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Tropical Depression FOUR-E Forecast Discussion

Home   Public Adv   Archive  

WTPZ44 KNHC 080244

800 PM PDT TUE JUL 07 2015

Visible satellite images late this afternoon indicate that curved
bands of convection have developed around the center of the
well-defined low pressure system located more than 1000 n mi
east-southeast of the Hawaiian Islands. Therefore the system now
qualifies as a tropical cyclone, and advisories are being initiated
at this time. SAB/TAFB Dvorak satellite intensity estimates range
from T1.5/25 kt to T2.5/35 kt, and the system has a pattern T-number
of T2.0, so the initial intensity is set at 30 kt.

The initial motion estimate is 300/15 kt. During the next 72 hours,
the cyclone is forecast to move in a general west-northwestward
direction along the southern periphery of a strong deep-layer ridge
located to its north. By 96 hours, the system is expected to weaken
and become more vertically shallow, and be steered westward by the
low-level easterly trade wind flow. The models are in general
agreement through 72 hours, but then diverge significantly after
that, with most of the NHC guidance moving the cyclone or its
remnants west-northwestward to northwestward at 96 and 120 hours.
The exception is the ECMWF model, which turns the system
west-southwestward by 96 hours and beyond. The official forecast
track is similar to the consensus model TVCE through 72 hours, and
then follows the ECMWF trend after that since this model maintains
a larger and more realistic vortex on days 4 and 5.

The SHIPS guidance indicates that the deep-layer vertical wind
shear is expected to remain below 10 kt while the system is over 26C
or greater sea-surface temperatures for the next 24 hours or so,
which should allow for gradual strengthening into a tropical storm.
By 48 hours, the shear is forecast to increase to more than 20 kt
from the southwest, which should cap any intensification and induce
a steady weakening trend after that. However, the GFS-based SHIPS
model is forecasting stronger vertical shear than the ECMWF model
and, as a result, shows complete dissipation of the cyclone by 96
hours. Given the reliability of the ECMWF, the official intensity
forecast has incorporated a blend of these two models' shear
computations, and maintains the system as a tropical cyclone through
96 hours, and a remnant low at 120 hours. This scenario seems more
likely given the rather large size of the circulation, which will
make the vortex more shear resistant and also take longer to spin
down and dissipate.

The depression has crossed 140W longitude as of the 0300 UTC
advisory time, so this will be the last advisory issued by the
National Hurricane Center on this system. Future information on the
depression will be provided by the Central Pacific Hurricane Center
in Honolulu Hawaii.


INIT  08/0300Z 15.4N 140.2W   30 KT  35 MPH
 12H  08/1200Z 16.6N 142.2W   35 KT  40 MPH
 24H  09/0000Z 17.6N 144.5W   40 KT  45 MPH
 36H  09/1200Z 18.6N 146.5W   45 KT  50 MPH
 48H  10/0000Z 19.6N 148.3W   45 KT  50 MPH
 72H  11/0000Z 21.5N 152.2W   35 KT  40 MPH
 96H  12/0000Z 22.7N 156.3W   30 KT  35 MPH
120H  13/0000Z 23.0N 160.0W   25 KT  30 MPH...POST-TROP/REMNT LOW

Forecaster Stewart

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Page last modified: Wednesday, 08-Jul-2015 02:44:53 UTC