Tropical Depression FIFTEEN-E

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TROPICAL DEPRESSION FIFTEEN-E DISCUSSION NUMBER   1
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP152014
200 AM PDT WED SEP 10 2014

Deep convection has increased in association with the area of low
pressure located southwest of Acapulco, and the system is now
designated a tropical depression. The convective pattern consists of
a large area of tops colder than -80C west of the center and some
curved bands forming to the north. The initial intensity of 30 kt is
based on data from a pair of ASCAT passes between 0300 and 0500 UTC.
The SHIPS model shows moderate easterly to northeasterly shear over
the cyclone, consistent with the location of the low-level center on
the eastern edge of the convective canopy as shown by the ASCAT
data.

The shear should continue for the next 24 to 36 hours, which
should result in gradual strengthening as the cyclone is situated
over very warm SSTs around 30C. By 48 hours, the shear is expected
to decrease, allowing the cyclone to take better advantage of the
favorable oceanic conditions and intensify more quickly. There is
considerable spread in the intensity guidance. The SHIPS model is
the most aggressive, showing the system reaching 90 kt by 72 hours.
The HWRF shows quick strengthening in the short term, but then
weakens the cyclone due to land interaction. The GFDL model also
shows land interaction and a weaker solution. Given that the NHC
track keeps the center offshore, the official intensity forecast
will be above the intensity consensus but below the SHIPS model,
showing the cyclone reaching 70 kt in 72 hours and peaking at 80 kt
at days 4 and 5.

The initial motion is a north-northwestward drift, as the depression
is situated in a region of weak steering south of a mid-level ridge.
The western part of the ridge will gradually erode over the next 48
to 72 hours, which should allow the deepening cyclone to begin
moving slowly northwestward to north-northwestward. Later in the
period, the cyclone should accelerate northwestward as the ridge
strengthens to the east. While overall the guidance is in general
agreement on this scenario, there are significant detail differences
that will determine how close the system comes to the coast of
southwestern Mexico in 72 to 96 hours. The ECMWF shows more
interaction with an upper-low retrograding westward across Mexico
and pulls the cyclone farther to the east and closer to the coast.
On the other hand, the GFS has a track farther offshore showing
less influence from the upper-level low and more interaction with
the disturbance currently situated about 1,000 miles west-southwest
of the depression. The NHC forecast is close to a blend of the GFS
and ECMWF models through the period and is a little slower than and
left of the TVCE consensus at days 4 and 5. Given the present weak
steering currents and and spread of the model guidance, confidence
in the details of the track forecast is lower than usual.

A tropical storm watch may be needed for a portion of the coast of
southwestern Mexico later today.

FORECAST POSITIONS AND MAX WINDS

INIT  10/0900Z 14.4N 102.5W   30 KT  35 MPH
 12H  10/1800Z 14.5N 102.5W   40 KT  45 MPH
 24H  11/0600Z 14.7N 102.6W   45 KT  50 MPH
 36H  11/1800Z 15.0N 102.7W   50 KT  60 MPH
 48H  12/0600Z 15.4N 102.8W   60 KT  70 MPH
 72H  13/0600Z 16.9N 103.6W   70 KT  80 MPH
 96H  14/0600Z 20.0N 106.5W   80 KT  90 MPH
120H  15/0600Z 23.0N 109.5W   80 KT  90 MPH

$$
Forecaster Brennan

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