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Tropical Depression HARVEY


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Tropical Depression Harvey Discussion Number  10
NWS National Hurricane Center Miami FL       AL092017
500 PM EDT Sat Aug 19 2017

The cloud pattern of Harvey has continued to decay during the day,
at least in part due to 15-20 kt of northerly shear.  The convective
area near the center is neither very concentrated or curved, and
overall the pattern more resembles that of an open wave than a
tropical cyclone.  Based on the decay and data from the aircraft
mission this morning, the cyclone is downgraded to a tropical
depression.  Another Air Force Reserve Hurricane Hunter aircraft
will investigate the system this evening to see if the circulation
still exists.

The intensity forecast is problematic.  The current shear should
subside over the next 24 h, and the statistical guidance responds
to this by forecasting significant strengthening.  On the other
hand, the structure of the cyclone has decayed to the point where
it may not be able to take advantage of the better environment, as
suggested by the ECMWF and GFS. The intensity forecast follows the
trend of the previous forecast, albeit with lower intensities, in
showing gradual strengthening until landfall in Belize or Yucatan.
However, an alternative forecast scenario is that the system
degenerates to an open wave and is unable to regenerate during the
next 72 h.

The initial motion remains 275/19.  There is no change in the
forecast philosophy from the previous advisory, and there are only
minor tweaks to the forecast track.   A low- to mid-level ridge
extending across the western Atlantic should keep Harvey, or its
remnants, on a fast westward course across the Caribbean Sea for the
next 36 h. Thereafter, there should be a weakness in the ridge north
of Harvey caused by a strong mid/upper-level low currently seen in
water vapor imagery over the Gulf of Mexico.  This pattern should
cause a turn toward the west-northwest and a decrease in forward
speed. The track guidance remains in good agreement that Harvey
should pass near or just north of northeastern Honduras, and then
cross Belize and/or the Yucatan Peninsula into the Bay of Campeche.

A Tropical Storm Watch could be required for portions of the
northern coast of Honduras and northeastern Nicaragua tonight.  At
the present time, there is enough uncertainty about whether Harvey
will actually be a tropical storm in 48 h that a watch is not
warranted.

FORECAST POSITIONS AND MAX WINDS

INIT  19/2100Z 14.1N  70.0W   30 KT  35 MPH
 12H  20/0600Z 14.3N  73.0W   30 KT  35 MPH
 24H  20/1800Z 14.6N  77.0W   35 KT  40 MPH
 36H  21/0600Z 15.2N  80.5W   40 KT  45 MPH
 48H  21/1800Z 16.0N  83.8W   45 KT  50 MPH
 72H  22/1800Z 18.0N  89.0W   45 KT  50 MPH...INLAND
 96H  23/1800Z 19.0N  92.0W   30 KT  35 MPH...OVER WATER
120H  24/1800Z 19.5N  94.0W   40 KT  45 MPH

$$
Forecaster Beven

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