength late in the forecast period.  The NHC forecast is a little
below the statistical guidance and is close to the intensity
consensus, ICON.

The initial motion is a somewhat uncertain 295/8 kt.  Polo is
forecast to turn northwestward today and continue in that
general direction during the next couple days while it moves around
the southwestern portion of a mid-level ridge over the northern Gulf
of Mexico.  Although the global models generally agree on this
scenario, there are differences in how close Polo moves to the
southwestern coast of Mexico in a couple of days.  The ECMWF takes
Polo inland while most of the remainder of the guidance keeps the
tropical storm offshore.  The NHC forecast is near the middle of
the guidance envelope during the first 2 to 3 days.  After that
time, a building ridge over northern Mexico should turn the cyclone
west-northwestward.  Confidence in the track forecast, especially
late in the forecast period, is lower than normal.

Interests along the southwestern coast of Mexico should monitor the
progress of Polo.  A tropical storm watch may be required for a
portion of the coast of southwestern Mexico later today.


INIT  16/0900Z 11.9N  98.4W   35 KT  40 MPH
 12H  16/1800Z 12.8N  99.9W   40 KT  45 MPH
 24H  17/0600Z 14.1N 101.3W   45 KT  50 MPH
 36H  17/1800Z 15.5N 102.5W   50 KT  60 MPH
 48H  18/0600Z 16.9N 104.0W   55 KT  65 MPH
 72H  19/0600Z 18.8N 107.0W   60 KT  70 MPH
 96H  20/0600Z 19.7N 108.8W   60 KT  70 MPH
120H  21/0600Z 20.5N 110.8W   55 KT  65 MPH

Forecaster Brown

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Page last modified: Tuesday, 16-Sep-2014 09:09:10 UTC