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TROPICAL STORM OLAF DISCUSSION NUMBER  11
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP192015
800 AM PDT SAT OCT 17 2015

Olaf's cloud pattern has not exhibited much overall change since
the last advisory in conventional satellite imagery.  The cyclone
is characterized by a small CDO with one primary band over the
western half of the circulation, though the band is detached from
the main convective mass and fragmented.  The convective
distribution is suggestive of some northwesterly shear, as shown in
GFS- and ECMWF-based SHIPS output, but a partial 1030 UTC AMSU pass
indicated a mid-level eye, perhaps not completely coincident with
the low-level center.  Dvorak intensity estimates are consensus
T3.0/45 kt, but the initial intensity is increased to 50 kt on the
basis of the greater organization seen in microwave imagery.

The initial motion estimate is 270/11.  The mid-level subtropical
ridge steering Olaf westward is about to weaken in response to the
long tail of a mid- to upper-level trough near the U.S. west coast
settling southward to the north of the cyclone.  The weaker ridge
should induce a slower forward speed with a gradual increase in
latitude during the next couple of days. After 48 hours, a large
anticyclone forming in the east-central subtropical Pacific near
140W is forecast to shift eastward while a weakness develops along
150W.  Global models show Olaf moving in the flow between these two
synoptic features, which should result in a northwestward and then
north-northwestward turn after 96 hours.  The official track
forecast is nearly unchanged through 36 hours.  After that time, the
track forecast is shifted westward in agreement with a leftward-
shifting multi-model consensus and a majority of ECMWF ensemble
members that were farther west than the operational ECMWF solution
that lies east of the multi-model consensus.  The new track forecast
is on the western side of the guidance envelope.

Except for the northwesterly shear, there are no obvious impediments
to continued strengthening for the next few days. Exactly how much
the shear is a factor, however, remains to be seen, but its most
likely effect would be to possibly slow the rate of intensification.
According to the SHIPS output, the northwesterly shear diminishes
after 36 hours, and with all other large-scale factors favoring
intensification, Olaf is likely to become a major hurricane in 2
or 3 days.  Late in the forecast period, once Olaf gains enough
latitude, a drier environment with an increase in southerly or
south-southwesterly shear should result in weakening.  The official
intensity forecast is near the multi-model consensus through 36
hours and generally above it after that time.


FORECAST POSITIONS AND MAX WINDS

INIT  17/1500Z  9.4N 129.5W   50 KT  60 MPH
 12H  18/0000Z  9.5N 130.9W   60 KT  70 MPH
 24H  18/1200Z  9.8N 132.5W   75 KT  85 MPH
 36H  19/0000Z 10.3N 134.1W   90 KT 105 MPH
 48H  19/1200Z 11.0N 135.8W  100 KT 115 MPH
 72H  20/1200Z 12.6N 139.0W  100 KT 115 MPH
 96H  21/1200Z 14.7N 141.4W   95 KT 110 MPH
120H  22/1200Z 16.9N 142.5W   85 KT 100 MPH

$$
Forecaster Kimberlain

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