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TROPICAL STORM OLAF DISCUSSION NUMBER  12
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP192015
200 PM PDT SAT OCT 17 2015

There appears to have been a temporary interruption to Olaf's
intensification this morning, which could be related to a couple of
different factors.  Microwave imagery from earlier suggested that
the cyclone ingested a tongue of relatively drier air.  There is
also moderate northwesterly shear still affecting the system.
Nevertheless, there are some signs that Olaf could be about to
resume an intensification.  The cyclone has maintained a small CDO,
but there is now greater definition to inner-core features than 12
to 24 hours ago, and the convective bands have lengthened and gained
greater curvature.  While satellite classifications remain a
consensus T3.0/45 kt at 1800 UTC, the initial intensity estimate of
50 kt from the previous advisory is maintained.

The initial motion estimate is 265/11.  The mid-level subtropical
ridge steering Olaf westward should weaken soon 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 west-northwestward course at a reduced forward speed
during the 2 to 3 days.  After about 72 hours, a large anticyclone
forming in the east-central subtropical Pacific near 140W is
forecast to shift eastward while a weakness develops along farther
west.  Global models show Olaf moving in the flow between these two
synoptic features, resulting in a northwestward and then north-
northwestward turn after 96 hours.  The guidance has shifted toward
the left again this cycle, especially later in the forecast period,
and the official track forecast is moved in that direction.  Since
most of the global models depicted Olaf farther north than where it
currently is, it would not be surprising if further leftward
adjustments are necessary.

Other than some northwesterly shear over Olaf during the next day or
so, the large-scale environment is conducive for Olaf to intensify.
The shear could slow the rate of intensification, but it would not
appear strong enough not to allow Olaf to strengthen at least at the
climatological rate of development of one Dvorak T-number per day.
A diminution of the shear by 36 hours, in combination with other
very conducive environmental factors, should allow Olaf to
strengthen to near major hurricane strength in 2 to 3 days.  Olaf
is also expected to become a much larger cyclone during the next
several days, with an impressive expansion of the wind field
depicted in global models.  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 new intensity forecast is identical to the previous one and is
closest to the FSU Superensemble output and near the upper edge of
the guidance envelope.


FORECAST POSITIONS AND MAX WINDS

INIT  17/2100Z  9.2N 130.5W   50 KT  60 MPH
 12H  18/0600Z  9.3N 131.8W   60 KT  70 MPH
 24H  18/1800Z  9.8N 133.5W   75 KT  85 MPH
 36H  19/0600Z 10.3N 135.2W   90 KT 105 MPH
 48H  19/1800Z 11.0N 136.9W  100 KT 115 MPH
 72H  20/1800Z 12.9N 139.9W  100 KT 115 MPH
 96H  21/1800Z 15.2N 142.0W   95 KT 110 MPH
120H  22/1800Z 17.3N 142.8W   85 KT 100 MPH

$$
Forecaster Kimberlain

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