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HURRICANE DARBY DISCUSSION NUMBER  20
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP052016
800 AM PDT SAT JUL 16 2016

Darby continues to have a well-defined 20 n mi wide eye, which is
completely encircled by convective cloud tops as cold as -70C.
Satellite intensity estimates at 1200 UTC were rather conflicting.
Subjective Dvorak numbers increased from six hours ago, but at the
same time objective ADT estimates have decreased.  Given this
discrepancy, the initial intensity is kept at 90 kt.

Darby is beginning to move over cooler water, with the eye having
reached the 26C isotherm.  Although vertical shear over the cyclone
is expected to be low for the next few days, SSTs will be dropping
to as cold as 23C by 72 hours.  Therefore, gradual weakening is
anticipated to begin soon, with Darby likely becoming a tropical
storm in about 48 hours.  Increasing shear should then contribute to
a faster weakening rate after 48 hours.  The new NHC intensity
forecast is very close to the IVCN intensity consensus during the
entire forecast period, and it is a little lower than the previous
forecast from 36 hours and beyond.

The initial motion is west-northwestward, or 290/9 kt.  The
subtropical ridge to the north of Darby is in the process of
weakening due to an amplifying trough along the west coast of the
United States.  Therefore, Darby is expected to continue moving
west-northwestward for the next 24 hours.  After that time, the
weakening cyclone will come under the influence of lower-level
steering, causing it to move westward through the end of the
forecast period.  There is very little spread among the track
models, and the NHC track forecast is just an update of the
previous one.

FORECAST POSITIONS AND MAX WINDS

INIT  16/1500Z 17.8N 123.9W   90 KT 105 MPH
 12H  17/0000Z 18.2N 125.1W   85 KT 100 MPH
 24H  17/1200Z 18.6N 126.8W   75 KT  85 MPH
 36H  18/0000Z 18.9N 128.7W   65 KT  75 MPH
 48H  18/1200Z 19.1N 130.7W   60 KT  70 MPH
 72H  19/1200Z 19.7N 135.2W   50 KT  60 MPH
 96H  20/1200Z 19.9N 140.0W   40 KT  45 MPH
120H  21/1200Z 19.0N 145.0W   35 KT  40 MPH

$$
Forecaster Berg

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