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Hurricane OLIVIA


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Hurricane Olivia Discussion Number  27...Corrected
NWS National Hurricane Center Miami FL       EP172018
200 AM PDT Fri Sep 07 2018

Corrected second paragraph about previous category-4 strength

Olivia's overall cloud structure in infrared and passive microwave
satellite imagery has changed little since the previous advisory,
except that the cloud tops have warmed significantly around the
well-defined, 20-nmi-diameter eye. As a result, both subjective and
objective intensity estimates have decreased markedly since Olivia
briefly gained category-4 strength on the previous advisory.
However, microwave imagery indicates that Olivia has evolved into an
annular hurricane, with only inner-core convection present and no
banding features. As a result, the initial intensity only been
decreased to 110 kt, which is a typical slower rate of weakening for
annular hurricanes.

The initial motion estimate is 290/13 kt. There is no significant
change to the previous forecast track or reasoning. Olivia is
expected to continue moving west-northwestward for the next 24-36
hours, followed by a westward motion as the hurricane nudges up
against a strong deep-layer subtropical ridge that stretches from
Mexico and the southwestern U.S. westward into the central Pacific.
The new NHC forecast track is essentially just an extension of the
previous advisory track and is down the middle of the very tightly
packed model guidance, closest to the speed of the TVCN consensus
track model.

As mentioned in the previous advisory discussion, the current
annular pattern indicates that Hurricane Olivia should weaken more
slowly than indicated by guidance, even though the cyclone will be
moving over slightly cooler SSTS of about 25.5 deg C by 36 h. The
low shear environment that the cyclone will be embedded in should
help to offset some of the negative effects of the cooler waters.
After that time, Olivia will move back over slightly warmer
waters and remain in a low shear environment, so a little leveling
off in the weakening process is forecast until 96 hours. On day 5,
however, southwesterly vertical wind shear of 15-20 kt is expected
to induce more significant weakening at that time. The official
intensity forecast is similar to, but higher than, the consensus
models, and more closely follows a blend of the dynamical models
HWRF and HMON.

FORECAST POSITIONS AND MAX WINDS

INIT  07/0900Z 19.6N 131.2W  110 KT 125 MPH
 12H  07/1800Z 20.2N 133.1W  100 KT 115 MPH
 24H  08/0600Z 20.9N 135.7W   90 KT 105 MPH
 36H  08/1800Z 21.4N 138.3W   80 KT  90 MPH
 48H  09/0600Z 21.7N 140.9W   75 KT  85 MPH
 72H  10/0600Z 22.0N 145.6W   70 KT  80 MPH
 96H  11/0600Z 21.9N 149.6W   65 KT  75 MPH
120H  12/0600Z 21.0N 153.5W   55 KT  65 MPH

$$
Forecaster Stewart

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