423 
 WTPZ44 KNHC 171446
 TCDEP4
 
 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