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Tropical Depression TWELVE-E


800 PM PDT SUN AUG 17 2014

Visible satellite images indicate that the large area of low
pressure located several hundred miles southwest of Mexico has
acquired a well-defined center of circulation along with enough
organized deep convection to qualify as a tropical cyclone.  The
initial intensity is set to 30 kt in agreement with a 28-kt ASCAT
pass from 1754 UTC and a more recent 30 kt estimate from TAFB.

Environmental conditions would seem to favor intensification due to
warm waters ahead of the depression during the next few days and
relatively low shear.  The biggest negative factor is the initial
structure of the cyclone, with a large radius of maximum wind and
overall large sprawling circulation envelope.  Tropical cyclones
with this type of structure tend to intensify only gradually, and
this is reflected in the NHC wind speed prediction below.  The
model guidance is also in relatively good agreement with the
NHC forecast, with only the SHIPS model showing the depression
reaching hurricane strength.  The system should begin to weaken in a
few days when it moves over much colder waters, although it could
spin down more slowly than average due to its large size and a
mid-latitude trough interaction.

The best estimate of initial motion is 275/7.  The subtropical
ridge to the north of the cyclone is expected to remain for the next
day or so, causing the depression to take a westward, then
west-northwestward, course.  However, an unusually deep trough for
mid-August is forecast to dig just offshore of the southwestern
United States, which will likely steer the tropical cyclone
northward in two or three days, followed by a turn toward the
northwest at long range as a result of a weakness left by the
trough.   Model guidance is in poor agreement at days 4 and 5 due to
varying predicted strengths of the trough, with the ECMWF taking
the cyclone significantly farther to the north of the rest of the
models.  Since the ECMWF ensemble mean is very close to the model
consensus, for now the ECMWF operational solution is thought of as
an outlier, and the NHC forecast is near the consensus at all
forecast times.

It is worth noting that while the official forecast is well
offshore of the southwestern United States, model guidance does
suggest a surge of moisture is possible in that region by Thursday.
Please monitor products issued by your local National Weather
Service office for more details on a possible heavy rain threat.


INIT  18/0300Z 16.1N 116.6W   30 KT  35 MPH
 12H  18/1200Z 16.3N 117.7W   35 KT  40 MPH
 24H  19/0000Z 16.7N 118.9W   40 KT  45 MPH
 36H  19/1200Z 17.6N 119.5W   45 KT  50 MPH
 48H  20/0000Z 18.4N 119.7W   50 KT  60 MPH
 72H  21/0000Z 19.8N 120.0W   50 KT  60 MPH
 96H  22/0000Z 22.4N 121.0W   45 KT  50 MPH
120H  23/0000Z 26.5N 124.5W   35 KT  40 MPH...POST-TROPICAL

Forecaster Blake