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HURRICANE OLAF DISCUSSION NUMBER  17
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP192015
800 PM PDT SUN OCT 18 2015

Olaf's inner-core convective pattern has improved significantly
since the previous advisory. A cloud-filled eye about 15 n mi in
diameter has become apparent in visible, infrared, and passive
microwave satellite data since around 18/2300Z, and cloud tops have
cooled to below -80C almost encircling the eye during the past hour.
The initial intensity has been increased to 85 kt based on a blend
of Dvorak satellite intensity estimates of T5.0/90 kt from TAFB and
T4.5/77 kt from SAB. This intensity is supported by NHC AODT
satellite intensity estimates of T4.8/85 kt and T5.2/95 kt using
embedded-center and obscured-eye patterns, respectively.

The initial motion estimate is 275/10 kt based on microwave and
visible eye fixes. Other than a slight southward adjustment due to
the recent eye positions, the previous forecast track and reasoning
essentially remain unchanged. Olaf is expected to gradually round
the southwestern and western portion of a deep-layer subtropical
ridge, turning northward and slowing down considerably on days 4 and
5 as the hurricane moves into a weakness in the ridge. The westward
shift in the guidance as mentioned in the previous discussion
appears to have settled down on this latest cycle with the UKMET and
the other models now having moved closer together. The new NHC track
forecast has been adjusted slightly to the left of the previous
advisory track due to the more southward initial position, and lies
between the consensus models and the more westward UKMET solution.

Although Olaf has made a sharp intensity comeback, the proximity of
a pronounced dry slot that has worked its way completely around the
small inner-core convective region of the hurricane precludes
forecasting rapid intensification at this time. However, with the
vertical shear expected to be near or below 5 kt for the next 72
hours while Olaf remains over 29C or greater SSTs, at least steady
strengthening appears to be in order for the next 36 hours or so.
Although the LGEM intensity guidance brings Olaf to category 4
strength (115-120 kt) in the 36-72 h period, the official forecast
levels off the intensity at 110 kt due to the likelihood of one or
more eyewall replacement cycles occurring during that time. By days
4 and 5, a gradual weakening trend is forecast to occur as Olaf
moves into a region of much drier mid-level air with humidity values
less than 50 percent, along with cold upwelling occurring beneath
the cyclone due to the hurricane's expected slower forward speed of
around 5 kt.

FORECAST POSITIONS AND MAX WINDS

INIT  19/0300Z  9.7N 135.4W   85 KT 100 MPH
 12H  19/1200Z  9.9N 136.8W   95 KT 110 MPH
 24H  20/0000Z 10.3N 138.6W  105 KT 120 MPH
 36H  20/1200Z 11.0N 140.2W  110 KT 125 MPH
 48H  21/0000Z 11.9N 141.6W  110 KT 125 MPH
 72H  22/0000Z 13.7N 144.1W  110 KT 125 MPH
 96H  23/0000Z 16.1N 145.4W  100 KT 115 MPH
120H  24/0000Z 18.2N 145.4W   90 KT 105 MPH

$$
Forecaster Stewart

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