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Tropical Storm POLO Forecast Discussion

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WTPZ42 KNHC 200834

200 AM PDT SAT SEP 20 2014

Polo continues to lose organization due to the effects of 25-30 kt
of easterly vertical wind shear.  The low-level center is exposed
to the northeast of the convection, and the convection has
decreased in both coverage and intensity over the last 24 hours.
In addition, water vapor imagery shows upper-level dry air moving
over the low-level center.  Satellite intensity estimates are 55 kt
from TAFB and 45 kt from SAB.  Based on these data and trends since
the last advisory, the initial intensity is reduced to a somewhat
uncertain 45 kt.

The initial motion is 315/6.  Polo should gradually turn west-
northwestward in the next 24-36 hours as a mid-level ridge builds
north of the cyclone.  Late in the forecast period, the dynamical
guidance shows the shallow remnant low turning south-southwestward
as a low-level ridge to the west becomes the dominant steering
mechanism.  During the first 48 hours, the new track forecast is
again a little to the north of the previous forecast, and it lies a
little to the south of the center of the guidance envelope.  After
that time, the new forecast is similar to the previous forecast.

The large-scale models forecast the shear to continue for the next
72 hours while Polo moves over cooler sea surface temperatures and
into a dryer air mass.  This combination should cause steady
weakening, with the cyclone forecast to weaken to a tropical
depression in about 36 hours and to degenerate into a remnant low by
72 hours.  The new intensity forecast is an update of the previous
advisory and is a blend of the SHIPS, LGEM, and intensity consensus

Based on the forecast track and wind radii, tropical storm force
winds are most likely to remain south of the Baja California
peninsula.  However, any deviation to the north of the forecast
track could bring tropical storm force winds to the southern Baja
California peninsula.


INIT  20/0900Z 20.5N 108.6W   45 KT  50 MPH
 12H  20/1800Z 21.1N 109.5W   40 KT  45 MPH
 24H  21/0600Z 21.8N 110.7W   35 KT  40 MPH
 36H  21/1800Z 22.2N 112.0W   30 KT  35 MPH
 48H  22/0600Z 22.5N 113.2W   25 KT  30 MPH
 72H  23/0600Z 22.5N 115.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  24/0600Z 21.5N 115.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  25/0600Z 20.5N 116.0W   20 KT  25 MPH...POST-TROP/REMNT LOW

Forecaster Beven

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Page last modified: Saturday, 20-Sep-2014 08:34:36 UTC