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Tropical Storm POLO Forecast Discussion

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WTPZ42 KNHC 170851

200 AM PDT WED SEP 17 2014

The overall organization of Polo has changed little in conventional
satellite imagery during the past few hours.  However, an earlier
SSMIS microwave overpass showed a low-level ring, which suggests
that the inner-core is becoming better defined.  The latest Dvorak
estimates from TAFB and UW/CIMSS still support an initial intensity
of 45 kt.  Recent satellite imagery shows that upper-level outflow
is becoming better established over Polo.  With the improved
inner-core structure and low shear forecast, steady strengthening is
expected during the next day or so.  The statistical guidance
predicts less intensification than previously, but this could be due
to the lack of strengthening during the past 6 hours and its affect
on the persistence factor in the SHIPS model. Therefore, the new NHC
intensity forecast is similar to the previous one through 24 hours,
but shows a slightly lower peak to be in better agreement with the
intensity consensus.  In a couple of days, strong upper-level
easterly winds are expected to cause an increase in shear, which is
likely to halt further intensification. Weakening is expected later
in the period, due to the shear and the cyclone moving over
decreasing sea surface temperatures.

The initial motion estimate is 315/10 kt.  Polo is forecast to move
northwestward during the next few days around the southwestern
portion of a mid-level ridge over northeastern Mexico.  Later in
the period, the cyclone is expected to turn west-northwestward while
a ridge builds to its north.   Although the model guidance is in
general agreement on this scenario, there are large differences in
how sharp Polo turns west-northwestward.  The ECMWF and GFS show an
earlier west-northwestward turn and are along the southern edge of
the guidance envelope, meanwhile the UKMET and GFDL are on the
northern side.  For now, the NHC forecast lies near the latest
multi-model consensus, which is also similar to the previous NHC
forecast track.


INIT  17/0900Z 14.8N 101.8W   45 KT  50 MPH
 12H  17/1800Z 15.8N 102.9W   55 KT  65 MPH
 24H  18/0600Z 16.9N 104.3W   70 KT  80 MPH
 36H  18/1800Z 17.7N 105.7W   75 KT  85 MPH
 48H  19/0600Z 18.5N 106.8W   75 KT  85 MPH
 72H  20/0600Z 19.9N 108.8W   70 KT  80 MPH
 96H  21/0600Z 21.1N 111.3W   60 KT  70 MPH
120H  22/0600Z 22.0N 113.5W   50 KT  60 MPH

Forecaster Brown

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Page last modified: Wednesday, 17-Sep-2014 08:51:54 UTC